Slant-Bed CNC Lathe Build

[JimDawson]

since I don't know what the speed limit on 25mm timing belts is.

Easily more than 5000 RPM, I have run 8x50mm belts faster than that with 10 to 30 KW drives.

Please let me know if hanging the headstock off the rails is stupid for some reason

Looks like it would work. As far as I know, the rails are rated the same in any orientation.

Please let me know if an inner & outer slip fit for the deep-groove pulleys handling the pulley load is a bad idea

That's the way most mill spindles work, so I don't see why it wouldn't work in this application.

 

[barnbwt]

Easily more than 5000 RPM, I have run 8x50mm belts faster than that with 10 to 30 KW drives.


Looks like it would work. As far as I know, the rails are rated the same in any orientation.



That's the way most mill spindles work, so I don't see why it wouldn't work in this application.

Fantastic, it sounds like I'm on the right path, then. The one concern I have with the headstock rails is that the spindle centerline will have to cantilever about 3" past the fixed carriage mounts; otherwise the mounts get in the way of the long tools toward the bottom of the toolplate (same reason fixed rails can't be used). Now, I expect a 3/4-1" thick mounting trunnion at the front/rear of the spindle casing tied to those carriages should be rigid enough for end-work when dropped down low, but it still gives me pause.

Maybe stagger the one guideway furthest from the toolplate so it hangs down lower? The tools could stick out up to 6.5" without any worries that way (versus the 'infinite' clearance they have now), and I suspect anything longer would require drilling past the collet nose simply to control run out; at that point there's no side-turning to do, and the other long tools would have to be omitted so the toolplate could drill to full depth anyway. I don't have plans to try and fit such a large boring job in this rig, but it never hurts to think ahead.

I accomplished today's mission of tracking down & ordering the connector for my 24V power supply*, but man it was hard fought. I was *this* close to saying "screw it" and ripping out the goofy pin headers & replacing them with something normal. Two hours of scouring the net for traces of the defunct Cherokee International company's specs, and two more scouring Digikey for the correct connectors & contacts, and in an available version without a 10,000 piece minimum. But in a week or so, I'll be able to power the motion controller on for real, and get it talking to the PC.

Jim, do the Galil communicate over a typical network cable, or do they need one of those crossover versions? The manuals don't seem to specify, so I'm guessing it's the normal type, but I wanted to ask on the off-chance I can avoid some frustration.

*The connectors that came with the unit have a very goofy V-notch style connection to the wires, and once I'd replaced them with the wire for my power cable, the connection was very unreliable. Solid enough to check voltages and pin-outs, but nowhere safe enough for delicate electronics

 

[JimDawson]

My concern with not having the spindle rigidity attached to the machine structure would be chatter issues. Since you're using NX, you should be able to run some vibration analysis on the system to see what it looks like. Having the spindle support structure cantilevered might be an issue. Additional mass might be your friend here.

I know the Galil can use normal cable, and I recall you can use a crossover also. I think it has an auto sensing port.

 

[barnbwt]

Yeah, it may be a good idea to use a fatter rail & screw on the headstock for that reason. Japanese ones, too.

 

[barnbwt]

I've come across a handful of Swiss style lathes that have sliding/articulated sub-spindles (X-axis, though); I will ask around about their capabilities as far as turning vs. just drilling.

My thinking is that if the headstock is more rigid than the tool plate (probably heavier, larger/higher quality rails & screws, possibly clamped or pinned into fixed positions vs. truly articulated on the fly for most things) it won't be the limiting factor for chatter & accuracy.

I will try to do some sort of flexure analysis; I'm curious if this arrangment is more or less rigid/dynamically undamped than three axes stacked on top of each other. It's kind of telling that a 3 axis Hurco turning center carriage is nearly as massive as the head stock casting above the base. If stacked 3-axes high, my spindle height above the base would go up by over 50% (since the tool plate would need to move up to utilize the drilling tools. Usually that increased swing capacity in a lathe is a win-win, but we all know that the longer load-path between tool & workpiece requires exponentially more mass to remain rigid. By the same token, minimizing swing to the minimum necessary should likewise reduce the amount of mass required overall.

