Are ballscrews necessary for PRECISE work on a cnc lathe?

[daytrader]

I have an old Atlas 10x36 that I'm going to convert to cnc. I want to make this as simple as possible, as this will be my first cnc. I do, however, have extensive manual machining experience.

I am planning on using all of the existing screws on the lathe. I will drive the threading leadscrew with a stepper motor and engage the half-nut. I will also drive the cross slide screw from the back side of the lathe with a stepper motor.

Question: As long as I only cut in one direction on both axes, will I have the same accuracy as when using the lathe in manual mode? I will mostly be making long curved tapers, where the screw in each axis would only turn in one direction for the lenght of the cut. I won't be turning things such as balls that require one axis to change direction mid-cut.


2nd question: I would also like to thread with this setup. I read through the CNC Lathe Threading thread. If I fool the machine into thinking the thread length is longer than it really is, the tool will retract more than necessary, taking up all of the backlash before cutting the next pass. Do you guys think this will work?

Thanks,

Nick

[balsaman]

Yeah, it will work. I don't see why you would need to make the threads longer than necessary. Just cut the threads, retract, back up, cut the next pass. No different than what you do manually.

Eric

[DareBee]

You must remember to always program extra back tracking to takeup backlash.
It will take bigger motors to deal with the friction of standard screws.
You are limited to the type of work that you did manually, contouring will be difficult without ballscrews.
IMO it is worth the extra money to do it upfront as opposed to wishing you had.

[HuFlungDung]

I really wonder how much life you will get out of the halfnuts. Maybe better have a few extras in stock I don't know if those things are all that cheap, either.

Having used a cnc lathe myself with a bit of backlash in X, I would bend over backwards to avoid it. It just rears its ugly head just when you don't expect it. For example, suppose you are turning a long slender piece and you want to program a slightly modified path to counter the flexure and get a straight cut. This will be an in and out motion with backlash only being a nuisance.

Suppose you want to turn an external taper, getting larger towards the chuck. In order to overcome the effects of backlash, you are going to have to make an initial movement in X+ to take up the backlash. Now as you turn the taper additional X+ movement is required. However, the force of the work acting against the tool will move the cross slide outwards in uncontrolled amounts, because it lifts the cross slide off the bearing side of the nut and across the backlash distance.

This is the same problem as you have with a manual taper turning attachement. It can be overcome by starting at the other end of the work and cutting inwards. This means as a hard and fast rule, that you must always turn external tapers from large to small, and internal tapers from small to large (if you are working in the conventional tool positions). Its just kind of a nuisance that backlash causes.

[daytrader]

This machine will be dedicated to build pool cues. The back half of the pool cue (called the butt) is just a straight taper. The front half of the pool cue (the shaft) has a "curved taper". But the curve doesn't go "in, out, and in again". Although it is curved, both screws will never change direction mid-cut. I had already considered that I would have to cut towards the headstock and the diameter would have to be smallest at the headstock to always be riding against the thread of the screw.

The reason I don't want to use ballscrews is because I want to retain manual use of the lathe. I don't know how I would get rid of backdrive in manual mode, without some sort of brake. And if I did solve the backdrive issue, I don't know how I would easily disengage the ballnut from the carriage, and then get it out of the way of the carriage movement in manual mode.

Another reason is this will most likely be a temporary conversion. Once I see that this works well (hopefully), I will build a dedicated machine for this operation, and restore the lathe to general purpose metal work.

Thanks for the replies, and let me know if this all makes sense. If you guys know of a good way to solve the backdrive and CONVENIENT cnc to manual mode switching, I would like to hear them.

Nick

[HuFlungDung]

If the machine is dedicated to pool cue turning, have you considered the tailstock setover method? This should be easily in the range of taper for a pool cue.

Or, why not use a taper attachment? Granted, you would have to build a full length unit, but that's not really so hard if you have an existing unit to copy/modify. No cnc required.

Another option worth considering, would be a temporary removal of the carriage hand wheel. Build yourself a reduction unit and add the stepper drive on there. This should cause minimum alterations to your machine. I think that using the half nuts on a continuous basis is going to wear out very quickly. Plus, its a dirty environment to use a generous amount of lube on the lead screw.

