New machine: small, sturdy, high performance. Input needed!

Hey guys!
It's been quite a while since I posted!

My last machine was a Probotix Fireball V90 that has since gone to a new home (and they've put over 500 hours of 3D carving on it since! wow!). I have been without a CNC machine for about a year, and have been finding more and more jobs in my new workshop that would be well suited to a smallish CNC router.

I currently make quite a number of semi-custom knives for customers all over. A new CNC router would be very handy in my shop for engraving (I used to do that on the V90) as well as 3D carving G10/Carbon fiber for knife handles... Currently I do basically everything by hand but I'd like to start looking at automating various parts of my process.

The materials I'd like this machine to work with are: wood, G10/FR4, Carbon fiber, aluminum. I'll also be using it quite a lot to engrave my logo and serial numbers into the knife blades after their completed. I've done this in the past with a carbide v-scoring bit and it's worked out very well so far.

One other potential use that I have for this machine that's a little outside the box is abrasive machining. Carbide tools don't last very long when cutting G10, but I run ceramic belts on my belt grinder that will last a very long time when cutting G10. For my handles for instance I think that it might work well to mount a small belt grinder in place of the spindle and use the abrasive belts to machine some of the simple contours on the handle. Kinda hard to explain just in text, but I think it has the potential to work out well.

The machine I have in mind would be fairly small with a work envelope of 12x24" to 24x48". I haven't decided yet whether I should concentrate on going small with the aim of getting extra performance out of the machine, or go a bit larger and hope the the extra flexibility will pay off.

Features I'm looking to incorporate:
* Ballscrews on all axis
* Profile rails on all axis
* Router frame constructed of aluminum extrusion and MIC-6 plate
* All rails/screws fully shrouded to protect from dust created by abrasive machining
* Water-cooled 80mm spindle and VFD
* Ability to use tooling up to 1/2"
* Home/limit switches on all axis
* 2x ballscrews and 2x steppers on X axis
* Gecko G540, NEMA23 steppers, and LinuxCNC for control

So, the questions I have for you guys:

1) Should I go big (24x48") or stay small (12x24"). My shop isn't large. Having the extra work envelope is nice, but if I can build a smaller and stiffer/more performant machine then that is also attractive as this machine *will* be used for production runs. The parts I'll be making will generally be small (on the order of 1.5x5x.25"), and the materials they're made from generally only come in small sheets (12x24" usually).

2) What kind of extrusions should I use for the machine frame? I can either get t-slot extrusions from Misumi, or I can simply use extruded rectangle tube from my metal supplier. I have a mill and lathe, so machining the tubing should be ok. The simplicity and cut-to-size nature of the t-slot is appealing, but at the same time I'm sure I can get more stiffness from 2x3" (or larger) rectangular tube.

If you guys think any of this sounds nuts please do speak up! The primary goal for this machine is to be performant and long-lasting. I'm less worried about it being the cheapest possible build as I know that once I get it running it will make my life better and help me get more product out of the shop.

I have a reasonable amount of experience fabricating parts, but I'm not an expert machinist by any stretch of the imagination. I am very familiar with all the parts necessary for the CNC though, as well as the required CAD/CAM workflows as I've built and run several kit CNCs in the past. I want this one to be a good deal more serious!

Any input it appreciated! Drawings and CAD mockups will start coming soon.
-Aaron

Check out the Momus design here on the forums it can be modified to suit the size you're looking for. I've been milling a lot of aluminum on mine lately and it's doing a fine job for my needs.


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Originally Posted by Jkountz

Check out the Momus design here on the forums it can be modified to suit the size you're looking for. I've been milling a lot of aluminum on mine lately and it's doing a fine job for my needs.


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Thanks for the response mate!

The Momus looks like a very nice machine, wish that had been around when I first started getting into CNC! However it's quite a bit lighter-duty than what I had in mind... Whatever machine I end up building, I'd like it to be able to take .125" DOC at least in Aluminum (with 1/8" endmill), and the Momus isn't built for that kind of thing.

Wow that's a big bite for any machine to take with just an 1/8" bit us it not? How do keep from snapping the bit taking a cut like that in al?


