Lathe taking a deeper cut then dialed in on the DRO.

[Just for fun]

Not sure the title explains it well. Here's the deal, I have been playing on the lathe today with a piece of 2" diameter mystery metal and noticed that if taking a light cut of .002 it matches the DRO setting. So, I have about four inches turned down to 1.914 and about an inch turned down to 1.866. Now I setup to take a .002 cut off of the 1.866 and continued on for about 1/2" onto the 1.914 section.

So, on the .002 cut I got what I thought and ended up at 1.864 but as soon as it started cutting the 1.914 section it should have been a .050 cut but ended up being .054 cut ending up at 1.860.

I have a theory, but I thought I would ask here to see what you guys with the experience have to say.

My thought is that when the cutter starts taking the .050 cut something in the tool post, cross slide or compound is flexing causing the cutter to cut deeper.

Maybe I have too much stick out on the cutter? Maybe the gibs are too loose? Maybe that is just the way it is? Maybe all I would need is to lock the compound if I could? (I have the compound lock covered up with a DRO scale). I'm going to fix that, but that is a different story.

By the way my setup is PM-1236T, BXA quick change tool post, 5/8″ Shank Left Hand Turning Tool with a cutter designed for aluminum. I don't know what RPM I was running at (Tach is not hooked up yet) and I don't know what my feet rate was, but it was pretty slow.

I hope that all makes since.

Tim

 

[682bear]

Is your tool on center?

If it is slightly above center, the heavier cut may be flexing the tool downward slightly... which may cause it to cut a little deeper.

Along the same logic, if the tool is angled toward the chuck, the tool pressure may be flexing the tool toward the tailstock... which would cause it to cut a little deeper.

A picture of your setup might help... also, does the surface finish change as it begins the deeper cut?

-Bear

 

[Just for fun]

I believe the tool is on center and square to the chuck. I will verify that tomorrow and will also take a photo or two.

And yes, the surface finish does change. Here is the only photo I have right now, the larger stuff on the left was cut with cutter designed for steel. I just changed the cutting bit to see how it would do. And on a light cut the finish was better than with the steel cutter. So that being said, there may be a good chance it might be a little high.

 

[compact8]

May be the the cutter was partially rubbing the metal in the shallow cut and really dig into the metal and pull the tool towards it in the deeper cut ? The unexpected extra 2 thou cut should be due to the backlash of the cross slide screw. Locking the cross slide should help.

The same thing happens in drilling. The dill bit tends to lift the workpiece and pull it towards the drill. The larger the rake angle, the easier it happens. The rake angle of the insert in your photo does look quite large.

That's why I always do a test cut of fixed depth, say 0.03 mm before measuring the resulting diameter of the workpiece. Then rough to close to the wanted final diameter before finishing. The depth of the final pass is always the same as that of the test cut , ie. 0.03 mm in my example.

 

[682bear]

Judging from the picture, I would guess that the tool is flexing to the right as it starts the heavier cut. This would tend to cause the tip to move into the material.

The flex could be in the tool itself, the qctp, the compound, cross slide, or (most likely) a combination of two or more of these...

You may be able to reduce or eliminate this flex if you can track down where it is coming from... I would start by simply checking the adjustment of the gibs in the cross slide and compound. Then double check to be sure the qctp is tightly clamped, and that the tool is tightly clamped in the tool holder.

I've also noticed on older lathes that the top of the compound gets worn where it is not flat, preventing the qctp from clamping rigidly. Your lathe looks to be newer, so that is probably not an issue, but it wouldn't hurt to put an indicator in the chuck and run it across the top of the compound if you can... just to be sure.

-Bear

 

[Lo-Fi]

I'd highly encourage learning the "balanced cuts method" which negates these kind of tool leaning in effects and makes precision diameters much easier to achieve. Stefan explains beautifully here:

Learning this was a hallelujah moment in my lathe work!

 

[mmcmdl]

I'd highly encourage learning the "balanced cuts method" which negates these kind of tool leaning in effects and makes precision diameters much easier to achieve.

I made a comment on here yesterday on a boring thread , and this is what I was talking about .

 

[mmcmdl]

FWIW , there is a sequence we follow when requiring tolerances down in the .0001 range .

I also encourage everyone to learn this . I brought it up yesterday and this does apply to turning or boring .

 

[Just for fun]

I'll check out the video after bit when I get on the computer.

 

[benmychree]

One needs to rough out workpieces to a relatively uniform allowance for finishing, then take fairly light finishing cuts to size, neither dials or DRO will guarentee accurate results; cut, measure cut ---