Drill bit advice

Post by **Peetee** » Sun Jul 16, 2006 4:28 pm

I'm trying to drill into a casting and the HSS bits I'm using just go blunt. I have a mains drill that only operates at high speed. Should I be using a low speed setting?

Post by **jonathon** » Sun Jul 16, 2006 4:32 pm

Yes, you'll cause less heat, and save your bits. Always drill undersize and build up to the final required size.

Post by **Onne** » Sun Jul 16, 2006 5:09 pm

and use some kind of cooling? I use oil when drilling stainless steel. Thin machine oil that is not the semi-fluid oil that goes in the car

Post by **Peetee** » Sun Jul 16, 2006 5:31 pm

Thanks Johnathon it does look like the casting is getting a bit overheated - signs of discolouring. Anyone in Southampton area got a slow powerful hand drill?

Post by **Alec** » Sun Jul 16, 2006 7:21 pm

Hello PT,

I presume that you mean cast iron? It is normally one of the easiest materials to drill, and the speed of the drill should be proportional to the diameter of the drill itself. (From a long time ago, I think the cutting speed should be about 80 ft\min, which is about 1200 rpm for a 1\4" dril) it also doesn't need a lubricant as there is a lot of carbon in it giving natural lubrication.

Try a new drill of a smaller diameter (or sharpen it if you have the means) and try again then go up to the rquired size. You may have been unlucky and caught a bit of casting slag but that is rare these days. Don't be afraid to apply a lot of pressure initially to get the drill to bite into the iron.

Alec

Post by **Peetee** » Sun Jul 16, 2006 8:12 pm

I'm having a go again with my low speed rechargeable drill. It is cutting, really slowly but I have to take it really easy because to much pressure and the bit bites, snatches and the clutch comes in.
It may take a while - but it is a lot quieter!

Post by **Dominic** » Sun Jul 16, 2006 8:23 pm

It sounds as if the bit is either blunt, or of a low grade steel. Get the best bits you can afford, and keep them sharp, you should find it quite easy then!

Post by **Alec** » Sun Jul 16, 2006 10:56 pm

Hello PT,

I don't know if you have grindstone but some materials like cast iron machine better with negative rake. What that means as far as drills are concerned is that you grind the leading edge slightly backwards from the axis, this will lessen the snatching.

What diameter hole are you trying to drill?

Alec