Today while attempting to peck drill a 9/32” hole in a piece of 6061, my Tormach chuck pulled off the arbor. I was using speeds & feeds calculated by FSWizard, but with the feed dialed down to 40% of the recommended: 4892 RPM and 375 mm/min (14.75 in/min) Z feed rate. The machine managed to drill about 4mm before the chuck came loose and I stopped it. I reset the chuck on the JT arbor with a light tap from a mallet and tried again with the feed reduced further, but the chuck came off again. I wound up finishing the job with a hand drill since the hole was already started and did not need to be precise.
What was I doing wrong? Should I not peck? Is the JT arbor the wrong choice for aluminum? Or could this JT arbor and chuck be damaged in a non-obvious way?
I’ve installed chucks 2 different ways. Any burrs or dings will interfere.
Cleaned both surfaces and used green loctite ( bearing retainer)
The other way was to heat up the body around 250 and tap it on the arbor. With this method I disassembled the chuck first and then regreased it when putting it together.
I’d suspect the JT taper/chuck fit before the feeds. A Jacobs taper should seat clean and dry with no oil, chips, or burrs, and the chuck needs to be fully seated on the arbor. If it keeps pulling off after reseating, I’d inspect both tapers for damage or wear. Peck drilling can increase the shock load, but it shouldn’t normally pull a properly seated chuck off the arbor.
With those parameters, I would suspect damage on one or both of the tapered faces. That’s not a heavy drilling operation, nor a terribly large tool. Usually, if JT chuck is going to pop off it’s because the chip load is way too high, or perhaps a big burr formed during breakthrough but neither seem to be the case here. Unfortunately, my experience is that once a chuck comes off, it gets progressively easier for it to happen again so you’re best bet is probably to replace the assembly, or just use ER collets as suggested above.