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Originally Posted by Jkountz

Wow that's a big bite for any machine to take with just an 1/8" bit us it not? How do keep from snapping the bit taking a cut like that in al?


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Taking a cut that size is done all the time on manual milling machines. It's more ambitious for a CNC router, but I've certainly seen machines that will do it!

Snapping the bit is usually caused by feedrates that are too slow in my experience. A 1/8" solid carbide endmill is pretty sturdy. If you're presenting it with the correct chipload it should fly through the aluminum. If you go too slow (on the feedrate) then the tool will rub, and chips will weld to the cutting edges, then you snap the tool...

That being said I haven't dealt with too much aluminum on a CNC router, so I could be wrong.

Interesting I did not know that!


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Originally Posted by Jkountz

Interesting I did not know that!


Sent from my iPad using Tapatalk

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I spent a lot of time watching YouTube videos of people running aggressive production toolpaths on big CNC machines (Vertical Machining Centers mainly). After watching a bunch of those I was always pushing my milling machine and old CNC router to see what they could do. Hard to keep up with a VMC though!

Here's a fun example of what the big machines can do:

https://www.youtube.com/watch?v=81DrNK6U0xo


The page here recommends that tools under 1/8" be kept to 0.25x diameter max DOC when slotting. Tools 1/8" diameter and above can go to 1.5x diameter DOC when slotting. 1.5x diameter DOC is recommended for all tools when profiling (only one side of tool touching workpiece). If the machine is rigid enough then it's good to go with the maximum DOC possible as that way you're using more of the tool and tool life will go up!

Solid Carbide Speed & Feed

Great post with lots of information.

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Originally Posted by aarongough

Hey guys!
It's been quite a while since I posted!

My last machine was a Probotix Fireball V90 that has since gone to a new home (and they've put over 500 hours of 3D carving on it since! wow!). I have been without a CNC machine for about a year, and have been finding more and more jobs in my new workshop that would be well suited to a smallish CNC router.

I currently make quite a number of semi-custom knives for customers all over. A new CNC router would be very handy in my shop for engraving (I used to do that on the V90) as well as 3D carving G10/Carbon fiber for knife handles... Currently I do basically everything by hand but I'd like to start looking at automating various parts of my process.

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A small router type machine is a good choice but not the only one. Another option would be a CNC like Tormachs small machine with the fast spindle.

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The materials I'd like this machine to work with are: wood, G10/FR4, Carbon fiber, aluminum. I'll also be using it quite a lot to engrave my logo and serial numbers into the knife blades after their completed. I've done this in the past with a carbide v-scoring bit and it's worked out very well so far.

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Place a big importance on your personal safety. Consider a fully enclosed machine to control dust.

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One other potential use that I have for this machine that's a little outside the box is abrasive machining. Carbide tools don't last very long when cutting G10, but I run ceramic belts on my belt grinder that will last a very long time when cutting G10. For my handles for instance I think that it might work well to mount a small belt grinder in place of the spindle and use the abrasive belts to machine some of the simple contours on the handle. Kinda hard to explain just in text, but I think it has the potential to work out well.

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My first thought is that this would require a substantially different machine. Thinking about it a bit I suspect that there are some possibilities if you have a lot of z clearance. Your z axis may need to wrap around gantry though and the likely need for a rotary on the X. I think the bigger issue here is software. You may have trouble finding CAM software that will work well with your grinding arraignment, especially in conjunction with a rotary axis. Hopefully somebody with experience here will chime in.

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The machine I have in mind would be fairly small with a work envelope of 12x24" to 24x48". I haven't decided yet whether I should concentrate on going small with the aim of getting extra performance out of the machine, or go a bit larger and hope the the extra flexibility will pay off.

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Consider no bigger than required to make enclosing the machine easier to control all of that dust. The last thing you want to be doing is to breathing in glass and carbon fiber dust. The flip side here is that a moving table design might be advisable. Personally I would want a machine larger than 12x24 and might suggest a square machine of approximate 24 x 24 in capability. Frankly I'd take you most common sheet size and add a couple of inches to each dimension.

