Mini lathe, has anybody converted to CNC?

[phantomcow2]

I'm coming to a close on converting my micro mill to CNC now, all i have left is get a computer (picking it up monday). So through it all I have 2 extra 127oz, a load of aluminum, a non working driver board which I am confident can be fixed, and a mini lathe.
Sounds like a good recipe for another CNC conversion project.
Has anybody here converted their mini lathe to CNC? If so, what are your thoughts of the final product? ANything i should know or "must do"?
Thanks

[acondit]

Have you looked at Jon Fettig's kits for converting a minilathe to CNC? He has some pictures at the following url.
Mini Lathe Tooling plate

Alan

[RotarySMP]

Mine is here:

http://www.wrathall.com/Interests/interests_index.htm

[JFettig]

phantomcow2: You can also see more and kits for sale at my website http://jfettigmachines.com


Jon

[phantomcow2]

I have seen your website JFettig, well done on the motor mounts, they look nice.
However I am in a technical program at school, so I plan to make all hardware there using their bridgeport.
A friend of mine gave me two Japan servo 127oz steppers, brand new. But they have a pulley to take a timing belt, so i might as well go that route.
I like the website you have, RotarySMP. I was thinking about enlarging the bore for the eladscrew support at the end, to make room for ball bearings since the leadscrew will be used so much more. Have you odne anything like this?

[RotarySMP]

I tossed the standard leadscrew as I felt driving the saddle through the half nuts was a waste of time due to excessive backlash. To be far, the Z axis of a lathe is probably the most tolerant axis for backlash on any machine tool, as you normally only cut in one direction (although is is pretty cool to cut Al both ways.

I used a metric acme screw with a nylon nut pressed into a tight hole. It has been a very good solution, with little (0.09mm) and consistant backlash, and has not worn. However I have ballscrews now and am in the process of fitting them.

I would guess that those 127 ozin steppers will need 3:1 reduction.

I now have the X axis drive belt completly enclosed which is a big improvement.

I wouldn't bother enlarging the bore of the C.I pillow block that the leadscrew runs in. You won't have enough meat to put a decent bearing in. Just make a replacement block.

I feel the motor on the mini lathe is too wimpy for the machine. A three phase 375W (1/2hp) motor driven with a VFD would make a huge difference to it.

The X travel increase mod really made the machine a lot more flexible.

At the end of the day, the only reason I have the 7x lathe is cause I have no space. As soon as I have more space I am getting a 10x lathe.

[phantomcow2]

a 3:1 reduction, that sounds good as there is a lot for that motor to move.
So about this backlash, is it the leadscrew itself that causes the excessive backlash? Or the half nut?

[phantomcow2]

Oh, one more thing. WHere did you buy your timing pulleys/belts?
SDP-SI?
The machine itself has impressed me, its got plenty of ridgidity for its size. The motor on mine is labeled 4/5HP i believe, its probably closer to 1/2 though

[JFettig]

Motor reductions on stepper motors is almost almost always a bad idea, unless your going for resolution, dont do it. If you find some torque vs speed graphs, youll find that other than your holding torque, your only loosing speed.

If you go with the stock screws, careful tweaking will get you running around 15-20ipm. with my 272oz-in motors(same with the 162?) from automation direct, I can get 20-30ipm out of those on the stock screws.

Jon

[phantomcow2]

So it sounds like my best bet would be to order a high torque motor to drive the leadscrew and carriage. Like 260+.

[JFettig]

I dont see any reason you cant try out your motors you have on it? If they dont satisfy you, depending on the drives you can upgrade by simply switching them out.

Jon

[RotarySMP]

I'd disagree with Jon's basic statement on the gearing issue. But do not disagree with his solution to the problem of converting a 7x lathe. You need to understand the difference between our machines.

On Z, Jon is direct driving a 1.5mm pitch leadscrew with 287 ozin steppers, while I am driving a 4mm pitch screw with 187ozin steppers through a 2:1 reduction. By my calculations, he should be seeing about twice as much thrust on the saddle as me.
Basically Jons machine is geared down more through the fine pitch screw relative to mine through the timing belt reduction and a course screw.

From an engineering perspective my set up is a little more "efficient" as I am geared to achieve the same rapids as Jon (power), have sufficient thrust for cutting (torque), but with smaller motors.

In the real world Jon's conversion is more "efficient" as it is simpler, he needed far less modification, and was certainly cheaper in time and money. His motors today cost the same as my weaker motors cost me.

On my lathe the Z axis has never stalled during a cut, so I must have enough thrust. Jon has about twice as much thrust, but more than "enough" is of no further use. He needs the big motors overkill on Torque, to get decent rapids as his lower gearing puts him further out the torque curve during rapids.

While Jon's set up is a rather inefficient use of motor power from an engineering perspective, the price of good steppers is low enough to make this the most practical solution.

I would venture that Jon could gear up (by replacing his 1.5mm leadscrew with a 13/64" ballscrew) acheiving roughly the same thrust as my conversion, but taking the 30IPM rapids out to around 80+IPM (partly through the gearing, and partly through the huge difference in transmission efficiency between the ACME (~35%) and ballscrew (95%+).

If you direct drive through the 1.5mm pitch standard lead screw with that 127ozin stepper, your machine should have about the same thrust as mine. Hence you should have no problem with cutting. Your rapids will be slow, but you only travel short distances on these small machines, so that may be acceptable to you. Since that motors are standard, try it, if it is too slow, buy the 287ozin motor. It is then plug and play.

The big problem with the 7X lathes is the uneven bed thickness. If I run the saddle snug enough in the thin part of the bed(luckily the bit you use most), it stalls on rapids on the thicker parts(near the tailstock end). I think this lathe needs this work done (i'll be doing it when I strip the machine to finally fit the ball screws):
http://warhammer.mcc.virginia.edu/ty...s/Bedways.html


I felt the timing belt drive also gave an advantage in that alignment becomes non-critical. This was important to me, as I had no mill or drill press and made everything for the conversion on this lathe. You already have a mill so making a standoff to hold the motor, aligned with the leadscrew, within the tolerances of an oldham coupling should be no big challenge.

If you are going to stick with the Sieg lead screw driving through the halfnuts, then direct drive. As standard you will have backlash in the leadscrew pillow blocks, and the halfnuts. You are already intending to address the pillow blocks. For much work on a lathe, backlash in the Z drive is not a show stopper, so you might want to take this easy route, as you can still convert to ballscrews if you ever find limitations in the leadscrew drive method.

X is a much thornier problem. Backlash on X is bad. Both from a control perspective, as you will get a mark on the surface when the axis changes direction during a curved cut, even if using backlash compensation in the software, and mechanical perspective as if leads to surface finish issues and makes cutoff more differcult.

While using my machine manually I found the misaligned cross slide nut method of backlash adjustment to be pretty hopeless, as the nut then wears quickly and you don't have constant backlash. Currently I have two brass nuts working in opposition on a 3mm AMCE leadscrew, and find that this solution is barely better than the standard misaligned nut set up from SIEG. I am fitting a small ball screw, but space is very tight. Will probably have to grind some of the nut and some of the saddle.

The little DC motor on the 7x is normally 250W (little chinese watts) or 350W if you have a 7x14 with the mini mill motor.

I bought my timing belts and pulley from Mädler in Germany.