Why did the lathe take off 6 thousands when the cross feed was indexed at 2?

I never really thought about it, but I can almost always get what I want with my mill just by dialing it in. But, I did have to get to know my lathe quite a bit before I had much accuracy. I did a lot of overshooting.
 
Like Rust Rescue says. Trying to machine just a skim is a pain. You had 0.150" of diameter to work with. Use your early cuts to sort out the speeds and feeds. Get it down to about 1.380". Measure, divide the remainder by 3 (it will be about 0.010" each) - so take that 0.010" cut (don't even retract the tool, just crank the carriage back pretty quick - lathe still running - don't worry about the scratch - you'll take that off in the next cut). Shut the lathe off, measure and go in half the remainder (which ought to be about 0.010" - but maybe a bit different - that's fine). Make the cut with your established speed/feed - again, crank the carriage back quick, lathe still running. Shut off and measure. Advance the tool to your target diameter (allow what ever you want for polishing - say 0.001"). Make the final cut, retract the tool before cranking the carriage back.

This will manage the spring and give the tool enough to get a nice bite, even with a little lathe. Of course if the lathe is solid and the work piece can handle it - you can take the same approach with heavier cuts.

If you loose track of your spring and are trying to start from just picking up the diameter - like you were, start with very small in-feed (like zero) - until you can figure out where you are at.
 
I forgot but what angle do you set the compound to get .0001 per .001 of indicated travel?
 
Looks like 5.77 degrees , but check my math as I'm not positive what you mean . I'm not sure my calculator has adjusted for todays inflation either . :)

You asking 10 on the compound to get one on the tool tip ?
 
My practice is to never make a final pass using a touch off as a reference. Even with a DRO to indicate cutter position, there os uncertainty in the touch off. If I have to touch off, as in changing tools, I will back the tool off slightly and advance until just skimming and measure with a micrometer. I try to arrange my cutting so that I have material for at least one pass prior to the final pass.

For critical dimensions, I will make one or more spring passes before measuring with a micrometer to determine final stock removal. Depth of cut definitely affects the reading.

Regarding backing the tool off when returning the carriage, I have found that if I pull back on the tool post as I return, there is enough flex to provide clearance. Doing this removes the requirement of resetting my cross feed dial and possible errors that come with it when making sub-thousandths adjustments.
 
RJSakowski said:
Regarding backing the tool off when returning the carriage, I have found that if I pull back on the tool post as I return, there is enough flex to provide clearance. Doing this removes the requirement of resetting my cross feed dial and possible errors that come with it when making sub-thousandths adjustments.
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This is genius!
Thanks RJ
 
I'm terrible at using dials so I have a DI on my xslide. It helps me be precise and also immediately flags any backlash issues. If you went both ways with the handwheel instead of pushing it to the part all in one direction you could have had some backlash introduced. If the tool digs in a bit sucking out the backlash could easily add 5 thou to the cut. The DI makes this obvious even to my 70 year old eyes


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Lots of excellent suggestions here. I have had the same problem before, (over cutting). I have learned to take spring passes occasionally while hogging off metal, sometimes 4-5. The heavier the cut, the more spring passes it takes before it stops cutting. On final passes I do a lot looking, feeling and hearing. Sometimes I can get it to .001 but not often. But I'm learning and it's fun.
Chuck
 
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