It looks like 5C faceplates are available up to ~6" diameter (and I'd be terrified of a fast servo running away with anything larger), and that's about what fits with the currently proposed Y-axis setup; anything larger is wasted space as I see it.

 

[JimDawson]

That's the way vibration is controlled in my lathe, everything is massive, and then damped with a few thousand lbs of special concrete. Even the small lathes , EMCO for instance, is filled with concrete.

......... (and I'd be terrified of a fast servo running away with anything larger)............

We're going to try to prevent that from happening We'll have encoder loss detection and excess position error shutdown. For initial testing, we'll limit the command signal to a very low value, using the TL command, until we know that everything is correct.

 

[barnbwt]

That's the way vibration is controlled in my lathe, everything is massive, and then damped with a few thousand lbs of special concrete. Even the small lathes , EMCO for instance, is filled with concrete.



We're going to try to prevent that from happening We'll have encoder loss detection and excess position error shutdown. For initial testing, we'll limit the command signal to a very low value, using the TL command, until we know that everything is correct.

I planned on using .25" wall box tube for the frame, and filling it with lead in most areas.

I'm an aerospace guy, so I'm always convinced you can get away with only making the right spots rigid

 

[barnbwt]

So, after looking at some Swiss Lathes (Maier ML20, for example), it appears they have a Y-axis sub spindle of very similar geometric layout to what I'm planning. Talking with some of our machinists, the sub-spindles are perfectly capable of turned side-work in addition to axial end-work, despite not being particularly massive compared with what you normally think of as a spindle headstock. Of course, what the sub can do is limited by the stick out of the material being completely unsupported, and the max stock diameter being small for a Swiss lathe.

(sub spindle is the one at the left; the spindle at right is the primary, but doesn't really do much rigidity-wise in the X/Y directions)

I did notice the bedways are rather wide compared to the small sub-spindle size; they're wider than the spindle is long. In that sense, the guideways are 'sized' like the footprint of a full size spindle headstock. I think I should try to maximize that spacing as much as I am able in my build.

 

[JimDawson]

Wow, there's a lot going on there. But I don't see a Y axis (headstock). What I find interesting is the headstock moves as the Z axis, never seen that before. And it appears that the sub spindle moves in both the X and Z direction.

 

[barnbwt]

Wow, there's a lot going on there. But I don't see a Y axis (headstock). What I find interesting is the headstock moves as the Z axis, never seen that before. And it appears that the sub spindle moves in both the X and Z direction.

Sorry, it's labeled +/-X2. My scheme is the same idea, just turned up a 30deg incline. The wide, flat striped thing in the bottom of the view is the bellows-cover for the sub spindle X axis ways.

All Swiss lathes feed the headstock along Z, since the tool plate (vertical wall section in the middle) only moves along X & Y, and therefore only cuts across a plane right in front of the rotating guide bushing. Think of it like a follow rest, only the rest of the lathe moves relative to it & the saddle for a Z move. A Z's are done via bar-feeding. Very, very rigid setup very, very, very resistant to chatter & workpiece deflection. Only trouble is you can only cut once, since you lose your bushing support if you pull the workpiece back in after a pass. Everything, even 1/4" radius reduction on a 5/8" bar, has to be done in a single pass. The machine above is about 4x4x8 in size plus the shrouds, so this scheme allows for incredibly rigid machines in a small space.

My proposed scheme is a hybrid, where the tool plate does the Z moves (which is nice in that it gives more flexibility for workpiece shape/size, as well as order of operations), but the spindle is still articulated along Y like a sub. I really like the concept since it should make setting up precise tool offsets much easier than without a Y axis, and of course it also packs a ton of tools into a small space just like a Swiss. Plus, I like the idea of only having 2 axes of backlash between the workpiece & frame, and tool & frame.