Cutting an accurate hole in GRP?

bumblefish

bumblefish

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I am planning on installing a new plastimo 101 bulkhead compass. how do you suggest that I cut the new aperture with the greatest degree of accuracy? Nice smooth edges and no erratic circle!
 
Assuming this compass is too big for a hole saw, I'd work on the basis that, knowing me, there's little chance of a tidy job. So I'd be thinking about what sort of trim I could use to cover up the imperfections, then go for it with a jigsaw, after covering the area with tape or something to prevent scratches.
 
As others have said use a hole saw if possible - reason for post is that if your are not deft with the saw, you can slam the hole saw into the GRP as the pilot goes through, and leave a rough edge (which would be covered by the compass flange anyway) but in order to be neat, clamp a piece of ply over the area and bore through that and on through the GRP - it leaves a smooth edge to the GRP, and any slippage or vibration from an imperfect arbor or pilot drill is taken up by the ply at the start before you hit your boat. Ideally clamp a piece on the far side to avoid splintering, as you 'burst' through.

Yours aye

JackIron
 
I've not used dremel.
I've found the flap wheel sander does a job
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I very much doubt if you will get a hole saw big enough. Drill a small hole in the middle. Scribe a deep groove in the gelcoat with dividers the size you want the hole using the hole you drilled as a centre point and run a pencil around the groove to make it stand out. Then either chain drill or jigsaw the hole out inside this mark and finish off with the biggest flap wheel you can get. If you use a jig saw a good tip is to use carbide tile cutting blade that won't crack your gel coat. If you scribe deeply enough it stops cracks anyway. Protect the shoe of the jigsaw with insulating tape to stop scratches.
 
I cut holes for a couple of ventilators (4" IIRC) with a jig saw (attachment on an electric drill) no bother at all and it was probably ony the second time I'd used it.

Use the reccomended blade for GRP and work steadily. Not available when I did that that but these days you can get narrow blades that enable you to cut smaller diameter holes.
 
It's also worth mentioning that fitting a new compass probably does not require that much accuracy. The design of fitting usually allows a few millimetres variation in size without it being apparent - well within the capability of any of the common cutting mechanisms (hole saw, jigsaw, hacksaw or even drilling a ring of holes then joining them up and smoothing off).
 
As previously said, getting a really acurate hole isn't necessary as the compass has quite a decent sized overlap on the trim piece.

More of a challenge is to cut out the funny rectangular bit for the compensator that sits below the compass...

Any which way they provide a full-size tempalte for you to stick to the bulkhead (use low-tack 'mounting' adhesive?)
 
My B & D router has a centre on trammel bars that I use to cut circles larger than a hole saw.

I have seen in the UK hole saws upto 8 inch dia 200 mm dia most common unto 6 inch 150 mm dia
 
An adjustable holecutter is the way to go. I first found them for cutting clean holes in plasterboard for downlighters. You have a mandrel with an adjustable arm which is moved in and out according to the exact diameter of the hole. The end of the arm has a tungsten carbide scribe which cuts beautifully through GRP as well as most other things like aluminium and does not leave any ragged edges. Adjusts in the range 125mm down to 50 mm - I think it is in the Screwfix catalogue
 
This is what I referred to as trepanning cutter. I too have used them frequently for cutting holes for circular buoyancy tank hatches and have always achieved excellent results.
 
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