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Author Topic: Cutting GRP hulls  (Read 7724 times)

BlueWotsit

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Cutting GRP hulls
« on: July 26, 2006, 05:15:01 pm »

What is the best method for trimming a GRP Hull.

Sanding is fine to a certain degree, but on a model Im working on, I have to trim off about a one inch line the length of it, and wonder if there is a "good" tool to use.

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wombat

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Re: Cutting GRP hulls
« Reply #1 on: July 26, 2006, 05:33:12 pm »

I used a "Dremel" with a carborundum disk. Seemed to work reasonably, but you have to keep the disk moving or it overheats the plastic and gums up.

My "Dremel" is actually a B&Q cheapy clone (£30 including a flexible drive shaft, stand and a range of tools)

Tim the Wombat
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kayem

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Re: Cutting GRP hulls
« Reply #2 on: July 26, 2006, 05:46:40 pm »

I've tried everything, and the best tool by far, in fact the only useful hand tool I've found, is a long discontinued Eclipse sheet metal saw, they come up occasionally on eBay, and are worth searching for. Use a 24tpi hacksaw blade, silicon steel works best on fibreglass. An ordinary hacksaw is no good, as the frame won't pass along the cut. A coping saw is OK, but it's a bit difficult to cut a straight line, so cut a safe difference away from the line, and finish off with a sanding block. A Dremel? Not unless you're uncommonly patient and have vast numbers of replacement blades. Using a Dremel type tool on fibreglass is alright for small areas, very useful in fact, I wouldn't be without one, but you it's not a suitable tool unless you only want to cut two or three inches. Mark the cutting line accurately with masking tape, and only ever cut from the gelcoat side, otherwise you'll make lots of chips along the edge.
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Martin (Admin)

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Re: Cutting GRP hulls
« Reply #3 on: July 26, 2006, 05:56:56 pm »

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BlueWotsit

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Re: Cutting GRP hulls
« Reply #4 on: July 26, 2006, 06:30:07 pm »

Thanks one and all for the great input
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Barthez

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Re: Cutting GRP hulls
« Reply #5 on: July 26, 2006, 09:25:36 pm »

Perma grit do a set of two small tungsten carbibe cutting wheels and mandrel. I have had mine for years and I have Trimmed  quite a bit of GRP with mine no problems at all.
You could use a diamond cutting disc but they are usually expensive, Carborundum cutting disks get too hot.
The link below takes you to the Perma grit site

http://www.permagrit.com/system/index.html
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Stavros

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Re: Cutting GRP hulls
« Reply #6 on: July 26, 2006, 10:59:28 pm »

If you have got a 4inch angle grinder use it wiht a cutting disc BUT FOR GOODNESS SAKE wear a MASK what ever way you do the job.GRP dust is very very dangerous stuff and will not od your lungs any good at all,preferably do this cutting operation OUTSIDE as the dust will be everywhere good luck
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Shipmate60

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Re: Cutting GRP hulls
« Reply #7 on: July 26, 2006, 11:04:09 pm »

I use a padsaw with fine toothed hacksaw blade, I use at a shallow angle cutting the gel coat first which acts as a guide.

Bob
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White Ensign

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Re: Cutting GRP hulls
« Reply #8 on: July 27, 2006, 09:20:20 am »

Doing patterns and huills on my own I found the best is one of these rotating blades for Dremels, which si covered with small diamond chips. The ususal carborundum Rot.blades produce heat which might start to melt the Epoxy and though the surface will be "greased" with Epoxy. A standard saw does not "cut" (in a technical way) the grp, it "tears off" the grp, which does not make a fine surface anyway.

Hope this was out of any help,
Jörg
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anmo

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Re: Cutting GRP hulls
« Reply #9 on: July 27, 2006, 10:12:25 am »

If you can't find that Eclipse sheet metal saw kayem recommended, shipmate's padsaw suggestion is the next best thing. You need something with a flat blade, makes it much easier to steer a straight line than with a coping saw. You have to be patient with this kind of thing, keep the saw at a shallow angle and don't try to rush it, and I'd never advise using a power tool for a job like the one BlueWotsit asked about.
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John C

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Re: Cutting GRP hulls
« Reply #10 on: July 27, 2006, 06:40:16 pm »

Now then BlueWotsit........It's me John C, I thought I'd add my twopenneth.
I really wouldn't use a power tool either (especially if you're not experienced with them, the slower you go the less of a mistake in this instance) and agree with ANMO whole heartedly on this. I also think that if you haven't done this sort of thing before, then a coping saw is pretty unwieldly (due to the size of the jaws) if you're not used to using one.
But.....and it's a big but, do you have a fret saw? (smaller version of a coping saw used for metal work). As you only need to remove an inch and I know the size of boat you're familiar with!!. If you have, once you've secured the blade in the saw, take a small nosed pair of pliers and twist the blade through 90 degrees a little way away from where the blade is held in the saw at both ends.
You now have no problem with cutting the length of the hull because in effect you're using the saw sideways, and you don't have the extra weight of the coping saw jaws dragging down on one side and putting you off your aim.
What was it? best ideas are the simple ones!!

Regards

John C
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BlueWotsit

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Re: Cutting GRP hulls
« Reply #11 on: July 30, 2006, 09:29:42 pm »

Well after careful consideration I went for the Dremel Clone - simply as I had one in my tool box.

Thanks to a tip I read elsewhere to do the cut in the opposite direction to the rotation of the disk, I actually got on quite well with cutting the GRP - totally surprised myself,

Thanks one and all for your valued contributions
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White Ensign

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Re: Cutting GRP hulls
« Reply #12 on: August 03, 2006, 01:55:44 pm »

BlueWotsit, glad that you had managed the Dremel- not the Dremel managed you!  ;D

B.t.w.- there was this modeler, who found it very difficult to cut a piece of wood of with a jigsaw. It took the hell of a time and wasn`t right-angled. Though he went back to the lad who sold him the machine and asked him how this could happen.
The lad in the store controled the machine, the blade and when he took out a piece of wood to have a test the customer stared at him saying: "What the `eck are you doing there? And where does this noise come from?"  ;D ;D
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