Thread: lead screw versus power feed on a lathe?

Thank you for all the good info now to keep on topic ish I always see people replace the lead screw to do a cnc conversion on larger lathes that have power feed why don’t you just hook a motor to the power feed and go?

Originally Posted by Jmamc711

I always see people replace the lead screw to do a cnc conversion on larger lathes that have power feed why don’t you just hook a motor to the power feed and go?

Because backlash, because imprecision, because doesn't work well.

Originally Posted by sfriedberg

Because backlash, because imprecision, because doesn't work well.

Yabutt !
The Home CNC guys run rack & pinons ....and Mach3...

And it ...uhm...works....

Originally Posted by digger doug

Yabutt !
The Home CNC guys run rack & pinons ....and Mach3...

I've seen some gantry mills with a rack and pinion ... don't know what the feedback loop was tho. Maybe laser ?

I am the owner of a South Bend 16 inch lathe that was manufactured for the WW2 effort. Whoever used this lathe for some production purpose used the lead screw for longitudinal feed. As fortune would have it, they only used the first few inches nearest the chuck which totally destroyed the threads in that area. I purchased a length of stressproof steel of the correct diameter and made a new lead screw, using the good portion of the threads on the original lead screw. Fortunately the lathe came with a follower rest and I was able to cut half of the threads in one setup, then flip the workpiece end-for-end and cut the remaining threads. I was able to pick up the timing to make the thread position to match longitudinally by positioning the tool in the cut thread at full depth and noting the depth reading on the dial, then cutting the remaining threads ato the same depth. As a repair machinist in a previous life for the largest taconite mine in northern Minnesota, I have used this technique to pick up threads many times.

After cutting the threads, I then cut the lengthwise keyway on my vertical milling machine. This way I replicated the combination LEAD SCREW AND FEED ROD. This lathe still resides in my shop with a Tru-Trace attachment mounted on it. I am going to experiment using this attachment in combination with the lead screw for withdrawing the threading tool in the same exact spot right next to a shoulder.

I was taught as an apprentice never to use the lead screw for longitudinal machining, and I never have. I have the opinion that a person who does so is a not a machinist, and should not be hired for this purpose.

Re-posting this subject.

Originally Posted by EmanuelGoldstein

I've seen some gantry mills with a rack and pinion ... don't know what the feedback loop was tho. Maybe laser ?

The encoders are on the rail servos. Large burn tables run on racks and vee wheels over track (Thompson). This is because it is not disaster when they get hit with forklift, and more common picking a plate up with crane that snags gantry.

The really good cnc hobbyist runs linuxcnc; somewhat more geekie. Which is why is is not predominate in the commercial world. There are a few commercial shops running linuxcnc plus a well known CAD and a commercial version of APT. One aerospace shop I'm familiar with also uses glass scales plus and incremental encoders on the X axis and maybe all three plus correction on all three axes. This is not a wimpy hobby machine. Only a 6' x 4' x 5' (or so) workspace.
Mach3/4 fits a given user group as does linuxcnc the groups rarely overlap. This is not a pissing match you use what gets your job done.
Make chips.