How should I cut fibreglass

[MidlandsOnSea]

I need to cut a square hole in a fibreglass panel in order to mount a monitor. It's a little bigger than a sheet of A4 and the fibreglass is about 6mm thick I'd guess. What is the best way to cut the hole? The monitor will overlap the aperture so the accuracy isn't critical, but I'd hate to see cracks originating from the hole or create big splinters etc.

I thought about using an angle grinder with a cutting disk, or perhaps drilling a pilot hole and then using a pad saw. Or perhaps a jig saw (but which sort of blade would I use?).

Thanks in advance for your tips on this one.

 

[snowleopard]

Use a jig saw with a straightforward metal cutting blade. There are other types but metal cutting is quite adequate for the job you want to do. Sticking masking tape over the cutting line will reduce damage to the gel coat.

A metal cutting disc in an angle grinder is very fast but ten times more likely to slip and damage you (or worse still the boat /forums/images/graemlins/wink.gif ). It also covers everything in highly irritant dust.

If you haven't worked with GRP dust before, wear rubber gloves as the stuff is very itchy and you will be sensitive to it.

 

[jimi]

.. and a mask is probably a good idea especially if you're in a confined space

 

[petecavo]

If using a jigsaw on fragile materials if Jigsaw has "pendulum action" make sure that this is set to "0" as this minimises cracking.
Also use a fine toothed blade that cuts on the UP stroke, this Must be combined with using a flat piece of wood i.e 16 - 18mm ply with a straight edge.
Firstly carefully mark out your area clear marker that can mark plastic or fibre glass not pencil or felt tip etc. The pens that you write on computer CD'S are good.
With extreme caution and brand new stanley knife score surface along outside edge of lines. Go back over lines until scored quite deeply 1-2 mm.
You then carefully drill a hole (9-12mm or so) in the inside of each corner of the marked out area(each corner of your A4 sheet) this allows you to have a start point to place the jigsaw blade into on each "run".
Then you place your piece of flat wood,ply, with the straight edge running exactly along the line from one corner to the next.
The wood must be resting on the outside of the area you are going to cut rather than resting on the bit being cut out, with the edge of the wood running exactly along the scored line.
Place jigsaw blade into starter hole with footplate of machine firmly resting on ply and start cutting carefully along what will be the inside (waste side) of the scored line. As the blade cuts on the up stroke it pushes the fibre glass upwards towards the ply. You have downward pressure on the ply through the jigsaw. The ply or wood acts as a "backer" stopping the fibre glass splintering upwards on the keep side of the scored line,(also why it is vitally important to use fine sharp "upwards" cutting blade)
It will splinter a bit on the waste side of the cut. Do not push the jigsaw through the material fast as this increases splintering. Slow and steady and sharp blade decreses splintering though using this method you should have none.
As the blade cuts upwards it throws waste up so as earlier poster says use dust mask definately googles (old wrap around sunglasses almost as good) and thin gloves the keep control of machine but minimise irritation to hands. Cover forearms as well.
Lastly if on curved section of hull proceed as before but "roll" piece of ply around shape as you cut, good if second person (with brain) helps.
Good Luck.

 

[gorb]

I've just added two larger windows to my Colvic Watson, which necessitated cutting larger holes. Like the first post says, a jig saw is perfect. I used a powered one with a metal cutting blade and it is frighteningly easy! Masking tape avoids marks on the exposed GRP from the "shoe" on the power saw as well as reducing surface cracking.

But remember the old rule - measure twice; cut once!

Gorb

 

[scarlett]

I recommend a hole saw at low revs for the corners. Reduces the stress and produces a neat job. Nice low speed strokes a jig saw has a mutitude of benefits.

 

[MidlandsOnSea]

Thanks for the guidance.

Using a jig saw blade which cuts on the UP stroke seems a bit counter-intuitive. If I used a blade which cuts on the DOWN stroke, would it not ensure that any splinters are on the underneath (hidden) surface?

 

[petecavo]

I realise that it does seem incorrect to cut on the up stroke,BUT, by using a piece of ply along the line as well as scoring described this prevents splintering upwards on every thing but the waste piece. (I was assuming that you can only get access from the outside,) If you use a blade that cuts on the down stroke that would SEEM logical the reality is that you have no way of supporting the rear of the fibre glass so the splintering does happen but only at the rear, and as you correctly suggest you cant see it. HOWEVER just because you cant see it its still there and thus weakens the cut edge as it will have thinned it out at the rear which leads to problems later.

 

[Norman_E]

Two important points.

