Amount Of Material Removal On Spring Cut

No I am not.. Consider the geometry. Take a 1" round bar and now place your tool 1/8" below centre. We know that when you get to the point where your cross slide dials say you are at zero there will still be a cylinder that is 1/8 in diameter, this would be our maximum error. As you progress out from there the error would reduce, but you still will have an error and your part will be bigger than what your dials or DRO say it should be. This may be a small error but if you are trying to work to thousands you will never be accurate enough.

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Ok I did a quick diagram and worked it out.

In this example you have a 2" bar and you put the tool 0.05 below centre. You now advance the cross slide 0.5" forward. You would expect the bar to be 1" in diameter now, but because of the lower tool height the radius will now be 0.5024". No matter how many times you transverse the cutter this will be your final dimension because this is how far away from the axis the cutter nose will be.

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atunguyd said:
Ok I did a quick diagram and worked it out.

In this example you have a 2" bar and you put the tool 0.05 below centre. You now advance the cross slide 0.5" forward. You would expect the bar to be 1" in diameter now, but because of the lower tool height the radius will now be 0.5024". No matter how many times you transverse the cutter this will be your final dimension because this is how far away from the axis the cutter nose will be.

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Click to expand...
I wouldn't advance that far without also raising the tool. In fact I'd only set below center when finishing.

In practice I don't do it at all because I don't have a fancy toolpost.
 
Its just a hobby forum work it out for one's self. Opinions and slop in our old machines,we all have them.
 
Well this post started out with someone saying that they did not send up cutting what they expected to cut. I am merely offering an explanation as to why one should strive to cut on center since that is what our machines are calibrated to do.

Sure you can lower the tool and measure and readjust your dials or DRO often but that would be too painful for me.

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atunguyd said:
Ok I did a quick diagram and worked it out.

In this example you have a 2" bar and you put the tool 0.05 below centre. You now advance the cross slide 0.5" forward. You would expect the bar to be 1" in diameter now, but because of the lower tool height the radius will now be 0.5024". No matter how many times you transverse the cutter this will be your final dimension because this is how far away from the axis the cutter nose will be.

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Click to expand...

Sounds perfect since I seem to always take .0025"to much cut.
 
To throw another wrench in the works, by moving the tool height down, the tool rake becomes less positive. If starting with zero rake, it would be introducing positive rake. I would like to see what the effect of decreasing the tool rake but keeping the cutter on center would be.
 
Even big machines will have some spring. One lathe I run at work is a 22"X 90 Kingston. Nice heavy machine. Use a 1" carbide bar. Had a guy running parts that needed a .003 tolerance on the id. He came to me and wondered why when he took the finish pass, it was always over size. What I do is retouch off with the tool and dial in from there for the finish, if needing a small cut. When you go just from measurement and dial it in from the last setting, it don't always work well. Try to take the same depth of cut each time, checking actual metal removal, and take that same cut as a finish cut. Usually will result in a better finish as well.
I would not run my tools below center at all. Changes cutting geometry, and the potential to break the tip of the tool increases as well. Just my .02
 
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