Plywood undersized router bits

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Forum topic by woodworkingdrew posted 06-18-2014 01:30 AM 1517 views 1 time favorited 12 replies Add to Favorites Watch
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189 posts in 1030 days

06-18-2014 01:30 AM

So I was all excited to use my new freud plywood undersize bits today. I used the 3/4 bit and the plywood didnt fit at all. Are there vaneer plywoods that are actual 3/4 thickness?

-- Andrew, California

12 replies so far

View sawdustjunkie's profile


342 posts in 1139 days

#1 posted 06-18-2014 01:34 AM

Not that I have ever used.
The undersize router bits are sometimes not correct either.
When I use 1/2” plywood I have 3 different undersize bits to try and make the fit.

-- Steve: Franklin, WI

View TheDane's profile


4938 posts in 3084 days

#2 posted 06-18-2014 01:40 AM

Steve is right … plywood thicknesses are all over the place. The only way I have found to get correct-width dadoes is to use an undersized bit and router dado jig that I saw in WoodSmith magazine …

-- Gerry -- "I don't plan to ever really grow up ... I'm just going to learn how to act in public!"

View RockyTopScott's profile


1184 posts in 2900 days

#3 posted 06-18-2014 01:47 AM

I use Whiteside plywood bits with good success but a jig like Gerry mentions above is almost fool proof.

-- “When you want to help people, you tell them the truth. When you want to help yourself, you tell them what they want to hear.” ― Thomas Sowell

View JAAune's profile


1614 posts in 1738 days

#4 posted 06-18-2014 01:51 AM

If you want a perfect fit on every panel, the simplest method is to mill a dado 1/16” thinner than the plywood then use a rabbet bit and router table to size the edge of the plywood to fit the dado. The rabbet bit needs to cut the top side of the plywood as it goes through the router table to ensure each piece of plywood comes out with a tongue the exact same thickness.

The problem with trying to get a perfect fit straight off the router bit is that a sheet of plywood often varies 1/32” in thickness from one end to the next.

-- See my work at and

View Redoak49's profile


1820 posts in 1410 days

#5 posted 06-18-2014 01:58 AM

I think the best way is what is already mentioned using the router dado jig. It is as fool proof as any method out there.

View knotscott's profile


7146 posts in 2797 days

#6 posted 06-18-2014 09:19 AM

Plywood thickness can vary a lot within the same panel depending on where you measure it, which makes it darn tough to find a bit that will fit perfectly. I used to spend a lot time picking the best looking veneer….now I spend time checking thickness uniformity.

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

View bondogaposis's profile


3969 posts in 1773 days

#7 posted 06-18-2014 12:45 PM

Are there vaneer plywoods that are actual 3/4 thickness?

No, not unless you get a hold of some really old plywood, thirty years ago plywood ran true to size, those days are over.

-- Bondo Gaposis

View TheWoodenOyster's profile


1275 posts in 1356 days

#8 posted 06-18-2014 01:48 PM

Andrew, the exact same thing happened to me about 2 weeks ago. I was really unhappy. I used a 23/32” thinking that was “undersized” and the plywood was still too thick. My initial reaction was just what you have stated – Is some veneer ply actually a true 3/4”? My conclujsion is yes. I have two different 3/4” veneer plys from different sources. Neither one fit the 23/32” dado. My guess is that the plywood itself is 23/32”, but when they add the veneer, it gets just a hair thicker, and ends up around 3/4”.

-- The Wood Is Your Oyster

View TheWoodenOyster's profile


1275 posts in 1356 days

#9 posted 06-18-2014 01:52 PM

That jig is nice and seems like it would work great. Thanks for posting that.

-- The Wood Is Your Oyster

View bonesbr549's profile


1137 posts in 2489 days

#10 posted 06-18-2014 02:01 PM

I’ve not seen 3/4” ply in a long long time. I run my ply over a bit on the table to take it down to a fixed width and leave a lip over the edge. Just like mortise and tenon, I cut the dado and cut the ply to fit the dado. Either take some off each side or leave a lip below. Mostly I do one side (bottom of ply) it’s much quicker.

-- Sooner or later Liberals run out of other people's money.

View BinghamtonEd's profile


2263 posts in 1791 days

#11 posted 06-18-2014 02:59 PM

I have the Freud undersized plywood bit, ran into the same problem as you did. I ended up cutting a rabbet along the shelf edge, using an edge guide, to get it to fit. Next time I need to do it, I’ll be making that adjustable jig.

On an related note (related to the dado jig), I can’t find any place in town that carries 1/4” hardboard. The BORG only carries 1/8”.

-- - The mightiest oak in the forest is just a little nut that held its ground.

View davidroberts's profile


1025 posts in 2907 days

#12 posted 06-21-2014 07:06 AM

I believe 3/4” Baltic birch plywood, and I mean true Baltic birch is manufactured as a true 3/4” thick, similar for 1/4” and 1/2” Baltic birch. It’s been awhile since I’ve had some in the shop. Of course moisture could affect plywood thickness, maybe just enough for “too” snug or just a hair loose, but so could the country of origin, manufacturing process, etc. I agree with the consensus that making an adjustable jig is time well spent.

I’ve seen free online plans for a dirt simple jig that works fine to a jig by Kent Shepherd and also featured in Fine Woodworking magazine
that still makes me drool. The link to FWW may be members only.

-- Better woodworking through old hand tools.

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