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A while back I ordered a three pack of undersized ply router bits. The brand is Whiteside and the part # 470. These bits are undersized alright, they are too small and too big. The 1/4" bit is 7/32 and my Baltic birch ply will not fit into the grove. The 1/2" bit is 31/64 and the Baltic birch ply fits loose. The 3/4" bits fits better than the others but in my opinion, it should fit snug.

So what gives here?
 

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The problem is that I'm not sure there is a standard for finished ply. It starts out rough and then it gets sanded smooth. That's why it winds up under size. To make matters worse a lot of what I've seen lately is metric thickness to start with I think. Your best bet is probably to use an smaller bit and go with one of the exact thickness jigs that have been posted.
 

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Discussion Starter · #3 ·
The problem is that I'm not sure there is a standard for finished ply. It starts out rough and then it gets sanded smooth. That's why it winds up under size. To make matters worse a lot of what I've seen lately is metric thickness to start with I think. Your best bet is probably to use an smaller bit and go with one of the exact thickness jigs that have been posted.
Charles that is what I was thinking of. The bad part is I spent $52 for the 3 bits and now I don't need them.

Someone got my money tree this winter. I may be wrong but it might be Theo that got it. >:) :laugh2:

Just kidding Theo.
 

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I have three undersized 3/4" bits, all cut slightly different width grooves. I have an 18 mm bit that I bought to use with the Sande plywood that I use a lot, sometimes the plywood doesn't fit the groove. I now have a block with four dadoes cut across it, identified to the bit used, and use it to check for the required fit with the actual plywood that I'm using. If none of them give me that fit that I'm looking for, it's time for a 1/2" bit and an exact-fit jig.
 

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undersized bits come in imperial and metric...
all slightly different sized...
 
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And plywood isn't necessarily consistent in thickness across or along a sheet. I was cutting dados for plywood recently and with the same dado setup, some pieces of the plywood fit, and others didn't. They were all cut from the same sheet!

I cut my dados a little wide to allow for such, and for wood expansion due to soaking up the glue.
 

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And plywood isn't necessarily consistent in thickness across or along a sheet. I was cutting dados for plywood recently and with the same dado setup, some pieces of the plywood fit, and others didn't. They were all cut from the same sheet!

I cut my dados a little wide to allow for such, and for wood expansion due to soaking up the glue.
very true for imported plywood due to poor QC...
 

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Charles that is what I was thinking of. The bad part is I spent $52 for the 3 bits and now I don't need them.

Someone got my money tree this winter. I may be wrong but it might be Theo that got it. >:) :laugh2:

Just kidding Theo.
Sure wasn't me, I'd have been bragging about it if so.

I work mainly with 1/2" plywood. The few times I need to cut a slot to fit plywood into it, I usually tack a strip to rout a straight line, then mark how wide I want it to be, tack down a strip, and rout that side. So far that gives me just the widths I want, with a snug fit. Used to do a lot of that when I was making my small figure banks. Best I can come up with, because that's the way I do it, it works for me, and not likely to change. I tack everything down, I don't worry about nail holes, because I make sure they will be hidden when whatever is put together. No need to replace tape when it runs out either, a box of nails should last me forever.
 

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In the process of building a pantry closet for the kid w/plywood panels. Picked up the plywood at big blue. Stated to be .453. (29/64"). Used a WS undersized bit (31/64") for dado's and the fit was perfect. I always measure and test fit first. You just never can be 100% confident on the PW sizing.
 

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time for a dado/grove guide...

..
 

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All nice jigs, but the downside to jigs like the first shown is that they're "mated" to a specific router - which may or may not be a problem. Another potential problem is that the axis of the collet may not be on the center of the base - an easy check if you have the time. The cure for that is to mark the router and jig so that the router is always used with a marked point on the base touching the guide surface of the jig - or make a sub-base, either square or round, where the bit is perfectly centered on the base, a fairly simple exercise. IMHO the jig shown last is the better choice as the use of a guide bushing makes it suited for use with any router in the shop - just mark the jig with the diameter of the bit and guide bushing used.

The T-square jig shown has the same drawback if the "notch" on the base leg is used to locate the groove to the layout mark in that the router used, and the orientation of the base in case it's not symmetrical, is important. Additionally, it doesn't address the need for a groove which doesn't match the diameter of the router bit being used. While still having the drawbacks stated above, this tip I saw in a recent copy of "Woodcraft" magazine would allow you to cut an exact-fit dado using a smaller bit and two passes as in the other designs. I built the T-square shown, but haven't had a chance to try it out yet.
 

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Probably the easiest jig is one that has two arms and either slides or scissors together so that you can use a piece of the ply you'll be building with to set the width and then use a pattern bit that fits in the slot and make a pass one way up against one side and back again against the other arm.
 

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All of my routing is with a table mounted router, and a flush trim bit. So for me, my method is perfect, AND I don't have to figure out where I left the guide last time I used it, just get a straight piece and tack it down.
 

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All of my routing is with a table mounted router, and a flush trim bit. So for me, my method is perfect, AND I don't have to figure out where I left the guide last time I used it, just get a straight piece and tack it down.
Some jobs are too big for a table Theo so you have to implement plan B in that case. If the work is small it's usually easier to take it to the tool (as in router table). If the work is large it's usually easier to take the tool to the work (as in portable router).
 

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As has been mentioned, baltic birch is from the Baltic, Europe, METRIC. The exact thicknesses aproximate ours. The Whiteside bits are made for American, inch, plywood. This problem is a vote for cutting dadoes with a dado blade set on a table saw. You can shim to an exact width regardless of where the plywood came from.

Sent from my SM-G900V using Tapatalk
 

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No one is going to pry me from my DadoWiz! Exact fit, runs on a straight edge clamp, uses a down cut spiral bit. No longer made, but what a great tool. Stopped dados are a breeze. Clamp the guide down and it delivers a very clean groove. I slide a piece of paper in with the sample piece of wood to give a little room for glue. Learned not to pull the router up when it's still running on this jig; the damage was minimal to the aluminum block of the jig, but the spiral bit was a goner. Other than that, I also like this type exact fit jig with a bearing guided mortising bit:
 

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Some jobs are too big for a table Theo so you have to implement plan B in that case. If the work is small it's usually easier to take it to the tool (as in router table). If the work is large it's usually easier to take the tool to the work (as in portable router).
Understood. But in my case, a table works fine for me, because of what I make, and the sizes I make them in. Just now anything and everything I make, my table router handles it well. But I have a future project (I hope), and I'm either gong to have a larger table (which I think would work, if I had a large space to build it in), or a portable router. Obviously, details not yet worked out, but being realistic, portable router.

So, remember, any future references are what works for me, not necessarily for someone else's work. Sorry about that.
 
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