Measuring Lathe Wear

[wlw-19958]

Hi There,

Have you done a two collar test on your lathe to check
the accuracy?

As to measuring the in or out movement of the saddle due
to wear, I don't have a "pat" answer for you. If you could
put a precision ground test bar running parallel to the bed,
then you could measure some of this but even this isn't
absolutely perfect. It would be hard to tell if you have
indeed mounted the test bar truly parallel to the bed.

Your measurements might be measuring inaccuracies in
headstock/tailstock alignment issues (or at least compounded
by them). If you had some custom jigs to work with, you
could use them to test it but anything you do will need to
have a known good reference point to measure from.

I think someone mentioned Connelly's Machine Tool Reconditioning
as a reference. I can recommend this book. It is the bible to
testing as well as reconditioning of machine tools but it is a
very "dry" read (if you need something to put you to sleep,
this will do it).

I'm sorry to read (you posted while I was writing my previous post)
that your experience with Joe was so poor. I think he is getting
ornery in his old age. I've dealt with a half dozen or so machine tool
dealers and they are like used car sales people. Joe isn't the worst
and is better than some but he isn't a saint. Part of the problem
is expectations. Some peoples are higher than others.

When I got my first lathe, I wanted it to be "perfect" but it was far
from that. I've owned at least two dozen different lathes and
none were perfect (the closest was a SBL 10K I had but I sold it
to get a bigger lathe). The biggest question is: will it do what you
want?

As to "seen little use" and needed to replace parts, well that just
can't be. But I have seen lathes that were pressed into service
that were used for one operation only (like used for cutting off
a piece of copper rod) and the gearbox had one worn-out gear
and all other gears were unused. The bed on that lathe was like
new but the cross feed screw was worn. So a handful of new
parts put that lathe back to near new condition. It kind of
depends, sort of.

Personally, I respond better to truthful advertizing. If a person tells
me, "this lathe was used only for a cut-off operation only and has
a few minor worn parts but the ways are like new," I would be
interested and could figure what I'm going to need to put the
lathe back to good condition.

Good Luck
-Blue Chips-
Webb

[Rex]

Best way I have found to test the ways:

Pull the carriage off altogether
Mount a DTI to the tailstock casting.
Run the indicator probe along the prismatic carriage ways, front, back, high and low.
You should also measure the rear of the bed where the carriage rides, both vertical and horizontal surfaces.

[sjames]

I don't think they measuring the ways will give you the full story, especially if they are hardened, in that scenario I would think the underside of the carriage would wear faster than the ways.

[ronm]

Remove shims or tighten the gibs to take all the slop out of the carriage at the point where it gets the most use-usually up near the headstock...then run it way back to the far end of the bed., or remove the chuck & run it right up close to the headstock where it rarely goes. If there's significant wear on the ways, the carriage will get tighter, possibly not even move as it gets to the unworn area...the same technique works with the crossfeed & compound.Wear, by nature, is hardly ever even...sure sign of a clapped-out machine is a carriage that gets tighter at the far end of the bed.

[Rex]

I don't think they measuring the ways will give you the full story, especially if they are hardened, in that scenario I would think the underside of the carriage would wear faster than the ways.

The carriage can wear some without affecting accuracy, as it will be fairly uniform. Not so with the ways.

[sjames]

I don't think they measuring the ways will give you the full story, especially if they are hardened, in that scenario I would think the underside of the carriage would wear faster than the ways.

The carriage can wear some without affecting accuracy, as it will be fairly uniform. Not so with the ways.

excellent point. I had something recently read fresh in my mind, carriage was worn to the point that it was resting on the tailstock vway and throwing things way off. but it would take a significant amount of wear to get to that point.

[Harold_V]

Yesterday I put calipers on the lead screw, checking the diameter at both ends. I figured a worn lathe would have a lead screw that was narrower near the chuck. I couldn't find any difference.

And you most likely wouldn't, even if the machine is seriously clapped out. There's a reason for that.

Most threads never make contact on the major or minor diameters. Pitch diameter (flank of the thread) is where one witnesses wear--a narrowing of the width of the thread (or a widening of the groove of the thread), not a reduction in diameter.

Harold

[wsippola]

How a bed wears can have more effect than how much it wears. If the bed is worn front and back so the carriage ends up low, it will have very little effect on the diameter of the shaft you're turning (more so with smaller diameters). If the majority of the wear is only on the front ways, the cariage tilts down and will cause a much larger error. In the end, every lathe will have some inacurracies, including new machines, unless you're spending a whole lot of money.

How much wear is a problem? Totally depends on what you're doing. To find out how your lathe is cutting, take a largeish diameter shaft and take a light cut down the full length (make sure your tailstock is on center first). Mark it our every inch, then mike the diameters at each inch and plot it out. That is what your lathe is doing. A constant slope in the plot could be from the bed out of level or tailstock misalignment. Bed wear will typically show up as a change in diameter mostly near the chuck.

Backlash will always be there. You have to compensate for it, so does it really matter if its 5 thou or 50 thou? Doesn't have to effect (or affect??) the accuracy of the work. Might be the threads of the screw and the nut that are worn, but it may also be just some adjustment required on the screw mounting system. I've found problems more often there than in the threads themselves (unfortunetely not the case in my surface grinder I just got going - nut is almost stripped out and the screw is quite visibly worn).

For leadscrew wear, look at the width of the thread crest - it will be narrower on the worn part.

Don't write off your machine as clapped out until you get everything adjusted properly and take some real life measurements. And in the end, if you didn't have a mic, would you even know there's a problem? Again it all depends on the parts you're making.

Wayne