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Turning a Spherical surface - I'm confused!!

Hoppy

Cast Iron
Joined
Feb 21, 2005
Location
Millington, NJ
I'm turning a part that has a spherical surface between two grooves. The grooves are 0.050" wide and I'm using a 0.043" wide round-nosed tool to cut the spherical surface. I set the tool length by toughing off and dialing in 0.0215" (its nose radius). Here's my code:

G01 G42 X2.1 Z-0.139 F0.1 (Invokes TNR comp; brings tool to center of starting groove)
X2. F0.005 (Moves tool to starting diameter)
G03 X2.012 Z-0.444 R1. F0.0015 (Cuts spherical surface; ends in center of ending groove)
G01 X2.1 F0.01 (Withdraws tool in X)
G40 Z0.1 (Cancels TNR comp)

The first line of code executes as I expected and the tool is centered in the starting groove. When the second line of code executes, however, the tool moves in both X and Z. Why? What am I doing wrong?
 
My guess is you're running a straight line to your start point, and then reversing back to the arc so it swaps sides of the line for the tool offset. It's looking ahead on the 2nd line of code?

I'd probably not try to comp that.

I'm not a lathe guy, yet, but that's what I'd do on the mill.
 
I'm turning a part that has a spherical surface between two grooves. The grooves are 0.050" wide and I'm using a 0.043" wide round-nosed tool to cut the spherical surface. I set the tool length by toughing off and dialing in 0.0215" (its nose radius). Here's my code:

When the second line of code executes, however, the tool moves in both X and Z. Why? What am I doing wrong?

Ditch the tnr. You don't need it, on a 2 axis lathe it's a pita and causes problems (such as what you are seeing.) Just figure out the actual geometry and use that.
 








 
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