Simple hole drilling question

Little Dorrit said:
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.
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With the hole saw adjust the pilot drill so that it sticks out well beyond the holesaw teeth. When the pilot breaks through on the far side, you do feel the difference, you an drill from the far side easily and hopefully keep a clean cut.
 
Boathook said:
With the hole saw adjust the pilot drill so that it sticks out well beyond the holesaw teeth. When the pilot breaks through on the far side, you do feel the difference, you an drill from the far side easily and hopefully keep a clean cut.
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I was referring to a situation where the drill bit of the hole saw does not make it through to the other side! You are pretty stuffed if you don't drill a pilot right the way through in this case.
 
Read somewhere that the tungsten carbide grit holesaws were particularly good for GRP. Bought cheapie version from Toolstation and it worked great drilling holes for through hulls. It was better than my cheap ‘regular tooth’ varieties and didn’t get melting hot, on a slow speed!
 
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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Vyv's thoughts that GRP may blunt Forster bits quickly are right, they blunt very rapidly when used on GRP, I found two holes were about the best I could manage before the bit was too blunt to cut correctly.
 
DownWest said:
You don't actually need a pîlot drill. Jusr predrill a bit of 10mm ply and clamp it to the work as a guide, or in this case, put your feet on it.
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Fair enough but sounds more complicated than simply running a pilot through. Perhaps we are in agreement I don't actually fully understand your method and why a pilot is not easier? But then again I'm simple and uneducated in these matters.
 
Little Dorrit said:
I was referring to a situation where the drill bit of the hole saw does not make it through to the other side! You are pretty stuffed if you don't drill a pilot right the way through in this case.
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Really? You can drill the pilot hole with any 1/4" bit and you can drill from both sides wth the hole saw, so unless the material is > 4" thick, you are good. Plausible in wood; there are different bits for that. Damn unlikely with most other materials.
 
Little Dorrit said:
Fair enough but sounds more complicated than simply running a pilot through. Perhaps we are in agreement I don't actually fully understand your method and why a pilot is not easier? But then again I'm simple and uneducated in these matters.
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This is done where there is nothing for the pilot bit. For example, enlarging a hole, overlapping an existing hole, or drilling over an irregulaly shaped opening (damage, for example). I've used this trick.
 
thinwater said:
This is done where there is nothing for the pilot bit. For example, enlarging a hole, overlapping an existing hole, or drilling over an irregulaly shaped opening (damage, for example). I've used this trick.
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Having read your reply I understand your point. I am reading the OP's question only and assuming a new hole deeper than the hole saw and the bit can handle. You can start the hole from one side but if the bit does not protrude through you are scratching your head!? I was thinking that the obvious solution is taking a pilot before you start right through? Your solution is a good one but only when you have a different problem to solve. BTW I have messed up on many occasions so thanks or the heads up.
 
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Little Dorrit said:
I am reading the OP's question only. In addition, I am assuming a hole deeper than the hole saw and the bit can handle. You can start the hole from one side but if the bit does not protrude through to the other side what then? Surely you are scratching your head? The obvious solution is taking a pilot before you start right through? Your solution is a good one but only when you have messed up (as I have done on many occasions) thinking you could go right through.,
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BTW, the length of the pilot protrusion is adjustable, from none to over 3 inches. I assumed you knew this.

Unless the GRP deck is over 3 inches thick--not implied by the OP--this is a non-issue.

And yes, you could drill the pilot first. Often this is a good idea, so you can check the location from both sides, and redrill if advisable.
 
There is another thing with pilot drills. I have occasionally had them break when drilling holes at an angle, if the saw catches (usually in metal.) So if this looks likely, I drill the pilot first, then put a bit of 1/4" rod in the mandril as a guide for the hole saw, not so fragile as a drill.

The plywood guide trick is also useful when the is a tendancy for the hole saw to wobble and drill oversize. Again, usually in metal, when the pilot enlarges its hole and allows the saw to wander. Thin SS is sometimes prone.
When driling a bunch of 50mm holes in 6mm SS, I made a jig with a metal guide for the saw, no pilot drill. Worked very well.
 
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