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  1. #1
    Join Date
    Apr 2012
    Posts
    134

    Steve's 3x4' Gantry Mill

    Here is the design for my 3' x 4' x 6" steel gantry mill. Any feedback before I start would be great!

    Attachment 183342 Attachment 183344 Attachment 183346 Attachment 183348 Attachment 183350 Attachment 183352 Attachment 183354 Attachment 183356 Attachment 183358 Attachment 183360 Vice to gantry distance:Attachment 183362
    Z Back Plate:Attachment 183364 Attachment 183366 Attachment 183368
    Z Front / Spindle Plate: Attachment 183370 Attachment 183372 Attachment 183374
    Y Closeup: Attachment 183376 Attachment 183378
    Exploded: Attachment 183380

    This machine will be able to mill steel, aluminum, wood, plastic. This is a hobby (not production) machine for making various one-off parts. I expect to be doing a lot of 3D work. I plan to add a 4th axis at some point.

    Why this design? I started designing an aluminum CNC router over a year ago, and just wasn't happy that you just can't cut steel with a router. I had several aluminum designs, but didn't want to build them. After a while I decided to just keep making it more rigid until this 9th design. Gone is most of the aluminum, for rigidity and thermal expansion reasons. I settled on a 3x4' envelope because that's about as big as you can get with ball screws. The 3' wide cutting area makes the gantry narrow enough to be a reasonable weight for this rigidity. The sides (x rails) will be raised so that there will be virtually no sideways gantry motion. The Y ball screw supports are attached to the gantry bottom beam, so the top beam gets no sideways force. I have incorporated triangles into the structure wherever I can.

    Travel will be 37 x 49 x 12". There will be 6" clearance under the gantry, and 3" additional clearance if the bed is moved back. I plan to be able to mount a milling vise on the front horizontal beam. The spindle can move 1.5" past the front end of the bed so that I can cut dovetails. There will be lots of space between the linear blocks, so there will be no multiplication of forces. The Y Rails are 8" apart center to center. The Z rails are 5" apart.

    Construction: Welded steel (stress relieved gantry), concrete filling inside the beams, self-leveling epoxy.

    Rigidity: Horizontal deflection on one of the two 3x5" gantry beams is 0.00043 with 100 lbs force. This is for a bare beam, so it will be better when fully assembled. The top and bottom beams of the gantry will be joined by welded spacers. It will have a sheet metal skin on the back.

    Dampening: Concrete in various places. Cast Iron Z if I can.

    Accuracy: Linear rail beds will have self-leveling epoxy. The Z plates and gantry support bars will be ground. Linear blocks will have preload. Ball screws will have double ball nuts and AC bearings in the end support. I expect the accuracy of the ball screw to be the limiting factor, so .003 per ft. I expect that repeatability will be excellent. I will be using a KFLOP, so I may do glass slides and feedback later at least for the Z.

    Speed (rapids): around 400 ipm (using 1610 (not 1605) ball screws, and low impedance motors).
    Acceleration: Well.. A pair of 960 oz motors put out about 1300 LBS force at lower speeds... I'm not sure exactly how much gantry weight matters.

    Materials:
    Base: Front horizontal beam 3" x 5" x 3/8" steel, concrete filled, the rest 3" x 3" x 1/4" steel (750 LBS) + concrete. I'm thinking of maybe filling everything in the base with concrete. There are some recipes that include some aluminum powder to make the concrete expand slightly.

    Movable Bed: 36x48x3" consisting of steel chanel around the edges, a rebar grid, 1" square stock in a grid pattern (drilled and tapped for hold downs), concrete, epoxy coating. 300 lbs. I haven't seen a bed exactly like this. It may be a crazy idea. This will probably be the first thing I make after I pour an epoxy pad.

    Gantry (Y): Two 3" x 5" x .25" x 48" steel beams (5" is horizontal), 4" tall spacers welded between, sheet metal skin on back. 150 lbs. I am considering going up to 3/8" wall. I'm also considering pouring 2" wide concrete or EG bulkheads in the gantry beams.

    Z back plate: 1/2" ground cast iron or steel plate. Girders up the edges to control flex.

    Z front plate: mostly 1/2" cast iron. Box section. Z total weight (back,front,spindle,motor) 45+ lbs.

    Linear Components:
    CPC AR20 profile linear rails from Anaheim Automation.
    CPC-AR20MNBZV1P blocks, light (middle grade) preload, "Precision" (middle) grade.
    1610 Ball screws with double nuts from Linearmotion2008 (2 on the Y).
    BK12/BF12 end supports (will replace bearings with AC).

    Spindle:
    Ah.. the spindle... That's an issue. I would like one that can do 800-15k with a couple of pulley ratios.
    At minimum I will get an x2 R8 mini mill spindle from Littlemachineshop, and upgrade the bearings for 10k.

    I'm also thinking of something like Mactec54's ER16 (But ER20). The current design shows this type.

    Or dwalsh62's ATC spindles http://www.cnczone.com/forums/produc..._cartidge.html

    Spindle Motor (so far) would be a 2HP treadmill motor
    Motor controller: KBMM125

    This is interesting but risky:
    2 2KW CNC Router Metal Work Spindle Motor and Inverter | eBay

    Tormach spindles are also a possibility. A full Tormach head seems kind of heavy at 120 lbs.

    I'm also thinking of bolting on a router for cutting wood if the spindle isn't fast enough.

    Coolant:
    I'm thinking of using mist coolant when cutting metal. I'm not sure how to control flood on this machine.

    Dust control:
    Harbor Freight 2 HP

    Noise control:
    I'm thinking of building a wall with a door around the machine, air compressor, and dust control.

    Electronics:
    G201x Drivers from CNC Router Parts
    Breakout board
    960 oz/in, 2 ohm impedance NEMA 34 motors from CNC Router Parts. I will look into a kit from CNCRP when I get that far.
    KFLOP
    I have an old Dell computer

    Software:
    Alibre Design (have it)
    Possibly Rhino
    Meshcam
    Kmotion. Don't know if I can get by without MACH3.
    FsWizard
    Free software for Vcarving

    Tools:
    I have a Chop saw, drill press, stick welder, metal lathe, small granite surface plate, height gage, dial indicator, test indicator, digital calipers, Master machinist level .0002 per 10". I will purchase a granite triangle for aligning the machine. I plan on getting an engine hoist. I will pour a 6' square pad of self-leveling epoxy to do layout on, build the bed on, and to put the machine on.

    Work distribution:
    I plan to have the gantry spacers and side plates water jet cut. I expect to have the Z back and front plates cut, ground, and drilled by someone.
    It would be nice if I can get the tubing cut accurately.
    I will probably make the smaller metal parts.
    My brother and I will do the welding, aligning, drilling, tapping.

