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threads in solid carbide

Bill D

Diamond
Joined
Apr 1, 2004
Location
Modesto, CA USA
This has been bugging me for a while, with solid carbide boring bars how are the inserts clamped in? are there threads cut/molded into the bar or is it a hole with a steel nut. Seems like carbide would be impossibe to thread and the theads would be brittle and break out with a shock load. Never seen one in real life.
Bill D.
 
Your first reply from finegrain is spot-on.

However, I'll have to get a photo of the threaded solid carbide end mill tips I have. Looks to me to be done 100% in the fired hard condition using a diamond grinding wheel.

I've a lot of "green condition machining" with pre-fired ceramic materials (SiAlON, mostly) and if it's anything like unfired carbide it's not easy to get fine shapes and you have to allow for a lot of shrinkage.
 
I also agree with Finegrain.

When it is done, and it is rare in boring bars, usually a tapered hole is put in from the bottom and a steel threaded insert is put in the hole. Carbide grippers for lathe chucks are done this way.
Decent sized holes can be done just like helical thread milling using a diamond wheel.

You can rough it in the "green" state but it still needs to be ground. As noted by Pixman you can't control the shrinkage well enough and the resulting surface is too rough.

OD threading is easy since you can use a reasonable diameter wheel.
Bob
 
Green state carbide is softer than sidewalk chalk. It can be machined. You can put in approximate threads, sinter and finish with diamond.

Due to the differences in cost and material properties generally you use as little carbide as possible and as much steel and then join the two by brazing, mechanical fixturing or other means.

Tom
 
Most carbide bars have the brazed on heads. One thing you can do if you wreck the bar and the carbide dose not break is replace the head . Most larger companies will replace the head or you can do it yourself
 
Somewhat off-topic, when I worked in a header die shop, I machined carbide header die inserts regularly. It was rough work, but it was machining. The inserts needed a press lead/radius on one or both ends, turned on a small bench lathe using a brazed carbide tool. The inserts were drilled too, using what amounted to a spade bit, both for size and form. So, it is possible, not too practical, not too precise, though you could hold 0.005 limits when drilling.
 








 
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