Need Help!- G76 Thread cycle Haas SL-30

[haaszard ahead]

I've never used this threading cycle before and would appreciate greatly if someone could help me.(Haas SL-30)
(1). The programmer at my new job uses this
G97 S400
G76 X2.0 Z-1.5 K.0768 D.02 F.125 A58
G97 S200
G76 X2.0 Z-1.5 K.0768 D.02 F.125 A58

He also always uses a single point 60 degree insert that is the same pitch as the thread.(8-pitch,8-UN full profile insert)

I know the second G76 is a spring pass...but does the A58 have to be A60 or does it make a difference?? And if so,how??? I've always used G92 or G33 for Fanuc and Okuma so I don't know.Also....can you change spindle speeds and still pick up the same lead???? I did'nt think you could.Please let me know.Very confused.

[Mike xyz]

You can change the spindle speed on the second pass. Just make sure that it starts at the same Z location. that is the key to synchronizing the spindle to the threads. I noticed your second G76 cycle has a D.02 that will make your flex pass have way to many passes. I would suggest using a number slightly smaller then the K number. In this case a D.075. This way your flex pass will only have two passes. The way I program my deburr pass is to run the G76 cycle then run the finish OD tool over the threads again then program a G92 cycle so it will only take one pass.

Most people use an A58 for a 60 degree thread tool. This will cause the tool to move in at a 29 degree ( 58 / 2 ) angle on each pass thus cutting mostly on one side and just a little on the other side. if you program an A60 the tool will feed in at a 30 degree angle just cutting on one side.

Best regards.

Mike

[sinha_nsit]

I have heard that if you change the rpm, the same helix is not guaranteed, though the pitch would be still correct.
Comments /confirmation invited.

[Geof]

Originally Posted by sinha_nsit I have heard that if you change the rpm, the same helix is not guaranteed, though the pitch would be still correct. Comments /confirmation invited.

I think you are contradicting yourself here; if the pitch is the same the helix has to be the same because the pitch is the axial movement for one turn of the helix.

When you change rpm, sometimes, the start point on the part may change because the machine needs more or less time to achieve synchronization. The way around this is to always leave more than enough distance to reach synchronization ahead of the part at the highest rpm.

[haaszard ahead]

Thank you for taking the time to help.I fully understand.And thank you Mike for the deburr tip...will definitely incorporate the advise.Look forward to talking more.

[sinha_nsit]

Originally Posted by Geof I think you are contradicting yourself here; if the pitch is the same the helix has to be the same because the pitch is the axial movement for one turn of the helix.

Not necessarily. For example, a double-start thread has two interlaced threads with the same lead but shifted helices.

Sometime back, I read a discussion on this issue which you may like to refer to:
http://www.cncci.com/pdf/os-win02.pdf

[Geof]

Originally Posted by sinha_nsit Not necessarily. For example, a double-start thread has two interlaced threads with the same lead but shifted helices...

This thread does not mention two start threads, but my comment is pertinent even to multistart threads because in this case the helix is related to the lead and if the lead stays the same the helix stays the same.

[sinha_nsit]

Perhaps I am not able to properly explain what I mean.
Pl read the given link on more information on what happens when we change RPM during threading:

"If you’ve ever tried to use two different
spindles speeds when threading
a workpiece on a turning center,
you’ve probably been very disappointed.
For whatever reason, most
turning center controls (including
Fanuc) cannot correctly adjust the
entry point for the threading tool after
a spindle speed change. While
the threading tool will cut the appropriate
pitch, the tool will
cross-thread to some extent – much
like what happens when a multiple-
start thread is machined...."

They go on to explain a method for determining the deviation:

"... The first task is to create a test program
to help the setup person determine
the amount of deviation between
the entry point at the roughing
speed (500 rpm in our case) and
the finishing speed (600 rpm in our
case)..."

"...So now we have a workpiece that
has a witness mark in the form of spiral
scratch. Using a colored ink
(commonly called bluing), the setup
person must now “blue-up” the
workpiece. They will also change
the S word in line N010 of the program
to S650, the finishing spindle
speed.
At this point, the setup person will
run the test program again. Since the
rpm in the program has been
changed, the threading tool will not
“track” in the same witness mark it
did when the program was run the
first time.
The deviation will show up in the
form of another spiral line around
the workpiece. Note that this new
line could be to the right (positive)
side of the original line or to the left
(negative) side. This polarity is important.
And since the setup person
has blued the workpiece, they can
easily tell which line was made first –
emphasizing this polarity..."

"...The production program must, of
course, deal with the deviation
amount and polarity caused by two
different threading rpms..."