Cutting fiberglass the right way

Hi guys.

So far every time I have needed to trim fiberglass, it's been non-critical. Either the cut edge was going inside aluminum skins, or I could clean up the edge with a vixen file to get it straight. I have used a die grinder with an abrasive wheel to do the cutting - the results are less than pretty.

Now I'm about to cut the wingtips for the hinge installation method. The edge needs to be nice and straight, and I don't want a gap - so as thin a kerf as possible would be good. So, how would you recommend cutting the tip? I have a Dremel, but don't have a whole lot of confidence in my ability to make a perfectly straight 3' long cut.

I've already got the wing and wingtip drilled for the wider P4 hinge half, and it's clecoed in place. I have thought about making the cut, doing any cleanup with a file as needed, then clamping the flange and hinge back in place before drilling the outboard hinge. Or maybe making some sort of guide that I could use to keep the Dremel aligned with the edge of the fiberglass while making the cut, or using a coping saw to try to keep the cut straighter. What have others done here? I don't want to screw up the wingtip. I think this will look great - but only if the cut is straight and there's not a big gap between the wing skin and wingtip., otherwise it will look like crud.

One way to do it

Make the cut however you want, but leave a little extra material. Get a long, straight board (1x2 usually works fine) and glue 60-grit sandpaper to it with super77 or some other spray adhesive. Use the long sanding board you just made to sand the cut edge down nice and straight to the desired line.

krw5927 said:

Make the cut however you want, but leave a little extra material. Get a long, straight board (1x2 usually works fine) and glue 60-grit sandpaper to it with super77 or some other spray adhesive. Use the long sanding board you just made to sand the cut edge down nice and straight to the desired line.

Click to expand...

You know, I read that and my initial reaction was, "Well, that won't work, the edges have to match up just right..." Then I thought about it a little more, and realized that you're exactly right. It doesn't matter if the cut-off flange under the wing-side hinge half is a little short, as long as the edge of the wingtip meets up nicely with the wing skin.

Some days I just feel like an idiot for not seeing the obvious. Thanks for the suggestion.

I kept a couple of strips of steel stock from Lowes around the hangar for straight edges. Trim the wing tip material just short of your drawn line, the clamp up the steel to your line and sand. It'll make a straight edge and keep you from sanding past it.

Kurt is right on. Once you get the hinges fit you will want to make a perfect (or close) fit using a sanding block. I did the hinge thing and love it.

When I put the wingtips on my Rocket using hinges I took magic marker made a wide line around the wingtip half inch or so wide. Then I shoved the wingtip into the end of the wing with a thin scribe while a helper held the wingtip. Scribe along that line and it becomes very easy to see with the magic marker.

Think machinist's ink. Along those lines...pun intended.

Cut with a pair of snips to within 1/4" and sanded the rest of the way to the scribed line with an orbital sander. My tips have virtually no gap.

Get a 4 inch wide desktop bench belt sander - the kind with the flat length and curved end. Rough cut near your desired line and the sander does the rest. You will have GREAT control and it beats the snot out of hand sanding to shape. You can find them cheaper than this one. http://www.amazon.com/Rockwell-RK7866-Belt-Disc-Sander/dp/B002CJLV0A
It is the right tool for the job!

I have been cutting my cowl in the last couple of days and began using the angle head die grinder with a 3-4" cut off wheel. It was windy and cold outside so I went inside and tried my Dremel with a diamond cut off wheel. Huge improvement, in controllability, narrow kerf (not needed here) and cutting ease. Yes, you could probably make a guide to cut better, but this method with a long board sanding block is working great.

I did mine this way two days ago. I used a Dremel and an Aluminium strap about 3mm thick. Clamped with cleco clamps, cut it along the edge of the recess and smoothed with a 200mm length of pine with 220 grit paper glued on with spray adhesive. They turned out real nice. There is a real good description on a builder's website, which I don,t have with me. PM me if you want the website address.

I used blue masking tape for the line. Cut just outside the line with the dremal then use perma grit sanding block to get just to the edge of the tape.

Skin itch is a common "I hate fiberglass" complaint. Any high-speed cutting wheel puts a lot of glass fiber in the air, so I avoid them where possible.

Most of my rough cuts are done with an ordinary pair of sheet metal shears. There is some laminate crush, so cut about an 1/8" outside the line and take the rest with a long sanding block.

I make a lot of cuts with nothing more than a bare hand-held hacksaw blade. For heavy laminates or a lot of cutting I pull out a jig saw or run the part through the band saw.

The best all-around sanding block is 3/4" smooth birch plywood cut 3 x 18" so it will take an entire sheet of 3x18 3M Green intended for an air sander. 80 grit is the most used, but I keep a 40 grit and 180 grit block handy.

The shop stays a lot cleaner if you seat yourself comfortably on a short stool, and place the business end of a shop vac hose on the floor between your feet....the poor man's dust collector. Use drywall bags in the shop vac, not the standard bags.

My favourite glass cutting tool is a Fein vibrating cutter. It makes a very fine, accurate cut and the dust tends to just fall to the floor rather then blow around.
It is THE tool for cutting cowlings. You can really follow a line with this tool. The kerf is not much wider then a sharpie mark!
There are other brands, I have not tried them.

+1 on the Fein

The Fein sander with the HSS offset blade is THE "go to" tool for the EZ/Cozy community when cutting glass. The HSS blade will last for years, the others won't last 5 min. The blade may be hard to find, but a Fein sells direct.
That make cheap imitations that also work, get the real Mc coy though, it's a nice tool you'll use on a lot of other projects.
Tim