John's Nautical Page
Brian Dixon's Easy Plywood Scarf Bevel Cutting Technique
Basic Steps to Cutting a Scarf Joint in Plywood:
What you need: - A strong table which is the same width as the plywood you are scarfing - Either a power hand plane, a belt sander, a jack plane, or even a smooth plane for removing the bulk of the wood in the scarf joints ('roughing in'). For the least work with the best results, the power hand plane is the best choice. The second best joice would be the belt sander. - Either a low-angle block plane, a belt sander, or a fairing block and sandpaper. Least work and best results will come from a razor sharp block plane. A belt sander can be used, but must be used very carefully and you will have to follow up with a long narrow sanding block (fairing block). 3M makes a good poly fairing block, or you can make one on your own with a very straight piece of wood, e.g. cedar 2x4 carefully cut to be straight. Note that if you make your own, that it'll only stay straight enough for awhile since weather and humidity will cause it to change..check it with a metal straight edge prior to using it. - 4 clamps with 6" clamping capacity. - Medium, fine, and possibly 'very fine' sharpening stones for your plane(s)
1. Determine which side of the plywood you'd like to use
as the 'face' or 'good' side.
2. Stack the plywood so it is face-to-face (good sides together).
3. Slide the top piece back to get it out of the way and draw lines on the edges of the plywood pieces to mark the bevel that you will be cutting, e.g. an 8:1 scarf angle line. The lower end of the scarf lines should intersect the bottom corner on the end of the plywood.
4. Draw lines across the top side of each piece of plywood to mark where the scarf will intersect the stop of the wood, e.g. connect the scarf lines where they intersect the top of the wood.
5. Position the lower piece of plywood so the end is square with the end of the table.
6. Carefully position the upper piece so that a) the leading edge is exactly parallel (*exactly*!!!) with the line you drew across the top of the lower piece and b) so the scarf lines on the edges of both pieces line up perfectly (*perfectly*!!!). Use a metal straight edge to make sure the lines on the edge of wood are perfectly line up. Note that lining up the lines on the edges will usually result in the end of the top piece of plywood being a bit back from the line drawn on the top of the bottom piece of plywood. Just make sure the end is parallel to the line and the scarf lines on the edge line up perfectly, and don't worry about it if the line on the top of the bottom piece of wood landed right on the end of the top piece of wood. (This will be clearer when you actually lay out the wood).
7. Tightly clamp the two pieces of plywood to the table, using all 4 clamps, 2 along each side, leaving room for operating your power tools but staying as close to the scarf area as possible. You really want the two pieces of wood held tightly together, and you should make sure the table is flat too. If the table is not flat, you may have to cut or plane a smooth curve into a 2x4 (opposite of the nonflatness in the table), then screw the 2x4 to the bottom of the table, tightening the screws (countersunk) to flatten the table. Check it with a metal ruler.
8. Now you need to 'rough out' the scarf cut to remove the bulk of the wood and to reduce the amount of effort you have to make with small planes or sanding blocks later. Remove wood to within about 1/8" of the scarf lines, being careful and going as slow as necessary. A very good hint to how well you are making your scarf is to observe the laminations in the plywood in the scarf cut. All the lamination lines should be parallel to each other and to the end of the plywood. You can use a block plane to remove a little wood in selected areas now and then to 'fix' areas that need it. I usually alternate between a power hand plane and a block plane, while many others will do all the work with a belt sander. Just make sure you check things often (visually and with a straight edge) as you go, and work conservatively...you can hurry on some other part of your project! The better you do at roughing in the cut, the easier your task will be when you 'finish' the cut.
9. Finish the scarf joint using a low angle block plane. Some people will prefer a belt sander, finishing with a long sanding block, but I will stand firm on my recommendation for using a sharp block plane. You can adjust the cut so very slim shaving will be removed, allowing gradual and forgiving (mistakes will be small) work as you finish the scarf. Also, especially for scarves that will undergo high stress in your boat, the plane has the slight advantage that it tends to keep wood cells open in the joint, allowing a slightly better bond when you glue the joint. Note that as you get close, the top and bottom pieces of wood will meld together, and you'll be working the whole scarf as though it were a single piece of wood. Keep checking the flatness of the cut with a metal straight edge, both across and along the scarf joint. And keep checking those plywood laminations to make sure they are parallel with each other and with the end of the plywood. Realize that if you mess up, it's no big deal, because you can always take off more wood, e.g. take the whole scarf on both pieces of wood a little deeper into the plywood. Just keep checking and don't stop until it's perfect. Finally, when you are really getting down to the feather edge of the wood, you really want to be careful not to tear out wood as you sand or plane. I found it easiest to plane towards the edge of the plywood to develop the fine feather edge, followed by removing wood above the feather edge until the joint passes my flatness checks. Finally, note that you want to take the wood down until the feather edge becomes irregular, e.g. very thin. For example, with an 8:1 scarf, if you leave a 1/32" thickness on the feather edge of the wood, this translates to a 8*(1/32") or 1/4" gap on the surface of the scarf when you glue things up. If you are painting, it's no big deal and you can be more lax about how far you go in producing the feather edge, but if you are clear coating the wood, you want to minimize the gap by producing a fine edge. Thickened epoxy will appear whitish in the gap and take away from the beauty of the joint. (All of this sounds harder than it is...remember my remark about messing up being ok!)
A Better Way to Cut a Scarf Joint:
1. Using scrap plywood that is at least 2 feet long, and the same width as the 'real' plywood that will go into your boat, use the instructions above to prepare a scarf joint in them as though you were going to glue them up.
2. Do the required marking of the 'real' plywood (B) as described above, but instead of just stacking these pieces, stack them with the 2 pieces of scarfed scrap plywood (A) instead. One scrap piece goes below the stack of good wood, and one goes above. When you line all the wood up and get the edge lines lined up (etc), use the scarfed surfaces of the scrap pieces as a guide, lining them up with the scarf lines too. The scrap pieces will act as a reference surface when you are checking your work during the roughing out of the 'good' scarves, and will also provide a surface for your plane to ride on while you are doing the finishing work on the good wood. Finally, the scrap below the bottom piece will help prevent tearing out the feather edge on your good wood.
3. Rough out and finish-cut the scarf joint in the good wood (C), using the scrap as a guide. I greatly prefer this technique over just scarfing the good wood by itself, and have produced very beautiful scarf joints in this manner.