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Features I'm looking to incorporate:
* Ballscrews on all axis
* Profile rails on all axis
* Router frame constructed of aluminum extrusion and MIC-6 plate

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I'm not a big fan of aluminum T slot extrusions mainly because of the cost and somewhat the reliability of the t slot connections system. There is a wide variability in T slot systems so we could pull this thread off track so I will let it rest here. Steel tubing even aluminum tubing can be much cheaper.

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* All rails/screws fully shrouded to protect from dust created by abrasive machining
* Water-cooled 80mm spindle and VFD
* Ability to use tooling up to 1/2"
* Home/limit switches on all axis
* 2x ballscrews and 2x steppers on X axis
* Gecko G540, NEMA23 steppers, and LinuxCNC for control

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Since you have unusual design requirements I'd reserve jumping on controls until you are sure about the the mass you will be moving around on the Y axis. I'm actually leaning towards suggesting two machines.

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So, the questions I have for you guys:

1) Should I go big (24x48") or stay small (12x24"). My shop isn't large. Having the extra work envelope is nice, but if I can build a smaller and stiffer/more performant machine then that is also attractive as this machine *will* be used for production runs. The parts I'll be making will generally be small (on the order of 1.5x5x.25"), and the materials they're made from generally only come in small sheets (12x24" usually).

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I'd go slightly larger than you common sheet size assuming you can cut out multiple parts in one setup. The reason to go slightly larger is to be able to support machining fixture plates and the like on your machine. One of the keys to leveraging a CNC machine in a production environment is being able to easily switch setups and like wise make those fixtures for your parts.

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2) What kind of extrusions should I use for the machine frame? I can either get t-slot extrusions from Misumi, or I can simply use extruded rectangle tube from my metal supplier. I have a mill and lathe, so machining the tubing should be ok. The simplicity and cut-to-size nature of the t-slot is appealing, but at the same time I'm sure I can get more stiffness from 2x3" (or larger) rectangular tube.

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For what you want I'd be looking at much stiffer tubing, probably in the 5 to 8" range.

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If you guys think any of this sounds nuts please do speak up! The primary goal for this machine is to be performant and long-lasting. I'm less worried about it being the cheapest possible build as I know that once I get it running it will make my life better and help me get more product out of the shop.

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There are a couple of good threads on gantry design from a few weeks back, one even has a spread sheet to help you understand what the gantry will do under stress. What you have to do is balance mechanical requirements with the much short distance between supports which in this case might be as little as 26". To build a nice stiff machine you will need to work with nice stiff tubing larger than what you are imagining right now.

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I have a reasonable amount of experience fabricating parts, but I'm not an expert machinist by any stretch of the imagination. I am very familiar with all the parts necessary for the CNC though, as well as the required CAD/CAM workflows as I've built and run several kit CNCs in the past. I want this one to be a good deal more serious!

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If this is the case I'd most certainly focus on a stronger machine than you have indicated here. The steel tubing to do the machine isn't that expensive really, so I wouldn't dismiss the idea on expense reasons. It may require machining capabilities you don't have but that is what machine shops are for.

I'm also having a hard time seeing how abrasive machining can be incorporated into a 3 axis machine cleanly without adding a lot of complications. Of course sitting back and thinking about the abrasive machining problem might help but I really thinking you will need two machines.

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Any input it appreciated! Drawings and CAD mockups will start coming soon.
-Aaron

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I'd love to see what you have in mind.

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Originally Posted by aarongough

I spent a lot of time watching YouTube videos of people running aggressive production toolpaths on big CNC machines (Vertical Machining Centers mainly). After watching a bunch of those I was always pushing my milling machine and old CNC router to see what they could do. Hard to keep up with a VMC though!

Here's a fun example of what the big machines can do:

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Nice video but just realize that building a router type machine to even come close to this performance will be a challenge and likely more expensive than it is worth. After all if you really needed that performance you would be shopping for a VMC not a home built CNC router. You most certainly won't accomplish these sorts of machining rates with a machine built out of 2" square tubing of any type.

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The page here recommends that tools under 1/8" be kept to 0.25x diameter max DOC when slotting. Tools 1/8" diameter and above can go to 1.5x diameter DOC when slotting. 1.5x diameter DOC is recommended for all tools when profiling (only one side of tool touching workpiece). If the machine is rigid enough then it's good to go with the maximum DOC possible as that way you're using more of the tool and tool life will go up!

