3/4" slot in plywood -- route or dado?

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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.
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wrote:

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.
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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.
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routing to that depth is _no_ problem, assuming you've got an 'adequate' router.
_I_ would use a narrower bit, and do two passes -- that way I can guarantee an exact fit for 'whatever' thickness the nominal 3/4" ply is. I've had some that would _not_ fit in an exact 3/4" slot and others that were loosey-goosey in a 23/32nds slot.
The type/class of the plywood, the manufacturer, the humidity it's been exposed to, can all make a difference. And they're all -guaranteed- to be in the direction that you'd rather -not- have them go. <wry grin>
This is especially important since you've ruled out the optimum solution of cutting a slot with a Freud 60-tooth blade. Which, as reported in another thread herein -- according to Freud's own advertising -- "always cuts to the correct length." :)
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On Sun, 17 Jan 2010 19:53:53 -0700, Robert Bonomi wrote

I've gone the router route ;^)
Problem with a dado set is you must make sure the wood is _firmly_ forced against the table or the resulting bump in the bottom of the groove will throw everything off. Also consider cutting more than a foot into the center of a board (side shelf supports). Either you need a really long miter gauge or a good panel sled setup.
-Bruce
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Good guide rail, and do it in two full-depth plunges, one for each side.
If you've got a really good guide rail, you could do it in one pass (for 1/4" depth) or multi-depth passes, cutting both sides simultaneously. That's a hateful process though, as the forces are pushing you both ways simultaneously, so are far harder to control.
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Don't try to get to the thickness of the plywood.
Cut a rabbet to a "known" thickness(1/2" is good) and then route a slot using a 1/2" bit.
Using this method, you can really get very close and not worry about weird sized plywood.
Actually, I would make my dado and sneak up with the rabbet.
You can do this with a table saw or a router table.
mkr5000 wrote:

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I've used that method when I've pre-finished the components. The rabits and dados then expose unfinished wood for the glue to bond to.
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Yes, they do make a <3/4" bit designed for plywood rabbits. And, if you're intending to do a lot of them, maybe it's worth the $$.
Otherwise, multiple passes.
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The response "b) cut a rabbet on the inset piece of a specific thickness and use the proper bit size for it instead. "
is they way I did it last time. It's forgiving and hides the joint nicely.
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