Broken End Mills

I'm used to breaking end mills. As one machinist put it, they're a consumable. They really do break. And depending on what you're doing it'll happen more or less frequently. My worst nemesis project was a big copper heat sink that was entirely made using a 1/8" end mill plunged almost half an inch deep. Recipe for disaster. I must've killed four before I finally got the thing finished. The end result? The customer decided copper was simply too heavy and switched to aluminum. ARGH!

For the most part the end mills I break are in the "tiny" category: 1/8" diameter and smaller. They're thin, they're flexible, they typically have a lot of length for their diameter, so they're fragile. Any little thing goes wrong and snap!

Today I broke a half inch end mill. ?! !!! I was surprised. These are typically beefy, thick, strong, and tend to dull out well before they break. This one wasn't even in bad shape. Upon reflection, though, I realize what I did wrong.

I was doing a technique called "chopping". I learned it from a tool and die maker. When he showed me the technique he said, "This is one you'll never find in a textbook." He's right! I've never seen it described anywhere. So here it is:

In most CAM systems, when you want to hollow out a large area, it'll tend to do it through a process called "pocketing", "countour pocketing", or somesuch. The idea is to run the end mill around at a given depth in the area you want removed, after which it'll plunge down slightly deeper, repeat, plunge down slightly deeper, and repeat (ad nauseum) until it reaches final depth. A good CAM system will then clean things up with one or more "finishing passes", shaving off a few thou to a few tens of thou to get a nice finish at the bottom of the pocket and on the side walls. Depending on the rigidity of the machine, the size of the end mill, and how far over you're willing to let it step as it runs around the pocket area, this can take quite a while.

Chopping takes things in the opposite direction: You set your quill stop so you can't cut too deep, move to the middle of the pocket, and for the first pass you pull down on the quill handle while moving one axis back and forth to give the chips somewhere to go, and to reduce forces on the cutting edge. But basically you're drilling a rough hole with an end mill. Retract the quill. Move over about 1/8" in one direction and pull down again, moving slowly back the way you came. This cuts a tapered wall, eating out about 1/8" of metal in that direction. Retract the quill, move over another 1/8" in that direction, pull down again, etc. Keep doing this until the cavity is mostly eaten out. Oddly enough on the same machine, same end mill, etc. this technique moves a lot faster.

"Sounds like chain drilling!" Yes, but it's not. For starters, it doesn't use a drill. (A drill, by the way, is still one of the most efficient ways to remove metal with a machine, which is why people use chain drilling to do pockets.) More importantly, chain drilling leaves lots and lots of thick/thin/thick/thin transitions that you then have to pass an end mill through. Want to talk about breaking end mills? Chain drilling to any appreciable depth is a great way to break end mills when you go back to make the cleanup pass. It's interrupted cutting at its worst.

Oddly enough, chopping seems to like a heavy cut. When you step over to start the next line of chops, you want to move over almost the entire width of the end mill. Contrast this with contouring where a 45% diameter step-over distance is typically a good number to use. Chopping really does go fast. And when you're done chopping out a cavity, it's clean. You may have some light scalloping on the walls, but the bottom is flat, the sides are straight (you do a straight plunge at the end of a line of chops), and things are generally really clean. A quick pass around the walls to clean things up, and you're ready for your finishing pass(es).

So how'd I manage to bust a half inch end mill? Simple: I didn't lock my axes. When you're chopping, the table really likes to move around. Lock the axis you're not stepping in. This keeps the end mill from pulling the table around in that direction. I failed to do this, so my end mill threw the table sideways by whatever backlash I have, and when it rotated 90 degrees so the next cutting edge engaged the material, it had now moved substantially, so all of a sudden the cutting force went up by a lot. SNAP!

I swore, I cursed, I flipped the end mill around (it was a double-ended one), touched back off the top of the part, and re-set my quill stop. I then locked my free axis before starting again, and had no more incidents. The cavity was cleaned out, and the rest of the work on the part began.

Like a lot of machining techniques, this probably won't make any sense without a diagram and preferably a video. I'm not up to shooting a video at the moment, but I hope this description at least gives someone an idea of what chopping is about. And don't forget to lock your free axis so you don't break end mills!

Tom