[awerby]

After having to put up with my criticisms of a lot of your designs, now is your chance to weigh in on one of mine! The idea is to build a sizeable, fairly heavy-duty and versatile CNC router which could either mount a spindle directly and a table to work in 3-axis mode, work in 4-axis mode with the table removed, or have a swiveling/tilting head fitted to work in 5 or 6 axis mode.

The idea is to weld up four 4-foot sections of 12"x 12" square pipe with base-plates so that these units can be bolted to the floor. They would be filled with concrete so that each one would have four sections of one-inch threaded rod sticking up. Some 10-foot lengths of heavy steel tubing 4" x 8" with a 3/8" wall thickness would be drilled to accept these rods, and then precisely leveled with nuts and washers. Then each one of them would have epoxy cast on top to provide a flat surface to mount 20mm profile rails.

On the side of each tube a piece of aluminum with a shoulder milled into it would be mounted and carefully shimmed to be straight and parallel to the one on the other side. This would have sections of Nexen rack attached to it, so it could be driven on both sides at once with roller-pinion wheels, a system that's supposed to eliminate backlash and the other inefficiencies of rack-and-pinion drives.

On top of these rails, a gantry built from two 72" lengths of 4" x 4" x 1/4" wall stainless steel tubing with an aluminum backing plate to unify them will ride on ball-bearing trucks. Between these tubes, a ball screw will transmit motion to the Z axis, rolling on a set of 15mm square profile rails. The Z axis will provide about 30" of travel, powered by a harmonic-drive servo actuator, and will itself be a linear actuator with internal ballscrew and profile rails. The spindle will either be attached directly to this actuator or to a B/C axis head which would provide the other 2 degrees of motion, powered also by harmonic-drive servo actuators.

The 4th axis would be a commercial rotary table, and the tailstock would be repurposed from a lathe.

This design is still in progress - there are lots of things I haven't figured out yet, like what's driving all these different servos, whether it's all powered by servos or some steppers should be used, whether to use a commercial control or build my own from a breakout board and individual drivers, how exactly I'm going to get everything to line up perfectly in all dimensions - but I figured it was far enough along to ask for feedback from my pals here in the Zone. So don't hold back - tell me what you really feel.

[ger21]

Is the floor a single piece slab, and can it carry the weight?

I have two Nexen pinions I'll sell you cheap, if you're interested.

[awerby]

Fortunately my shop is slab-on-grade; I don't think it's going to sink into the ground. And as long as the parts don't have to move, I figure the heavier the better, right? I'm still wondering about the gantry beam, though - I was intrigued by the fellow who's making his out of carbon-fiber composite, but it seems that takes a lot of skills I don't have. Making it out of stainless tubing seems like it would be stiffer than regular hot-roll steel (let alone aluminum) and probably straighter as well, while weighing about the same, although that will still be a lot.

Were you considering building a machine with roller pinions but gave up? I got a pair of them, but had to wait a year before the racks came up on Ebay. They were still quite expensive, although not quite their weight in gold, as Nexen prices them. Are yours the large A series (22mm) or small B series (18.5mm) type? I was considering using it for the Y axis, but I already have a ball screw that will work.

[ger21]

Yes, I was planning on using them, but changed my mind. My original plan was to make my own rack from phenolic.
The part numbers on mine are RPS25P-BISO050.00MMP3
Part No.966673.

They are pretty big, about 5" across.
I have two of them, I paid $180 each. I'll sell them both for $180, delivered.

[1Jumper10]

Dam ambitious design! Don't see any show stopping problems but I do have a suggestion. Instead of drilling your 4"x8"x 3/8" wall tubing to fasten to the tops of your corner stands, drill 8"x8"x1/2" plates and weld them to the tube. It's just easier to drill accurate holes on plates versus the tubes since you can use a drill press. You could also use the plates as a jig to accurately locate your threaded rod in your concrete.
Also, I can't quite tell but it looks like you're using a harmonic drive to drive your ballscrew?? Typical gear reduction on a true harmonic drive is about 100:1. Too slow for a ballscrew.


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[Jim Dawson]

Looks like an interesting project. I question the need to use stainless tubes on the gantry. I don't think you will gain anything over structural steel. 4x4x1/4 steel tube is pretty stiff and normally is straight except that it may have a very small amount of ''cup'' on the sides. I think you will find the same thing in stainless.

Should be really fun to do the controls and system programming, but thinking about the G code makes my head hurt.

