best practices for cutting plywood from a pattern

i plan to use a jig saw (probably a metal cutting blade) and a small circular saw with carbide-tipped blade (3 3/8 inch blade, 20-tooth carbide tipped).  i want to do what i can to minimize tear-out of the plywood.

should i apply masking tape to the plywood where the cuts will be, then use 3M spray adhesive to attach the pattern to the plywood?  i'm thinking i won't need masking tape on the botom of the plywood since both the jig saw and circular saw will cut on the up stroke.

thanks in advance

ken


3 replies:

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RE: best practices for cutting plywood from a pattern

Don't bother with the masking tape, this might interfere with adhering the pattern to the wwood and isn't needed anyway Applying the pattern in a way that allows proper alignment is critcal. If the pattern has a datum line, great. Establish a straight line on the plywood and then match lines. (Chalklines are NOT precise enough) If the pattern doesn't have a datum line you must devise a way to make sure the pattern is transferred to plywood in a way that preserves the original lofting alignment. Regardless, do not cut the pattern along the profile line. Leave at leatst a half inch buffer outside the profile.

Once the pattern is adhered to the plywood take an Exacto knife or really sharp utility knife and carefully and deeply score the pattern profile. To cut to shape, the circular saw is preferred, especially for long shallow curvves. Set the blade depth so the teeth just emerge from the bottom. Cut to the profile scoring cut within 1/32" if you can. On tighter curves use the jigsaw but beware of blade deflection. The blade will deflect to the inside of the curve on the bottom of the plywood, where you can't see it, so stay 1/16" away from the line on tight curves. The deep scoring cut previously done will "absorb" any tearout. After you have cut the panel, cut exactly to the scoring line with a sharp plane.  You now have a panel as good as and maybe better than any CNC cut panel.

 

 

 

RE: best practices for cutting plywood from a pattern

   thanks for the detailed reply.  very helpful.  

to make sure i understand:  adhere the pattern to the plywood (3M wi spray adhesive, i assume); score the outlines of the pieces with xacto; remove the pattern and rub the adhesive off the plywood then cut as you directed?

one more question:  what do you mean by "...in a way that preserves the original lofting alignment"?

thanks

RE: best practices for cutting plywood from a pattern

Whenever I'm transfering a paper pattern to wood I leave the pattern on the wood until I plane to final dimension. It just seems easier to plane to the line with a white contrasting pattern. BTW, don't try to sand off the adhesive. Liberally coat the panel and pattern with lacquer thinner, wait a couple minutes and remove the pattern. Wash the panel again with lacquer thinner to make sure you've removed all of the adhesive, which may interfere with epoxy. Do this outside with a respirator because lacquer thinner has nasty fumes.

As far as alignment goes, your roll of panel patterns will usually have several patterns in close proximity. If you cut these out you now have a long floppy pattern that you can quite easily transfer inaccurately to the plywood stock. CLC uses a connect-the-dots method, which is accurate but I find tedious. I roll out the pattern master sheet onto a suitable surface and then make sure the datum line on the master is straight. String gauge, laser, etc, take your pick. Then, on each individual pattern, I cut out a series of small triangles that have one angle aligned with a straight line. On the destination piece of plywood I make another straight line. I cut out the pattern and then align the angles on the pattern with  the straight line on the plywood. Viola! the pattern is now aligned according to the original lofting. I hope this clarifies my previous response.

 

 

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