Large hole in steel

[Safran]

I need to drill a 90mm diameter hole in a 4mm steel coaming to take a Raymarine autopilot housing. I have a holesaw suitable for steel. The position does not allow for anything better than a handheld electric drill.

Experience in wood suggests the most likely outcome is being thrown across the cockpit as the holesaw locks and the drill rotates. Does anyone have a strategy for cutting a hole of this size in steel with a holesaw?

 

[TheoSr]

The problem will be that your average drill will run too fast for the saw. If you want to keep to a cutting speed of 30m/min you should not exceed 100 rpm. At that speed the normal drill wil not have the power to turn the saw.
I would scribe the hole and then another centric circle with a diameter of 84 mm and then punchmark this circle on 5 mm intervals and drill about 50 5 mm holes along this circle.
Then drill the center hole so that you have a good guide for your hole saw.
The hole saw will now cut easily because the 5 mm holes have removed most of the material. Use cutting oil or even water to keep your saw as cool as possible, because you will have to run it at a too high speed.
Use the extra grip on your drill if you have it. I used to have a mate hold the plug ready for disconnecting in case of a jam. Still have all fingers so method is n ot so bad.
Good luck,
Theo

 

[TrueBlue]

A recent post on this board suggested that the centre drill (once that hole had been drilled) be replaced by a longish bar, so that the cutter can't (easily) jump out.

In any event I'd only use a large slow speed drill with a side handle; lock the throttle so the drill can't possibly go fast, and cut very, very lightly.

It might be worth considering
(a) cutting from both sides if possible
(b) Not cutting right through (which is where the saw will snatch)and finishing off with a jig- or hand pad-saw.

The first hole is the worst.

It is possible to do the whole job with an old fashioned bit and brace drill, but very tedious..

 

[AndrewB]

As a rough-and ready approach, I've used a similar method of marking out a slightly smaller circle and drilling holes right around it with the drilled holes overlapping or almost so, then punching out the centre. Tidy up with a file-bit or small grinding wheel mounted in the drill and finally hand-file to clean the edges. Won't produce a perfect looking circle, but if the hole edge is going to be hidden under a fitting this may be adequate.

 

[SlowlyButSurely]

Yes, I have a strategy. I must have drilled dozens of holes of varying sizes with holesaws in the process of fitting out our steel boat, all with an ordinary Bosch variable speed drill.

The way I do it may be slightly unorthodox and definitely not what my metalwork teacher taught me at school but it works for me:

1. Use the drill on full speed. This is the only way I can get it to produce enough grunt to turn the hole saw.
2. Apply plenty of oil to the job.
3. Stop frequently to allow the drill to cool down.
4. Hold the drill at a slight angle so that the hole saw is not contacting the steel all around its circumferance. This will prevent it grabbing and breaking your arm.
5. Go through the steel slowly.

Best of luck!

 

[PaulJ]

I too have drilled/sawed several holes, some of 110mm dia in 4mm steel in the course of my fitting out. I used a two speed, 850watt B&D "Quatro" which also has a variable speed trigger. I agree with Slowlybut Surely, use oil and "wobble" it gently so that it is not cutting all the way round at the same time. Don't press too hard and just let the saw do do the cutting. It will get through eventually!

 

[Safran]

Thanks very much all. Think I can see what to do using the tools available. Main point seems to be to keep it as slow as possible and keep the bit (only HSS) cool and lubricated.

 

[mikejames]

I once did a 30 mm hole in a steel tank by using a stack of socket adaptors and bars from the hexagonal drive shaft of the hole saw I was using and then using a ratchet spanner to provide the turning power, with my foot resting on the inner end of it to provide pressure.
My cordless drill couldnt turn it at all.

A morning later and I had a hole saw, a hole and a sore arm :-(