Drilling through GRP

[nwclegg]

I want to drill through the GRP transom of my Nelson 34. It is not a sandwich but fairly thick 1969 GRP. It is above the waterline but access on the inside is poor. Holes are 8mm for bathing platform. The hull above the waterline is "framed" by a wood overlay about 15mm thick.

Does anyone have any recommendations ref bit type, procedure etc.. I am concerned about splitting the GRP layers and will try to get a block on the inboard side into which I can drill but doubt whether I will achieve it. The external wood layer will add a certain amount of bracing.

Help appreciated.

Thanks

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[MLBURGE]

Shouldn't be a problem any metal drill bit will be ok, drill a pilot hole first if you want to, if im drilling directly on the gelcoat then a sharp drill will chip the gelcoat and so i start with a bluntish drill first but if it's covered with wood it probably won't matter.

use large penny washers and if needed wooden backing plates on the inside of the transom to take the load if you can pref hardwood or good exterior ply. For a really neat job glass these over with a layer of fibreglass mat and paint or gelcoat.

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[snowleopard]

stick a bit of masking tape on the gelcoat and make your drilling mark on that. it stops the bit skidding and reduces chipping of the gelcoat. any sharp drill bit will do, they tend to go blunt if used regularly in GRP but will be fine for a few holes.

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[tugboat]

My experience concurs with snowleopard. Use masking tape and cover the surroundings in case of a slip. Invest in a new drillbit and start drilling with very little pressure so that the cutting starts at the very tip of the bit first and gradually works outwards to full diameter. This prevents chipping. Stop from time to time to prevent too much heat buildup. Obviously make sure your stance is comfortable with a firm footing.

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[MLBURGE]

yes good with the masking tape, sounds like the grp is overlaid with wood so the gelcoat won't be visable.

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[nwclegg]

Thanks to all so far. Yes the wood overlay should stop slippage and may prevent chipping but certainly it won't be visible. I can see the point of a very slow approach allowing the drill bit to cut its way through rather than force it through which tends to support the sharper bit theory rather than the blunt one. We'll see on Sunday. If I can I will add a photo of the finished article - having said that not sure how people have been actually inserting photos into theor posts - will have to experiment.





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[MLBURGE]

I only use a bluntish drill on the very first bit you must use a sharp drill to go through.

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[theia]

don't forget to countersink the holes before you put sealant on otherwise you won't get a watertight seal. just as important above the waterline.

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[plegare]

I have found information about a special drill for drilling Fiberglass. This one is carbide tipped and I believe the cutting faces are at different angles to the normal. "These carbide tipped twist drills need less thrust for fast penetration and to prevent delamination on exit." The link to the information follows. Good luck with your project!

http://www.icctool.com/webdoc35.html

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[nwclegg]

I spoke to a tools/fasteners wholesaler and he suggested a bit type that has about 3mm of spike on top of the drill and the spiral of the main bit causes the cut to pull the material inwards rather than force it out like an ordinary HSS which seemd to make sense to me.

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[davidwf]

The wood type with a 3mm spike as described are what I've found to be best for GRP. Still use masking tape as well to stop initial slippage.

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[MLBURGE]

we used a spade spiked bit from wickes's for our main trim tab ram holes and it did go through easy - think they were 25mm, although conventional bit should be ok for 8mm.

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[Guest]

Drills ....

Never had problem using standard drills and even at times when hole is needed to be REALLY big - extra long Hole cutters ..... I have never had to resort to the spade wood borers that some advocate - so cannot comment as to whether they are better ..... standard drills did the job for me fine.

<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[Phoenix of Hamble]

Re: Drills ....

Also,

Run the hole cutter/bit backwards to start with until the surface layer is scored... this prevents chipping/splintering particularly well... this is the technique used by fibreglass car body repairers/modifiers..

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[nwclegg]

Thank you one and all for the help. Job finished and despite a bit of a learning curve looks pretty good. The platform is split either side of a transom boarding ladder and the first is not quite as tidy as the first. The wood drill bit worked really well but was getting a bit blunt by the last hole.

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