    Budget: $5-6k US

    Time frame: I will start after I get some feedback. I expect this to take up to year to build, mainly for financial reasons.

    -Steve

  2. #2
    Join Date
    May 2005
    Posts
    3920
    Quote Originally Posted by steve123 View Post
    Here is the design for my 3' x 4' x 6" steel gantry mill. Any feedback before I start would be great!
    Interesting design! I hope you don't take the feedback the wrong way.

    This machine will be able to mill steel, aluminum, wood, plastic. This is a hobby (not production) machine for making various one-off parts. I expect to be doing a lot of 3D work. I plan to add a 4th axis at some point.
    Will you have enough Z clearance for a fourth axis? Obviously that depends upon your intentions but you can quickly run into issues here with most router designs.
    Why this design? I started designing an aluminum CNC router over a year ago, and just wasn't happy that you just can't cut steel with a router.
    This is also a router is it not? As for cutting steel you need a machine stiff enough for the tooling you will be running and a spindle that can run at the right speed. It is a matter of design, routers can be designed to Handle steel.
    I had several aluminum designs, but didn't want to build them. After a while I decided to just keep making it more rigid until this 9th design. Gone is most of the aluminum, for rigidity and thermal expansion reasons. I settled on a 3x4' envelope because that's about as big as you can get with ball screws.
    You can get some pretty big ball screws so I'm not sure why that is a limitation.
    The 3' wide cutting area makes the gantry narrow enough to be a reasonable weight for this rigidity. The sides (x rails) will be raised so that there will be virtually no sideways gantry motion.
    That is only the case if X support is stiff enough. I'm puzzled a bit by why you went with a moving gantry design if your goal is stiffness. The common reason is space but yo seem to be concerned about the stiffness of your machine.
    The Y ball screw supports are attached to the gantry bottom beam, so the top beam gets no sideways force. I have incorporated triangles into the structure wherever I can.

    Travel will be 37 x 49 x 12". There will be 6" clearance under the gantry, and 3" additional clearance if the bed is moved back. I plan to be able to mount a milling vise on the front horizontal beam. The spindle can move 1.5" past the front end of the bed so that I can cut dovetails. There will be lots of space between the linear blocks, so there will be no multiplication of forces. The Y Rails are 8" apart center to center. The Z rails are 5" apart.
    I'm concerned about the Z capability and your interest in a fourth axis. The moving table might help there but then it limits what you can do with the fourth axis. Look at it this way add a fourth axis and you effectively have cut your usable Z in half. Actuall it is worst than that because of clearances. An alternative might be to have several beams under the table onto which you can mount vises or a fourth axis. With the fourth mounted on the table you could find yourself coming up short with capacity and suffering from excessive frustration.
    Construction: Welded steel (stress relieved gantry), concrete filling inside the beams, self-leveling epoxy.

    Rigidity: Horizontal deflection on one of the two 3x5" gantry beams is 0.00043 with 100 lbs force. This is for a bare beam, so it will be better when fully assembled. The top and bottom beams of the gantry will be joined by welded spacers. It will have a sheet metal skin on the back.
    Nice but will in vibrate? Deflection is one thing but it isn't in many cases a key issue.
    Dampening: Concrete in various places. Cast Iron Z if I can.

    Accuracy: Linear rail beds will have self-leveling epoxy. The Z plates and gantry support bars will be ground. Linear blocks will have preload. Ball screws will have double ball nuts and AC bearings in the end support. I expect the accuracy of the ball screw to be the limiting factor, so .003 per ft. I expect that repeatability will be excellent. I will be using a KFLOP, so I may do glass slides and feedback later at least for the Z.
    Feed back on KFlop will be greatly appreciated. I like the approach they have taken with the hardware and software but haven't seen one implemented locally and have yet to build my own machine.
    Speed (rapids): around 400 ipm (using 1610 (not 1605) ball screws, and low impedance motors).
    Acceleration: Well.. A pair of 960 oz motors put out about 1300 LBS force at lower speeds... I'm not sure exactly how much gantry weight matters.
    It matters.
    Materials:
    Base: Front horizontal beam 3" x 5" x 3/8" steel, concrete filled, the rest 3" x 3" x 1/4" steel (750 LBS) + concrete. I'm thinking of maybe filling everything in the base with concrete. There are some recipes that include some aluminum powder to make the concrete expand slightly.
    3x3 might be a bit on the thin side especially when it comes to getting good concrete fill. It isn't impossible though but it will be tedious. Think about how you will ram the concrete into long 3x3 sections or even 3x5 sections for that matter.

    This isn't so much a structural issue as it is a process issue. You will want good stiff concrete to avoid shrinkage associated with looser wet concretes but you also need to get good fill to avoid gaps. Further all of this crap gets heavy real fast and as such you need to be equipped to handle the parts. If you can answer the question of how do you pack concrete in to a 2.5 inch tube then go for it.
    Movable Bed: 36x48x3" consisting of steel chanel around the edges, a rebar grid, 1" square stock in a grid pattern (drilled and tapped for hold downs), concrete, epoxy coating. 300 lbs. I haven't seen a bed exactly like this. It may be a crazy idea. This will probably be the first thing I make after I pour an epoxy pad.
    Well as stated I'm not sure why you are going movable gantry in the first place. But then you also want a movable bed at 300 + pounds. Something just doesn't seem right.

    On the other hand you do have an interesting design. The interesting task will be getting it cast flat. One thing to look into is a book written a few years ago that covers making concrete counter tops. Sorry I don't have the author or title but such a book might give you a few ideas. Honestly though making the whole bed epoxy concrete might make more sense.
    Gantry (Y): Two 3" x 5" x .25" x 48" steel beams (5" is horizontal), 4" tall spacers welded between, sheet metal skin on back. 150 lbs. I am considering going up to 3/8" wall. I'm also considering pouring 2" wide concrete or EG bulkheads in the gantry beams.
    A number of pieces of equipment make use of welded steel beams for gantries. It isn't a bad approach but you will most likely have to have it stress relieved and finished milled for the linear rails. Structural steel isn't all that straight and any welding builds in stress that will come out sooner or later. This is probably the biggest negative with respect to steel structures.

    I'd also consider a square beam to allow for easier fill of the beam with concrete.

    Z back plate: 1/2" ground cast iron or steel plate. Girders up the edges to control flex.

    Z front plate: mostly 1/2" cast iron. Box section. Z total weight (back,front,spindle,motor) 45+ lbs.

