Rigid tapping on MX mills

A colleague of mine broke a spiral flute M5 tap when rigid tapping aluminium with 1100MX using a code posted from Fusion360. He said that the tap didn’t hit the bottom and the tap broke during retract. That is all the information I’ve got.

I have used code generated by Solidworks CAM and GoEngineer postprocessor for successfully rigid tapping 200 blind M6 holes using a spiral flute tap without any issues:

N60 ( Tap1 )
(T507 tap [M6x1.0 blind] dia:6.0 loc:15.0 3FL HSS [Yamawa SE6.0MALEN])
M6 T507 G43 H507
S705 M3 M8
G0 X6.2 Y15.
G84 G98 R78. Z63. F705.
G0 Z100.
M5 M9

However, out of curiosity it made me search for more information about rigid tapping on 1100MX and I found somewhat contradicting information.

First of all, the Supported G-codes Reference on tormach.com lists G33.1 for rigid tapping and does not include G84 at all although Tapping Cycle (G84) is listed in the menu on the right side of the page:
Quite obviously that is just a mistake on the web page because PathPilot does support G84 and I have used the code above for rigid tapping on 1100MX.

states “G84 is designed for use with rigid tapping; however, it can also be used with success when utilizing a tension/compression head. A dwell time is automatically calculated when using.” I find this statement very questionable because any dwell at the bottom during rigid tapping is a big no-no. So I’d put that the other way round and say that G84 is designed for use with T/C head but can also be used for rigid tapping if dwell is zero.

The TAPPING CYCLE (G84) reference documentation

states that “This cycle uses a P word, where P specifies the number of seconds to dwell. The P word is optional – if it is not included, PathPilot calculates a dwell for you (half of a second per 1000 RPM).”

Which brings me to the actual question - does PathPilot calculate a default G84 dwell also for the MX machines that have spindle encoder and support rigid tapping? For the code snippet above that would mean 0.35 s dwell time and 4 mm z movement which doesn’t sound realistic without immediately breaking the tap. The goengineer postprocessor does not post the G84 P parameter neither with the default 0 s value of Solidworks CAM tapping cycle nor with any other manually entered dwell time.

Don’t think I can answer the question exactly, but I can tell you that G84 definitely operates differently on the MX machines compared to the non MX machines.

On anything non MX, you need to have a T/C head and the code works by making some assumptions about the deceleration the spindle will achieve as the direction changes. On an MX machine, there’s a different implementation of G84 that turns on rigid tapping. The idea is that the same code should be able to run on both machines without modification, but will use rigid tapping when the machine has the capability.

G33.1 is supported on the MX machines and can be used to perform peck tapping. I’m reading between lines here, but it’s very likely that the MX version of G84 uses G33.1 internally. G33.1 always finishes back at the starting height, so you cannot implement all of Fusion’s peck tapping options, but you can get pretty close.