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Drilling fiberglass
I have an SPF windshield mounted mirror that I want to change out for a cowl mounted mirror (the windshield mirror is just as useless, I just want accuracy).
I spoke to a guy at Hillbank and he says it's just as simple as drilling wood, you just use a metal type screw (that comes with the mirror, of course) instead of a wood screw. My concern is tightening the screw and having a chunk of the dash come off. Are these concerns valid? Any tips or tricks? Thanks |
First you want to use some masking tape where you drill and start off with a smaller size bit and work up to the size you want. glass tends to splinter so you must go slow.
joeg |
in addition to the above, get yourself a new (or at least very sharp) bit.
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bit size?
Any idea on what the size bit should be?
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I had the same question and if you search the threads you'll see the answers. In general..it's all been said...mask the area, start with a small diameter hole, work up to the size you need, drill slow...some one even recommend drilling backwords...it's more scary then anything but if you take your time it will cut through like "buttah" as Linda Richmond says (from SNL). I got a scrap piece of glass from Peter at ERA and practiced on that to get my confidence up. It is really simple and easy. I also took it a step further and reamed the hole with a reaming bit.
As for starting sizee you can go either by sight or the best is to use a micrometer to see you ending circumfrence then work down from there. Hope this helps. |
Also,
I don't know if it makes a big difference but after drilling the hole to the final size, I like to use a countersink bit to just ease the top of the hole a bit. This way you have a small bevel around the drill hole. I find you don't get as much splintering and it allows for the screw to go right in. I believe it also stops the paint from chipping beyond the drilled hole. |
Best practice is to not rely on the fibergass to hold the screw. Use a backing plate of sheet metal and thread the screws into this, not the fiberglass itself.
I've yet to see fiberglass not form stress cracks over time from the weight of items screwed to it, especially on boats that take a lot of pounding. I'd also recommend fabricating a rubber gasket under the base of the mirror. |
Use 10-32 machine screws with oversize (fender ?) washers with nylock nuts. Ronbo's suggestion of a rubber gasket is on target.
The drill bits for use on FRP have a sharper point than those normally used for metal and drill easier. |
"some one even recommend drilling backwords"...Do it!
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Drilling in fiberglass is amazingly easy. I'm not saying don't be careful but it's not at all difficult to do. What's more important is to make sure you get the hole the right size. That is, don't undersize the hole and then "force" the screw in. You can easily break the head off of screws when screwing them into fiberglass if the screw is of the cheap "chinese" variety. I speak from experience. I had to move my cowl mounted mirror over 1/2 inch to cover up a screw that I broke off in the cowl. I'd even suggest replacing the screw that comes with your mirror with a stainless steel one if you can find one at your local hardware store.
Chris |
Another option
I wasn't fond of like the idea of a sheet metal screw in fiberglass, so I epoxied stainless steel T-nuts to the underside of the holes, with the sleeve of the T-nut inside the hole. T-nuts can be purchased with holes punched in the T-nut flange and that permits the epoxy to "ooze" up through the holes and,I believe, give it could make a stronger bond.
I have also had good success using sheet metal step drills for making holes in fiberglass. Beats step drilling with individual drill bits. Oh,... yeah. I forgot to mention, that the reason I liked using T-nuts is that the sheet metal screws can then replaced with machine screws. Then they can be removed and reinstalled al much as you wish and not eventually produce "a loose screw in a sloppy hole" situation. |
When determining what size drill to use if you use a drill bit that is the same or a few thousands larger than the minor diameter of the thread you will be good to go.
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Quote:
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Plug in "plastic stress cracking". It explains the whole thing. In boat work we often drill the hole slightly oversize, wrap the machine screw with 1 or 2 wraps of teflon tape, and set the screw in thickened epoxy. When the epoxy sets the stress is allowed for, and the teflon allows you to back the screw out to install the fitting. Do it all the time in big ocean racers.
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use a uni-bit. Thru bolt using machine screw and nylocks. anything mounted to a painted surface should have rubber under it.
http://www.irwin.com/irwin/consumer/...l/10231_sm.jpg |
tapping
all of my boats were repainted with awcraft. We always taped the holes, most places on a sailboat (the deck) can't be reached for a washer and nut.
NEVER had on pull on me, always countersunk the hole and always tryed to do this before painting...paint never cracked..good luck |
Although it's not relevent to this application, in a blind hole situation use well nuts if possible. Always isolate with rubber to prevent vibration shock being transferred to the fiberglass substrate.
Big problem with fiberglass is the gelcoat / paint surface is far more rigid than the fiberglass underlayment. BTW carbide burrs are the best thing I've ever used to drill into glass, the more cutting edges the better. |
Nuts
Take a look at the large flange rivnut, they come in fractional and metric, steel and aluminum. May be good for other uses too.All my aluminum panels will be held on with rivnuts instead of regular rivets.
Al |
Best solution I have found for fiberglass threading and occasionally in aluminun are slotted body rivet nuts. Very secure and easy to use, either in a blind situation or exposed. See page 3214 at http://www.mcmaster.com/.
Same type of fastener as used in dry wall. Unit mushrooms on the backside for a large surface area. Any Good nut shop should have them. |
Thanks for all the help. Lots of handy folks here.
I never expected this response! |
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