82 degree counterbore is kicking my butt. I could use some reliable ideas...

I've got hundreds of wearplates to drill and counterbore from A2 steel decarbed and ground, but not yet heat treated, and some aluminum Bronze. The material for both came in plate form and was waterjet cut to size. Here's a picture of the A2 part and the fixture for reference...

The screw hole in the center is .3125" in diameter with an 82 degree counterbore that goes to the edge of a 3/4" counterbore tool. I suspect the issue is the disparity between the center hole and the edge of the counterbore, and trying to get the feeds and speeds right. So far this is proving to be a dismal experience.

I started off with a Kennametal SEFAS tool, running a Kentip 5/16 drill and the 41 degree carbide wings. Screwed like a son of a gun on the first couple holes at the suggested feeds and speeds so I backed off. It sounded good until the 8th hole then the tool just gave up the ghost. Completely melted the head off the Kentip drill and buggered up the start of the carbide counterbore wings. Lousy pictures but it looks like this...

Kennametal rep was kind enough to send me another tool, but it died before making another couple parts. So I switched to 2 tools. A Kentip drill to blow out the center hole, and an M.A. Ford carbide 6 flute 82 degree countersink tool. Kentip drill works great as expected. The M.A. Ford factory reps suggested I use 80 SFM @ .014 IPR for the chamfer/counterbore, and assume the average diameter of the tool is .55". So 556 RPM and 7.7 IPM was tried. It resulted in chipping the cutter in the center of every 2nd flute.

So I backed off the feed and tried 5 IPM at the same RPM. It also resulted in chipping

So I called back to the factory and they now suggest I try 278RPM @ 3.33 IPM feed. The results were a totally wiped out tool on the 8th hole...

So this is starting to get frustrating and expensive. MY guess is the large area of metal being removed in the counterbore is causing the issues. It has to be done to clear the fastener after heat treating and grinding the wear plate. I have thousands of these counterbores to make...

Anybody have a good suggestion or tip. Feeds and speeds? A tool that you prefer because you know it works really well?

Thanks,

Stu

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