Another: Lathe Test Bar

^i think that’s what mostly the RDM is for, although the 2 collars test is also related. I’m sure RJ can clarify.
 
ddickey said:
Perpendicularity to?
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A bad choice of words on my part. The spindle axis must be perpendicular to the cross slide ways which in turn are perpendicular to the bed ways. Otherwise, you will cut a dish or a cone. On my lathe, the perpendicularity of the cross slide to the bed ways is not adjustable so the adjustment has to be made between the headstock and the ways.
 
RJSakowski said:
A bad choice of words on my part. The spindle axis must be perpendicular to the cross slide ways which in turn are perpendicular to the bed ways. Otherwise, you will cut a dish or a cone. On my lathe, the perpendicularity of the cross slide to the bed ways is not adjustable so the adjustment has to be made between the headstock and the ways.
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Are there lathes that this is actually adjustable?
 
ddickey said:
Are there lathes that this is actually adjustable?
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Not that I know of but I don't have intimate knowledge of many lathes. Someone had suggested scraping the cross slide ways to correct any issues. In my particular case, I had realigned the headstock to correct cutting a taper. It corrected the taper but now I couldn't cut a flat face so my first order of business was to correct my previous error. The inspection record for my lathe shows the original error to be .015mm/200mm or.0006"/8". My "correction" had increased that error to .004"/6".
 
But at least the error still produced a concave face, correct?
 
RJSakowski said:
A bad choice of words on my part. The spindle axis must be perpendicular to the cross slide ways which in turn are perpendicular to the bed ways. Otherwise, you will cut a dish or a cone. On my lathe, the perpendicularity of the cross slide to the bed ways is not adjustable so the adjustment has to be made between the headstock and the ways.
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The headstock has to be square to the bed for cutting parallel to the axis of the spindle.
If the cross slide is not perpendicular to the bed ways and is cutting a dish or cone when facing, the compound could be set square to the bed ways and used to feed straight across the face. To me a cross slide that is too far off perpendicular to be serviceable is defective and should be replaced for one that is to spec. I would not compromise the importance of having the headstock in line with the bed ways at all times.
 
mickri said:
I haven't tried this because I don't have a test bar. I understand how this works to measure headstock alignment. The far end of the bar will wobble and the amount of the wobble when compared to the measurements at the chuck gives the amount of misalignment. But not for tailstock alignment. If the bar is held between centers the bar isn't going to wobble. The bar will spin on the centers and the DI reading won't change as the bar is spun. This of course assumes that the centers are truly in the center of the bar. Zero the DI at the headstock. Move it to the tailstock end. The reading on the DI should be the amount of the offset. If you hold the bar in a chuck then you have to rotate the chuck to find the halfway point in the chuck runout. Zero the DI at this point on the chuck. Move the DI to the tailstock end. The reading on the DI should be the amount of the offset.

Am I missing something here?
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Yes...
The wobble is only related to how out of round the bar is with respect to the center. Using an average reading compensates for this. The wobble has nothing to do with the headstock or tailstock alignment. The indicator (average) will read differently at the headstock than the tailstock and this will tell you how far off the tailstock alignment is.
What I can't figure is how this could help in any way with headstock alignment? I feel like the bushing effectively makes it as though you are turning between centers so the headstock alignment is not in play? Imagine a headstock that is 10 deg misaligned to the ways (a lot) but perfectly aligned to the tailstock. If you turn between centers the test bar will be perfectly straight and will not register a taper.
RJ?
Robert
 
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Tozguy said:
The headstock has to be square to the bed for cutting parallel to the axis of the spindle.
If the cross slide is not perpendicular to the bed ways and is cutting a dish or cone when facing, the compound could be set square to the bed ways and used to feed straight across the face. To me a cross slide that is too far off perpendicular to be serviceable is defective and should be replaced for one that is to spec. I would not compromise the importance of having the headstock in line with the bed ways at all times.
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When I first set my lathe up, I didn't have a machinist's level and the bed was twisted as a result. This caused cutting a taper. In ignorance, I changed to head stock alignment to remove the taper. This resulted in the cross slide no longer being perpendicular to the ways.

A few years ago, I purchased a machinist's level and set about a thorough top to bottom alignment. Aligning the headstock to the ways as I had originally done was not correct, the reason being that bed twist also affects the ways and it is possible that they can be set up so their effects cancel. As the saying goes, two wrongs don't make a right.

By aligning the headstock to the cross slide, any twist of the ways is fairly well removed from the picture as the twist will be proportional to the distance from the headstock. With the headstock aligned, the bed twist can be checked and corrected for. This will result in a lathe that will both cut a flat face and turn without a taper.

I would not want to have to use the the compound for cutting a flat face. It has limited travel and is more difficult to advance uniformly. If I couldn't correct the cross slide alignment, to within a reasonable amount, I would want to replace the saddle as that is what determines the perpendicularity of the cross slide to the ways.
 
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