Can't hold tolerance with finish cuts

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

Post by Glenn Brooks »

BadDog wrote:What you describe is indeed severe wear, but I'm still not seeing where the force moving the cross slide deeper into the cut is coming from. If you approach the cut properly (feeding inward to take up the slack in this case), then the thread should be seated against the far side of the nut, presumably in the wear surface that it has created. Even if there is a ramp ramp that would be traversed thus pushing the nut and attached cross slide toward the axis should the screw be forced sideways in the nut, what force is pushing it sideways hard enough to overcome friction AND the step in order to shift it outward and up onto the step?

It still feels like there must be something else going on here because nobody has explained what force is acting to overcome both the cutting forces that generally push the tool away AND friction in the slide. And apparently doing so simply by starting the spindle before the cut is entered. The only guess I have at this point is that the maybe the excessive wear is somehow causing the screw to bind and "wind up" somewhat, then unwinding when vibration causes the friction to drop? Or maybe the screw has a bind that rides in the wear groove as you wind in the cut, then likewise the vibration lowers the friction allowing the screw to drop down (or up, or sideways) onto that step you describe in the nut?
Me either! :shock:

I suppose another possibility is that the worn shaft at the taper housing allows the screw to be forced offcenter a degree or two, forcing the cross slide to move obliquely forward due to tool pressure. Or some lateral moment drives the outside diameter of the screw threads into the ramp action Spro mentioned. This would be equivalent to pivoting the tool into the work, deepening the cut. But the tool itself isn't moving in the tool post holder, it's the cross slide -driven by whatever mysterious force is set up in the nut's uneven thread shape, that is causing this.

Also, remember, during the cut itself, thetool floats several thou in and out on the work. Again indicating the threads and the nut aren't holding a constant pressure.

I like Harold's early comment that weird, unpredictable reactions occur when the outer diameters of thread and nut come in contact with each other, because of wear. I've done a bunch of other cuts with smaller and larger depth of cuts. The results sometimes are truly bazarre. E.g. Dial .005 on the machine, cut .020, etc.

Thanks !!
Glenn
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10KPete
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Re: Can't hold tolerance with finish cuts

Post by 10KPete »

The information Glenn has presented recently has make it clear, to me anyway, that until a few worn out parts are replaced and or repaired this machine won't be able to do any decent work. My earlier comments on the TA and screw thrust bearings were made before this information became clear to me. The mechanics of the TA and its requirements for slack still apply but even with tight thrust bearings this machine is shot in the cross slide department. :shock:

Pete
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spro
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Re: Can't hold tolerance with finish cuts

Post by spro »

Very interesting, 10KPete. From that info, the screw isn't actually "bent" but appeared to be, by the wear. It was wallowing in the nut and riding the threads. Totally understandable now with your additional info. Guys here arrived at same conclusions as others but there is always another possibility which can be thawed out later.
Glenn B. You already Did it ! In a few days, you will have the answer. It will be great having the proper screw and nut, not screwing with your brain.
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10KPete
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Re: Can't hold tolerance with finish cuts

Post by 10KPete »

I really like the open minded nature of the folks here. A bunch of brains sorting through the information as it comes in and staying focused on meaningful diagnostics. An absolute winner!!

Just waitin' and whistlin' for Glenn to get the parts.....

Pete
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stephenc
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Re: Can't hold tolerance with finish cuts

Post by stephenc »

Thank you Pete .
I have a lot clearer picture on what all is going now . Plus it always nice to find out I wasn't completely wrong . Even if I wasn't exactly right in how I came to the conclusion
Glenn Brooks
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Re: Can't hold tolerance with finish cuts

Post by Glenn Brooks »

Yes, truly an interesting journey so far. Rather like the old days in police work. The facts are always present. It is just the discovery and understanding of them that is confusing....
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Harold_V
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Re: Can't hold tolerance with finish cuts

Post by Harold_V »

Glenn Brooks wrote:Also, remember, during the cut itself, thetool floats several thou in and out on the work. Again indicating the threads and the nut aren't holding a constant pressure.
Not necessarily. Some materials don't machine well (carbon steel, for example). It isn't uncommon for the cut to vary, often leaving trenches, for lack of better description. It can often be attributed to chip welding, but a slightly dull tool will readily skip.

I'm not suggesting that this is your problem, but if you're using mild steel for your tests, I highly recommend you lose that idea, as it won't be conclusive. If you must use steel, use a leaded alloy, one that machines well. That said, I'd suggest the use of 2024 aluminum, say a T351 condition. It machines exceedingly well, in spite of yielding a frosty surface. Tearing isn't an issue, so the cuts will truly represent the cutting condition of the machine.

Harold
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10KPete
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Re: Can't hold tolerance with finish cuts

Post by 10KPete »

A few things are running around in my head; can't sleep until I clear them up.

Without a TA the screw is pushing the tool. With the TA the screw is pulling the tool.
Both conditions only allow the tool to move in to the work; that's the only direction it can move as the screw prevents it moving away from the work.

Without a TA the screw thrust bearings are at the operator end of the screw.
With a TA the screw thrust bearings are at the TA end of the screw.

But here is where I was wrong in part of a statement I made on the SB board:

I said that the backlash could not be removed in the 'normal' way with the TA on the machine. NOT TRUE. The way to compensate for backlash in either case is to always turn the dial to the right to come up on the mark.

Having a TA does not change any thing about the operation/manipulation of the cross slide screw.

So, any issues with the tool jumping around are due to the usual and normal problems of clearance/slack/backlash.

Only in this case the poor machine is so worn out in the cross slide screw department the jumping around is caused by the inability of the screw/nut/bearings to control the slide/tool.

Am I missing something here??????

To sleep.

Pete

Harold was typing while I was but he brings up a very important point: While the tool is usually pushed away from the work there are certain tool geometries and materials that want to suck the tool into the work. This condition will cause the tool/slide to move into the clearance/slack/backlash "space".

Yep, I think that's it.
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ccfl
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Re: Can't hold tolerance with finish cuts

Post by ccfl »

How about checking for movement between the saddle and the bed? Typical wear pattern makes the saddle banana shaped, high in the center and low at the outboard ends (left-right ends, viewed from operator's position). When the saddle wears like that it'll not only do the rocking chair thing, left to right, but that wear also allows the saddle to rotate clockwise-counterclockwise if viewed from above. Movement in that axis will pull the cutting tool closer to C/L when feed direction is towards the headstock, and will move the tool away from C/L when feed is towards the tailstock. Usually.

Stick the mag base to the saddle and measure down to the ways, in both directions. Check with the indicator vertical for the rocking chair movement and then reposition the indicator 90* to check for saddle rotation.
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Glenn Brooks
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Re: Can't hold tolerance with finish cuts

Post by Glenn Brooks »

CCFI,

how do you fix saddle wear, aside from grinding, and building back up with moglice? Is it possible to affix a shim to the rear gib somehow?

I just got back in the house from checking Saddle movement left and right. I noticed no deviation, not even a bump on the dial indicator. So it looks rock solid. HOwever can't take a cut at the moment as the cross slide and compound are removed. Should have the new screw and nut early in the coming week.

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

Post by ccfl »

I think that's OK, if your previous tests showed the unwanted movement without actually cutting on anything.

Apologies if I missed it, but could you make the extra-DOC issue stop happening if you locked the x-slide gib down tight? If you have the problem with the gib adjusted normally, and it goes away with the gib locked, then you can safely ignore all the other potential goofiness we're throwing at you.
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John Hasler
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Re: Can't hold tolerance with finish cuts

Post by John Hasler »

Please let us know it the new screw and nut fix it.
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