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In this video, I make an adjustable drill bit. I start by using the metal I cut from the M12 bolt. Then I am cutting a Spade Drill Bit. Then I combine the two.
Making Video :
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Did you see this ? Countersink Drill Bit making
During compression, the ruler slipped and I could hardly get the size I wanted. To solve this problem, I designed a new ruler marking gauge.

I decided to do it myself. For this, I used a 10mm leaf drill bit that I had at hand. The result was even better than I expected.