Hardinge Cobra 51 & Fanuc 21i-T Question

[CNCBahamas]

Hello guys, I'm new! I am in need of a little help.

I've been trying to make a 7/8-6 ACME thread on this 4140 shaft. My hole program runs great, except for one problem. The first two to three threads do not seem to be exactly the right pitch... If I cut that end off, the rest of the threads are great... They screw into the mating piece no problem... I've spent the last 2 days rechecking everything in the programing and mechanically. Everything checks out.

Anyone have any advice?

[leggazoid]

There is a formula to determine how far Z+ to allow Z axis to get up to speed before starting the cut. If you start Z+ about the amount you cut off plus a little, it should work fine.

[CNCBahamas]

My threading pattern starts around 1" away from the thread:

G97S1200M4
G0Z-1.1M8
X.845 (1ST PASS .015 DOC)
G32Z-5.275F.16667
G0X1.2
Z-.9
X.815 (2ND PASS .015 DOC)
G32Z-5.275
G0X1.2
Z-.9
X.785 (3RD PASS .015 DOC)
G32Z-5.275
G0X1.2
Z-.9

etc...

I guess that I could back it up some more, but I think that it would hit the live center.

[DouglasR]

Back up your Z axis start point by an additional .25" in the positive direction and you should be ok. Prove out accordingly.