    Linear Components:
    CPC AR20 profile linear rails from Anaheim Automation.
    CPC-AR20MNBZV1P blocks, light (middle grade) preload, "Precision" (middle) grade.
    1610 Ball screws with double nuts from Linearmotion2008 (2 on the Y).
    BK12/BF12 end supports (will replace bearings with AC).

    Spindle:
    There is no perfect spindle. So I'm gaining to suggest interchangeable spindles as A low cost solution. For example one head using an off the shelf router and another that is a more substantial spindle for lower speed work. Or you can co mount the spindles and switch as needed.

    I suppose if budget is of no concern you could install an industrial do it all spindle.
    Ah.. the spindle... That's an issue. I would like one that can do 800-15k with a couple of pulley ratios.
    At minimum I will get an x2 R8 mini mill spindle from Littlemachineshop, and upgrade the bearings for 10k.

    I'm also thinking of something like Mactec54's ER16 (But ER20). The current design shows this type.

    Or dwalsh62's ATC spindles http://www.cnczone.com/forums/produc..._cartidge.html

    Spindle Motor (so far) would be a 2HP treadmill motor
    Motor controller: KBMM125
    You can't be serious? After all the time and money that will go into this you want to use a tread mill motor. To be polite that is nuts. If you have spent all the time and effort to design this machine, take the time to design in a 3 phase motor at the very least or a more substantial servo / spindle drive.
    This is interesting but risky:
    2 2KW CNC Router Metal Work Spindle Motor and Inverter | eBay

    Tormach spindles are also a possibility. A full Tormach head seems kind of heavy at 120 lbs.

    I'm also thinking of bolting on a router for cutting wood if the spindle isn't fast enough.
    Yes! That is what I was alluding to at the start of this spindle section. Do a robust spindle for lower speed work and use a generic router for high speed work. It saves the expense of an high performance spindle.

    Coolant:
    I'm thinking of using mist coolant when cutting metal. I'm not sure how to control flood on this machine.

    Dust control:
    Harbor Freight 2 HP

    Noise control:
    I'm thinking of building a wall with a door around the machine, air compressor, and dust control.

    Electronics:
    G201x Drivers from CNC Router Parts
    Breakout board
    960 oz/in, 2 ohm impedance NEMA 34 motors from CNC Router Parts. I will look into a kit from CNCRP when I get that far.
    KFLOP
    I have an old Dell computer

    Software:
    Alibre Design (have it)
    Possibly Rhino
    Meshcam
    Kmotion. Don't know if I can get by without MACH3.
    FsWizard
    Free software for Vcarving

    Tools:
    I have a Chop saw, drill press, stick welder, metal lathe, small granite surface plate, height gage, dial indicator, test indicator, digital calipers, Master machinist level .0002 per 10". I will purchase a granite triangle for aligning the machine. I plan on getting an engine hoist. I will pour a 6' square pad of self-leveling epoxy to do layout on, build the bed on, and to put the machine on.

    Work distribution:
    I plan to have the gantry spacers and side plates water jet cut. I expect to have the Z back and front plates cut, ground, and drilled by someone.
    It would be nice if I can get the tubing cut accurately.
    I will probably make the smaller metal parts.
    My brother and I will do the welding, aligning, drilling, tapping.

    Budget: $5-6k US

    Time frame: I will start after I get some feedback. I expect this to take up to year to build, mainly for financial reasons.

    -Steve
    Lots of details that don't deal with the bigger design issues above. What you ultimately select control wise isn't all that important as upgrades here are easy and frankly the tech changes rapidly. The one nice thing about KFlop in this regards is the steady stream of improvements.

    From my standpoint I'd look a little Harder at the hard parts of the machine and define better your future wants such as that fourth axis. Six inches of Z axis clearance is great for many router applications but I see that fourth axis requirement as a potential problem. Also why the moving gantry if stiffness is an important goal. Nail down what is import at here before worrying about controls.

  3. #3
    Join Date
    Apr 2012
    Posts
    134
    Thanks for taking time Wizard.
    Interesting design! I hope you don't take the feedback the wrong way.
    It's all good.

    Will you have enough Z clearance for a fourth axis? Obviously that depends upon your intentions but you can quickly run into issues here with most router designs.
    It's a good question. How much is enough? I'm planning on pushing back the bed, and mounting the 4th axis on the front beam. That spot has 9" clearance under the gantry. The Z can go as high as 6" above that. I would love to hear from people that have a 4th.

    This is also a router is it not? As for cutting steel you need a machine stiff enough for the tooling you will be running and a spindle that can run at the right speed. It is a matter of design, routers can be designed to Handle steel.
    You could call it a router, but it's supposed to be much stronger than a wood router. I want it to be truly multi-purpose, not "gee I hope it can maybe cut steel".

    You can get some pretty big ball screws so I'm not sure why that is a limitation.
    The next size-up ball screw that doesn't have a tiny lead is 2510. Those require massive motors just to accelerate the screw. The 1610 size is limited to about the lengths I am using before you get too much wip.

    That is only the case if X support is stiff enough. I'm puzzled a bit by why you went with a moving gantry design if your goal is stiffness. The common reason is space but yo seem to be concerned about the stiffness of your machine.
    The moving gantry allows the 3x4' envelope. You see me talking about stiffness because I want to be able to cut steel. I want it all: space and stiff enough. I'm very aware that most of the advice on CNCZone is to avoid gantry mills. I still wants it.

    I'm concerned about the Z capability and your interest in a fourth axis. The moving table might help there but then it limits what you can do with the fourth axis. Look at it this way add a fourth axis and you effectively have cut your usable Z in half. Actuall it is worst than that because of clearances. An alternative might be to have several beams under the table onto which you can mount vises or a fourth axis. With the fourth mounted on the table you could find yourself coming up short with capacity and suffering from excessive frustration.
    I wasn't clear about the moving table/bed. The table can be moved back, but it isn't automated. The idea is to be able to push it back when I want to have more Z, lets say for the 4th axis, or for more clearance with a vice. These things could be temporary, or could be left there if other jobs don't need the 3x3' bed that's left. I'm thinking I might be lazy and leave a vice/4th mounted on the front beam.

    Nice but will in vibrate? Deflection is one thing but it isn't in many cases a key issue.
    I agree, but you have to start with stiffness. Then comes materials and wall thickness I think.

    Speed (rapids): around 400 ipm (using 1610 (not 1605) ball screws, and low impedance motors).
    Acceleration: Well.. A pair of 960 oz motors put out about 1300 LBS force at lower speeds... I'm not sure exactly how much gantry weight matters.
    It matters.
    Agreed. This is one of those things I have had problems finding an answer to. I know that the whole machine has to be strong enough to handle huge pushes of the motors when slinging the gantry around. I know that others are making much wider gantry routers, and some of them use servos. I *think* these motors will be enough. I may have to cut down the accelerations.

