TC22 tapping fail

Started machining up a nice fixture plate for my TC22. It is 24x12, with about 350 1/2-13 holes. I purchased a 1/2-13 form tap (OSG, steam oxide), and did a half dozen tests to dial in the proper drill/bore/tap cycle. Got the hole dialed in perfectly in the middle of the recommended size. Tapping seemed to work great. When I went to tap my real plate (MIC 6 Aluminum) it snapped the tap on the 3rd hole! :mad: :(

Now I'm nervous to try this again. The spindle sounded a tad different when it tapped this time. Maybe a bit more load, although I'm not sure why this would be. The plate is 1" thick, so the tap was fully engaged. Tap broke soon after the spindle reversed. The first two holes it tapped look great.

I used a standard ER32 collet holder. I don't see any sign of slippage on the body of the tap. Using flood coolant with the same synthetic coolant I use for everything.

I checked through the G code, and it is using an M29 G95 G84 cycle. This is "feeds per revolution" which the DX32 programmers manual calls "FANUC mode".

The programmers manual also says:
"Note that the G95 works only with desired spindle speeds, it does not self adjust if the spindle speed varies due to load or operator actions"

This kinda implies that G95 is not closed loop? Or maybe it means that it isn't closed loop without M29?


Now I'm worried something is up with the machine. I have a floating tap holder but it doesn't go up to this size. Maybe the steam oxide was inappropriate for aluminum?

Anything I should check?

As noted in this thread, I did a long test tap (10"). S-Axis POSERR was stable around 2 on the way down, and -2 one the way up. I think this is in degrees. This would be a vertical error of .0004", which seems fine. This was unloaded. I suppose the acid test here is to try it when the spindle is loaded up (ie during the tapping cycle).

After not finding any indication of problems with the machine I did some research, talked to my coolant vendor, and bought a refractometer.

I read in a few places that using form taps that don't have "oil grooves" in deeper holes is a no no. This makes sense to me. The form tap I was using did not have any grooves, so once it is in the hole there is no way for coolant to get in there. As well, my coolant % was about half the recommended for "heavy duty" work. The tap broke during the "reverse" at the bottom of the hole.

So I bumped up my coolant %. Bought a new tap that is longer that has an oil groove. Machine tapped 350 holes without breaking a sweat!

Incidentally I was completely unable to remove the broken tap from my part despite having good access on the backside. Once a form tap decides it is stuck no amount of persuasion will convince it to move again!


James