Cutting on the upstroke helps you control the jigsaw. Downstroke blades try to push the saw away from the surface and need careful handling with firm downward pressure maintained on the saw.

Vitally you should NEVER cut an opening with sharp square corners in fibreglass panels. Such corners act as stress raisers and will lead to cracks forming from them if the panel is subjected to vibration or flexing. Corner radius should be as large as practical, certainly never less than panel thickness, and preferably at least double.

This phenomenon is well known in engineering, but sometimes ignored with consequences. A maker of large marine diesels (big ship sizes) made a series of engines with flake cast iron bedplates. The main bearing locations were milled out with square corners. All those engines failed sooner or later with fractured bedplates, the cracks radiating from the square internal corners. This was told me a marine accident investigator who had seen several. Some of his other stories almost beggered belief. The best was illustrated by a photo of a ship in drydock with its huge (I guess 18/20 feet diameter) single propeller hanging down on a shaft bent through 90 degrees. Some idea of scale was given by men looking up at prop. Huge chunks had been bashed out of the sterpost and the prop. Apparently the ship had been re-engined, and before re-launch the new stern gland bearing had been assembled dry (no grease). On sea trial to prove designed maximum speed a regular banging noise developed, and got louder and louder. Instead of stopping the engine the ship pressed on at full power till the engine was stalled by the final collision between prop and sternpost, accompanied by a massive bang. The prop shaft had been red hot from friction and the weight of the prop bent it, even though it was about two feet thick!

 

[MidlandsOnSea]

Thanks. I do appreciate your detailed replies. I will tackle the job with more confidence as a result.

 

[boatmike]

While it is possible to use an angle grinder this is not in fact the way I would do it as there is a risk of slipping. With a conventional jig saw this is reduced but conventional blades can cause the gel coat to flake. A fine blade that cuts on the down stroke is much better but better still is a diamond edged blade. These are normally used for cutting ceramic tiles and are available in DIY stores. If you stick masking tape around the hole to stop the shoe of the saw marking the GRP it helps too. Dont lean too heavily on it. there is no need and it will wander. Just let it cut. Easy, safe, and does not produce fine dust as grinding does,

 

[PuffTheMagicDragon]

Pardon my disagreeing! A diamond edged metal disc on an angle grinder is the way to go. It helps you to keep a straight line when joining the holes that you will have made at each corner. A spade bit works well for these (cheap) if you do not have a carbide cutter (expensive).
Very important: although the diamond disc produces much less dust, do remove all your bunk cushions from the boat: otherwise you'll regret it all summer long!
Good luck.

 

[Avocet]

You can buy "laminate cutting" jigsaw blades now at most DIY shops. They are designed for lamibate flooring and worktops where the same sort of problem exists - i.e. you don't want to burst little pieces off the gell-coated side of the panel. If it was me, I'd drill 10 or even 12mm diameter holes in each corner and join them with the jigsaw. This will give you radiused corners which, as has been said, will minimise cracking although if the panel really is 6mm thick I'd be very surprised if the loads a monitor can put into it will cause any cracks! It's worth protecting the visible side of the panel with masking tape BUT NOT JUST ALONG THE CUT LINE!!!! Make sure the whole area that the sole of the jgsaw is in contact with it covered as it is easy to put scratches on the surface with the jigsaw sole.

 

[VicS]

It's so easy with a jig saw you'll wonder why you asked. If you use a blade that cuts on the down stroke you'll need to keep much firmer pressure on the machine to keep it flat on the work.


Gosh thats post number 1001. I dont really believe it!

 

[William_H]

My reaction to your question was to ask is there no other way to mount the monitor. Historically electronics in boats (and aeroplanes) tend to be replaced every few years while the hull just goes on and on. In fact there doesn't seem to be a limit to the life of a fibreglass hull except redundancy.
So what I am saying is that the monitor may well be thrown away in a few years leaving you with a great big hole which of course can be filled but never the same and if youu sell the boat the monitor will have to be working perfectly and what the new omner wants otherwise it will be a detriment to the sale.
Can't you mount a LCD flat panel external to the bulkhead? Just a few thoughts olewill

 

[Kyle2]

I recently cut a small rectangular hole for a NASA Clipper display unit, and used a pad saw (i.e. a hacksaw blade with a handle on one end). The cuts were started off by drilling a hole and then starting the cut with a junior hacksaw blade. It is amazingly easy to cut through GRP like this, and I suppose that by using hand tools you are less likely to damage the surrounding material. You could try this way first - there's nothing to stop you switching over to the power jigsaw part way through. However you do it there will be plenty of dust!