    3x3 might be a bit on the thin side especially when it comes to getting good concrete fill. It isn't impossible though but it will be tedious. Think about how you will ram the concrete into long 3x3 sections or even 3x5 sections for that matter.

    This isn't so much a structural issue as it is a process issue. You will want good stiff concrete to avoid shrinkage associated with looser wet concretes but you also need to get good fill to avoid gaps. Further all of this crap gets heavy real fast and as such you need to be equipped to handle the parts. If you can answer the question of how do you pack concrete in to a 2.5 inch tube then go for it.
    I see what you mean. I was thinking of mixing my own with mortar and pea gravel to avoid large gravel. I need to learn more about this.

    Well as stated I'm not sure why you are going movable gantry in the first place. But then you also want a movable bed at 300 + pounds. Something just doesn't seem right.

    On the other hand you do have an interesting design. The interesting task will be getting it cast flat. One thing to look into is a book written a few years ago that covers making concrete counter tops. Sorry I don't have the author or title but such a book might give you a few ideas. Honestly though making the whole bed epoxy concrete might make more sense.
    You are right. It isn't right. The bed is only "movable" in that you will be able to push it back. The mechanism to move a 300 lb thing back is something I'm thinking about. Maybe some wheels underneath that can be brought up by a lever.

    Let me talk about the bed. If I could get one made of cast iron or granite with t-slots, I would. I talked to some granite dealers, and I could get counter-top material, but then I would have to drill lots of holes in it for hold-downs. Blah! I can't find one for a price I want to pay, so I'm thinking of making this design. Here is how I propose to make it flat:
    1. Pour a pad of self-leveling epoxy. Wax it or some such mold release.
    2. Set the frame (with rebar grid) face down on the pad, and fill with concrete. Vibrate it. Allow to dry.
    3. Pour self-leveling epoxy on top (actually that's the bottom). Allow to dry.
    4. Flip it over, apply self-leveling epoxy to the top. This also seals it.
    Both surfaces should be flat and parallel. The hold-down holes would be drilled and tapped later when installed on the mill.
    I haven't seen anyone do this before, so it may be a great or crazy idea!

    A number of pieces of equipment make use of welded steel beams for gantries. It isn't a bad approach but you will most likely have to have it stress relieved and finished milled for the linear rails. Structural steel isn't all that straight and any welding builds in stress that will come out sooner or later. This is probably the biggest negative with respect to steel structures.

    I'd also consider a square beam to allow for easier fill of the beam with concrete.
    I agree. The gantry will be stress-relieved. I think that structural steel will warp if you try to finish mill it. I plan to pour self-leveling epoxy for the bearing mating surfaces instead. I'll consider your square beam idea. I'm not sure if it matters if there are some voids in the concrete.

    There is no perfect spindle. So I'm gaining to suggest interchangeable spindles as A low cost solution. For example one head using an off the shelf router and another that is a more substantial spindle for lower speed work. Or you can co mount the spindles and switch as needed.

    I suppose if budget is of no concern you could install an industrial do it all spindle.
    I have thought about interchangeable spindles. The steel-cutting one is the one that's hard to find. Can't afford a $5k spindle. My budget is probably < $1800 for the spindle.

    You can't be serious? After all the time and money that will go into this you want to use a tread mill motor. To be polite that is nuts. If you have spent all the time and effort to design this machine, take the time to design in a 3 phase motor at the very least or a more substantial servo / spindle drive.
    Heh. This is up in the air. I't something I know I can get. I have spent many hours looking at this, and reading the PMDC motor threads. I'll keep your concerns in mind. I'm more worried about finding a spindle, then I''ll find a motor to match.

    Thanks again for the feedback.
    -Steve

  4. #4
    Join Date
    Jan 2004
    Posts
    56
    Steve & Others,
    Good start on an ambitious and worthy project!
    (I'm working slowly on the build of a medium-duty wood & aluminum cutting router/miller of my own design)
    I agree with most of the feedback comment/suggestions you have received thus far.
    Don't take this the wrong way...because you asked for feedback......
    1) Spread-em and Stagger-em! The linear bearing blocks that is! This is the cheapest way to get the additional stiffness you need in the y and Z axes.
    I believe that staggering the X-axis bearings may reduce the propensity for binding or for lack of a better word, "pre-binding" vibration.
    2) Connect or cap the ends of the z axis "U-shape" beam assembly, with solid plate.
    3) Your bed/structure probably ok, but the y & z do not appear to be comparably stiff. For cutting steel you have 2 weak-links in the Y&Z axes.
    4) The contributory stiffness and damping may not worth the toil to do it, nor the potential mess it may make. Unless you use polymer concrete or epoxy-granite.
    If when the concrete cracks and starts to break down, that could make matters even more messy. Another option is to stuff the tube structures with precast elements (think patio pavers) or granite counter-top scraps laminated together use a polymer grout or RTV or cast silicone or just regular old polyester resin typically used for fiberglass.

    5) If your work envelope requirements are negotiable, consider designing your machine around HIWIN "Bridgeport" Milling Machine replacement screws and bearings. They are ground to accuracies beyond the capabilities of the other elements of your machine.
    For the money these screw are exceptional value. I have not checked price in years but I recall them being around $1,000US for a x&y set.

    6) You are going to need a STIFF bed material. I would consider Granite Slabs (laminated), or a used or imported surface plate. I don't know if or how well this would work, but have considered bonding interlocking patio pavers together layer by layer to build a slab. In case you ever have to move the machine, it would be nice to be able to detach and the slab and move it independent of the machine.

    I believe milling steel successfully with a machine of this construct may be tall-order, but I do wish you good luck! I also believe that you are exceedingly wise to maximize the research and planning phase of this extreme project.

    That's all for now.
    Good luck!
    Dan

  5. #5
    Join Date
    Apr 2012
    Posts
    134
    Hi Dan,

    Thanks for the feedback. I'm glad to get it.

    1) Spread/stagger bearing blocks.

    I don't know about this. I understand spreading them. Staggering them will cut down on envelope. The 20mm bearing blocks have over a ton of load capacity each.

    2) Connect or cap ends of Z axis U-shape beam.
    I think the idea here is to keep the z axis back plate stiffeners from vibrating sideways, right? I lengthened the blue stiffeners to the full height of the z, and enlarged the top motor plate so it can tie into the stiffeners. This should take care of the top. The bottom is a harder problem. I don't want to limit the z travel to put a plate there. I'm currently arguing with myself about this.
    Click image for larger version. 

