I need to expand out the holes in some seat belt mounting brackets. They are currently 9mm and need to be 11mm. I've tried using a nice good quality new 11mm HSS drill bit, with a pillar drill on low speed with cutting fluid, and pulling down nice and gently, but it just won't cut through the steel and very quickly knackers the drill bit.

So what are my options here? Never come up against anything this hard before.

maybe its stainless

amazed such a tiny bit of metal is being such a pest


[Edited on 18/2/09 by Mr Whippy]

How about a small grinding stone in a die grinder?

Ran into a similar problem with wing stays. Broke 4 bits. Keep at it until your arms want to fall off. It'll work eventually.

Re sharpen the drill bit, set your speed on the pilar drill to its lowest at most 400rpm then try again. If still no luck you may have got the piece you are drilling too hot and hardened it slightly so then get a 11mm TCT masonary drill bit, make sure its sharp (use a green grit wheel if you have one to sharpen it) then try again.

How about a carbide burr? Gets my vote.

If the parts arent connected to the chassis, and you dont mind getting them hot, you could try 'normalising' the steel - basically you heat it up then allow to cool again, but I'm not sure how hot you need to get it. I'm sure someone will come along with the relevant information soon
Regards
Hugh

Normalising requires a 'cherry red' heat - which is aout 660 Celsius with most steels.

I would simply resharpen the drill, set the drill to it's lowest speed, bathe the whole area in good quality cutting fluid and go steadily - should be fine

how was the orignal hole formed, plasma cut or something? i had problem on my manifold blank, it was just surface hardness when cut, i filed the hole to start with to remove the initial surface then was easier to drill.

I've sometimes had trouble with things like this.
Agree that it can be caused by the drilling heat-treating the metal.

I've sorted it before by using smaller graduations of drill bit.

Try a 9.5mm then 10, 10.5 then finally your 11m.
As said above, make sure you drill on low speed.
Alternatively, a good file will do the trick.

HTH,
James

Carbide burr, that did the trick - forgot I had one - cheers chaps

If you go to an engineers' merchant you will be able to buy a drill suitable for hard materials. Alternatively, use a masonary drill. This will need to have to be re-ground with cutting angles suitable for steel.