Drilling holes for self tapping screws

[MikeJ42]

I want to fit a bimini frame and it requires drilling holes and using self tapping screws. I am concerned that I am going to end up with star crazing in the gel coat.

How do you self-tap into fiberglass/gelcoat safely? Should I countersink the hole to remove the gelcoat? Will the fibreglass itself star craze? Ta.

 

[burgundyben]

In the past I have found a little countersink to be effective, the added bonus is you get a little cavity around the screw filled with sealant to keep the water out.

 

[Firefly625]

I want to fit a bimini frame and it requires drilling holes and using self tapping screws. I am concerned that I am going to end up with star crazing in the gel coat.

How do you self-tap into fiberglass/gelcoat safely? Should I countersink the hole to remove the gelcoat? Will the fibreglass itself star craze? Ta.

Having screwed a few things into FG a few tips from my experiences. But basically screwing self tappers is fine I just follow the technique below, if you can gain access to the underside then use nuts & bolts....

BTW- no warranty given or implied with this, its just what I do!
Drill size to screw size needs to be very close. If your screw is a little too big for the hole you have drilled you will stress the FG around the hole and the gelcoat will chip off around the hole. So whenever I am screwing something in a hole I have just drilled I always do it by hand (don't use a powered screw driver), and work slowly, if there is too much resistance I will ease the screw out a tad and then back in so in effect cutting a thread, however if too tight then do not put too much effort on the screw, just remove and go one size up on the drill to give better clearance. Hope this helps.... I am sure someone else will have better advice!

Countersinking should remove some pressure on surrounding Gelcoat, so probably a very good idea...

 

[MikeJ42]

Having screwed a few things into FG a few tips from my experiences. But basically screwing self tappers is fine I just follow the technique below, if you can gain access to the underside then use nuts & bolts....

BTW- no warranty given or implied with this, its just what I do!
Drill size to screw size needs to be very close. If your screw is a little too big for the hole you have drilled you will stress the FG around the hole and the gelcoat will chip off around the hole. So whenever I am screwing something in a hole I have just drilled I always do it by hand (don't use a powered screw driver), and work slowly, if there is too much resistance I will ease the screw out a tad and then back in so in effect cutting a thread, however if too tight then do not put too much effort on the screw, just remove and go one size up on the drill to give better clearance. Hope this helps.... I am sure someone else will have better advice!

Countersinking should remove some pressure on surrounding Gelcoat, so probably a very good idea...

Thanks!

 

[DAKA]

Good advice so far plus

Cobalt is the technically correct drill material for fibreglass

Use insulating tape to help the drill bite

Stainless self tappers can snap off if you apply too much pressure and you cant get them out.

Slowly screw in by hand so you dont over heat the screw.
if its too tight take it out and put the drill back in to wiggle it about a little and then use a brand new screw, the old one that didnt fit is hot and now weak, it will shear off in the hole, if you are too tight to throw the old screw away then only use it in a hole that has had a new screw in it first.

Use thread lock on the screw to stop it coming out and to stop water getting in.


PS

remember to check what is behind the GRP, an engineering mate of mine ( who frequents the forum so best remain nameless ) drilled and self tapped through the grp and into his aluminium fuel tank

 

[Major Catastrophe]

remember to check what is behind the GRP, an engineering mate of mine ( who frequents the forum so best remain nameless ) drilled and self tapped through the grp and into his aluminium fuel tank

I think it would be helpful to all of us if you named the person.

If nothing but for the comedy value.

I usually countersink and then pump in a blob of sealant to keep the screw tight and the water out.

 

[VicS]

More or less as above.

The hole needs to be a little larger than perhaps you would use in thin sheet metal.

I counter-bore a clearance hole through the gel coat.

 

[Firefly625]

I think it would be helpful to all of us if you named the person.

If nothing but for the comedy value.

I usually countersink and then pump in a blob of sealant to keep the screw tight and the water out.

...and the fuel in?

 

[Coaster]

My method is largely as stated by previous posters but the first step may be different. After putting a pencil cross to mark the spot:

1. Use a metal punch to make an indentation about 1 - 2mm wide and deep (pushing hard is usually sufficient to dig into the gelcoat). This reduces the likelihood of the drill bit skating.

2. Drill a 2mm pilot hole.

3. Use a sharp countersink to widen the pilot hole to a diameter of screw size plus 1 - 2mm.

4. Drill a hole of 1mm diameter less than the screw size, or 0.5mm less if the fibreglass being screwed into is more than 5mm thick.

Then insert the screw.

 

[rafiki_]

I use masking tape on the gelcoat, and mark on this. Helps to resist "skating" of the drill bit. As others have said, if you can get to the backside, use a nut and bolt. Countresinking is good, as is a blob of mastic sealing.

 

[Ripster]

In my experience, gel coat damage is done when the screw is first turned in to bite - the surface cracks and when it is over tightened causing the crazing. So all good advice so far, but dont be tempted to overtighten - just pinch it up and then an 8th/qtr turn more to lock it. Bit of sealer is always a good idea as are washers/plates to spread any surface point loading you may have.

 

[carlton]

I want to fit a bimini frame and it requires drilling holes and using self tapping screws. I am concerned that I am going to end up with star crazing in the gel coat.

How do you self-tap into fiberglass/gelcoat safely? Should I countersink the hole to remove the gelcoat? Will the fibreglass itself star craze? Ta.

I'd be wary of relying on self-tappers to take the strain of a bimini frame. Can you access the back of the panel and slap some chunky lumps of wood/ply behind it? It would help distribute the stress.

 

[GrahamHR]

Using self tappers into a GRP substrate is a pretty crude way to do things, even if you countersink the (more brittle) gel coat. The threads on the screws are trying to push the GRP apart in it's weakest orientation; interlaminar. I don't think you'd ever come across an aircraft with self tappers into the GRP ! Either use nuts and bolts (with washers etc) so the stresses are compressional (where GRP is very good) or bond in threaded inserts.

 

[jfm]

Using self tappers into a GRP substrate is a pretty crude way to do things, even if you countersink the (more brittle) gel coat. The threads on the screws are trying to push the GRP apart in it's weakest orientation; interlaminar. I don't think you'd ever come across an aircraft with self tappers into the GRP ! Either use nuts and bolts (with washers etc) so the stresses are compressional (where GRP is very good) or bond in threaded inserts.

Agree all that. Another alternative is to buy bimini fitings with a big footprint (or get them wellded onto plates to increase foot print) then use the strong adhesive sikaflex (I've forgotten the number) as well as self tappers. The self tappers really just hold it while the glue dries