Backlash Or Just Poor Technique?

[great white]

I've got an atlas 10F. Pretty good shape for it's age.

One thing I've noticed if I make a cut towards the headstock and then run the carriage back without backing the tool off the work, it will shave more off on the way back. Mostly just chips, but sometimes enough to make a curl.

Is this an indicator of wear, backlash or something out of adjustment?

Or should I be backing the tool away from the work every time I run it back to the starting point?

It seems to me if I make my cut at a certain depth, the tool should clear the piece on the way back.....

 

[Ed ke6bnl]

better chance it is flex in the tooling must add flex in the part also, too much stick out, lack of support?

 

[great white]

better chance it is flex in the tooling

possible I guess.

I use an AXA wedge style QCTP and HSS tooling kept as short as I can to the holder face. Seems like it shouldn't flex that much to me?

 

[RJSakowski]

possible I guess.

I use an AXA wedge style QCTP and HSS tooling kept as short as I can to the holder face. Seems like it shouldn't flex that much to me?

Flex or lost motion is always there on a small lathe. (it's there on large lathes as well, just not as much). The whole lathe contributes to it. With the exception of cutting with a laser or edm, cutting processes need to exert pressure to cut. The pressure results in an opposing force on the entire lathe structure tending to move the cutting edge away from the work. This is particularly true for heavy cuts. Also for dull tools. That is why a spring cut is frequently used as the final pass; no additional advance, just a second pass.

I have developed a technique of lightly pulling the tool post toward me when returning the carriage on finishing cuts. The spring in the lathe is enough to allow me to clear the just completed pass.

 

[JimDawson]

Almost every cut I have ever made on any lathe does that, so you are not alone. The cause is tool/work spring. I have managed to get rid of it by setting the tool BELOW center. I know this flys in the face of everything we have been taught, but it works. I discovered this by accident a couple of years ago when I had the tool set too low. Try different heights as you are taking roughing passes. I have set the tool as much as 1/8 inch below center. You have to be careful when setting below center on long thin work because the work may want to climb up on top of the tool. I have used this technique on parts as small as 3/16.

 

[T Bredehoft]

I use that 'back cut' to finish the cut, (generally on short cuts), I make the forward cut using feed, then hand feed it back, measuring after the back cut. Granted, this is on a Turret Lathe, and in no more than 1/2" aluminum, so no matter how coarse I feed it, there's gonna be thousands of revs before I get off the piece.

 

[joshua43214]

This is normal, the heavier the cut, the worse it will be.
It is more than just backlash, the whole carriage is forced toward you and will climb up the V-way when you cut. On the return, the cut is not heavy enough to push the carriage around and it cuts that ugly spiral groove. The lighter the carriage the more pronounced the problem.
Tool grind plays a really important role as well. Top rake, side angle, and nose radius play huge roles in the amount of pressure the tool requires to cut. If you can perfectly balance all these variable, the tool will be pulled into the work with the same force that is being applied to it to make the cut (good luck doing it though). I get best results by roughing as heavy as I am in the mood for, followed by a pre-finish cut of ~0.015 deep, with a final pass of 0.003"- 0.005". The pre-finish cut lets me make the final measurement so the finish cut is to size.

 

[Bob Korves]

I have developed a technique of lightly pulling the tool post toward me when returning the carriage on finishing cuts. The spring in the lathe is enough to allow me to clear the just completed pass.

Interesting idea. I will have to try that.

 

[great white]

Work spring makes lot of sense to me as I'm usually cutting tube and such around 3-5 inches out of the chuck.

I'm not discounting tooling and lathe movement, but this might be and easy source to eliminate/mitigate.

I wonder if a follower rest would help cut down on the "back cut" tendency in my application......

 

[great white]

Egads!

Scratch that idea. I just had a little internet surf for follower rest prices.....