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Old 20-09-2007, 15:39   #1
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Making holes in GRP bigger

Hi All,

I have two compasses mounted either side of the washboards which I wish to replace. After taking one out, I discovered that the new compasses need me to increase the hole diameter by about 2-4mm more for the glass to fit .

I have thought about making up a template and using a router to do a clean job, but I have read that GRP ruins woodworking tools/bits.

Any suggestions!
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Old 20-09-2007, 16:36   #2
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You could use a template and drill a series of holes carefully to the inner edge of it - then by "joining the dots" with a jigsaw or router, you would achieve a proper enlargement. I've done this to form a hole from scratch to fit a windvane - glass was about three quarter inch thick (before builders discovered how thin they could make them) - be careful of inhaling any dust - a cheap painters mask would be advisable.
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Old 20-09-2007, 17:49   #3
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Aloha Talisker,

If it is just 2 MM I would use a sanding drum on a power drill with heavy grit to enlarge the hole. If 4 MM then probably a Bosch jigsaw (saber saw). Definitely use a template and you are right about fiberglass dulling router bits.

Kind Regards,

JohnL
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Old 20-09-2007, 18:02   #4
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I would just use a dremel tool or similar to file that much away.

http://www.dremel.com/en-us/tools/to...=66981&I=69724
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Old 20-09-2007, 18:32   #5
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We use a Roto Zip here at our boatyard for cutting nice holes in fiberglass.

http://www.rotozip.com/

Cheers, Don
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Old 20-09-2007, 19:17   #6
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If you have access to the proper sized hole saw, screw a piece of wood to the back side of the bulkhead for the pilot drill to bite into and drill away. Once the hole saw has cut away a bit of material, the pilot isn't absolutely necessary but it certainly is safer to keep it.

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Old 20-09-2007, 20:36   #7
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I vote for the sanding drum in a power drill, as large as you can find that will still fit inside the hole. Hard to make a clean circle with a jigsaw, especially it there is other stuff that will get in the way, and dremel/rotozip is also difficult to make a clean circle unless you use a radius-arm attachment. Whatever tool you use will probably wear out the bit by the time you are done, fiberglass is murder on tools, even carbide!
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Old 20-09-2007, 21:12   #8
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I've not used roto zips but have been curious about them. Are they like a mini router? Are the bits inexpensive?

JohnL
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Old 20-09-2007, 21:14   #9
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Roverhi has the right idea if the hole is of a size that a hole saw can fit. I've got up to 4" but I kind of thought that a compass would require a larger and perhaps odd sized hole.
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Old 20-09-2007, 21:24   #10
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OK! My turn

If you go to Sears or a wood working shop you'll find a tool bit that is like a rotary file but with very large burrs on it, like a rasp file. Grinding Points - Cylinder



It's max RPM is only 5K for wood, but for glass I run them at about 2K or less. A 3/8" drill motor works great. And to help keep the dust down get a squirt bottle with water and keep it just moist. That also helps preserve the bit as well, keeping it cool.

And do wear a dust mask, goggles, gloves and long sleeves taped around the wrists/gloves. And a hat too. High speed milling/grinding of GRP is very itchy and gets EVERYWHERE.

Enjoy......................................_/)
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Old 20-09-2007, 21:25   #11
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Rotozip is like a dremel on steroids - a little bigger, a lot more power, but not much in the way of attachments. Mostly used by drywall guys when the put a sheet of drywall on the wall and run around the electrical boxes, windows, etc with a bit thats smooth at the end to act like a pilot bearing.
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Old 21-09-2007, 06:44   #12
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I'd probably haul out my dremel tool with a sanding drum, make a pencil line with the new compass mount, and then sand to fit--Its very controllable. But I'd also have the the shop-vac pointed at the dremel tool to pick up the fiberglass dust before it goes everywhere.
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Old 21-09-2007, 09:08   #13
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Easiest way I know of

Use a taper wood plug, the kind you all have hanging by your through-hulls...

Tap it in, if it's too long cut it reasonably flush. Drill with standard hole saw. No, you don't have to be precisely centered in the plug.
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Old 21-09-2007, 09:22   #14
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They are like a large Dremel. The handle is removable which helps get into tight corners. The original zip bit works well in fiberglass even though it was designed for drywall. You will burn the bits if you try and cut on too high of a speed, use the lowest speed possible that cuts well and don't push hard; let the tool do the work. The bit with the guide point works like a bearing on a router and can be used with a template. I think we pay about eight bucks for a five pack of bits. They have expanded the accessory line and offer bits for fiberglass now also. I have never used the fiberglass bit as the original works just fine if you don't push too hard or use too high of a speed.

http://www.rotozip.com/Shop/Category...=188064&BID=18

Cheers, Don



Quote:
Originally Posted by SkiprJohn View Post
I've not used roto zips but have been curious about them. Are they like a mini router? Are the bits inexpensive?

JohnL
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Old 22-09-2007, 10:53   #15
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Thumbs up

Thanks for the advice guys.

I found a rasp bit and used it with a battery powered drill, was a bit sceptical at first but it worked a treat.

Compasses fitted!
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