Odd diameter errors in using CNC with lathe/indexer

[johnm]

Folks -

Okay, I've posted this trouble before, but perhaps I was overly complex.

Here's the trouble. We are turning tapered legs on a well mount lathe, like Wayne Locke has. I have checked the alignment of the spindle along "X" to the center of the headstock and to the tailstock. I used a 3 jaw chuck with a centering bit, and used a leeeeeeedle "V" bit in the spindle. I brought the points as closely together as I could and recorded their coordinates for my lathe/spindle "TDC"... I did the same at the tailstock and didn't need to move the lathe - everything was within a fraction of a fraction.

The problem? My actual (measured) diameter on the "live" end is smaller than what the machine indicates, and the actual diameter at the tailstock is larger than what is indicated - the difference is 1/8"

The lathe is tight, the 'bot is tight... I could see an error if the CL of the lathe were skewed in relation to "X" axis, but it isn't.

Sure could use some help on this one.... Any takers?

John & Rose
4736

[john_david]

is the lathe level with the "X" rails??
I would check that

[johnm]

John -

As noted above, I didn't need to move the lathe - it is plumb and square with the spindle cutter to within 1/32 or less.

Like you, that was one of the first things I checked.

John

[richards]

John,
Assuming that the lathe is running along the X-axis, there are only two things that could cause a difference in diameter (assuming that the Z-axis is not creeping up or down). Either the lathe is skewed in the Z direction or in the Y direction. You could also have a little of both. A 'little' is exactly that. If the radius of your Z was different by 1/32 of an inch, the Z diameter error would be 1/16 inch. If your Y radius was different by 1/32 of an inch, the Y diameter error would be 1/16 of an inch. Both errors totaled together would be 1/8-inch.

I run into that kind of problem with my metal cutting mini-lathe. I sometimes forget that if I shove the cutter in 0.005 inches, that I'm reducing the radius by 0.005 inches and I'm also reducing the diameter by 0.010 inch.