mach 3 turn thread pitch changing

[jasminder]

hello friends,
i am a beginner to mach 3 threading. i am using g76 thread cycle to cut 0.750 inch long thread 1/4 bsf type.
Internet says that the pitch for the thread is 0.039 inch. But when i began to cut thread, 0.052 pitch matches the taps i have of 1/4 bsf thread.
i am running the spindle at 900rpm.

The problem begins after a couple of hours of run when the spindle looses some rpms due to thermal expansion. it reads 890 now and the pitch of the thread changes.
Whrere is the problem? a friend at cnczone said that it is longitude gain problem and i should use cut/rev mode. But i am already using it in the beginning of the program.
ex:
t0101m6
gox1 zo
g95
m3s900
g76 x0.950 z-1 blah blah blah

where could be the problem?
I am scraping a lot of parts that i make after an hour of run of the machine.
thanks hood in advance for the previous and future help.

jasminder

[RICH]

it is longitude gain problem
don't know exactly what is meant by that....

Any change in rpm will change the pitch, it's just a matter of degree. 1% is not a lot and normaly should provide an acceptable thread, but...not sure what tolerance you are trying to achieve. If mach sees a change in rpm, after the first thread pass, planning is done to provide a correction to the next pass so the pitch is mantained.

Later , got to go,
RICH

[jasminder]

dear rich,
longitude gain means: feed not changing with the change of rpms.
Do you thread on mach?
if yes then please post an example code
g76 then what after and before?

i didn't get an example any where.
thanks for help,
jasminder singh

[RICH]

G32 Code for a 1/4-20-1" long on a 1" shaft @ 400 rpm from the wizard.

For  G76 parameters see the Threading on the Lathe Manual in members docs.

RICH