Three different drill bits

September 17, 2018

When I shattered a 7mm drill bit the other week, Dormer Pramet got in contact and offered me a replacement. I’ve been following them on twitter for a while and particularly like their machining tips and Monday quiz from which I’ve learnt a thing or two. So I took them up on their offer.

They sent me an A100 ground bit and an A002 bit. I also bought a cheap as possible “Rolled” bit. The rolled bits are formed by forging i.e. it’s heated and then squashed between some large rollers. The ground bits are formed by machining away all the parts of a metal blank that don’t look like a drill.

When I looked at the rolled drill there was a slight lip on the trailing edge of each drill. This could rub on the hole and make it drill oversize or heat up the drill or work.

Another difference in the drills was that the rolled drill was unmarked. Typically these are stamped with the size. That stamping process can cause burrs which cause the bit to misalign in the drill. Which I expect is why the Dormer bits have laser etched markings. It will be interesting to see how well this lasts but you might need to come back on 5 years…

Another aspect that might cause misalignment is the end of the drills. My really cheap rolled drill had a malformed end, both Dormer drills had a bevelled end.

There are a few differences at the other end of the drills. The A100 and rolled drill had a regular tip and the A002 has a split point. This means that the drill is harder to sharpen but it self-centres. This makes a big difference on harder materials and convex surfaces.  This also gives the drill it’s “four facet” appearance which means there’s a reduced surface in contact with the work.

The A002 also has a TiN coating which is a hardened surface and hence will improve the drill’s lifespan. The combination of the coating and split point should also mean you can use higher feed rates. The A100 has slightly smaller facets than the rolled drill too so I’d expect less heat and more cutting from this too.

As a simple test of the three drills, I drilled a hole in a 5mm mild steel plate using a drill press. No pilot hole was used and a simple centre pop was provided for starting. The results below: left to right Rolled, A100, A002.

The rolled drill had a noticeable wobble in the drill chuck, but it managed to drill the hole without issues. No wobble from either drill and the A002 had a very smooth start.

Both Dormer bits suffered from the same issue,  they snagged when breaking out the back of the metal. I repeated the experiment with a backing plate and the problem was resolved.

Looking at the retail prices, the rolled drill was £1, in bulk (10) the A100 is £2 and the A002 is £3, prices are more for singles and I’ve seen the A100 up to £5 each. In general does seem to be a case of you get what you pay for, but watch out for excessive retail markup.

So given this experiment, I won’t be intentionally buying that cheapest drill bit again. The A002 is quite a bit more (once you multiply this by all the drills you use) but I think for specialist cases it would be worth investing in the split point geometry. So for general purpose use, I’d recommend the A100.

By Andy from Workshopshed Add Comment

  Posted in: Drilling