Real confusing issue with milling machine

David G

Well-known Member
I clamped 2 pieces of steel together so I could ensure the bores were identical. When I broke through the first piece it quit cutting completely. I thought the new bit had gone dull. I chucked up another bit and it behaved the same. I then started poking around in the hole. The last few thousands of the top piece had broken loose and was creating a bushing. It looked like a washer. I picked that piece out and it cut like new again.

I bet I stared at that for an hour.
 
I"ve had that problem happen quite a few times. The funny thing is that it doesn"t even have to be a "washer" like you run into. I"ve had it happen when drilling through a really thick piece, and using cutting oil, where small chips would get caught in the oil, and then get under the cutting edges of the bit and keep it from cutting. Usually it happens when you back off the pressure to stop a long, stringy chip from forming, or for any reason the cutting edge leaves the metal for that matter.

In the case of the "washer" it"s just one of those freak things that can happen when the two pieces are even a few thousands apart vs tight together. It can be quite confusing the first time it happens, but now you"ll know what to look for the next time it happens.
 
Not really all that confusing, provided that you know that the end of an endmill isn"t ground to a plane that"s square to the shank, but instead is ground to a slightly-concave cone. Being ground that way, it"s the outside edge that guides a center-cutting endmill when it"s being plunged into the workpiece, without wandering like a typical twist drill will.
 
Some mill cutters will plunge cut but not all. In the many thousands of holes we have drilled thru multiple steel plates building molds we never use a mill cutter. Do some counter bores for bolt or pin heads but drill the pilot hole first.

An apprenticeship normally takes longer than a few days for the many learning mistakes it takes to complete it. Look for more confusing issues in the future. I had an apprentice that it took two mill cutters for every set up. He had to break the first one to find out which way the cutter was suppose to turn. At least that was the way I had suspected.
 

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