Lead screw vs. ball screw

[RayJohns]

Here's a question I have been wondering about:

I have a Clausing 8520 milling machine now - currently taken apart for cleaning and restoration - and I was giving some thought to seeing about maybe trying to add CNC to it at some point.

Anyway, in doing a little preliminary research, it seems that ball screws work well for stepper motors, but maybe not so well for manual cranking. My original plan was to use some dual shaft motors, so that I could retain the hand cranks.

So I guess my question boils down to this: is there a ball screw pitch that would work well for both stepper motors and hand cranks? Or would just adding stepper motors on top of the existing "ACME" style lead screws be a possibility? I was thinking maybe I could do something with the lead screw nut to reduce the backlash on the stock lead screw/nut setup and/or use Mach3's backlash compensation to offset it.

I don't have much experience, but I do have a good background in computers and electrical engineering, so the CNC part shouldn't be much of an issue. My main concern is in the area of the lead screw vs. ball screw and how that may affect wanting to retain manual crank handles.

Any input would be most welcome. Thanks!

Ray

 

[Blue Steel]

Ball screws have the huge advantage of zero backlash. If you want to do stuff manually you can on CNC just by using an electric hand control that is part of your controller. You can even put the tool where you want to and putting a bit of code using MDI to run a pass if that suits you better. Have a look first and see if it is economical to rework your machine or if it is better to buy one that has all the bits already.
Steve

 

[RayJohns]

Ball screws have the huge advantage of zero backlash. If you want to do stuff manually you can on CNC just by using an electric hand control that is part of your controller. You can even put the tool where you want to and putting a bit of code using MDI to run a pass if that suits you better. Have a look first and see if it is economical to rework your machine or if it is better to buy one that has all the bits already.
Steve

okay thanks! I do hear people say you can use the controls of the CNC for manual work (and I have seen it in action also), but it would be nice to still have the cranks I think.

Is it impossible to use the cranks with ball screws?

The main issue I have with CNC'ing the 8520 Clausing is that the Z axis would have to be driven either off the spindle (with only 3" travel) or the knee (with perhaps a bit more complication in the mounting area).

Ray

 

[PJRitz]

If you're going with CNC, then definately go with the ballscrews, My CNC Bridgeport has ballscrews and handwheels with a DRO mode, it works fine but I never really use it manually, thought I would when I got it, but works so much better just telling the CNC what to do.

Also we put a set of ballscrews in a manual Bridgeport at work to replace the worn out leadscrews and it works great, these ballscrews were the same pitch as the original leadscrews.

PJ Ritz

 

[camaro_dan67]

Ray, I have a supermax with the anilam m control. I have used it in manual mode to do some hand milling it and it works ok. When milling like this you have to be careful how much you cut at one time. Mills set up for cnc have easier sliding ways and the ball screw also moves easier then a manual set up. I always watch my readout when manual milling to make sure none of the axis are drifting.

 

[scudzuki]

Problem with ballscrews when manual machining is you have to lock the table whenever you take your hand off the handwheel.
The table will run away very easily without the static friction that a conventional leadscrew adds.

Joe

 

[MattW]

I'd say start your conversion and leave the lead screws for now. Since this is your first cnc conversion you'll have plenty on your plate. Changing to ball screws is a project in itself, and you'll find you need your disassembled mill to make the parts you need. Work out your controls, compensate for the backlash as best you can, and program around the limitations. When you get it working, then look into the ball screw conversion. and maybe add mpg's for your handwheels.

 

[ions82]

...it is better to buy one that has all the bits already.
Steve

What he said.

 

[Gordon Long]

Joe (skudzuki),
What kind of machine have you used where the table would run away? I use retrofit Bridgeports with ball screws in my shop with college students running them manually without locking the table and I've never seen even a hint of the table moving. The standard acme screws on the other hand will definitely jump if a student tries to climb mill a roughing cut.