Solid Carbide Speed & Feed

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For most home machine builds the other big limitation is the real capability of the spindle. It isn't like you will have twenty horsepower available to you.

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Great post with lots of information.

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Hey wizard! Long time no type! Thanks for the detailed response!

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A small router type machine is a good choice but not the only one. Another option would be a CNC like Tormachs small machine with the fast spindle.

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Small Tormach is indeed also an option. My worry with those machines is just that they're not setup to deal with abrasive dust from composites. I'm worried I'd kill it quickly. That being said I'm sure I would find many uses for a machine like that.

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Place a big importance on your personal safety. Consider a fully enclosed machine to control dust.

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Absolutely. I wear a full-face P100/OV respirator when I'm in the shop. My last machine (V90) was also fully enclosed with a HEPA dust collector, and I intend to do the same for any new machine. I'm not a fan of dust especially seeing as I work on the belt grinder for hours at a time with toxic materials...

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My first thought is that this would require a substantially different machine. Thinking about it a bit I suspect that there are some possibilities if you have a lot of z clearance. Your z axis may need to wrap around gantry though and the likely need for a rotary on the X. I think the bigger issue here is software. You may have trouble finding CAM software that will work well with your grinding arraignment, especially in conjunction with a rotary axis. Hopefully somebody with experience here will chime in.

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I agree that CAM might be a bit of a nightmare. I'm a programmer by trade though so I'm not adverse to the idea that I would have to write g-code by hand, or that I would create a program to output the required g-code. The toolpaths I have in mind would be deliberately simple to facilitate this. I would also only use the machine for parts that are well finalized, so hopefully not too many changes required.

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Consider no bigger than required to make enclosing the machine easier to control all of that dust. The last thing you want to be doing is to breathing in glass and carbon fiber dust. The flip side here is that a moving table design might be advisable. Personally I would want a machine larger than 12x24 and might suggest a square machine of approximate 24 x 24 in capability. Frankly I'd take you most common sheet size and add a couple of inches to each dimension.

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24x24 is definitely an option. The reason I was thinking 12x24 was to minimize the gantry span and help maximize stiffness that way. I have enough space in my shop to fully enclose a 24x48" machine though, and if I was going to have the gantry span to 24" I would be tempted to go to a quarter-sheet size for future flexibility.

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I'm not a big fan of aluminum T slot extrusions mainly because of the cost and somewhat the reliability of the t slot connections system. There is a wide variability in T slot systems so we could pull this thread off track so I will let it rest here. Steel tubing even aluminum tubing can be much cheaper.

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I was thinking aluminum tubing with thick walls, that way I could machine the ends to accept dowel pins and bolts. I agree about the t-slot being a lesser option, obviously for many people it's necessary as they don't have the machinery to work with plain tubing. I'm perfectly fine going with plain tubing though, and it was the direction that I was leaning in already.

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Since you have unusual design requirements I'd reserve jumping on controls until you are sure about the the mass you will be moving around on the Y axis. I'm actually leaning towards suggesting two machines.

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The only reason I mentioned the G540 was that I already have one and the appropriate steppers and power supply. They were bought from Ahren when I was planning to convert my small mill to CNC. I used the G540 on the Fireball V90 and was impressed with how well it worked. If it's not appropriate for the build at hand then I'll happily put it aside for another project.

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I'd go slightly larger than you common sheet size assuming you can cut out multiple parts in one setup. The reason to go slightly larger is to be able to support machining fixture plates and the like on your machine. One of the keys to leveraging a CNC machine in a production environment is being able to easily switch setups and like wise make those fixtures for your parts.

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Agreed. Fixturing is something that's been on my mind. I was thinking that I'd make the machine base out of a plate of aluminum with some dowel pin holes spaced across it. That way fixture plates could be removed and replaced quickly while maintaining alignment. I saw a fixture plate like this made for the Tormach by NYCCNC. It was basically a 1" grid of alternating 1/4-20 bolt holes and 1/4" dowel pin holes.

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For what you want I'd be looking at much stiffer tubing, probably in the 5 to 8" range.

