I am making a router table that calls for 3/4 birch ply shelving attached to the sides (same material). Plans call for dado in the sides to support the shelves. I know that 3/4 ply has not been 3/4 forever but, I purchased a 23/32’nds plywood router bit at woodcraft today and when I put the dado groove in some test material, it was still very loose. When I put the calipers on it, it was only 22/32’nds Is there such a thing as a 22/32nd bit? if not what do you suggest? I saw a product called dial a dado that might do the trick for 45.00 bucks.
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Replies
by dado blades for your saw
Bones,
You need a good set of dados...probably $140 and up..and then sneak up on the dado by adding sheets of paper to the end of you panel.
Something I heard when I first cut dado's, is use all of the shims and the right chippers, etc on a first test pass. Test that groove on your plywood. THEN use the shims to fill out the space. The shims you use to fill in the test dado? Leave them off, and put the rest of the shims back on the saw. Perfect Dados on your second pass.
FWIW.
http://www.MLCS.com router bits show a 21/32 and a 23/32 but no 11/16.
Use a 1/2" bit and two fences. Set up the two fences so that you hold the router against one of them going one direction and the other one going the other direction.
Much cheaper than dado blades for the table saw.
Edited 7/18/2004 10:30 pm ET by Uncle Dunc
I vote for the two fence method with a router. You can guide the base of the router, or if both straightedge guides are smooth on top, you can use a template guide (collar guide) to make the setting fast for any thickness of dado:
Just subtract the diameter of your router bit from the diameter of your template guide, and find a drill bit or two of that size. Now clamp on one straightedge, put wood of the thickness of interest against the straightedge, and clamp on the second straightedge on the other side of the wood, using the drill bit as an extra spacer. Now if you rout up one side and down the other, you'll get a perfectly fitting dado. You can adjust the tightness of the groove by varying the drillbit spacer, and the adjustment will carry over to dados of other thickness. There used to be an online video of Pat Warner showing this technique, but it's now gone from the FWW website. He might have used a flush trim bit instead of the guide and spacer, come to think of it.
One feature of this method is that you can modify it slightly to make sliding dovetails.
many times baltic birch is actually measured in MM. 18mm i think. Most reputable router bits have plywood sets. CMC and whiteside i know for a fact sells them as im sure amana and others do as well. they are made for plywood and are undersized and measured in 32nds
Thanks to all who replied. WmP you were correct on the 18mm. I went by the cabinet supplier where I bought the ply and he told me that plywood has been in metric for some time and 3/4 is indeed 18mm. I put the caliper on it and it's dead on the money 18mm. The bit I bought was for plywood (23/32), but just a tad to big. None of the local suppliers had an 18mm bit in stock, so I will have to order one.
If your curret batch of plywood is consistent, consider yourself lucky. I have had 3/4 inch plywood that was actually only 5/8 inch thick (and it was stamped 3/4 on the edge). My current batch is outstanding quality (in terms of the face veneers) but thickness varies by more than 1/32 within a sheet. I am talking $100 per sheet furniture grade stuff, not seconds from Home Depot.
Given this variation between batches, or even within a sheet, I either make two cuts with a router, or one cut with a 1/2 inch bit and then cut a shoulder on the "tenon" part with the table saw, or a combination. I discovered the variation within the sheet in the current batch when the shoulder depth went from about 1/32 to 0. I checked with the calipers, and confirmed the reason was the plywood, not the operator. I don't recommend building a collection of router bits since the plywood varies so much from batch to batch.
(so I am currently cutting shoulders to match the dado). ________________________Charlie Plesums Austin, Texashttp://www.plesums.com/wood
Plywood, like a lot of lumber is shrinking.
As others have suggested, you can try to find a router bit that will correspond to the thickness of your plywood, but there is a much easier solution.
I routinely cut a 1/4' or 3/8" dado in one piece, a tongue of corresponding size on the other piece of plywood. I use the fence to cut the dado, a a jig that fits over and then slides along the fence for the tongue.
The system is very fast and efficient for building any sort of plywood carcase -- including drawers made from Baltic birch.
Much like nikkiwood said bones. Cut your groove or housing (dado) with a choice of router bit from about 1/4" to 1/2" (depending upon the structural needs) using a straight edge clamped to the carcass side and running the base of the router of the edge. It's an offset cut easily calculated for grooves along the length of the carcass side.
Many people make up a wooden T square type guide for this task to guide the router base to cut housings or dado's. The first cut will take a notch out of the cross bar of the T which makes lining up all the cuts to the marks simple.