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    3) Your bed/structure probably ok, but the y & z do not appear to be comparably stiff. For cutting steel you have 2 weak-links in the Y&Z axes.
    This gets us to how much structural loop stiffness we need. I have been going by page 26 of "Principles of Rapid Machine Design" which says 10 - 25 N/um deflection are considered adequate for machine tools. That translates to 57 - 144 lbs per .001 of deflection. Putting it another way, 100 lbs cutting force should give between .0018 and .0007 deflection.
    For the Z complete axis (back plate assy. and spindle plate assy.), I have calculated .00021 deflection per 100 lbs cutting force when cutting near the table with a short cutter. There are two reasons for this. 1. In the lower range of the Z, the Z spindle plate top and bottom blocks are very near the Y rails. Thus the Z back plate doesn't bend much at all. 2. The box section z front/spindle plate is extremely stiff with .00009 deflection per 100 lbs. The z back stiffeners are really only for very high Z where the back plate is a cantilevered beam. I really think that the z is stiff enough. I will be making the Z last. I plan on cutting the blue stiffeners on the machine, then taking the Z back apart and installing them.

    The Y full twisting is harder to calculate, because it is made of two beams tied together. I used the MyCNCuk cnc stiffness calculator, and if you assume the two beams welded together with the spacers become one 10" tall by 5" wide beam, you get .000051 with 100 lbs cutting force. That would be really small if I calculated it correctly.

    4) The contributory stiffness and damping may not worth the toil to do it, nor the potential mess it may make. Unless you use polymer concrete or epoxy-granite.
    If when the concrete cracks and starts to break down, that could make matters even more messy. Another option is to stuff the tube structures with precast elements (think patio pavers) or granite counter-top scraps laminated together use a polymer grout or RTV or cast silicone or just regular old polyester resin typically used for fiberglass.
    I'm not worried about the concrete breaking down. I don't see how it matters if it turns to sand over time. I'm going to be looking at formulations before I start mixing.

    5) If your work envelope requirements are negotiable, consider designing your machine around HIWIN "Bridgeport" Milling Machine replacement screws and bearings. They are ground to accuracies beyond the capabilities of the other elements of your machine.
    For the money these screw are exceptional value. I have not checked price in years but I recall them being around $1,000US for a x&y set.
    This is interesting. I will look into it.

    6) You are going to need a STIFF bed material. I would consider Granite Slabs (laminated), or a used or imported surface plate. I don't know if or how well this would work, but have considered bonding interlocking patio pavers together layer by layer to build a slab. In case you ever have to move the machine, it would be nice to be able to detach and the slab and move it independent of the machine.
    I can get granite, but don't like the idea of drilling 64 holes in it for tie downs. Concrete is a good material for dampening vibration. With good structure underneath, and rebar inside, it should be nice and stiff. I can also make the concrete bed 4" thick (another 100 lbs). I'm considering that to allow more z clearance when the bed is pushed back.

    I believe milling steel successfully with a machine of this construct may be tall-order, but I do wish you good luck! I also believe that you are exceedingly wise to maximize the research and planning phase of this extreme project.
    Thanks. I this it's possible. I am most worried about dampening vibrations in the gantry.

    Thanks again for your feedback. It gives me a lot to think about.

    -Steve

  6. #6
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    I have started looking into self leveling epoxies

    My entire design relies heavily on self-leveling epoxy. I started looking at sources. In reading threads here I see that very low viscosity, and long potting/working times are important.

    Here are some of the sources I have found:

    West Systems 105 epoxy with 209 hardener Example thread: http://www.cnczone.com/forums/cnc_wo...monster-3.html There can be problems with this epoxy if you don't use the slower 209 hardener.
    mixed 3:1 1000 cps viscosity (high compared to the others). Will probably go with one of the following:

    Precision Epoxy Precision Epoxy Products
    These guys do self leveling for a living. 13,500 psi. I am getting pricing.
    FS-190 sealant, applied with a roller.
    FP-80 Floor plate epoxy - clear (2:1 mix ratio) Someone said 400 cps.
    FP-85 pigmented
    FP-90 "...very slow rate of cure to achieve the most level and flat surface area possible."

    US Composites
    Epoxy :*Epoxy Resins and Hardeners
    635 Thin epoxy resin system Viscosity 600 cps
    2:1 with #556 Slow hardener. Drying time: 24-48 hours, 2-3 day drying time at below 70f
    Pot life: 35-40 min at 80f
    Set time: 5-6 hours
    $75 for 1.5 gallons
    This is very inexpensive.

    Epoxy.com
    They have a couple of products that look interesting:
    #899 primer/sealer
    #303 Low viscosity 300-350 cps. 10,000 psi compressive strength. 30 min pot life. Tack free 5.5 hrs.
    I have requested a quote.

  7. #7
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    WOW! Can't wait to see this project get off the ground! You going to have a bed "pan" and pour concrete, then top it with self leveling epoxy? Is that how I understand it?

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    Quote Originally Posted by nateman_doo View Post
    WOW! Can't wait to see this project get off the ground! You going to have a bed "pan" and pour concrete, then top it with self leveling epoxy? Is that how I understand it?
    One should strive to keep "bed pans" out of the shop!

    On a more serious note here are some links to books about concrete counter top design and build:
    CHENG Concrete Exchange - "Concrete Countertops" Book
    Concrete Countertops Made Simple - ISBN#1561588822
    QUIKRETE® - Countertop Mix
    CHENG Concrete Exchange - Concrete Countertops Concrete Countertops. By Fu-Tung Cheng

    I can't remember which one but I read a book some time ago on concrete counter top design and frankly there is a lot of material in the books that can be inspiring for a machine tool builder. Further quickrete now sells "counter top" concrete in a bad. This is a concrete mix with a plasticizer and other goodies already mixed in for good results in molds.

  9. #9
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    Yes, I will pour a self-leveled surface plate (bed pan ).
    Then I will set the channel + rebar grid on that (face down), then pour concrete. I have been looking at videos of concrete counter tops (I realized that those are very similar). One technique is to make a cement/sand mix first and put that down (pack and vibrate), then put a pretty dry concrete mix on that (pack/vibrate).
    When that is cured, I will pour a self-leveling epoxy layer on that. That becomes the very flat/level/parallel bottom.
    I then turn it over and seal the concrete top with epoxy (probably not a full self-leveling layer).
    When it is on the machine, I will drill and tap for tie downs.
    I will be looking at concrete formulations and additives after I get the epoxy sorted out.

