Scarf joints 1. Plywood.
Blog time : it’s almost the end of May 2021 , it’s been a horribly wet and cold month with more rain than the rest of the year so far ; this morning though the cloud rolled out and it’s been a great day to have a go at another job that iv’e never done before…..a scarf joint. As with marking out and cutting pieces i’m going to have to make a lot of scarf joints to make up the long stringers for the boat and just one long scarf joint to make the long bottom panel – today i decided that the whole day would be set aside to learn how to do that.
On the bench.
Once again this is our old garden picnic bench put to another use, so far it’s been my principal marking and cutting bench after i attached 4 plywood width lateral beams in cheap 2 x 4 PSE softwood. Because i put one cross beam at either end and set them square to the bench it made for a hard ‘sharp’ edge that would exactly support the feather edge of the bottom sheet as i planed away at the plywood.
My set-up was , as above, to place the bottom sheet with it’s short side exactly aligned with the end cross member of the bench beam and then lay the next sheet over that but 72 mm back from that edge : John’s plans specifying a 6 : 1 scarf ratio and the plywood being 12 mm thick. I then clamped the 2 sheets together at the opposite end and clamped the 2 sheets to the next cross beam back from the edges i wanted to plane. The large blue Irwin clamps weren’t quite keeping the 2 sheets close together at the scarf edge so i added a pair of small G clamps as near the edges as i could and then marked the line of the scarf on the plywood edge and marked the ‘stop here’ line on the upper sheet.
Tools and technique.
For taking off most of the stock i used an 18 volt battery powered planer set very fine and i started with the edge of the upper sheet first , only moving onto the bottom sheet when i started to catch it with the edge of the blade. Even though the plywood is only 12 mm thick i thought it would be a long job but less than a half hour’s steady work and i had to swap to a smoothing plane to take the edges down to a sharp feather edge and then finished the job with a few passes of a longboard with rough (60 grit) paper. Most of the time i was able to check the cut just by eye as the laminate edges gave me a good guide….as long as they were running square and clean across the boards then i knew i must be close. At the end i just used a straight edge placed on the angle of the cut to help me find the highs and lows….the few highs i took off with the longboard and the lows are so fine that they will fill easily when i glue the boards up. If i made any mistake it was in tearing the fine edge of the top board slightly which probably means that my plane was a little off sharp by then and maybe set a tad too deep. For a first attempt it looks ok to my eye ….the proof will only come when i set it up to glue.