Retro with acme screws

[flash319]

So I got a very good shape clone. I would like to convert it to a 2 or 3 axis cnc machine (hobby). Has anyone left the acme threads in the mill and used the DRO scales as feedback to stepper/servo control? Do the motors oscillate because of the backlash? Thoughts?

[Regnar]

I have started looking converting over a manual mill to cnc. In my search I did find someone that didnt change to ball screws but he did mention that he has about .004 and .006 backlash in each axis. For hobby use and if you Setup the controller to make up for this backlash I feel that it should be more than feasible to do. Something that you do have to about is that ballscrews are around 90% efficient where acme is around 40%. I just means that you might have to buy bigger motors.

[BTD]

I'm also looking into modifying my 10 x 54 BPwanese clone.
From what I've read it seems that most who started out using the ACME screws changed over to ballscrews. I plan to use ballscrews (at least X and Y) from the get-go.

As far as DRO feedback is concerned, the answer I recall is that the backlash in the machine will cause the controls to lock up...basically sit there and vibrate just like you said.

I ran across a thread that mentioned a company named Galil (I wonder if they're related to the Israeli machine gun mfg by the same name?). I went to their webpage and found lots of rather high-level technical articles on just this subject. They do use absolute position feedback but it's not as simply as I would have thought. What they do (if I understand correctly) is this. They use the normal encoder feedback to position the servo then they use the "DRO" signal to adjust this position. The basic control scheme is called PID..Proportion-Integral-Derivative. It's used in thousands of applications...in my work, we use PID control schemes for liquid level control, temperature control, pressure control, you name it. In Galil's case, the Proportional part is the servo encoder feedback while the Derivative part is the DRO signal. Apparently, there is no Integral component.

Bottom line is that, for what we do, (modify cheap mills) this type of sophistication is probably out of reach. The consensus on the forums seems to be...tighten up the machine with ballscrews, precision bearings, Z-axis air assist, good lubrication and properly adjusted gibs. Then, use motors with the correct torque for the application. Then, spend the time to tune up the system...oscilloscopes, lots of phone calls, trial and error. Finally, make cool stuff and sweep up lots of swarf.

Good luck and keep us posted...

[chevdrgtrk]

I used my acme screws for years till I couldn't adjust the nuts any tighter, and the backlash got to big to really machine with, so I went ahead and installed ball screws and a HD yoke from Elrodmachine.com. let me tell you it's night and day difference. I got my machine from a shop it was sitting in not even wired up for years, a little dirty, but mechanically in great shape. forget air assist on the knee, I went with 4 130# lift supports from liftsupportsdepot.com at around $36 each. after I get done with my set of parts I'm running now, I have a VFD, spindle indexer (rigid tapping), and a power drawbar to install.

[BTD]

chevdrgtrk...Good point about the air assist (assuming you mean pneumatic cylinders and precision air pressure regulators)...way too big...difficult to mount...can be very expensive. I read a post from chick2 who used the cylinder/regulator approach. Very nice work and obviously a lot of thought went into it but I just don't think I want to go thru all that.

I've just ordered two 150 lb x 15" stroke gas springs from Guden complete with .51" dia ball mounts. Total cost was around $50 and that included shipping.

I actually need a total of 4 gas springs but I wanted to play around with various mounting configurations before ordering the other two.

I weighed my knee/saddle/table/vise and it came up to around 660 lbs. So I figure 4 gas springs at 150 lbs each holds up 600 lbs which leaves 60lbs plus the workpiece weight on the knee lift screw. If the springs are still too light, I can order a couple more with a higher force....after a while I'll end up with several gas springs of various force and can mix and match depending on the work piece weight and mill dynamics...still cheaper and less of a hassle than the pneumatic cylinder method. See attached Solidworks images showing the four springs.

Could you post a couple of pic showing how you mounted your gas springs?

[chevdrgtrk]

yes I mean air cylinder. I actually have 2... 4"X24" throw cylinders that I thought about using, but could not figure out how to regulate the pressure as the cylinder was being compressed. These are for sale if anyone needs them. It was easier to just order the lift assist cylinders, and put them on, no need to hastle with figuring out different pressures. Just to let you know......on mine I actually measured the same distance forward and back from the lifting screw, and put them there......then the table started to chatter up and down. I started to do some checking and it went away when the table was all the way in Y+. so I actually ended up moving the cylinders as far out on Y- as I could get them. Chatter went away, and I can get 35 in a min travel out of the table. I would be able to get more out, but with the 4:1 pulley setup, that is the max that MACH3 will let me do.

[Andre' B]

I actually have 2... 4"X24" throw cylinders that I thought about using, but could not figure out how to regulate the pressure as the cylinder was being compressed.

A regulator will dump air to maintain the output pressure, with cheap regs. the difference in pressure between sourcing air and dumping can be large, more expensive ones can do a better job.

If you happen to have a large tank many times the volume of the cylinders you can connect that to the cylinders and then you can use any cheap regulator and the pressure does not change much (depending on the cylinder to tank volume ratio) and you reduce air consumption to just the leaks.

[chevdrgtrk]

I have a 60 gallon air compressor that I was using for the source. when I put 15 psi on the cylinders with my regulator, it wasn't blowing off till the cylinders reached 75 psi.....that is enough to rip the Z screw out of the machine, not a risk I was willing to take.