I really need to route it because I need to plunge into it and don't want the slot the total length of the board.
Been a long time since I've done this, so can't remember if I can route that wide a slot (3/4") -- just 1/4" deep or even less, just enough to fit another piece of 3/4" plywood into.
Do they even make a 3/4" bit or would I have to do multiple passes for both the width and depth? (yuk).
Altho w/ today's ply you'll more than likely want a 23/32" bit specifically for the purpose.
Even there you'll want to make a test cut and see if that matches the specific piece sufficiently well.
So, all in all, you may well want to consider one of two options -- use the narrower bit and make the cut in a couple of passes to ensure the proper fit or,
b) cut a rabbet on the inset piece of a specific thickness and use the proper bit size for it instead.
As mentioned, Leon... Oh that's me. I have designed a jig to route a slot to be exactly the width of any thickness piece of wood that you would want to fit the dado.
But! If you simply want to do the deed and be done you can use the principal that my jig uses and be done with it pronto.
What you will need,
A "Top Bearing" straight cut pattern bit. I was able to pick up cheap ones 1/2" in diameter for around -6 each a few years ago.
Two straight edges the length of the dado you want to cut and preferably at least 1/2" thick. Plywood works well for this.
What to do.
Clamp one straight edge exactly where you want the edge of the dado to be located.
Use a piece of the actual material that is going to fit in the dado and stand it on edge next to the board you just clamped.
Clamp the second straight edge up next to that piece of scrap. Basically what you end up with are two straight edges defining exactly where the dado will be and exactly the width of the material that will fit in the dado.
Remove the scrap material.
With the top bearing bit in your router plunge down between the straight edges until the bearing is below the top surface of the straight edge. Use both straight edges to guide your cut. The top bearing will ride along the inside edges of the straight edges. You will have to make a couple passes but do not move the straight edges until the dado is complete. When finished the scrap should fit perfectly into the dado.
Assuming the OP is not using cabinet grade plywood you are correct. However, if the project does involve cabinet grade plywood then a 3/4" piece of plywood is 3/4" thick.
Gordon Shumway
Our Constitution needs to be used less as a shield for the guilty and more as a sword for the victim.
After working with probably 50 sheets of cabinet grade 3/4" plywood in the last few years, specifically Oak veneer plywood, I can tell you that 3/4" cabinet grade plywood is not 3/4" thick. It is very close to 23/32" as witnessed by my tape measure just now. At least in Texas it is that way and had been for years.
Now I will say that recently I have seen cabinet grade 1/4" oak veneer plywood that appeared to be a full 1/4" thick at Lowe's. I was shocked.
As an example, the last 3/4" "hardwood plywood" I bought for a large kitchen project was "A-1 grade" Red Oak and was just a tad over 23/32" thick (.73) ... on a good day. :)
Since hardwood ply varies in thickness from lot to lot, even from the same manufacturer, I try to buy plywood for a full sized kitchen all at one time, and from the same stack, so that I can at least try to get an even average thickness.
AAMOF, and Leon will verify this, I ALWAYS carry a "project dado guide" in my truck for the projects that are currently in progress ... this is nothing more than cutoff stile with the project dado size cut in it that matched the thickness of the first lot of plywood bought for that particular project:
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way, and because I make all the face frames first, I can insure that any extra plywood that needs to be bought, from a different lot, stack, or retailer, will actually fit the pre-cut dadoes in the face frames already made up.
I do wish that this thickness variable wasn't in play, but it must be taken into account when doing any production type fabrication. :)
If you can get to it, mark the slot and hog out some of the midsection with Forstner bits on a drill press. Then (with suitable guides clamped to the work) route full-depth with a smaller bit (maybe 1/2").
It's easier to get the bit into the cut if you dont have to plunge-cut (and you can use non-spiral bits), it's easier on the ears to remove the bulk of the wood with a Forstner bit, and the heat buildup (and wear) on the expensive router bit is minimized if you just clean up the edges with it.
I wonder the length of your board. A dado blade in the table saw will work and is accurate but you will need to chisel out the end(s).
Another method that works well... Clamp on a straight-edge and make a hardwood strip that is taped to the straight edge. Make the first pass with a 1/2" , remove the strip, make the second pass. You may need 3 or 4 test tries to get the groove width you need. After the gauge strip is correct, the setup is repeatable.
A router sled jig is another consideration, good or not depending on your setup.
I have not seen a 3/4" straight router bit, but that sounds hefty (probably pricey), and you may need better at a slower speed.
I got one in a cheap set of bits at Harbor Freight awhile back. I recently used it to make dadoes for 3/4" shelves in a bookcase made of poplar, worked fine despite not being el primo quality.
I haven't used 50 sheets of plywood in these last years, but I can tell you that's it's been well over 30 years since I've actually seen plywood that's exactly 3/4" thick. Between imperial and metric measurements, plywood in Canada has been every conceivable thickness except 3/4".
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