Painless hole drilling?

[Flynny]

I'm sure this has been covered here before, but the search feature was not doing me any favors.

What is your method for painlessly drilling pilot holes for screws in fiberglass? What kind of bit do you use, and how do you correctly size the hole for the screw? I know its best to go slow, and I've seen a few recommendation to use HSS bits.

The issue I usually run into is that I drill the hole too a little too small. The screw starts to go just fine, and then turning the screw in becomes extremely difficult, and the head usually ends up breaking off.I'm afraid to go too big, because then I would have to refill and drill again.

 

[airshot]

Well that can be difficult for someone that is not familiar with machining. The proper way is to measure the root diameter of the thread that you are going to install. That would be the solid diameter of the screw not including the threads. Fiberglass is highly abrasive and will dull a good drill bit quickly so if you have many holes to drill then a really good quality bit such as a cobalt steel drill should be used and yes even that will dull after only a couple holes. Do not push hard on the drill and let the bit cut, trying to force it thru with pressure will cause the back side of the glass to breakout leaving a thinner area for the threads to bite on. If we knew for sure what screws you were going to use we could be of better help. As a toolmaker I keep precision measuring tools around so I have no problem getting the measurements I need but without those measuring tools I would need to know the exact screw you were pilot drilling for.

 

[Scott Danforth]

Airshot covered it. need correct pilot drill size for the fastener that you are using. Cobalt bits are great. I have a set of titanium nitride that I use. they work great until they need sharpening. then they work like any HS drill. My dad usually brings his drill doctor sharpener down here with him.. pretty sure he uses it as an excuse in the winter to leave WI and come to FL.

 

[roffey]

not meaning to hijack, but how do you fix the hole if and when to break off the screw? Epoxy and slightly bigger screw?

 

[airshot]

well you get the idea just have to drill thru with a bigger drill or sometimes you can just punch the broken screw thru but you will definitely open up the hole. This is why it becomes more critical to get the proper pilot drill for the intended screw or the next thing you know....you will have one heck of a big hole!

 

[Georgesalmon]

FYI, if you are drilling through gelcoat on the surface be sure to countersink the hole. If you don't you may chip out the gelcoat when you put in your screw.

 

[shrew]

FYI, if you are drilling through gelcoat on the surface be sure to countersink the hole. If you don't you may chip out the gelcoat when you put in your screw.

This is one of the most important steps. it'll reduce cracking as well. I also cover the spot I'm drilling with a good sized piece of blue masking tape. It reduces scratching if you get some accidental bit-walk. it tends to reduce chipping from the bit initially biting into the gelcoat. Then finish with a counter sink bit.

I've used a large drill bit held in my hand, rotated slowly to act as a counter sink bit if I don't have an actual countersink bit handy.

 

[Flynny]

This is one of the most important steps. it'll reduce cracking as well. I also cover the spot I'm drilling with a good sized piece of blue masking tape. It reduces scratching if you get some accidental bit-walk. it tends to reduce chipping from the bit initially biting into the gelcoat. Then finish with a counter sink bit.

I've used a large drill bit held in my hand, rotated slowly to act as a counter sink bit if I don't have an actual countersink bit handy.

This is great advice. I'm going to make sure to do that. The manual for the speakers I'm installing call for a 1/8" pilot hole. It doesn't list the size of the screw. So I suppose I'll go with 1/8".

1/4" bit to counter sink?

 

[spoilsofwar]

I always use tape as well, and I start the drill in reverse. Never had the gelcoat chip using this method.

 

[airshot]

Glad someone brought up the countersink as I missed that in my reply...that said go to your local hardware and look for a carbide "ball" burr. Get one that is around a quarter inch in diameter and it makes a really good countersink in fiberglass and being carbide holds an edge well. I have seen some folks use a carbide tear drop burr to start a hole in fiberglass to keep from chipping the edge of the hole. Dremmel tools does make a small carbide countersink tool with multiple cutting flutes that is not very expensive. Starting the drill in reverse works well, but really dulls the tool bit quickly, so have a few on hand.