Enlarging a hole from 55mm to 70mm in fibreglass

[Ubergeekian]

I have a drill bit made with sharpened flutes so that it can also be used sideways, like a slot mill. Works beautifully in GRP.

 

[Bosun Higgs]

Can I piggy back on this thread to ask for some related advice?

I have been chickening out of cutting some 150mm diameter access holes in the inner lining of my boat ( Sadler type construction with moulded glassfibre headlining and maybe 30mm clearance between headlining and deck). Question is, the best approach?

a/ holesaw
b/ jigsaw
c/ something like geeky describes
d/ drilling lots of little holes round the circumference

 

[Deleted member 36384]

.... 150mm diameter access holes in the inner lining of my boat ..... maybe 30mm clearance between headlining and deck). Question is, the best approach? ...... d/ drilling lots of little holes round the circumference

If depth control is critical I would use the small holes approach and set the depth bar on the drill (or make up a depth limiter and tape it to the drill). Then clean up the cut with a small diameter sanding drum, flap wheel. I would use a rasp to take off any peaks between the small holes first.

 

[Ubergeekian]

Can I piggy back on this thread to ask for some related advice?

I have been chickening out of cutting some 150mm diameter access holes in the inner lining of my boat ( Sadler type construction with moulded glassfibre headlining and maybe 30mm clearance between headlining and deck). Question is, the best approach?

a/ holesaw
b/ jigsaw
c/ something like geeky describes
d/ drilling lots of little holes round the circumference

A bodger writes ...

By a strange quirk of fate I had to fit three 150mm diameter access hatches to the buoyancy tanks of GRP dinghies last year. The first one I did with a jigsaw. I found it difficult to control and ended up with something like a lopsided 50p piece - luckily the flange of the hatch hid the horribleness.

For number two I drilled lots of little holes and then used a padsaw to join them up. Worked fine, but time consuming and the postage-stamp-perforation edge might be an issue in some cases.

For the last I was given the sideways cutting drill thing. I put a few holes round the circumference and then went for it. Lovely smooth hole in about 2 minutes. Light sanding only required for a beautiful smooth finish.

This is the sort of thing:

and Mr Google says eight quid for four. I don't think I'd want to try them on wood or metal, but on GRP they work a treat.

Edit: This thread has prompted me - I've just ordered a set from Amazon.