Lathe: control compensation during lead-in movement

[SlaveCam]

There is some discrepancy in the lead-in behavior in Mastercam. In mill, the control's cutter compensation on/off (G41/G42/G40) is applied in the lead in/lead out move, whereas in lathe, the cutter comp is not applied in lead in, but in the rapid movement to the position before the lead-in movement. The lead out works as expected. The backplot shows it incorrectly and the default post processor posts it like this (we have modified the post in our case to make it work as in the latter example):

 

Wrong:

G42 G0 G54 X30. Z2. <- not here
G1 Z0.  <- lead in
G3 X40. Z-5. K-5.
G1 Z-20.
G40 X44. <- lead out
 

Right:

G0 G54 X30. Z2.
G1 G42 Z0.  <- lead in, here
G3 X40. Z-5. K-5.
G1 Z-20.
G40 X44. <- lead out

 

This makes a huge difference as the arc will be machined incorrectly (undercut) in the first case. There should be an option in 2017 to define the lead-in behavior in control's cutter comp options.

[crazy^millman]

Really comes down to how you have defined you profile. I always add a lead in line to my cuts and never let Mastercma just decide. I am and old school pencil and paper programmer. I know what I need from my tool to allow my tool to give me t4he code I need. Yes it should not be needed you have a valid point, but you either like you did alter the post or be mindful of the issue and make the geometry you are using allow you to adjust for it. The other trick is to add a line in the lead in/out options if you are against adding geometry. Either way you can accomplish what you are after. Again you have a valid issue not saying you don't, I am just trying to give you a way to go about it in a different method.

[SlaveCam]

Everyone has a preference Indeed, adding a line in the lead-in is a way to circumvent the undercutting problem

 

G42 G0 G54 X26. Z2.
G1 Z0.
X30.
G3 X40. Z-5. K-5.
G1 Z-20.

G40 X44.
 

Also, adding a line in the lead-out is needed to avoid overcutting when G40 is read.

 

G42 G0 G54 X26. Z2.
G1 Z0.
X30.
G3 X40. Z-5. K-5.
G1 Z-20.
X44. <- added line
G40 X46.828 Z-18.586
 

In our Fanuc controls, its possible to define a vector using I and K to continue the "imaginary" profile to avoid overcutting. I don't know of any post that does it, but it could be implemented easily with misc variables..

 

G42 G0 G54 X26. Z2.
G1 Z0.
X30.
G3 X40. Z-5. K-5.
G1 Z-20.

G40 X44. I1 <- prevent Z overcut