SB Fourteen - Can't Hold Tolerance with Finishing Cuts

[Glenn Brooks]

Hi all,

I am asking for help figuring out why my new to me 1985 Southbend Fourteen can't hold decent tolerance on the X axis when making finishing cuts. E.g. Reducing diameter of work Parallel to the spindle axis.

Spent the day making mucho test cuts. Short of it is:

- dial in .020" depth of cut on the crossslide, the lathe cuts .024"

- dial in .010" depth of cut on the cross slide, lathe cuts anywhere from .014" to .019"

- dial in .005" depth of cut, lathe cuts .008" up to .015".

It seems to cut .004" to .006" over my selected depth consistently, with smaller cuts showing greater variation. Sometimes twice what I dial in. My goals is to consistently hold a thou when reducing diameter of work. This is very frustrating as I don't see any obvious movement or deflection in the tool post or cross slide when making the cut. I dial in the selected depth of cut. At the end, the tool bit is deeper than it should be.

All of this even with the gibs tightened up and the cross slide locked down to the taper attachment to prevent movement. Work supported by the tailstock. And also making a variety of test cuts with both HSS and brazed Carbide tooling.

This looked like a nice tight machine. Good original paint. Gears show little wear. Bed is near perfect. The tool post is a stout QCTP. The spindle has around .001" of play, maybe a bit less.

I could understand dialing in a cut, and coming up oversize due to tool pressure. But consistently and widely over the dialed in amount has me baffled.

What could I look at to tighten this thing up? Iam sort of stumped.

Thanks much for,any suggestions anyone could offer.
Glenn

 

[FOMOGO]

I guess I would be looking at the tail stock alignment, and make sure the chuck is running true first off. Doesn't sound like the tool slipping in the tool holder, or it would be cutting shallower, not deeper unless its being drawn into the work somehow. I can see why your somewhat stumped. Spindle play seems a little excessive, but shouldn't make for the differences your seeing. I'm sure someone will be along with a better answer than mine. I'm sure you'll figure it out. Mike

 

[Billh50]

That is strange for cut to be more than dialed. I assume by the dimensions you typed you have a taper as well as cutting deeper. I would check tailstock alignment first. Have you checked the cross slide for any up and down movement at both ends?

 

[Glenn Brooks]

Bill and FoMogo, no taper to speak of and the tailstock and headstock are dead on with each other. I releveled the bed tonight as the headstock end was off a bit. Only took less than a quarter turn on the leveling screw. Still have the problem.

Now, late tonight, setting up a dial indicator, I do find movement of the cross slide inwards, toward the bed when taking a cut. Measured .oo2 to .003", which would translate into. .004 to .006 radius reduction on round stock. It's also possible the compound moves as well. Need to check again tomorrow when I have more steam.

Glenn

 

[mikey]

I agree, Glenn, that this is unusual. The only thing I can think of is that the tool is being drawn into the cut, perhaps due to backlash in the cross feed screw/nut. Have you tried dialing in and then back out to your depth of cut? If that cuts on size then it is likely a backlash issue. Does your cross slide nut have a backlash adjustment feature? If so, that might handle it.