There are a couple of good threads on gantry design from a few weeks back, one even has a spread sheet to help you understand what the gantry will do under stress. What you have to do is balance mechanical requirements with the much short distance between supports which in this case might be as little as 26". To build a nice stiff machine you will need to work with nice stiff tubing larger than what you are imagining right now.

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I will have a look for those threads. As I understand it stiffness goes up more quickly with increasing cross-section than it does with increasing density within the same space. ie: a 5x10 tube with 1/4" walls would be stiffer than a 2x4 tube with 1/5" walls. I was aiming to leverage that by using aluminum tubing with larger cross-sections. Something like 3x6" with 1/2" walls was what I had in mind, but I'm fine to go larger as well.

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If this is the case I'd most certainly focus on a stronger machine than you have indicated here. The steel tubing to do the machine isn't that expensive really, so I wouldn't dismiss the idea on expense reasons. It may require machining capabilities you don't have but that is what machine shops are for.

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The reason I had thought about ALU tubing rather than steel was mainly because I was worried about straightness and distortion during machining. I know a place where I could get steel tubing surface ground, but as I understand it most grades of steel tubing would likely distort after such machining.

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I'm also having a hard time seeing how abrasive machining can be incorporated into a 3 axis machine cleanly without adding a lot of complications. Of course sitting back and thinking about the abrasive machining problem might help but I really thinking you will need two machines.


I'd love to see what you have in mind.

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Understandable! To help build a picture in your head here are some photos of the parts I'll be machining. In this case it will be just the handle section (the brown part) which is made in two sections, both from 1/4" brown G10 sheet:

http://i.imgur.com/6jFUFSql.jpg

http://i.imgur.com/1fmg4wkl.jpg

It's a little hard to see, but the shape is mainly a radius perpendicular to the long axis of the knife, along with several chamfers that follow rounded paths. Here's a 3 view sketch that hopefully makes the shape a little easier to discern:

http://i.imgur.com/8tJMqAEl.jpg

The hatched sections are all at approx 45º which means I could likely machine them with a large chamfer bit. The outline could be done with an endmill. Then once all that's done the abrasive machining could go ahead like so:

http://i.imgur.com/LTvDLJpl.jpg

The wheel would be parallel to the Y-axis of the machine and would basically follow a 'humped' toolpath over the block of G10. Obviously I'm not at all sure that this would work, but it's something I've been considering. If it works then it also has the potential to allow me to abrasively machine the bevels on some kinds of blades, but again that's purely speculation. Ideally I'd like to build a machine that allows experiments of this nature though... It's good fun and also has the potential to make my life a lot easier.

If it possible to get reasonable tool life from a ball-nose tool when 3d-carving G10 then I'd be open to doing that as well. From what I understand though that's not likely to be the case. I've heard that even good quality carbide tools generally have a lifespan of around 30 minutes when cutting G10. That's from an unsubstantiated source though, so it's possible that gent just didn't have his toolpaths setup right.

Note that I expect to still do fit and finish work on these parts by hand. Blending sharp edges and so on. I'm totally fine with that as it help ensure the quality of the product. I just want to make the gross shape much more repeatable both for productivity reasons, and also so I can interchange sheaths between the knives (which is not possible right now because each handle is so different).

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Originally Posted by wizard

Nice video but just realize that building a router type machine to even come close to this performance will be a challenge and likely more expensive than it is worth. After all if you really needed that performance you would be shopping for a VMC not a home built CNC router. You most certainly won't accomplish these sorts of machining rates with a machine built out of 2" square tubing of any type.

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Yes, I absolutely understand. The video was solely for the purpose of illustrating to Jkountz that solid carbide tooling is quite capable of taking very big bites, provided the machine is sturdy enough and has enough power.

