Enlarging a hole from 55mm to 70mm in fibreglass

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Ric

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I need to enlarge an existing round hole in fibreglass from 55mm to 70mm.

What is best way to do this?

If I use a hole-saw, I suppose I will need to glue over a piece of plywood so that the pilot drill has something to bite into, cut through both layers, and then have a lot of messy cleaning up afterwards.

I have seen in hardware stores carbide hole cutters that don't have pilot drills but how do I stop these from skidding about until they have bitten into the fibreglass?

The hole does not have to be clean-edged, so I suppose another option might be to use a jig saw, but again I can see that getting messy.

Any tips welcome.
 
There was a thread about this not long ago. If you do a search you might find it. One way is to plug the hole and drill a pilot hole in the centre.

Another way is to bore a 70mm hole in a piece of scrap wood and secure it in place on the hull and use it as a guide
 
You've obviously identified the problem: no centre. And it's a bit big for a step-cutter. Securing a piece of ply to work from makes sense, if you can bond it securely enough. If you go that route, best to drill the pilot hole in the ply before you offer up the hole-cutter, rather than asking the hole-cutter's pilot to do it.

If you have access to a Dremel or similar, that would work. I did it myself in 1/2 inch GRP not long ago (using a rotary sanding drum, rather than the steel rotary file). But do find something to practice on first.
 
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I recently had to do exactly the same. I ended up buying a set of four grinding drill bit type thingies from Homebase for about a fiver. Worked a treat!
 
Bolt two pieces of ply together either side of the original hole using three sets of nuts and bolts just inside the original smaller hole with a fourth hole for the pilot in the middle.

This will allow you to use a larger hole saw to cut the new hole.

Iain
 
OK, thanks for suggestions.

As I actually have to make another virgin hole of 70mm near to the existing 55m hole, I will probably have to buy some sort of hole cutter anyway.

But the question remains, how do you stop those carbide hole cutters that don't have a pilot drill from skidding about before they bite? I've examined them in the shops and just imagine them creating a catastrophic whirly mess!
 
easy method

the easiest way to do this is to use a 70mm holesaw with a 55mm blade also screwed in the arbour at the same time which acts as a guide these are obtainable from most electrical or plumbing wholesellers
 
Ric said:
OK, thanks for suggestions.

As I actually have to make another virgin hole of 70mm near to the existing 55m hole, I will probably have to buy some sort of hole cutter anyway.

But the question remains, how do you stop those carbide hole cutters that don't have a pilot drill from skidding about before they bite? I've examined them in the shops and just imagine them creating a catastrophic whirly mess!
Click to expand...


Pilotless holecutters are normally only used on very thin materials, or in a drill press/pillar drill.
 
Are the holes concentric? The other option is simply to use an abrasive flap wheel on an electric drill to bring it out to the new mark, the enlargement is not that great provide the grp is not more than about 8mm. though I would take precautions against breathing any of the dust.
 
Pad Saw

Pad Saw will do both jobs. Why make it complicated. There is no risk of getting it wrong and you can buy one for about £2. Ok it may take you 10 minutes to cut your hole.
 
christhefish said:
the easiest way to do this is to use a 70mm holesaw with a 55mm blade also screwed in the arbour at the same time which acts as a guide these are obtainable from most electrical or plumbing wholesellers
Click to expand...

I have quite regularly had to do this when fitting a larger size halogen downlight into a smaller/older hole. Works a treat.
 
I agree with Quandary's option of using a 2" dia flap-wheel, a dusty job but easily shaped to the 70mm you require and no backing timber needed for centering, most that I've had in my toolbox have been the 2" variety. Mask and glasses and Hoover operating near the work are a necessity.

ianat182
 
Piece of wood across the hole. Doesn't need to cover the whole thing, an inch-wide strip will do provided it crosses the centre. Screw it into position (quicker than waiting for glue to dry), placing the screws through the band of material you're going to cut out. Since you only have a narrow band you'll need to use fairly small screws, so two each end of the wooden strip will probably be needed. Drill the holes in the fibreglass to the outer diameter of the thread, the screws should be able to slip through without turning, then bite into the wood. Pilot holes in the wood since you'll probably be near the ends and it might just split otherwise (depends on the wood you have to hand, as well as the screws).

Once the holesaw gets going in the fibreglass it'll stay in its own groove provided you're holding it reasonably straight-on. The loads on the wood won't be high.

Anything involving sanding requires turning all that waste material into dust and blowing it round the place - fine if you enjoy dusting and hoovering. The saw will create some, but it's bigger than sanding dust so doesn't travel as much, and you don't get as much since most of the waste will be the solid ring of GRP you've cut out.

I've been cutting a lot of holes in my boat lately.

Pete
 
Flap wheel

Quandary's suggestion of a flap wheel is the quickest and easiest, done it 6 times over the last few years and works every time with perfect holes each time
 
I did a similar job last year, going from 60mm to 65mm. I used a sanding drum in my Dremel, holding the nozzle from the vacuum cleaner with the other hand to keep the work dust free and stop the mess going everywhere. Didn't take long at all.
 
Oops arbor!

As the others have said it's okay to used a firmly held piece of wood to take the central drill.... or just drill lots of small holes (doesn't take long) and join them up with a file. A small padsaw also doesn't take long..

BUT.. if you're really after a tool.. Google the oops arbor!

It's the central part of a holesaw but allows the use of two holesaws instead of a holesaw and a drill...

One here for example... http://www.cromwell.co.uk/STT0508029T
 
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