How to cut a 10" round hole in 5mm mild steel?

[Chris Green]

I need to replace a porthole on our little boat because the existing one has bad corrosion around it. So I need to enlarge the hole from the existing 8" to about 10".

The porthole is simply mounted in a hole in the 5mm steel superstructure, what can I cut it with? I can cut straight lines in steel with a cutting disk in my angle grinder but I don't know what to do for cutting curves.

I suppose a jigsaw might manage it but it would have to be a good one with good blades, both my current jigsaws are pretty ropey.

So, is there any other way to do this that I haven't thought of? Or can anyone recommend a jigsaw and blades that will cut through 30" of

5mm steel without too much effort (or cost!).

This is probably just going to be one off requirement so I don't mind spending a bit more time to avoid having to buy anything I'm not going to use much.

[Andy Burns]

I'd have thought a jigsaw with decent blades and take it steady would do the job, if it finishes either or both of them off ... well you've got a chance to treat yourself to a new one!

[Colin Bignell]

I used to have a tool that would cut variable diameter holes in steel. It comprised a single tooth cutting bit, mounted on an arm. The arm passed through a spindle, where it was held by a screw, so that the diameter could be adjusted. Very effective, quite dangerous and best used at slow speed. However, it needed a pilot hole to run in, so you would need to do something like screw a couple of pieces of ply together, on either side of the steel, putting the screws through the existing hole, then put the pilot hole through those, in the centre of your new hole. The cutting bit would also need to be long enough to cut through one layer of play plus the steel.

[Fredxx]

A 10 inch diameter hole? 5 inch arm?

How many rpm are we talking about? Sorry, it doesn't seem feasible except for small diameter holes.

[Fredxx]

I would get my plasma cutter out and a angle grinder to dress the hole.

However I don't see what is wrong with a jig-saw and a few spare blades. Sure it will take a while but it would work in a slow, methodical way, but be quite accurate with minimal finishing.

[Colin Bignell]

The slowest speed on our pillar drill, which I think was 50 or 60 rpm.

Sorry, it doesn't seem feasible

The whole point of it was to be able to cut larger holes than most hole saw sets could manage. From memory, the arm was about eight inches long, but I don't recall having to cut anything more than about six inches diameter. I wouldn't be surprised if it wasn't something my father had made up years before.

I have, however, got a similar, commercially made, cutter for putting holes through ceilings for PAR 38 downlighters, but that comes with a built-in rubbish catcher and is intended for use with a hand drill.

[Fredxx]

OK, this for the OP's boat which means it's not very conducive to being cut with a pillar drill.

Most DIY style drills won't go down to these low rpms or apply enough torque to cut with a tool at a 5" radius.

[Chris Green]

Not practical for cutting a hole in the side of my boat I don't think!

[Chris Green]

Yes, thanks, I'm beginning to think that maybe a jig-saw is the way to do it. I'll just be prepared to take a while and use a few blades.

[Colin Bignell]

I think it would be possible, but take quite a while to cut through 5mm steel. You would get a very neat hole though.

Have you asked whether your local tool hire has a power nibbler capable of dealing with 5mm mild steel?

[Michael Chare]

I would also have to use a half round file otherwise the hole would not be very round.

[John Rumm]

For mild steel, a decent blade in a jigsaw with a low to medium speed. Apply some cutting oil every so often, and the blades will last much longer. Dress with a grinding wheel or flap disc in an angle grinder when done.

(you could do it with an AG and a pile of thin kerf slitting discs. Making plunge cuts close to the diameter to create a multi faceted approximation of "round". You could then grind to the line once the bulk of the metal was out of the way)

[John Rumm]

Or just hire a plasma cutter with a circle jig - that will make a very neat hole quickly.

[Sam Plusnet]

I had a mental image of a huge Q Max cutter.

What do they do in boat building/ship building yards?

Plasma torch? (dunno if they can be hired).

[Paul]

5:25 oxyacetylene torch

Paul

[Colin Bignell]

Not a tool I have any experience with.

[Rod Speed]

Just needs a solid enoiugh arm. Even 3x3" RHS would be fine for a 10" hole.

[The Natural Philosopher]

Need a few tonnes to punch a 10" hole in steel. And a well made cutter.

I think the biggest I ever had made was around 4" for a small loudspeaker

What you use depends on how often you use it. Obviously if you are punching standard portholes out of mild steel you end up with a punch and a fly press or hydraulic press.

For a one off, a fly cutter is, if you take your time, a pretty good result.

[Robert]

If the porthole seals and overlaps the hole by a reasonable amount, then a good finish or even an accurate circle may not be necessary.

[Colin Bignell]

On 22/04/2023 20:10, The Natural Philosopher wrote: ..

Thank you, the name had completely escaped me.