I'm not aiming to run a 3/8" endmill at 1.5x D through ALU, but I would love to be able to run a 1/8" endmill at 1x D through aluminum on the machine I build! I plan on converting my mill sometime in the future, so that will be available to deal with the jobs that require more grunt. (Or I may end up buying something like a Tormach)

You should check out Dbsharp's build (http://www.cnczone.com/forums/cnc-wo...num-cnc-4.html). He also decided to go an aluminum frame and the result is one heck of a rigid machine! Not sure what feeds he is using but you can really see how much material is being removed in some of his videos

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Originally Posted by crazydiamond

You should check out Dbsharp's build (http://www.cnczone.com/forums/cnc-wo...num-cnc-4.html). He also decided to go an aluminum frame and the result is one heck of a rigid machine! Not sure what feeds he is using but you can really see how much material is being removed in some of his videos

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Hell yeah! Thanks for the link, that machine seems to have exactly the sort of capabilities I'm looking to replicate...

A couple of points. Square aluminum tubing isn't guaranteed flat anymore than steel is. Extruded T slot might have specs on it but it isn't absolutely flat either. This is why some suppliers offer to machine rail mounting surfaces flat. Even Aluminum can move after machining, it is something we have dealt with at work. As for steel tubing yes there is a chance of movement after flattening the surface. If it makes you feel better you can have it normalized.

As for your grinder I didn't even imagine anything like that at all. Rather I was thinking of a vertical spindle that would somewhat approximate an end mill. Well at least for the part of the spindle the grinding belt wraps around. The obvious problem here is that in a conventional 3 axis machine you would only address one side of the piece. This you would need a rotary axis setup to revolve perpendicular to the X. So your part could be machined similarly to the way and endmill might cut it. Given that the part to be machined would sit on a pedestal you should be able to machine the top, sides and maybe even the ends. The radius of the grinding spindle working roller would dictate feature size.

This may be hard to imagine. However I see it as fairly simple to implement because the bulk motions would be just like milling with the side of a mill cutter.

You might also look into PCD, (Poly-Crystalline Diamond), tooling to machine the garolite. They're expensive, but you sometimes see them come up surplus on Ebay. I remember someone saying that the tool life was way better than carbide.

Luke

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Originally Posted by wizard

A couple of points. Square aluminum tubing isn't guaranteed flat anymore than steel is. Extruded T slot might have specs on it but it isn't absolutely flat either. This is why some suppliers offer to machine rail mounting surfaces flat. Even Aluminum can move after machining, it is something we have dealt with at work. As for steel tubing yes there is a chance of movement after flattening the surface. If it makes you feel better you can have it normalized.

As for your grinder I didn't even imagine anything like that at all. Rather I was thinking of a vertical spindle that would somewhat approximate an end mill. Well at least for the part of the spindle the grinding belt wraps around. The obvious problem here is that in a conventional 3 axis machine you would only address one side of the piece. This you would need a rotary axis setup to revolve perpendicular to the X. So your part could be machined similarly to the way and endmill might cut it. Given that the part to be machined would sit on a pedestal you should be able to machine the top, sides and maybe even the ends. The radius of the grinding spindle working roller would dictate feature size.

This may be hard to imagine. However I see it as fairly simple to implement because the bulk motions would be just like milling with the side of a mill cutter.

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I'm open to using either steel or aluminum, I think either would work fine provided I can tame the warp during any machining. I am honestly not too sure what the straightness/flatness tolerances are on either the ALU tube or the different grades of steel tube. I'll have to do some research on that.

Given that I don't have a surface plate or straightedge big enough to check surfaces of that size I was vaguely contemplating the idea of using 3 pieces of tube all the same size (2 sides and gantry) and then scraping them together to get flat surfaces on all. It would be a bit of work, but would likely ensure the flattest possible surface. The piece for the gantry could be cut to length after scraping.

Alternatively I could simply farm the tubing out to a machine shop and have them fly-cut both sides of the tube. Am I correct in thinking that skim cutting both sides should help even out any warp? It would be fun to make as many of the parts myself as possible though.

Regarding the grinder idea: I actually have no need to machine both sides of the part. Each handle is constructed of 2 pieces that are only machined on one side each. I get what you're explaining in terms of the rotary axis, but I'm not sure it's needed for what I had in mind... We might be misinterpreting each other's explanations also, always a bit hard in text :)

Some quick looking around give size variation tolerances for 6061-T6 square tube at around 0.025 - 0.050" for 6-8" tubing. Not as good as I thought it would be to be honest.

