Greetings All:
I’m trying to select and buy the correct router bit for some veneer plywood I bought. All the web sites I’ve been to list the “plywood/undersize bits.” For the 3/4 inch oak ply I bought, the bit they say to use is 23/32 inch. But when I measure the wood, with an accurate caliper, I get more like 11/16 inch. If I use the 23/32 inch, the dado would be too large and not a good fit.
Did I buy an isolated, undersize, piece of ply? Is veneer ply a different dimension than regular ply? I’ve never cut dado’s in ply before and could use some help here before I spend some money on the wrong size router bits and waste a good piece of lumber. I don’t have a dado set up for my table saw.
Thanks for any info you can pass this way,
Mapmaker
Replies
Plywood which is nominally 3/4" is typically now .71" or so, and it varies from mill to mill. You'd need a whole slew of router bits to get an exact fit on each sheet with one pass of the bit. There are other solutions. One is to use two passes with a smaller bit; one bit can make any dado width you want.
Mapmaker, Jamie got it right. Use a 1/2" bit and make two passes. After making the first pass running the router base against a clamped down straight edge, make another pass with a custom made spacer strip between the router and the straight edge. Play around with some scrap until you get the spacer exactly the right thickness.
Thank you BldrBill and Jamie Buxton,
The two pass idea, with a smaller bit is what I'll probably do. I can't really afford a whole new set of bits, at present, so one high quality, smaller bit seems like a great idea. Thank you both for your feedback!!
Mapmaker
You can also simply make a 1/2" groove or housing (dado) and work a very slightly shy half inch tongue on the end of the shelf or divider.
You're not restricted to using a 1/2" router bit to make the groove or housing. You can use anything between about 1/4" to 5/8".
You simply make the tongue on the end of the male part to suit the groove worked.
Whilst the job done this way requires two passes of the router, and not one, the result is actually mechanically stronger due to the extra returns in the joint and increased glue line. Slainte.Richard Jones Furniture
Perhaps a sketch would help. Of course there is no need to run the tongue or the groove all the way to the front or back. With a plunge router it's quite easy to stop the groove short of the end by 1/4" or 3/8". Slainte.Richard Jones Furniture
I'm not sure if this is the case for you, but sometimes the factory edge on plywood is thinner than the rest of the board. If you measured the thickness on this factory edge you may want to rip 1/2 in. from the sheet and measure again.
Matt Berger
Fine Woodworking
Sir, I read a tip a few years back for dados with a pattern making bit with the bearing at the top.
Clamp a taller straight edge wide enough for the router base to sit on.The Height depends on the depth of the pattern making bit you use and the depth of the dado. Clamp the first guide to your index marks. Take a sample of the plywood you are using and and squish it in between the first straight edge you have made and the second one on the other side. Remove to plywood sample and this is the exact thickness of the plywood.
One pass one way and then, one pass back and it is done. I hope I've explained it well enough for you to understand.
There is a picture of this in Taunton's "Router Tips" book.
Vance
I can vouch for this method, it really works great.
Another way I read about recently using a 1/2" bit (or any bit smaller than the intended groove) is to make your first pass with a spacer that is the same with as the bit (e.g. 1/2", then replace that spacer with a scrap of the stock to go into the groove. I haven't tried it yet myself, but it seems quick if all you have is a straight edge and a straight bit.
Can't quite remember where I read it either... may have been here.Jeremy
The "tongue and groove" method described in this discussion is the only way to dado in my opinion. It also eliminates concerns about a board of varying thickness. So just be sure your dado bit is clearly undersized.
Do note, though, that the technique does actually just shift the emphasis, time, energy, sweat, etc. from the vertical to the horizontal board. Dadoing becomes easy (as it is virtually hidden). Cutting the shelf ends for a perfect fit then becomes the precision challenge. Now you have to be certain the vertical member to be dadoed is flat (whereas before it was the horizontal that was the worry). For a perfect fit, the top and bottom rabbeted edges need to be precisely aligned. And that calls for finely tuned tools, a steady hand in some (router)cases, and a degree of patience.
Many of us (yourself included apparently) delight in having our wood joinery appear as though it is highly milled metal. This is a good trick when considering the variations in even just one board. Assuming you're using a clear finish, plywood is more challenging, as the surfaces can be only lightly milled. Great recreation. jc
I thank you for the valuable input.
Strange.....the more I learn about wood and woodworking, the more I realize how very little I actually know. And how far I've got to go.
But this forum, and others like it, help a lot. As do the more experienced ww like yourself.
Thanks again.
Mapmaker:
I, too, share your frustration. Measure the thickness of your plywood in millimeters and check for a bit in that size. I have not required a bit for 3/4 ply, but find that a 5 mm bit makes a nice groove for 1/4 ply.
Cheers,
Joe
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