[daytrader]

The tailstock setover method only works for the butt, not the shaft. The but is a straight (constant) taper. But the shaft actually has a curve to it. I have tried the tailstock method in the past for shafts. I set it over to 4 different tapers, then blended the 4 straight sections together with sandpaper. The results were disappointing, as each section was not exactly concentric with its adjacent one. I was using a 60 deg center drill. I now have a curved center drill that I think will give better results. But, even if the results turned out to be perfect, it takes forever to do the setups.

Most cue-builders do use homemade taper bars. But, you need several of them. One for the butt, and a few for different shaft profiles depending on the need of the customer. Since I'm just starting, I figure it will be best to use CNC. That way, I just change programs when I get a customer who wants something different. PLUS, I do want to join the CNC community

As far as the leadscrew wearing out, I'm not that worried about it. It is already worn out quite a bit near the headstock. I might replace it, before I start anything. On the Atlas lathe, the leadscrew is used for carriage travel and threading. It does not have a separate drive mechanism for power feed and for threading like modern lathes do. So the wear from CNC use should be the same as wear for manual use when using the power feed. Finally, because I'm cutting the full length of the bed, if I do get wear, it will be evenly distributed along the length of the screw, and not focused near the spindle.

The dusty environment will definitely be a concern. I haven't spent much thought on addressing that issue yet.

Thanks,

Nick

[HuFlungDung]

Nick,
Does that lead screw not have a full length keyway in it? I'm not "up" on the old Atlas lathe construction. Does it not have power cross feed either?

[daytrader]

Actually, the lathe has never run yet since I got it 2 months ago. And the guy I bought it from never used it. It was his uncle's who last used it 20 years ago. The leadscrew does have the longitudinal slot in it. When I bought it, I turned the spindle by hand, to check the feed. But I could only get the cross feed to work. I couldn't get the carriage feed to work unless I engaged the half-nut. Its been sitting in my basement all this time. Today I'm actually going to do some work on it....clean it up and see what needs repairing.

I could very well be wrong about the carriage feed only working with the half-nut. I too am not familiar with old lathes. I will try to determine that today.

[HuFlungDung]

Ok, then that machine does have power longitudinal feed, I'll bet. You may try jogging the hand cranks while trying to shift the selection lever from "cross" to "longitudinal" because there's some gears that have to mesh.

The power longitudinal feed actually drives off the keyway in the leadscrew, and then an internal gear train inside the apron carries the power up to the rack gear under the bed, or, to the cross slide screw gear.

The carriage handwheel is working off the same gear train as the power feed, and the primary power feed is against the rack gear. That's why I suggested the stepper drive on the handwheel because very little wear will be caused using the rack for feeding.

As far as X backlash is concerned, you could rig up a crude "preload" system just by attaching a weight and cable hanging down off the back end of the cross slide. This will tend to eliminate the backlash for you. For cutting wood, this should be adequate.

[3t3d]

If you had a ball screw on the X axis, and it was a stepper (low cost) drive, why couldn't you use an encoder for the X axis handwheel?
Curiously, an encoder can generate step and direction directly...
Use the A signal for step, and the B signal for direction. Reversing these two lines will reverse the direction.

That way, when you are using the handweel (encoder) the stepper drive will hold the X axis steady. When you are using the CNC drive, the stepper could be swithched over with a DPDT switch. Switch the step and direction between the CNC control to the encoder.
Unless the CNC controller has a simple easy to use handwheel for manual operaton, then you could use that for the X axis handwheel.

Pete

[balsaman]

I am not sure you can do that. The direction signal needs to go high for one direction and low for the other. An encoder pulses the a and b channel 90 degrees out of sinc with each other. At no point does the b channel ever stay low, regardless of the direction you turn it.

As I said before, you can, and people have, sucessfully run lathes on cnc using the stock lead screws, and the half nut. It's not ideal, but it works. Go to www.dakeng.com for an example.

Eric