DBSharp solved this in his build in an interesting way: he poured 'footings' of self-leveling epoxy for all the rail beds. Reminds me a lot of the epoxy-granite thread that was going round a while back.

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Originally Posted by Trotline

You might also look into PCD, (Poly-Crystalline Diamond), tooling to machine the garolite. They're expensive, but you sometimes see them come up surplus on Ebay. I remember someone saying that the tool life was way better than carbide.

Luke

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Thanks, I'll have a look into it!

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Originally Posted by aarongough

Some quick looking around give size variation tolerances for 6061-T6 square tube at around 0.025 - 0.050" for 6-8" tubing. Not as good as I thought it would be to be honest.

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Well that is what they quote for a spec, what you actually get """MAY""" be better than that. Note the highlighted 'may'.

At work we have a number of special purpose machines built by various manufactures that have gantries. These gantries are made in a huber of different ways, some custom extrusions, some square steel tubing and so forth. No matter who made the machine nor the materials it is made out of the mounting surfaces for the rails have been machined flat. These machines don't require the ultimate accuracy of a CNC router either. but a good mounting surface is imperative to long rail life.

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DBSharp solved this in his build in an interesting way: he poured 'footings' of self-leveling epoxy for all the rail beds.

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That is a unique way to correct the surface. I have to wonder how well self leveling epoxy has held up for him. A more traditional machine tool rebuilding approach would be to use something like Moglice. By the way Moglice (Devitt Machinery in the USA) has an updated web site with a nice downloadable manual, so Dave just managed to kill oh about an hour of time looking at pictures and drawings.

The problem for us as home builders is that we don't have the tooling to replicate from especially if we are interested in larger machines. However there may be some advantage of building up a surface this way and machining afterward. The idea here being that for stability reasons we may not want to machine the actual steel tube. In any event go kill an hour or a day looking at some of the other ways to do things.

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Reminds me a lot of the epoxy-granite thread that was going round a while back.

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Epoxy granite might still be in your future as a fill to dampen the tubing. Depending upon the results you are after epoxy fill can go a long ways to improving a machine built with tubing. It works well in a fix gantry system because you can add all the mass there you want without problem.

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Well that is what they quote for a spec, what you actually get """MAY""" be better than that. Note the highlighted 'may'.

At work we have a number of special purpose machines built by various manufactures that have gantries. These gantries are made in a huber of different ways, some custom extrusions, some square steel tubing and so forth. No matter who made the machine nor the materials it is made out of the mounting surfaces for the rails have been machined flat. These machines don't require the ultimate accuracy of a CNC router either. but a good mounting surface is imperative to long rail life.

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I figured as much... I have some 4" 6061T6 tubing with 1/4" walls just kicking about the shop. I'm going to try a few methods to flatten a surface on it and see what I come up with...

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That is a unique way to correct the surface. I have to wonder how well self leveling epoxy has held up for him. A more traditional machine tool rebuilding approach would be to use something like Moglice. By the way Moglice (Devitt Machinery in the USA) has an updated web site with a nice downloadable manual, so Dave just managed to kill oh about an hour of time looking at pictures and drawings.

The problem for us as home builders is that we don't have the tooling to replicate from especially if we are interested in larger machines. However there may be some advantage of building up a surface this way and machining afterward. The idea here being that for stability reasons we may not want to machine the actual steel tube. In any event go kill an hour or a day looking at some of the other ways to do things.

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I like the simplicity of the epoxy idea, but as you said I worry about longevity and also worry that the softer epoxy in between the rails and the structure would kill off some of the advantage of building a sturdy machine.

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Epoxy granite might still be in your future as a fill to dampen the tubing. Depending upon the results you are after epoxy fill can go a long ways to improving a machine built with tubing. It works well in a fix gantry system because you can add all the mass there you want without problem.

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Absolutely! I have been toying with ideas regarding other, lower density, damping mediums in order to damp out the gantry...

I bought all the components for the control computer last night. I think I'm going to try to go for hardware step generation using a MESA 5i25 & 7i67 combo. I think it's worth it just for all the extra I/O alone. Let alone improved step rate and consistency. It will be interesting to see how that pans out, I'll be calling MESA later today to get their recommendations and place my order.