Drilling GRP

[reddragon44]

HI
I need to drill some 30 mm holes in GRP .I have a 32 mm flat bit and the flange on the fitting, will cover the bigger hole .But will a flat bit do the job? or should I buy a hole saw?.GRP thickness 10mm
Cheers bobt

 

[mikejames]

That will produce a lot of fibreglass dust, as all of the GRP that makes up the hole will be turned to dust and the flat wood drill bit will get somewhat blunt cutting the glass.

I would go for the hole saw , as this makes less dust and provides you with nice core samples of the laminate instead of turning them to dust.

 

[bigwow]

Buy a 30mm hole saw and cut from both sides, if a jobs worth doing etc. /forums/images/graemlins/smile.gif

 

[Lakesailor]

Flat bit will hammer the grp and possibly spread crazing in the remaining gel coat. Get a GOOD hole saw. (ie one with a central arbor and correctly sized barrel, not one of those cheap multi-size ones with spring blades)
Like this

Or drill a circle of small holes in side the marked circle and knock out the middle then use a coarse half-round file (or a Stanley) to achieve the correct shape.

It's a good idea to tape over the area around the proposed hole to avoid marking the gel coat with unplanned slips and cock-ups.

 

[Danny Jo]

[ QUOTE ]
Flat bit will hammer the grp and possibly spread crazing in the remaining gel coat

[/ QUOTE ] Yes. Even if you drill from both sides, breakthrough with a flatbit will create unnecessary stress (in the GRP, the bit, the drill and your wrist). With a decent hole saw, drilling each side in turn, you are able to stay in control and don't risk damaging the GRP. (I say "are able" because pressure on the hole saw still needs careful application. Make sure your own position is secure and that you cannot be thrown off balance if you get it wrong and it binds.) See if you can find an old piece on a skip to practice on first. Mark the centre and drill a pilot hole with a twist drill smaller than the pilot on the hole saw. Check periodically as you are drilling that dust is not building up and choking the teeth.

I agree also about getting "one with a central arbor and correctly sized barrel, not one of those cheap multi-size ones with spring blades". You generally have to buy the arbor and saw separately, but the arbor will fit other sizes. B&Q have them, or electricians' suppliers, or builders' merchants. Spending over a tenner on a saw that you may only use once may seem like a lot of money, but it's a lot cheaper than a repair job on damaged GRP.

 

[jeremyshaw]

I agree with the other posters, but if I was unable to buy a hole saw (I see you are somewhere in Greece) I would try the flat bit. The thing would be to work slowly with minimum pressure - and of course from both sides, and avoid overheating. But the bit will need to be sharp or you'll just get a lot of heat and no cut. I did it once and it was OK - but I would have much preferred a hole saw and now carry a decent range.

 

[boatbuilder]

Grp will quickly blunt an ordinary bimetal holesaw. So for repeated use I use TCT hole saws.

 

[Refueler]

As others say - decent hole cutter. Note that GRP is extremely hard on cutter teeth - you'd be surprised how easily it blunts drills, hole cutters, jigsaw blades etc. So get a decent hardened teeth job.

 

[Lakesailor]

An echo in here? /forums/images/graemlins/grin.gif

 

[Ubergeekian]

[ QUOTE ]
HI
I need to drill some 30 mm holes in GRP .I have a 32 mm flat bit and the flange on the fitting, will cover the bigger hole .But will a flat bit do the job?

[/ QUOTE ]
No. It will make a dreadful mess and a ragged, torn hole. Hole saw.

 

[Captain Coochie]

A flat bit uses a point to direct the cutter and the point gets bigger the deeper you go , once you have broken through it will go mental and do worse damage than 32 mm of hole . Buy the right tool and do the job properly .