Reference: Scarfing Plywood at BoatBuilder.org
Joel,
A couple of questions:
Would one use a bit of cabosil to thicken the epoxy in a scarf joint or only un-thickened?
Also, I have seen / heard of a couple "jigs" sold as a tool for scarfing. One utilizes a power planer and another that uses a belt sander. Have you experimented with any of these scarfing jigs?
From your pictures and description, you use the power planer and grinder "freehand". You are obviously skilled at this process. Any tips for shortening the learning curve?
Has anyone else out there experienced a scarf jig?
-Kruez-
I have used unthickened epoxy and epoxy thickened to a honey consistancy with cab-o-sil and /or wood flour. I use low viscosity MAS epoxy and if it is warm it can be fairly thin in colder weather it will be a bit thicker. I want the epoxy to squeeze out easily, but if the scarf are not good it may need to be thicker to it does not drain out of voids. I think the free hand method works the best, because even with the jigs not all the scarfs are the same and when the plywood is stacked like I do it, you use the entire stack as a guide. By watching the plys as you take off material everything can be kept uniform as you go. The key to scarfing the way I do it is to keep the ends of the plywood down, if they start to lift the scarfs can be uneven and the grinder has a tendancy to tear at the edges. I 'm not trying to poo poo jigs, I just personally like the freehand method. I'm a big believer in "whatever gets the job done", there plenty of ways to skin a cat. ---Joel---
ReplyDeleteI found a project file some ten years ago that used a router to do the shaping for plywood scarf joints. With any luck, I'll post the pictures to this site on this message. Otherwise, I'll send them to anyone who needs them. (Or, you can see them in a picture file called "Jigs and Things" at Cedarstripcanoes@yahoogroups.com). The jig uses a fixed-base router and a planer or rabbeting bit. The router rides on a sled. The router sled is a 3' length of 1/2" MDF, wide enough to span the router base, with enough extra space to allow two rails exactly the width of the router base apart. Handles at each end serve as stops. A slot is cut with the router bit the entire length of the sled, to permit clearance, and to allow the bit to extend below the surface 1/4" when the router sits between the rails. The sled rides on two angled ramps, screwed to the workbench or a backing board of 3/4" material. The angle of the ramps can be any between about 6:1 to 12:1 (I used 12:1 for scarfing 1/4" plywood used for side planks on a 16' sailboat.) These ramps are spaced exactly the width of the plywood pieces to be scarfed, and aligned perfectly square to the desired joint. The two pieces of the joint are anchored between the ramps (I used screws to anchor these pieces, far enough away from the joint to avoid the router bit when planing). Then the sled is placed on the ramps, so that the bit protruding through the bottom of the slot will barely touch the corners of the two pieces. Planing takes place as the router and sled slide up and down and from right to left on the ramps. When routing the two surfaces, only 1/16" or 1/8" will be taken off at a time, with the router bits adjusted between each complete pass. This is a very simple and lo-tech approach to getting matching angles on both sides of the joint. When complete, the surface will require some touch-up sanding prior to glue-up with very coarse sandpaper. Glue-up should be with thickened epoxy including a filler to avoid joint starvation. While I was waiting for the joint to set up, I held the joint in place using waxed paper-lined cawls, and applied pressure with drywall screws (patching the screw holes later with thickened epoxy during the sealer coat and painting prep). JR Sloan Spokane, WA
ReplyDeleteAttachment: Scarf2 08 02 02.jpg
Well, I tried to upload some pictures of the jig, and posted instead some illustrations of using scarf joints to "stretch" redwood 2x4s for use in making hull strips for some small boats. When I tried to upload the JPEG files of the jig to correct my error, MSN refused to accept the files. If you would like some clear diagrams, go to Cedarstripcanoes@yahoogroups.com, find the "Photos" section, select the folder entitled "jigs & things", and select any one of the first three diagrams there. Those are my drawings of the jig with instructions. Sorry I couldn't make the system work.... JR Sloan, Spokane
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