Simple hole drilling question

stuartwineberg

stuartwineberg

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Need a 20mm hole in a very thick glass deck. Looking at pilot hole then 19mm wood boring bit, then a bit of filing (haven’t got 20mm wood bit). Any better ideas as I am sure this is a really common one.

edit: got it Forstner bits. Ain’t google wonderful. £7.99 Screwfix
 
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One millimeter is of no consequence. The hole will probably end up 20mm if you waggle the drill a bit.
 
stuartwineberg said:
Need a 20mm hole in a very thick glass deck. Looking at pilot hole then 19mm wood boring bit, then a bit of filing (haven’t got 20mm wood bit). Any better ideas as I am sure this is a really common one.
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You'll need the arbor as well, but they aren't expensive.
 
A holesaw would be easier but you will have to buy an arbor as well. If you drill from both sides you will get a cleaner finish with no breakout splits. The GRP will probably blunt the saw so you might as well get a cheap one and discard it afterwards.
(beaten to it by JD)
 
I sometimes find the gelcoat cracks so (if I remember!) I start the hole with the drill going anticlockwise which scores through the gelcoat without cracking it. There may be other ways and means of achieving this perhaps with the use of masking tape. Start with a single hole right through so you have a guide for the hole saw and can drill from both sides, otherwise you will have to drill right through with the hole saw which could damage the fibreglass on exit.
 
Just a note, I found that if the edge on a hole saw gets blunted, they can be 'touched up' with carefull use of an angle grinder.
Re the OP, Forstner are not much good in GRP and then useless for wood.
 
vyv_cox said:
I have never used a Forstner bit in GRP but they are superb in wood. Could maybe blunt quickly in GRP but should be good for just one hole.
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They can also be very difficult to keep centered. They are impossible to use if there is any angle unless using a press. I only use them when I need a flat-bottomed blind hole in wood.

Another nice feature of hole saws is you can drill from both sides, eliminating chipping. Also nice for through hulls; you can start them on the inside, to get the location right and scratch the surface, and the do the messy part all from the outside. (Drill perbindicular to the outside, then adjust the angle, if any, with a tapered block on the inside.)
 
I bought a cheap 70mm hole saw from toolstation and managed to cut through 3mm 316.
It still feels sharp !
 
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