Chipped a carbide end mill because my touch plate trick failed

Broke a 3/8 roughing bit almost the same way last fall, except mine was on oak. I touched off on a piece of painter's tape sitting on top of the stock, planning to subtract the tape and glue stick thickness, then fat-fingered the number and left it in. First plunge buried the bit about a quarter inch into the part. Snapped two flutes and left a crater where my pocket was supposed to start. That one hurt more than the bit did because the oak blank had already taken me two hours to joint and cut to size.

What finally fixed it for me was boring repetition. I touch off on the waste board now, then measure the stock thickness with calipers and let the CAM handle the rest. No mental math at setup, so there's nothing to forget at 11pm when I'm tired and rushing. I'll say the washer idea itself is not crazy, plenty of folks touch off on a gauge block or a known thickness shim. The failure mode is doing the subtraction in your head at the machine. If you ever try it again, write the number down before the bit moves, or set the offset in two steps so you can see the value on screen before you hit go.

Also, you're right about the scrap can. Mine has a mix of aluminum and mystery steel chunks from similar incidents. It adds up faster than you'd think.

I gave up on paper years ago becuase I could never tell if I was compressing it or not, and my zero would wander a couple thou between parts. Now I use a homemade touch block I milled from a chunk of 6061 and measured with calipers, then wrote the height on masking tape stuck to the side so I never have to remember it. Clip one alligator lead to the bit, one to the block, and I jog down until a cheap multimeter beeps. Zero there, type in the block height, done. Takes about as long as the paper dance and I stopped second-guessing myself.

The bigger habit change was stopping all freehand math at the machine. Anything I need to add or subtract gets written on the setup sheet first, before the spindle ever spins. My memory has proven it can't do arithmetic while I'm standing there listening to a router whine at me. And yeah, those aluminum chips do add up. Mine goes in a five gallon bucket and the scrap yard guy pays me enough twice a year to cover a bit or two.

One thing I'd add about that chipped end mill, don't toss it in the scrap yet. I keep a little coffee tin of crashed tools myself, and a carbide bit with one or two chipped flutes still cuts fine for roughing in soft stuff like the 6061 I mess with, as long as you take light passes and keep the rpm down so it doesn't grab. Mine came out of a similar incident a couple years back and it's still opening slots in fixtures where a nice finish doesn't matter. Not a substitute for the real thing, obviously, but it softens the sting of a thirty-something-dollar mistake.

Same here on the touch block idea, that's what finally ended my crash streak too. I cut mine from a scrap of micarta I had lying around and it never squishes like paper does. The other thing that helped was sticking a sticky note right on the controller that says 'zero set? check it twice' because half my mistakes happen when I skip ahead in the routine. I keep meaning to wire up a proper electrical touch off but honestly the block plus calipers has been zero drama for over a year now.