[KH0UJ]

It`s sure is a very strong set-up it`s like a "Hulk" version of a CNC, if I had just the extra money im sure im gonna assemble mine too but personally I like a bigger table (4 ft. X 8ft.) 24 inch Z travel, I dont really need too much torque on all axis since I can program a gradual CAM even on stainless steel, with a 4 x 8 table I can load a whole barret 50 caliber gun without cutting it by sector, it`s a pain in the ass dividing it in sectors then manually zeroing it in each section, right now im cutting it in sections to fit on the table because the CNC is too small for the job, gets the job done but it takes a lot of time re positioning it for another CAM program.

[johncarpenter]

Two things to maybe check. 1. Sag in the middle of your 8 foot span with gantry weight and machining loads. Might be surprising. 2. It is unclear what the fore and aft spacing is between the bearings that attach the gantry to the rails on the bed. Spacing looks narrow. The more space between these bearings then the better the moment resistance of the gantry to tilting forward or aft.

[bostosh]

I'm fascinated with the desire to build your own machines.

We had a 6ft dia horiz mount rotary t able on the floor and did f18 intake ducts.
it was big enough to do a F5 Fuselage.

it was fun to get paid to mess with this stuff for R&D
I'll bet that this 7axis robot is in the scrap iron metal pile now

[awerby]

Dam ambitious design! Don't see any show stopping problems but I do have a suggestion. Instead of drilling your 4"x8"x 3/8" wall tubing to fasten to the tops of your corner stands, drill 8"x8"x1/2" plates and weld them to the tube. It's just easier to drill accurate holes on plates versus the tubes since you can use a drill press. You could also use the plates as a jig to accurately locate your threaded rod in your concrete.

[Thanks for the suggestion, but what would I gain by adding those plates? I was going to use a mag drill on the tubes, which works about as well as a drill press, and I didn't want to warp my tubes by welding on them, if I could avoid it. Would doing it that way give me anything
more than holes that are possibly better aligned? I was considering putting some plates like that on top of the vertical tubes, though, to align my threaded rods better when I'm casting them in the concrete.]

Also, I can't quite tell but it looks like you're using a harmonic drive to drive your ballscrew?? Typical gear reduction on a true harmonic drive is about 100:1. Too slow for a ballscrew.

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Actually, I haven't entirely decided what to use to drive that ball screw. It's a very fast screw, with 1" pitch, so some kind of reduction will be required if I use a servo (in my drawing, I'm picturing a servo with a small pulley on the shaft that turns a large one on the screw, for about a 3:1 reduction). But steppers like going slowly, so I figured I could use one and drive it directly at 1:1. The Z axis, which also has a 1" pitch screw, will be driven by a harmonic drive, according to the current plan; I was hoping that the gear ratio would be enough to keep it from backdriving and falling to the table when the power's turned off. I've got these little harmonic drive units already (Hi-T Drive servo actuator RH-14-3002-E100AL-SPA822) but haven't determined what the reduction on them actually is. Would 100:1 really be too slow, with a servo motor and a 1 pitch screw? I have 3 of these things, and planned to use 2 of them to run the 5th and 6th axes, where speed shouldn't be a big issue.

[awerby]

Looks like an interesting project. I question the need to use stainless tubes on the gantry. I don't think you will gain anything over structural steel. 4x4x1/4 steel tube is pretty stiff and normally is straight except that it may have a very small amount of ''cup'' on the sides. I think you will find the same thing in stainless.

[My rationale on using stainless instead of regular hot-roll was that I thought it would be stiffer, and I could save weight by using a thinner tube. But I could be talked out of it - especially when they tell me how much it's going to cost...]

Should be really fun to do the controls and system programming, but thinking about the G code makes my head hurt.

I'm with you there - I've been able to wrap my head around 4-axis milling, and have never attempted more. But it would be nice to be able to reach more areas than are currently possible, and I figure I can start by indexing the extra axes - even just positioning the tool horizontally rather than straight up and down opens a range of possibilities. What would you suggest for a control?

[awerby]

Two things to maybe check. 1. Sag in the middle of your 8 foot span with gantry weight and machining loads. Might be surprising.

[I've been worrying about that as well, but this tube is pretty massive. I suppose I could go for one that is even larger in the Z dimension, if it seems that 8" isn't enough. Originally I was planning on using smaller tube and 6 pillars rather than 4, but I wanted to be able to protect the whole thing with bellows, and the middle pillars would get in the way of that. I guess I'll have to do some FEA and see how bad it really is.]

2. It is unclear what the fore and aft spacing is between the bearings that attach the gantry to the rails on the bed. Spacing looks narrow. The more space between these bearings then the better the moment resistance of the gantry to tilting forward or aft.

Right; I may not have spaced them adequately in my drawing, but I'll be sure to put them as far apart as possible when I build it.

