Experiences with Acme Leadscrews on CNC Lathes

[cjtoombs]

I have been considering converting a lathe of mine to CNC. I had originaly intended to convert it to ballscrews, but leaving the original Acme leadscrews would greatly simplify the installation. This will leave some backlash on both the x and z axes. I know this is very undesirable on a milling machine, but I can see much less reason for not using them on a lathe. Lathes do not climb cut, and there are few instances where the z backlash can cause a problem, and even less with the x axes. I expect there are some members here who have tried this, and I would like to get your take on what was problematic, if you found a solution and what it was and what was just plain anoying about using the Acme screws and dealing with the backlash. This machine will not be used for production, so within reasonable limits, speed is not a major concern, so the additional speed that a ballscrew would provide is not an issue for this machine.

 

[dave2176]

Hi CJ,
There are a number of guys running the original acme screws on their CNC mill, moving to ball screws as they can. Mach has backlash compensation that can help minimize the effect of the backlash. You can move to ball screws later if desired. Your lead screw controls threading, wouldn't you need to maintain the same pitch or change all your change gears? My (non CNC) lathe is I believe 8tpi on the lead screw, I've never seen ball screws in that pitch. How would the half nuts work? Wouldn't you just put steppers/servos in place of the hand wheels and leave the lead screw alone?
I think I'm rambling.
Dave

 

[cjtoombs]

Dave,
I was thinking something along the lines of the conversion that this fellow did on a myford:

http://www.jeffree.co.uk/modelengineering.html

I was planning on leaving the feed gearbox on the headstock end of the lathe, driving the existing lead screw from the tailstock end. This would mean that when the half nut was engaged, the lathe could be either under the control of the feed gearbox, or the gearbox could be put in neutral and the stepper motor used for control. I had also planned on motorizing the x axis, but from the back side, so that I don't have to mess with new graduated dials or handles. As for threading, the CNC doesn't care what the pitch is, as you will program the amount of movement per step on the motor into the software, and it will account for it there. Of course, changing the pitch would affect the use of the original gearbox. The places where I see backlash being a problem are in ball turning, turning anything that looks like a spool. Basically, anything that has to be turned or faced from opposite directions. So if you turned a spool shape without any backlash compensation, it would be too narrow by the amount of your backlash. Depending on wear on your lead screw, the backlash could be different on different parts of the lead screw.

 

[jumps4]

hello cj
I used acme screws on my 9x20 hf conversion with mixed results. the machine works fine and will make nice parts but slows considerably on cv cuts like convex or concave. on a cv cut the slowest axis controls the speed of the machine. my x axis screw is 20tpi and my z is 10tpi. the X axis motor cannot spin fast enough, with any power left to keep up with the z so the motor tuning for x's max velocity setting is the fastest the machine will run. for straight turning and threading this is not a problem because x moves into the next pass position then z makes its pass, x retracts z returns and so on they are not moving at the same time. my machine does not control the spindle speed so as the cv move is being made the spindle does not slow to reduce heat build up in the bit due to rubbing. so slower spindle speeds are needed from the start.
I have purchase all the parts to convert to matched pitch ball screws on x and y but I'm procrastinating....
saved project for another day.
steve

http://www.hobby-machinist.com/showthread.php/8692-9x20-Lathe-CNC-conversion?highlight=9x20;+cnc

 

[cjtoombs]

Thanks Jumps. I looked over your build on that 9x20. I had already planned to use an indirect drive on the back side of the cross feed, so that should eliminate the problems that you are having with slow feed due to the 20 TPI screw (plus I think the leadscrew on this machine is 10 tpi). The lathe I plan to do also has very low speed capability, so course threading shouldn't be a problem, either. I think for the few items that need attention to backlash (like ball turning or turning a spool-like shape) I can deal with that in the planning stages. Most of the stuff I would like the CNC capability for is very simple, but multiple parts, so I will give it a try with the current leadscrews and see how it turns out. Plus threading and turning tapers would be a breeze, since I don't have a taper attachment currently.