Thread cutting

[willamcar]

Hi all got an LB15 and my machine seems to be doing some strange stuff in regards to thread cutting. Today i am trying to cut a simple M20x2.5 pitch external thread and the machine is cutting the incorrect pitch probaly 2.2mm.
I am using G71 X16.95 Z-25 B60 D0.5 U0.05 H3.05 F2.5 M32 M73.
Bit stumped here any suggestions?
Will

[txcncman]

Assuming G97 constant RPM programmed prior to G71 call.

What is you starting Z? Most machines like at least 3 times pitch to sync axis.

[willamcar]

Thanks for the reply, yes I call up G96 with the tool prior to the G71. I am using the face of the job as Z0 and only placing the tool at Z+5mm. So yes I will move the starting point to Z+10 and see what happens.

[underthetire]

?? Never use G96 with threading. Your always going to be waiting for the spindle to speed up/slow down, especially on a LB15.

[broby]

Not only are you waiting for the spindle to arrive at the correct speed, unless you have one of the latest generation of machines and you do not, you will be having problems with the pitch on your thread... oh looks like you are
You MUST use fixed spindle speed G97 for threading on the earlier machines for them to maintain thread pitch and spindle speed.
Change the spindle speed after you start cutting and you will change the timing between the tool point and the existing thread path.
Make sure you start approx 10mm before the start of the thread to allow the machine to accelerate the tool up to speed along the Z axis also. If you start too close to the end of the thread you will get pitch errors.

[willamcar]

Thanks all yes problem solved moved the Z strart position and used G97 very happy!!