[awerby]

It`s sure is a very strong set-up it`s like a "Hulk" version of a CNC, if I had just the extra money im sure im gonna assemble mine too but personally I like a bigger table (4 ft. X 8ft.) 24 inch Z travel,

[I'm glad you think it looks strong enough, but the travels on this thing are bigger - it's about 9'X, 5' Y and 30" Z.]

I dont really need too much torque on all axis since I can program a gradual CAM even on stainless steel, with a 4 x 8 table I can load a whole barret 50 caliber gun without cutting it by sector, it`s a pain in the ass dividing it in sectors then manually zeroing it in each section, right now im cutting it in sections to fit on the table because the CNC is too small for the job, gets the job done but it takes a lot of time re positioning it for another CAM program.

[Yes; I was hoping that with this thing I wouldn't need to worry about machining things in sections and trying to register them for that - it's never worked perfectly when I've tried to do that. But fortunately, I'm not planning to cut stainless with this machine, although I was thinking I might try carving stone, once I had it all protected with bellows.]

[awerby]

I'm fascinated with the desire to build your own machines.

We had a 6ft dia horiz mount rotary t able on the floor and did f18 intake ducts.
it was big enough to do a F5 Fuselage.

[Wow. What did that huge rotary table do for you that the machine couldn't do on its own? Was it just for more accuracy in making large circular cuts, or was that the only way it cut, in radial mode?]

it was fun to get paid to mess with this stuff for R&D
I'll bet that this 7axis robot is in the scrap iron metal pile now

It sure would be more fun to get paid for doing this instead of having to pay! If the head from that thing is in the scrap heap, could you pull it out and send it to me? I'll be glad to pay for shipping...

[1Jumper10]

It looks like your servo actuator is rated at 30rpm at the output shaft. 1" pitch screw would get you 30IPM. Seems a little slow. I think it's a planetary gear reducer and not a true harmonic drive but it's zero backlash so it's a moot point.

My suggestion for the plate was made before I knew you had a mag drill. Carry on

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[Jim Dawson]

I'm with you there - I've been able to wrap my head around 4-axis milling, and have never attempted more. But it would be nice to be able to reach more areas than are currently possible, and I figure I can start by indexing the extra axes - even just positioning the tool horizontally rather than straight up and down opens a range of possibilities. What would you suggest for a control?

Well, mine of course And the best part is that it is absolutely free (for now), but does require a Galil motion controller. Right now it will do full 4 axis profiling, but could do up to 8 axes with some minor tweaks. I just implemented and tested the 4th axis on my mill on Saturday. It's in beta testing right now running on 4 machines, 3 mills and 1 router. If you want to be a guinea pig, I'm game. But you get to generate the 6 axis G code

[johncarpenter]

If you assume for the sake of time saving that the weight of the beam is concentrated in the middle of the span and that the gantry weighs 250 lbs, then the deflection of the main span in the middle might be almost be 1/16 of an inch. Assuming the internet calculator is correct. That might or might not affect your machining.

[awerby]

It looks like your servo actuator is rated at 30rpm at the output shaft. 1" pitch screw would get you 30IPM. Seems a little slow. I think it's a planetary gear reducer and not a true harmonic drive but it's zero backlash so it's a moot point.

[Thanks for finding that; I agree it sounds a bit slow for the Z axis, but they should work okay for the pivot and tilt.]

My suggestion for the plate was made before I knew you had a mag drill. Carry on



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Yes sir!

[awerby]

Well, mine of course And the best part is that it is absolutely free (for now), but does require a Galil motion controller. Right now it will do full 4 axis profiling, but could do up to 8 axes with some minor tweaks. I just implemented and tested the 4th axis on my mill on Saturday. It's in beta testing right now running on 4 machines, 3 mills and 1 router. If you want to be a guinea pig, I'm game. But you get to generate the 6 axis G code

That's an interesting offer, Jim -"free" is a very attractive price. What does your system consist of? Does it require a particular model of Galil control, or can I find an old one someplace that will work? No problem about it not being quite ready yet, since this thing will take me a while to construct. And since I'm a VisualMill distributor, the six-axis CAM is something I have access to, and really should become more familiar with.

[awerby]

If you assume for the sake of time saving that the weight of the beam is concentrated in the middle of the span and that the gantry weighs 250 lbs, then the deflection of the main span in the middle might be almost be 1/16 of an inch. Assuming the internet calculator is correct. That might or might not affect your machining.

It would, I'm afraid - thanks for checking that. The gantry probably will weigh about 250 lbs with the Z axis, heavy ball screw, spindle and the 5th/6th axis assemblies. But don't I get to divide the weight by two, since it's supported on two sides? I suppose when all that stuff moves to one side or another, it starts weighing a lot more on that side, though. Fortunately, I haven't ordered the steel yet. What if I went up to 4"x 10" - would that improve things significantly?