    Nateman, I have been watching your build, and am really looking forward to seeing it cutting! It looks really good. One question for you: What are the dimensions of the mating surface of the Tormach mill head (where it connects to the z)? I'm thinking that head might be bigger/heavier than I want, but would like to see if it could fit. I will change some dimensions if it is close to fitting my Z.

  10. #10
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    Quote Originally Posted by wizard View Post
    One should strive to keep "bed pans" out of the shop!
    Heh, yes. I'm actually a bit worried about stepping in it! If I build it where the machine goes, fine, but now it's covered, and I can't use it for a surface plate. If I put it somewhere else, I will be stepping on it. I think it will end up under the machine.

    Quote Originally Posted by wizard View Post
    On a more serious note here are some links to books about concrete counter top design and build:
    CHENG Concrete Exchange - "Concrete Countertops" Book
    Concrete Countertops Made Simple - ISBN#1561588822
    QUIKRETE® - Countertop Mix
    CHENG Concrete Exchange - Concrete Countertops Concrete Countertops. By Fu-Tung Cheng

    I can't remember which one but I read a book some time ago on concrete counter top design and frankly there is a lot of material in the books that can be inspiring for a machine tool builder. Further quickrete now sells "counter top" concrete in a bad. This is a concrete mix with a plasticizer and other goodies already mixed in for good results in molds.

    Thanks for the links. That's all good info.

  11. #11
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    Here is a quick update on the epoxy front.
    I sent emails to Precision Epoxy and one other place to get quotes. No response. After three days, I called Precision. I nice guy called me back, and said they had been busy. We talked for a while and he said he would quote a kit with everything I need. I'm a bit afraid of what the price will be. I'm also worried that my little sale is too small for them. We will see. I'm still waiting.

    Here is a few things I learned from the Precision Epoxy guy:
    1. The FP-90 epoxy is their "Aerospace Test Bed" product. It makes an extremely flat surface, but he said it isn't intended to be touched! It isn't that strong. He said I don't want it. He recommends the FP-85 product.
    2. This is the important thing he said: A self-leveling epoxy layer should be a minimum of 1/4 inch deep, with an absolute minimum of 3/16.
    3. To self-level my 5 foot rails, he recommends three bridges between them: two end ones 6 inches from the ends, and one in the middle.
    4. The bridges should be the same width as the rails, so if the rails are 3" wide, the bridges should also be that wide. That way everything is the same and it will level well.
    5. He said that he has had people pour self-leveled rails that are 25 feet long!

    I will give Precision a few more days, then I'll start talking to other suppliers.

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    I'm a bit afraid of what the price will be.

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    Quote Originally Posted by steve123 View Post
    Here is a quick update on the epoxy front.
    I sent emails to Precision Epoxy and one other place to get quotes. No response. After three days, I called Precision. I nice guy called me back, and said they had been busy. We talked for a while and he said he would quote a kit with everything I need. I'm a bit afraid of what the price will be. I'm also worried that my little sale is too small for them. We will see. I'm still waiting.
    There does seem to be an issue with suppliers looking for the big fish and ignoring the little fish. Hopefully they will get back to you. I see the same problem at work when you just need one of something usually sold by the thousands. This might be a good business opportunity for a specialist.
    Here is a few things I learned from the Precision Epoxy guy:
    1. The FP-90 epoxy is their "Aerospace Test Bed" product. It makes an extremely flat surface, but he said it isn't intended to be touched! It isn't that strong. He said I don't want it. He recommends the FP-85 product.
    Long term I'm not convinced that any self leveling epoxy will hold up well. I'd like to be proven wrong though.
    2. This is the important thing he said: A self-leveling epoxy layer should be a minimum of 1/4 inch deep, with an absolute minimum of 3/16.
    3. To self-level my 5 foot rails, he recommends three bridges between them: two end ones 6 inches from the ends, and one in the middle.
    Bridges? I'm trying to imagine this.
    4. The bridges should be the same width as the rails, so if the rails are 3" wide, the bridges should also be that wide. That way everything is the same and it will level well.
    5. He said that he has had people pour self-leveled rails that are 25 feet long!
    Cool! At least they understand what you are doing.
    I will give Precision a few more days, then I'll start talking to other suppliers.
    Bug them! It sounds like they understand what you are doing and what your needs are.

  14. #14
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    I used Quickcrete countertop mix in my build. mixed 10 bags by hand. backbreaking. Very soupy stuff, but it is nice.

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    I'm going to skip through some stuff here, just wanted to provide you feedback on your machine in the areas I'm most familiar with. It sounds like you're asking the right questions for your design.


    Quote Originally Posted by steve123 View Post
    Thanks for taking time Wizard.

    It's all good.


    It's a good question. How much is enough? I'm planning on pushing back the bed, and mounting the 4th axis on the front beam. That spot has 9" clearance under the gantry. The Z can go as high as 6" above that. I would love to hear from people that have a 4th.

    Here's the thing about 4th axes... the ones people add that are essentially a rotary table or spindle on their mill bed are less accurate than a 4th axis could be. This is because the axes are stacked... any positioning error in either axis under the rotary is multiplied by any angular error in the rotary. For a more accurate 4th axis, use a tilting spindle.

    Quote Originally Posted by steve123 View Post

    The next size-up ball screw that doesn't have a tiny lead is 2510. Those require massive motors just to accelerate the screw. The 1610 size is limited to about the lengths I am using before you get too much wip.
    There's not that much difference between 1610 and 2510 as far as accelerating the screw. What matters is whether the screw has the rigidity to handle the force you're placing on it without deflection. "Whip" isn't generally an issue for screws this size unless they're being used for insane rapids.. which you probably won't get without bigger motors (more below). FYI, I don't know where you're looking at screws, but you can essentially get them in any size with any lead, you just have to look. There are 40 meter (yes, meter) ballscrews with 1 meter diameter and 10 mm lead if you explore the internet a bit.

    For the machine to be as rigid as possible, I'd go with as large a screw diameter as you can fit to avoid problems like stretching of the screw, wear due to being near its force limits, and flex. I'd also go with a low lead screw like 5mm or smaller. The extra mechanical advantage will help your accuracy and help you get the fast acceleration you want.

    Quote Originally Posted by steve123 View Post
    The moving gantry allows the 3x4' envelope. You see me talking about stiffness because I want to be able to cut steel. I want it all: space and stiff enough. I'm very aware that most of the advice on CNCZone is to avoid gantry mills. I still wants it.


