Can't hold tolerance with finish cuts

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Glenn Brooks
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Can't hold tolerance with finish cuts

Postby Glenn Brooks » Sun Feb 26, 2017 12:12 am

Hi all,

Just discovered my new to me 1985 Southbend Fourteen can't hold decent tolerance on the x axis when making finishing cuts, e.g. Turning down the radius of a shaft. Spent the day making mucho test cuts. Short of it is:

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

- dial in .010" depth of cut on the compound, 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 the greatest variation. SOmetimes twice what I dial in.

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 cuts with both HSS and 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
Glenn
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Harold_V
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Re: Can't hold tolerance with finish cuts

Postby Harold_V » Sun Feb 26, 2017 2:54 am

Cross slide or compound? One normally feeds with the cross slide, with the compound locked down. Helps avoid unwanted movement.

Are you taking in to account any possible backlash?

Some dials are marked such that a thou implies a thou of movement, so it's two thou overall. I'm not implying that's what's happening here.

Does the slide have a zero backlash screw/nut assembly? When they wear to the point of the major diameters making contact, they become erratic.

Interesting problem. Hope you find the cause.

Harold
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Magicniner
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Re: Can't hold tolerance with finish cuts

Postby Magicniner » Sun Feb 26, 2017 7:16 am

I'm trying to get my head around this, Z travel is along the bed, but then you're talking depth?
Are we talking about facing cuts?

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GlennW
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Re: Can't hold tolerance with finish cuts

Postby GlennW » Sun Feb 26, 2017 12:37 pm

Put a dial indicator on the cross slide and use it to check dial reading.

Do the same with the compound.

Report back.
Glenn

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Glenn Brooks
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Re: Can't hold tolerance with finish cuts

Postby Glenn Brooks » Sun Feb 26, 2017 1:42 pm

Thanks Guys, a couple of points of clarification: I was mistaken, it is the x axis movement - I Feed with the crossslide to reduce the radius of an axle or shaft. the compound is locked in place. Also yes, I backed off the dial and take the backlash out prior to setting DOC. Haven't measured facing cuts yet.

Harold, so far as I know, the lathe is supposed to make a .001" depth of cut for each .001" dialed in on the dial... (although one would never know know it trying to make .005" cuts!)

I will try the dial indicator approach this afternoon and report back.

If the bit is set a hair low, would that cause increased depth of cut?? Say .004" to .010" for light cuts??

Glenn
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Glenn Brooks
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Re: Can't hold tolerance with finish cuts

Postby Glenn Brooks » Sun Feb 26, 2017 2:35 pm

Hi Harold, re reading your post, you mention checking a zero backlash screw assembly. I don't know what that might look like. Where might it be located? Also, what are the major diamaeters you mention? Are these the diameters of the screw assembly and ID of a retaining nut?

Thanks
Glenn
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ctwo
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Re: Can't hold tolerance with finish cuts

Postby ctwo » Sun Feb 26, 2017 3:42 pm

I'm thinking a brass split nut or assembly under the cross slide. The threads could be worn out.
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John Evans
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Re: Can't hold tolerance with finish cuts

Postby John Evans » Sun Feb 26, 2017 4:07 pm

Look on the cross slide past the compound to the rear. Adjustable nuts will have several screw/Allen heads visible.Usually if 2 visible one retains the nut the other adjusts,if 3 two retain . A few lathes you have to access the adjuster from the back end of the slide from underneath . A picture of the top of the slide showing what is three screw wise would help.

Harold_V
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Re: Can't hold tolerance with finish cuts

Postby Harold_V » Sun Feb 26, 2017 6:11 pm

The nut on my Graziano is a zero backlash nut. It's split, much like the half nuts used for single point threading. The chief difference is the cross slide nut is spring loaded, keeping the nut in firm contact with the screw. When enough wear occurs on the flanks of the thread and nut, it starts contacting on the major diameters of the nut and screw. When that happens, movement of the slide is no longer linear. It may or may not respond properly to a dial setting, and it might go both ways, either taking too much, or too little, depending on the relationship between the nut and screw on the last pass as compared to the current pass.

The cutting tool below center offers the opportunity for the cut to attempt to climb the tool. A thou or two make little, if any, difference, but on a loose machine, an extreme amount can result in cutting pressure pulling the cross slide in to the cut. Generally happens only on small and very light weight machines, however. The condition is exacerbated as the diameter of the material is reduced.

Harold
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SteveHGraham
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Re: Can't hold tolerance with finish cuts

Postby SteveHGraham » Sun Feb 26, 2017 7:36 pm

Harold_V wrote:Cross slide or compound? One normally feeds with the cross slide, with the compound locked down. Helps avoid unwanted movement.


I can't believe I've been doing this since 2009 and never heard that before.
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BadDog
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Re: Can't hold tolerance with finish cuts

Postby BadDog » Sun Feb 26, 2017 8:15 pm

Mill or lathe (or related), I always lock any and all axis not in active use. Not sure where I learned it, but it's something I always do (unless just hacking at something, rough work).

I wonder at the bit and material as well. High positive rake and ductile materials can often suck up any backlash that is present (much like climb milling), and can be particularly problematic for zero backlash setups that don't fully restrain the adjusted nut.

When it really matters and I've moved things around (for instance, backing off for another pass), I generally pull back manually on the tool post to as I crank it forward to somewhat simulate cutting pressures. Otherwise the slide moves real easy with the screw, and work loads may push back even if you take it up in the right direction, but that results in the opposite observation.

But as stated, finding out what your feed it relative to dial reading is the first step. If the dial matches the indicator, and you still remove more than dialed in, then it has to be sucking in somehow.
Russ
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Glenn Brooks
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Re: Can't hold tolerance with finish cuts

Postby Glenn Brooks » Sun Feb 26, 2017 10:50 pm

Spent some more time cleaning and resetting the lower gibs on the cross slide and at the back of the machine. Also releveled the headstock - it was out a bit, now dead level with the tailstock. Also adjusted the tool bit to be dead on center. No difference in cutting stock diameter undersize.

Also set up a dial indicator to measure possible movement of the crossslide and toolpost. Toolpost doesn't move relative to crossslide. However, the crossslide moves toward the bed a couple of thou relative to the bed, when taking a .010 cut. This would account for maybe an additional .005 to .008" overage on each cut.

So locked down the compound tight with the gib. Feed in .010" depth of cut with the COMPOUND set at 90* to bed (measured with the dial indicator) and got a .010" and .011" measured radius reduction on two cuts. The compound screw looks good, so the nut must be worn somehow, allowing the whole assembly to be drawn into the work while cutting.

Now it's cold and late, so am packing it in untill tomorrow. If this holds true for a few more cuts,
Tomorrow, then I think the problem will be with excess wear in the compound nut, or someplace else in the compound base plate.

Also, Harold, there appears to be no adjustable zero backlash nut on this machine.

Glenn
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Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

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