Several outfits supply undersize router bits for forming dados for plywood. These are supposed to take into account the amount by which the actual dimension is less than the nominal. The rule appears to be 31/64 for 1/2 and 23/32 for 3/4 plywood. BUT – there are a variety of sizes sold for 1/4 plywood: 13/64 (MLCS), 14/64 (Woodworkers Warehouse), 15/64 (Rockler). That’s a range of 1/32!
They cost less than 10 bucks each. Maybe I should get all three and experiment? Has anyone had exerience with these?
Replies
As you have found, quarter-inch plywood isn't always the same thickness. It varies from supplier to supplier, and even sheet to sheet. If you're really intent on getting a tight fit, you're probably better off buying a bit you know will be smaller than any plywood you'll buy, and making two passes with the router to get exactly the dado width you want.
Thanks -- but the variation in size I cited was for ROUTER BITS from different suppliers, not plywood! I mean router bits that are supposed to cut a dado to fit 1/4 PW. You are right about measuring the PW -- that should be my first step.
Everyone says I should use a tacking dado set on the table saw. I'm sure they are right - except my table saw is a multi-ton cast iron, tilting table monster running a 14 in blade on a 1 in arbor. A dado set to fit would cost nearly as much as a new saw.
Javier,
Jamie is correct, your better off to go with one and adjust. I usually sand the male end of the plywood when I have too tight a fit.
The beauty of of stack dado set on a TS is you can take a paper thin cut very easily. ...but that is not your question.
Javier,
I make most dadoes with a router. I have used Eagle America bits that are 1/32" undersize. I have also used Jesada bits that are also 1/32" undersize. I have had good results with both.
I also have a Freud stacked dado set for my cabinet saw, but I can make dadoes with my router much faster with my routher and a TruClamp as a guide. I have even been able to make off-size dadoes by making multiple passes. I have been able to do this with pretty good precision.
Mike
Good question. A lot depends on how old your plywood is, as well as the thickness. I bought all three of the bits, as I was unsure of when I could get to this, with other projects ongoing too. I also wanted to build my collection of bits to save time for future building.
Since I was not able to rout these dadoes soon, I found that the thickness changed due to aging of new plywood because of the humidity and time. I chose the closet dado size with a veneer caliper, made the pass, tried fitting the wood to it, then made an incremental pass with the router again to have the right fit.
As the earlier threads showed, a smaller bit will allow the right size dado, though it may take several passes through it to accurately fit.
javier,
Another option for dadoing 3/4 plywood, such as cabinet bottoms for instance, is to cut a 1/2 or 3/8 dado and rabbet the bottoms to fit using a bearing equipped rabbeting bit. By making dado slightly deeper than the rabbet, you get a perfect seam inside the cabinet every time regardless of variations in the plywood (even in same sheet of the shop maple we have been getting the past 6 years).
If you have a shop and make a fair number of cabinets, I think it is worth it to have a couple cheap routers dedicated to this task alone.
Clampman
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