If you look at the tip of a traditional, twist drill you will see an area called "the web". The material being drilled beneath the web is not removed as chips in the traditional sense. The material is spalled or pushed out of the way.
That works great for brittle materials like gray iron.
It works TERRIBLE for material that work-hardens; materials like 300 series stainless steel. 301 is the worst. 302 is next, then 304. Progressively higher grades have less carbon and more nickel and don't work-harden to the same degree.
This is all very academic until you have a project where you are working with a 300 series stainless.
My project is to install a fireplace insert and a 6", stainless steel chimney liner.
|20 inch, square aluminum plate, 1/8" thick. Stainless bracket that holds liner.|
|The bottom of the aluminum plate has 5/16", galvanized carriage bolts projecting downward. The will be mortared into place to secure it against wind-load.|
My project is complicated by the fact that the folks who built this house specified an 18", square tile flue for the chimney. That is not a common flue size and commercially available caps are "custom made" which is pricey. Consequently, I decided to fabricate my own cap for the top of the flue.
I was crying in my beer after breaking several drill bits and I only had two holes in the stainless piece to show for my pain.
Then I call Jack. Jack is a problem solver.
I described my problem and he said he had a SPECIAL tool for exactly this problem.
Probing further, he informed me that he had a special 5.5mm drill bit called the M-855. I used to be pretty familiar with molybdenum based, high-speed tool steel but I was not familiar with M-855. I assumed it was a super, high cobalt alloy.
|"Put the hole on the mark" I told Jack.|
|And then there was a hole. Afterward, I had no problem running a larger drill bit through it to increase the diameter. Remember, the problem was the material beneath the web of the drill. Pilot holes solve that problem.|
|The back of the plate.|
|Jack's 5.5mm, M-855 drill bits.|