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AngWood
11-26-2003, 10:13 AM
If a person were to use an angle grinder to scarf plywood (e.g. 8:1 scarfs in 3/8" doug fir ply), what would be the best grinder wheel to use? A 50 grit sanding disk? A 36 grit sanding disk? Some sort of metal-grinding wheel? TIA.

Paul H
11-26-2003, 10:30 AM
A 36 grit wheel would remove the wood sufficiently fast, but, I'd be seriously concerned about getting a decent joint.

I picked up a Ryobi power planer last month, for all of $69 at Home Depot, it was the only planer in stock. Anyhow, I've scarfed together the six sheets that make up the bottom of my Tolman skiff, and it worked great. I didn't use a jig, just offset the sheets the right amount, and took multiple passes freehand, checking with a straightedge.

I'm hardpressed to see how one could produce a scarf edge that didn't look like a flintnapped arrowhead if using a grinder. The amount of epoxy needed to fill such a joint would likely pay for the planer!

AngWood
11-26-2003, 11:20 AM
The difficulty is that I must make one of the scarf joints on the boat, due to some poor planning. With a power planer, I would run into the keel before I could complete a pass. I have heard that an angle grinder can be used pretty effectively to shape ply.

Ron Williamson
11-26-2003, 11:33 AM
50 grit should be alright.I think small diameter would be better than large as large discs aren't as flat(in my experience)and the small disc is better in tight areas.
A light touch is required,as is a straight-edge for testing flatness.
R

NormMessinger
11-26-2003, 11:46 AM
Thirty six would be pretty agressive. I think I'd start with that, move to 50, maybe even 80 and finish with a hand plane of appropriate size

Bob Smalser
11-26-2003, 11:47 AM
I do similar all the time in a pinch by freehanding...using 24 or 36 grit for epoxy and 50 grit for resorcinol joints.

Just don't cut your add-on piece to length until you are done and stop often to test your grind with a trysquare.

http://pic3.picturetrail.com/VOL12/1104763/2594266/27259451.jpg

AngWood
11-26-2003, 12:01 PM
Thanks, gentlemen. Did I mention this is a handheld angle grinder? That one in the photo is scary.

Paul H
11-26-2003, 12:15 PM
Ah, well I can see for this application, it being the tool of choice. Just be aware that a 36 grit disk will remove wood with haste! Then again, with smooth steady sweeps, you ought to be able to do a fine job, and and little dips will be filled with epoxy and filler.

Bob Cleek
11-26-2003, 07:28 PM
Never use an abrasive when an iron will do the job. If I had to identify one single thing that separates the men from the boys, and a decent job from a crappy one, it is the tyro's overdependence on sandpaper. Sandpaper, and most all abrasives are for FINE finishing surface preparation, or the removal of coatings (but please don't take your 40 grit to the bottom paint!) They aren't designed for shaping wood, even though all sorts of salesmen will say otherwise and be happy to sell you scads of sandpaper to do the job. The task described here would undoubtedly be done much more efficiently, cheaply (disks ain't cheap), and precisely, with a properly sharpened and tuned block plane and, if need be owing to space limitations, a chisel. Dontcha think?

john welsford
11-26-2003, 07:30 PM
I have a 110mm Ryobi angle grinder pretty much permanently set up with a 40 grit sandpaper disk on a flexible backing, I call it my "power spokeshave" and with practice it is a surprisingly accurate tool. I have done ply scarf joints with it, and no problems but do prefer to cut them with a something with a longer base like a power plane and then finish them with a hand plane. Much easier to get a full 4 ft scarf dead straight that way.
John W

warthog5
11-26-2003, 08:45 PM
I talked my buddy in Slidell,La thru this using a grinder w/80gt. He had to scarf stringers in the boat. It came out vary nice.

http://www.classicmako.com/projects/ed_ring/images/tran68.jpg

http://www.classicmako.com/projects/ed_ring/images/tran67.jpg

http://www.classicmako.com/projects/ed_ring/images/tran69.jpg

http://www.classicmako.com/projects/ed_ring/images/tran75.jpg

[ 11-26-2003, 09:48 PM: Message edited by: warthog5 ]

John Blazy
11-26-2003, 09:37 PM
Although I respect Bob's experiences quite a bit, I find it hilarious that Bob elevates manual edge tools to a somewhat absurd exclusive level, as if only professionals ("Men" as opposed to "Boys") knew how to sharpen and when and where to use them, just to be immediately followed by a post from an ACTUAL professional boat builder that seems to have the common sense that, at times, abrasives are quite useful. Too perfect of timing John :D

BTW - When I scarfed the ply for my boat I used all the tooling that works, hand and power (power plane followed by a well tuned jack). In a case like described here, kindof a "blind" scarf, abrasives are wonderful. I routinely use 36 and 60 grit on my 4" hand angle grinder for this type of removal. Its not the tool that always dictates "fair", its the eye and standard for flat, plumb, straight etc. A good, experienced craftsman can use a grinder and get even better results than a chisel in some cases, like not getting grain tearout due to abrading a surface down.

T.KAMILA
11-27-2003, 05:16 AM
The problem ,correct me if I am wrong, is that the ply is already attached to the boat near the keel. You can do two things in my mind. The chisel and bull-nose or block plane route. In this case you have to form the scarf surface near the interference with a chisel. Then use a bull-nose plane or a block-plane to form the scarf to that point. You could very carefully use a angle grinder-sander (slower speed) to replace the chisel part. If you were to do the whole thing with the angle grinder-sander then you need a special pad that is ridged and flat using stick-on sandpaper of about 60 or 50 grit. Either way will take you to the top of the mountain, and neither one is a reflection on the size of your "we-we". <

AngWood
11-27-2003, 05:26 PM
Ask a simple question....

Yo, I've gone the block plane route in the past, but given the fact that for half of this scarf I'll be lying on my back on the garage floor, working overhead, I thought a grinder might work better. Since asking my initial question I've practiced on scraps (rightsideup) and find I can manage with the grinder. Not as neat as a block plane or power planer, but it'll do.

Thanks for all of the advice and help (and for defending the almighty sandpaper). Do I get a prize for luring Cleek out of his lair? ;)