    I wasn't clear about the moving table/bed. The table can be moved back, but it isn't automated. The idea is to be able to push it back when I want to have more Z, lets say for the 4th axis, or for more clearance with a vice. These things could be temporary, or could be left there if other jobs don't need the 3x3' bed that's left. I'm thinking I might be lazy and leave a vice/4th mounted on the front beam.
    It actually doesn't matter if it's automated or not. If it's not rigidly fixed to the frame(bolted), it's a source of error and potentially vibration and inaccuracy.

    A moving gantry design capable of cutting steel with any speed (other than 1cm/minute feed) and accuracy better than 0.01 will have to be *heavy*. Heavier than you're designing I think. There are good reasons you don't see many gantry type mills for steel... for rigidity and vibration damping you need lots of mass, to have a lot of mass you need big motion controls (motors, slides, etc) which adds more mass, and the cycle eventually concludes with a very heavy yet accurate gantry with a small work area (like 2 feet cubed for a 3 ton machine).

    Putting it another way... if you define your work area as 3x4 feet and want to cut steel, the mass of a gantry that doesn't use a very expensive (high speed/low vibration) spindle will be something like 2000-3000 lbs if you don't want it to vibrate like a bell. Even filled with concrete, a 300 lb steel gantry just doesn't have enough mass. For comparison, my full size mill weighs 3800 lbs, and can hold 0.001" accuracy provided I turn the feed rate down enough to keep it from vibrating. If I try to go too fast, it loses a ton of accuracy. Every machine has limits.

    While we're on the subject, what steel are you talking about? There's a world of difference between cutting mild weldable steel, stainless, tool steel, etc. Huge differences in cutting forces.

    You may be asking at this point why a mini mill weighing 150 lbs or so can cut steel (even slowly) if your design can't. The answer is that the mini mill isn't a gantry. The rigid vertical column that is aligned with the axis of the spindle (Z) plus the material (cast iron) help a lot. FYI, the "concrete" that is the best material for damping vibration isn't gray masonry material... it's polymer concrete, basically epoxy plus gravel and sand.

    A mini mill can cut steel... slowly if you want it to work well. It has nowhere near the performance numbers you want for rapid movement when CNC'ed, and can only hold decent accuracy if used by someone who tunes it, modifies it, and knows how to run it well.


    Quote Originally Posted by steve123 View Post
    I agree, but you have to start with stiffness. Then comes materials and wall thickness I think.

    Speed (rapids): around 400 ipm (using 1610 (not 1605) ball screws, and low impedance motors).
    Acceleration: Well.. A pair of 960 oz motors put out about 1300 LBS force at lower speeds... I'm not sure exactly how much gantry weight matters.
    Not actually "stiffness" but rigidity (which includes both deflection of materials and motion of machine joints) and mass. Materials are intimately part of this. Wall thickness is a component of materials... it matters if you're A) Using composite components with filler/cores or B) Trying to increase mass by using more material. Since you're going to fill the beams with concrete, the wall thickness only matters as far as tension/compression (because with a core the beams are composite trusses) and the deflection of the steel itself doesn't matter at all. If you pack beams with concrete, make sure you do it while they hang vertically. Otherwise the weight of the concrete will cause them to deflect and they'll harden permanently curved.

    Aside from whether your gantry is heavy enough to cut with accuracy and not vibrate, there are other concerns about your motors.

    First, you don't say whether these are steppers or servos. Since you imply lower speeds have more force I'll guess steppers. A 960 oz-in stepper provides 960 oz-in of holding torque at max. That's 60 lb-in or 5 ft-lb. Coupled to a ball screw the torque is multiplied by the ballscrew like a transmission and rpm is reduced.

    You have to spin your motor at 1000 rpm when turning a 2.5 tpi (10mm lead) ball screw to get 400 inches/minute of movement (2.5 turns to move 1 inch) . At 1000 rpm your motor is going to lose most of its torque, so you're going to need to "gear up" between motor and ballscrew to keep the stepper in its power band. That will cut your torque down. RPM is important to have high rapids, torque is more important because it's what lets you reach that high speed in a short time.

    There's another problem here related to driving the ball screws with the stepper. Let's assume you use timing pulleys to spin the ball screw at 1000 rpm. Assuming a "normal" stepper that means you've got about 1.8 degrees per step if you're running full step, which becomes half that through the pulleys. Per step the ball screw turns at 0.9 degrees per step or 1/400th of a turn. So one step on your 10 lead ball screw is 1/400 of 10mm or 0.025mm. That means you can only address your axis in 0.025mm or 0.000984252 inches... right around 0.001 or 1 thou, so if you want to be able to address coordinates without rounding to the nearest thou the stepper can address, you need a microstepping drive, which further reduces speed and torque.

    For moving a 200+ lb gantry, I'd recommend servos instead of steppers, and 5 lead ballscrews. The higher RPM of the servos lets you get the rapid speed you want, and their ability to deliver huge torque for short times will give you the acceleration you want.


    Quote Originally Posted by steve123 View Post
    Agreed. This is one of those things I have had problems finding an answer to. I know that the whole machine has to be strong enough to handle huge pushes of the motors when slinging the gantry around. I know that others are making much wider gantry routers, and some of them use servos. I *think* these motors will be enough. I may have to cut down the accelerations.
    You might not be able to without cutting your desired speed. You need a lot of torque to reach your 400 ipm rapid in a short enough time to make it worthwhile... it does no good to hit that speed just before you have to stop moving.


    Quote Originally Posted by steve123 View Post
    I see what you mean. I was thinking of mixing my own with mortar and pea gravel to avoid large gravel. I need to learn more about this.
    There's a lot of info on polymer concrete in various threads here... what works and what doesn't.

    Quote Originally Posted by steve123 View Post
    Let me talk about the bed. If I could get one made of cast iron or granite with t-slots, I would. I talked to some granite dealers, and I could get counter-top material, but then I would have to drill lots of holes in it for hold-downs. Blah! I can't find one for a price I want to pay, so I'm thinking of making this design. Here is how I propose to make it flat:
    1. Pour a pad of self-leveling epoxy. Wax it or some such mold release.
    2. Set the frame (with rebar grid) face down on the pad, and fill with concrete. Vibrate it. Allow to dry.
    3. Pour self-leveling epoxy on top (actually that's the bottom). Allow to dry.
    4. Flip it over, apply self-leveling epoxy to the top. This also seals it.
    Both surfaces should be flat and parallel. The hold-down holes would be drilled and tapped later when installed on the mill.
    I haven't seen anyone do this before, so it may be a great or crazy idea!
    In theory it should work. The problem is that with all the steps and transfers, you're going to end up with lots of chances to warp or otherwise make a non flat surface.

    I'm not sure why you don't want to drill a flat granite pad when you're talking about drilling the concrete one later... some coordinate measuring machines and commercial mills are made from granite slabs, and they work very well.

