Best tool for enlarging a hole in steel (10mm to 15mm 'ish)

[tinnews]

I need to enlarge a hole in the hull of our boat which is 5mm thick mild steel. I've previously cut quite big holes (like two 40mm ones recently) using a hole cutter but there are two problems in this case:-

I only need around 15mm diameter, hole cutters (the round saw type) seem to start at 16mm or so.

There's a hole there already, of about 10mm so I'd need to do something clever to locate the arbor of the hole cutter.

Would a taper drill or a step drill manage this for me? It's probably only a one-off so if I wear it out on one hole I won't be too sad. Because of the one-off requirement I really want a cheap solution.

I suppose I could just take a round file to it but that sounds a bit like hard work! :-)

[dennis]

Toolstation do a blacksmiths drill of 14mm dia that is close for about £3.50.

[ss]

I had a similar situation when abroad and limited tools at hand, it was for a ceramic basin and I needed to enlarge the hole for a new tap. I carefully marked the outline for the `new` diameter and used a conical shaped stone in the elec drill, I took it as close as I dared to the outline and then finished off with a round file. The conical shape sort of self centers itself and it took care of the bulk of the enlargement.

[Dave Plowman (News)]

I'd say a step drill would do it just fine if you keep the speed down and don't let it overheat. You might have to do it from both sides due to the distance between the steps, though.

[Bert Coules]

I've read that a tapered five-sided tool will enlarge a hole accurately, but I'm afraid I've no idea if one is available in the sort of size you need. I've seen truly conical file-type bits which would possibly be OK.

Bert

[John Rumm]

A reamer is the tool for enlarging holes to accurate sizes - although in this case one would need to drill closer to final size first.

[John Rumm]

As an aside, in case its of use...

My holesaws thread onto the arbour, and once in place there is enough arbour still poking through into the saw to thread a second holesaw on. Hence if you want to go up a hole saw size, you fit both to the same arbour - with the smaller one "inside" the larger. That then acts as a centre guide in the existing hole.

You could probably just drill with the right size drill...

[critcher]

critcher said....................... you need a 15/16 inch tapered reamer, just make sure you use plenty of lubricant and go easy on the pressure

[fred]

As 15mm seems to be the smallest, perhaps a dowel or a steel tube over the centre drill would do the same job. Given it's only the outer surface of the guide that needs to be hard, any gap between the drill and tube could be made up with a build up of tape.

Proper 15mm one here for just over a tenner + vat but I'm sure there will be cheaper if you just want it for a one off:

> Would a taper drill or a step drill manage this for me? It's probably

Before John's suggestion I would probably have given it a go with a well lubed step drill from both sides but I do have an expendable one here to try.

[Dave Liquorice]

A hand reamer would be damn hard work in 5mm thick steel... It would also leave a slighly tapered hole if that is a problem.

A step drill generally has maximum thickness between steps of less than 5mm as well so that would leave a stepped hole. Unless the final step was the desired finished diameter.

Think I'd be tempted to just use a couple of sharp larger twist drills but not sure how easy that would be with a hand held drill.

Failing that a half round file.

[John Rumm]

You can get parallel reamers... but then you need a target hole not far from finished size.

[Nightjar]

Plenty of 15mm hole saws turn up when I do a Google search.

You could tack weld a plate over the other side of the hole.

A taper drill would be the better option, as few step drill have 5mm steps.

If you need a parallel hole, that is the way to finish off, after using the taper drill from both sides to minimise the amount you need to remove.

Colin Bignell

[Dave Plowman (News)]

I've just measured one of mine and it's exactly 5mm. So you would have to take care not to put a chamfer on the hole. Or do it from both sides.

[Nick]

Possibly a mag drill with broach cutter. Would certainly do the job if you could adequately seat the drill and centre the cutter. This is presuming it's above water line. HTH, Nick.

[F Murtz]

Why all the complications, just do as Rumm says, drill it with a 15mm drill. (you can get them with step down shanks and probable manage it with a big battery drill on low speed.)

[grimly4]

Air-driven die grinder and use a conical carbide cutting bit.

[tinnews]

Yes, there are some down to 14/15mm, but not so many.

A bit difficult since the hull is lined with expanded polystyrene there and it's just by the bed in the aft cabin! :-)

It's not that critical, it's to mount a bulkhead fitting to feed a

10mm pipe through the hull so a bit of taper won't matter.