Drilling holes in steel or alloy

[Neeves]

When a question arises over drilling holes in metal common advise is, I think, to drill a succession of small to large holes - until the desired size is achieved.

I follow this practice but with no discernable logic or pattern.

Is there a rule of thumb on what size is small/large enough to commence and what size of increments - or is it simply suck it and see? I do use a cutting oil. The drill bits are nothing special but I do keep them sharp. I have a large bench drill which I operate at its slowest speed.

I would commonly drill 'mild' steel, say around 400 MPa, often 800 MPa, aluminium 5083 and 316 stainless. Normally I'd be drilling upto 15mm plate, but the alloy is 30mm plate. The largest hole would be 14mm (that's my largest drill bit currently).

Its not a critical issue - but if there is a well known increase in increments - it might be sensible to follow.

Jonathan

 

[John the kiwi]

As an apprentice tradesman i was taught to use a pilot hole where the final hole diameter was greater than 10 mm.
The pilot hole should be the same diameter as the thickness of the web on the larger drill. If you look at a standard twist drill there is a short edge in the centre where the cutting edge is not fluted. This part of the drill bit is an obtuse angle and therefore needs a lot of force to enable it to cut. By sizing the pilot hole appropriately the cutting is only done on the acutely angled edges and the amount of force needed is much reduced.
Using this methodology a single pilot hole is all that is needed up to quite large drill sizes.
Actually stepping up in sizes can cause an effect called "lobing where the drill chatters and you end up with a "triangular" hole.
Of course the speed needs to be adjusted to the diameter of the drill bit to keep the cutting speed correct for the material to be drilled.
Google can find you lots of tables on this subject.

 

[Daydream believer]

As an apprentice tradesman i was taught to use a pilot hole where the final hole diameter was greater than 10 mm.
The pilot hole should be the same diameter as the thickness of the web on the larger drill. If you look at a standard twist drill there is a short edge in the centre where the cutting edge is not fluted. This part of the drill bit is an obtuse angle and therefore needs a lot of force to enable it to cut. By sizing the pilot hole appropriately the cutting is only done on the acutely angled edges and the amount of force needed is much reduced.
Using this methodology a single pilot hole is all that is needed up to quite large drill sizes.
Actually stepping up in sizes can cause an effect called "lobing where the drill chatters and you end up with a "triangular" hole.
Of course the speed needs to be adjusted to the diameter of the drill bit to keep the cutting speed correct for the material to be drilled.
Google can find you lots of tables on this subject.

+1

 

[vyv_cox]

Yes, I fully agree and had formulated a similar response in my mind as I read Jonathan's question. No need to post further as it has been answered comprehensively.

 

[Neeves]

Thanks, Kiwi John and for the endorsements from your peers.

Its nice to have agreement.

I did not quite understand 'lobing' and specifically a triangular hole (I've obviously not been there - and I will not be going).

Jonathan

 

[RichardS]

Actually stepping up in sizes can cause an effect called "lobing where the drill chatters and you end up with a "triangular" hole.

That is so true!

I had to drill out two steel fittings last week, one was a seized in sheared bolt and the other drilling down a 10mm shaft and then tapping the hole to take an M6 bolt to replace the peened over shaft end which I had to drill out. After drilling a pilot hole and then stepping up with one and having bad juddering and a triangular hole, I decided to do a pilot and then go big with the second, which was a better approach. In both cases the steel was not that hard and the actual cutting with cobalt drills was quite quick. The most important issue was accuracy in hand drilling the hole exactly in line and central with the shaft/bolt.

Richard