    There's no reason to do steps 1-3 if you're using self leveling epoxy after that. They gain you nothing, might as well pick up a spare piece of sidewalk from outside because step 4 will level it just as well.

    Quote Originally Posted by steve123 View Post
    I agree. The gantry will be stress-relieved. I think that structural steel will warp if you try to finish mill it. I plan to pour self-leveling epoxy for the bearing mating surfaces instead. I'll consider your square beam idea. I'm not sure if it matters if there are some voids in the concrete.
    Whether or not any steel will warp when milled isn't due to the type but rather the internal stresses in the piece. If you're going to use a stress relieving oven on it, it doesn't matter.

    For the mating surfaces, self leveling doesn't matter, epoxy with a thickener will work. Look up "moglice" or "replicated joints".

    Quote Originally Posted by steve123 View Post
    I have thought about interchangeable spindles. The steel-cutting one is the one that's hard to find. Can't afford a $5k spindle. My budget is probably < $1800 for the spindle.
    I'd recommend one of the better quality chinese spindles on e-bay... the ones that are brushless motors powered by a VFD, and run at 24k rpm and water cooled. Get as big a unit as you can afford... 5 or more kw. No need to find a motor, it's integral.

    Most metal milling machines nowadays use high speed strategies and small cutters to work fast... don't buy a surplus mill head to get a spindle, get a decent high speed head instead.

    Also, put something in your budget for a decent cam package. That's the most expensive part of your software toolchain. The free packages mostly suck for those people with true 3d machines. It really sucks to have a full 3d mill available and to hate using it because the tools suck and you have to hand code parts.

    Erik

  16. #16
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    I got the quote from Precision a few days back, and I have been mulling it over. To pour a 5x6' surface plate + self-leveled rails would be 10.5 gallons @ $89, and cost $952 + shipping. That isn't going to happen. The reason it is so expensive is that they think I would need to do two pours at 1/4" each. I'm Scrapping that idea now that I know what it costs! I next thought I would just pour a plate small enough to pour the concrete bed on - let's say 38x50". That gets it a lot cheaper. Then I started reading about how concrete continues to shrink for years. (nuts) So I would have to be somehow re-milling/grinding/sealing the top surface every so often. I don't know if that's even possible. It made me stop and think that drilling holes in granite is not so bad! I am going to go visit some granite places and see what I can find. I'm thinking 36x48x4". Maybe they can drill the holes also!

    FYI, here is what Precision Epoxy would supply:
    7.0 gal. SC-15P A Steel casting epoxy @ $89.
    3.5 gal. SC-15P B Steel casting epoxy (part B) @ $89
    1 5" mixing blade @ $15
    1 10" wood handle spatula @ $2.40
    All of this comes in pre-measured kits for each pour. The part A is in a container big enough to also hold part B. Just pour in part B, mix, and pour.
    They also said I could call any time day or night for tech support (you can see they are geared up for big jobs).

    The odd thing about this is epoxy is that their web site says SC-15P has "a reduced pot life and rapid rate of cure". Exactly opposite of what I think I need.
    Here is the full description:
    SC-15 (mix ratio 2:1 / A:B)
    Steel Coat Epoxy was formulated to protect steel plates subjected to submersion in boiling water from corrosion. It offers unparalleled adhesion at high temperatures without delamination. SC-15 is very durable and chemically resistant with a reduced pot life and rapid rate of cure. Available in clear or pigmented, thin or thick viscosity liquid, paste or gel versions.
    I just sent them an email asking for clarification.

    I am going to move forward with other parts of the build. I have a surplus metal supplier in the area. I'm going to head down there soon and see what they have.

  17. #17
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    Quote Originally Posted by nateman_doo View Post
    I used Quickcrete countertop mix in my build. mixed 10 bags by hand. backbreaking. Very soupy stuff, but it is nice.
    More information would be nice, I'm trying to learn all about successful concrete uses in machine tool building.

  18. #18
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    Quote Originally Posted by steve123 View Post
    I got the quote from Precision a few days back, and I have been mulling it over. To pour a 5x6' surface plate + self-leveled rails would be 10.5 gallons @ $89, and cost $952 + shipping. That isn't going to happen. The reason it is so expensive is that they think I would need to do two pours at 1/4" each. I'm Scrapping that idea now that I know what it costs! I next thought I would just pour a plate small enough to pour the concrete bed on - let's say 38x50". That gets it a lot cheaper.
    The more you look into it the more cement technologies you will find out there. These guys need mention Kerneos Inc. due to a products called Ciment Fondue. I'm not certain the material is suitable for machine tools but any company that has a products called Cument Fondue deserves mention.
    Then I started reading about how concrete continues to shrink for years. (nuts)
    I don't know about shrinking but standard concrete takes years to reach ultimate hardness. Shrinking is a function of the type of concrete.
    So I would have to be somehow re-milling/grinding/sealing the top surface every so often. I don't know if that's even possible. It made me stop and think that drilling holes in granite is not so bad!
    Granite isn't a bad idea if you can handle the mass. The bigger problem is how do you fasten things to it reliable. Even epoxy granite with molded in inserts will have problems with those inserts pulling out.
    I am going to go visit some granite places and see what I can find. I'm thinking 36x48x4". Maybe they can drill the holes also!
    That would be very heavy.
    FYI, here is what Precision Epoxy would supply:
    7.0 gal. SC-15P A Steel casting epoxy @ $89.
    3.5 gal. SC-15P B Steel casting epoxy (part B) @ $89
    1 5" mixing blade @ $15
    1 10" wood handle spatula @ $2.40
    All of this comes in pre-measured kits for each pour. The part A is in a container big enough to also hold part B. Just pour in part B, mix, and pour.
    They also said I could call any time day or night for tech support (you can see they are geared up for big jobs).

    The odd thing about this is epoxy is that their web site says SC-15P has "a reduced pot life and rapid rate of cure". Exactly opposite of what I think I need.
    Here is the full description:


    I just sent them an email asking for clarification.

    I am going to move forward with other parts of the build. I have a surplus metal supplier in the area. I'm going to head down there soon and see what they have.
    Have you considered castiron, steel or aluminum plates. Considering the prices you are being quoted tooling plate might be a good deal. Steel plate can be had for 39 cents a pound as drops or cut offs. Obviously this isn't finished steel in any way. These plates get Hevesy real fast too. However if you are thinking concrete you already have a lot of weight to deal with.

  19. #19
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    I'm currently studying concrete, and doing research on Erik's suggestions.

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    Why not try sand mixed with resin? its dense and waterproof, and easy as hell to mix. I have used it in both my CNC builds.

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