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Beware cheap drill bits

November 20, 2023 — BarryK

I learnt something very important today. Continuing the custom trike project, previous two posts in what is turning out to be a very long saga:

There will be 10mm shafts to mount the wishbone swing-arms, inserted at each end in an aluminium plate. So the aluminium plate will have holes, that the shafts go through, with locking nuts on the outside.

I decided to insert bushings in the holes, so as to have a wear surface between shaft and plate. So, bought some of these, 10mm ID, 12mm OD: 


I have access to a workshop, but the set of drill bits kept near the drill-press did not have a 12mm size, so I bought this one from Bunnings: 


...a cobalt drill bit, price AU$14.26. Bunnings also have 12mm cobalt bit, Kango brand price AU$41, Sutton brand price AU$61. So, a considerable cost saving!

Today at the workshop, drilled some test holes in some scrap 5mm thick aluminium. Used the drill-press, and the aluminium was held in a vice and was immovable. Drilled the hole, but the bushings were a sloppy fit. In other words, the drill bit cut with a slight wobble. Until the last about 1mm when it broke through the other end of the aluminium plate, then it cut the last 1mm correctly at exactly 12mm diameter.

I found another 12mm drill bit at the workshop and tested that. It cut a nice 12mm hole and the bushing was a snug fit.

So, why did a brand new drill bit cut with a slight wobble?

There is a guy at the workshop who explained the cause of the problem. He explained that the cutting edges of the drill bit are not exactly equal on each side of the central point. He used a grinding wheel to improve it, but explained that it might take some experimenting to get it right. He also explained how to test that both sides have been cut evenly.

The lesson is that those cheap drill bits at Bunnings are not necessarily a cost-effective buy.   

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