typical newbie question I guess - what do people do to cut a pretty large hole (or any hole over 20mm) in 3mm mild steel? I've got a set of circular hole cutters but they're like plastacine against the steel probably made for plasterboard - how's it done ?

You can get hole saws for use on steel, a well known name is Starret. Made in the UK too unless they've moved!
You need to have a drill with a fairly slow speed or they dont last long. Not all that accurate but ok for most fabrication jobs.

The hard way is to drill a lot of small holes just inside a circle of the diameter that you want. Knock out the centre then file and file and file until the jagged edge of the hole is round.

Or you could use a jigsaw.

Either way is slow and hard work.

Personally I'd find an engineering company and get it laser cut.

Various methods:

A proper hole saw - expensive for a one-off job, and requires a hefty drill to drive it. You'll also need a mandrel to hold the saw.

Lathe - perfect, if you can swing the piece of metal, that is.

Hole boring head - Perfect, but you need expensive kit and a mill.

Gas cutting torch (or plasma cutter) - quick, expensive, needs a lot of skill. You have to file or grind it round afterwards.

Chain drilling - drill small holes just inside the circumference, cut between them, then file or grind it round. Hard work, but cheap and easy.

HTH
David

I have used a tank cutter in the past with reasonable results LINKY

At 64mm, a jigsaw should be able to cope. Buy a narrow (metal cutting) blade to enable it to turn with a tight enough radius. Drill a conventional hole close to the circumference to enable you to start.
The tank cutter (trepanning tool) is a good oldy-worldy option that you can pick up cheaply in markets and boot sales and can be adjusted to any diameter within it’s range. You'll need a slow drill and some patience but it gives a neat finish.

As has been said, a high speed steel holesaw is an ideal way to cut the hole you need but only if you have access to a pillar drill running at the speed recommended for that size of hole cutter and can securely clamp the job down. Use plenty of lubricant and all will be easy. I would definitely NOT attempt it with a hand held drill.

John

Thinking about it, if this is a one-off hole I'd probably chain-drill around the circumference, cut between the holes with a cold chisel (easier than it sounds) and finish off the hole with a grinding stone in a hand drill.

David

quote:

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Originally posted by David Jenkins
Thinking about it, if this is a one-off hole I'd probably chain-drill around the circumference, cut between the holes with a cold chisel (easier than it sounds) and finish off the hole with a grinding stone in a hand drill.

David

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quote:

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Originally posted by 02GF74
dunno what stones you have but mine dremel ones would disappear long before the metal so I use a large half round file.

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