Self Tapping Screws in GRP.

[jakeroyd]

I need to screw a plate onto my gelcoat , it's not loadbearing.
I have found if you drill a small pilot hole you can crack the gelcoat driving the screw or shear the head off the screw. If you drill too large the screw will just ream out the material and will then not hold.
Should I always go through the gelcoat and screw into the fibreglass , ie step drill the hole ?
Have any forumites got any pilot hole sizes or reccomendations ?
I have googled it and thet does bring up some sensible info but I would like to know what others have found.

(I could always glue it on !)

Thanks in advance.

 

[VicS]

I always drill a good clearance hole through the gel coat then a somewhat larger pilot hole than one would for other materials into the laminate. The trouble is if you only have a standard set of metric drills you are rather limited in sizes although in between sizes are available. Fortunately I have an old fashioned set of "number" size drills and a set of fractional drills with every size fro 1/16" upto 1/2" in 1/64" increments

I sometimes/ often/usually/ put the screw in with a dab of Araldite (Slow setting not the Rapid rubbish).

 

[tcm]

drilling into plastics is better with a sharper-angled (ie more pointy) drill bit if you can find or make one. I spose techincally the top of the hole should be clearance as the gelcoat is brittle, so the self tapper actually grips on the fibres below. Hiding to nothing really though, much better if u can thru-bolt it...

 

[johnalison]

You shouldn't have any trouble drilling carefully with a hand drill through the gelcoat with the right bit for the screw and enlarging the hole in the gelcoat with a slightly larger bit. This will take a tiny section of glass with it but there'll be plenty left to hold the screw which should be tightened just so. Usually the fitting will cover any untidiness though occasionally I've needed to fit just a screw and washer.

 

[earlybird]

If the hole drilled into the laminate is a bit on the tight side, I find it helpful to put a dab of lubricant on the screw and work it backwards and forwards. It cuts a thread for itself, making for a strong attachment, and much reduces the risk of shearing off the head. Silicone grease is good if you have some.

 

[pappaecho]

When you have done all that put Sealastic under the plate and screw it down

 

[FAITIRA]

The VicS method is good imo. Bill.

 

[VicS]

[ QUOTE ]
the VicS method

[/ QUOTE ] To which I'll add: if necessary carefully remove the screws when the Araldite has set (they come out easily enough if you don't clean them initially) and refit the fitting on sealant of your choce.

 

[FAITIRA]

I,ll do it your way next time Vic, sounds good.Bill.

 

[chwarae]

Reversing the drill direction and drilling slowly through the gelcoat I have found prevents chipping the gelcoat.
Harry.

 

[philip_stevens]

If you are drilling into gel-coat, you are also drilling into the grp.

If so, drill and tap, using the correct size drill bit, and a metric tap. Fit using a stainless steel machine screw with some sealant on the thread. It will hold just as well as a self-tapper.

If you can also get to the other end of the machine screw, you can add a washer and nut.

 

[RMC]

After drilling the pilot hole, use a countersink bit to countersink the outer gel coat, this should prevent it splitting when inserting the self tapping screw.

 

[snowleopard]

I have fixed almost all low-load hardware to my boat with self-tappers. In my case the structure is foam sandwich with skins around 3mm so not a lot to get a grip on but they hold just fine in one skin. My handrails are made with 5 holes per foot and you can swing on them without them pulling out. It is important to drill the right size hole, just big enough to allow the core of the screw to pass through so you get the whole of the thread biting into the laminate.

To avoid breaking off flakes of gelcoat you can use a much bigger size drill, e.g. 5mm for a no 8 screw, to countersink, stopping when you get to the laminate (best done with a hand drill).