Work a tongue on the end of each shelf using a side fence on the router to set the width of the cut. The depth is set as normal with the depth stop on plunge routers and by locking the motor housing on fixed base routers. As a bonus, you get a structurally sounder construction becasuse of the extra returns and interlocking nature of the joint. Slainte.
You can't always get your work up on a saw, and stop dados are even harder. The various specially sized router bits seldom fit any plywood I use. There are two easy ways to handle the problem. The first is to cut an undersize dado and shoulder the piece. The other is to make a rectangular base for the router. Cut the sides of the base so that one or more edges have a different relation to the edge of the bit. The variations allow an undersize bit to be used. You make one pass with side "A" against the fence, turn the router to side "B" and the cumulative cuts are set up to match the material you are using. You have to make the cuts in proper sequence to avoid the bit pulling. For example: if you set the guide fence to the left and push the router, you want to make the far right side cut first, otherwise the bit rotation will pull you away from the guide fence. The straight edge of a rectangular router plate is much more consistent when running up against a straight edge than the circular ones. Mark your baseplate clearly so you know which side is which.
Take your bit to a sharpener and have them grind it to the right size and re-sharpen. It shouldn't cost you more than a couple of bucks and definitly less than a new bit.
Sincerely;
The Tool Guy
Bones,
If you were/are looking for a metric bit, it's a non-standard size also (17.5mm)
I'd cut a 16mm trench and then trim back the excess as Nikkiwood and Sgian suggested, but one side only - a barefaced tongue and trench.
It's usually done by setting a slotting cutter in an inverted router/router table, leaving a 16mm gap between the bottom of the bit and the table (or whatever thickness matches your trench width if you have a router table.
More food for thought.
Cheers,
eddie
Edited 7/20/2004 7:12 am ET by eddie (aust)
Lots of good ideas from folks and you could always get a Freud 04-138 -----11/16" or 22/32" as you call it for $14.95
Amana also makes one. I'd prefer one with a 1/2" shank so I'd be using a double pass method with a bit I already have if it was me.
Edited 7/20/2004 12:29 pm ET by rick3ddd
They do make an 11/16" bit, but did you check to see if it was 18mm ply? There is alot of imported stuff coming in that is 12, 18, 19mm..
Use a barefaced housing joint instead of a full dado. You simply cut a rabbet on the shelf end and then a matching dado in the carcase side. Do this with your router. This joint is just as strong, if not stronger than a full dado. You can blind nail the tongue from underneath or nail from above, set the head with a nailset and putty. This would certainly be fine for a router table.
The depth of the rebate joint can be varied to produced a tongue that fits perfectly (very snug) in your 3/8ths or perhaps 1/2" straight bit. I bet you have both. Try the method on some scrap and you'll never cut a full width dado again, and you can continue to use your router - a much safer tool than a tablesaw with a stacked dado cutter.
The BB ply you bought was probably measured in mm's which explains the sloppy fit you are getting with your plywood router bit set. Again, the cure is the barefaced joint. You get a snug joint every time and all you need is a rabbeting bit and straight bits - items you should already have in a power shop. There is almost always a solution rather than going shopping. Although I will recommmend you spend a little money - buy the book Good Wood Joints.
Cheers.
Edited 7/22/2004 1:18 pm ET by cstanford
Thanks for the idea. I could not find an 18mm bit in stock anywhere locally, and found out they are not cheap. I checked and the book you suggested is not currently in print, but I found a used one on Amazon for $13.00. Looks interesting and will pick up a copy for the library.
Someone else gave me a good idea today as well. He suggested that I put the shelf against the side and clamp a strait piece of material on both sides. Remove the shelf and use a 1/2 inch pattern bit to ride against the scrap and make a similar pass on the other side. Should make a tight fitting dado. Someone earlier suggested a tongue type of arrangement. Taking material off both sides of the shelf to get the perfect fit. Your idea seems to accomplish what I want with the smallest amount of effort. I'll give it a shot tonight on some scrap, but I think might just to the trick.
My method is how hand tool woodworkers have achieved tight fitting joints in intermediate elements for hundreds of years. It translates well to plywood and router methodologies as well. All this straight piece of metal stuff - make a pass on this side, make a pass on that side is not worth the effort. You'll get a better joint, with the tools and bits on hand, than you ever will with all that jigging and other stuff. The only 'jig' you'll need is whatever straight piece of material you've always used to cut dadoes. In my power tools days this was a shopmade router t-square.
Once you've tried this once or twice you are very likely to never cut a 'full' dado again. There really aren't that many reasons to. You also won't have to worry about whether your sheet material was spec'd for the metric system or Imperial, whether the manufacturer has figured out a way to clip another 64th off and get it past grading and all that rot.
Get the book from Amazon.
Edited 7/22/2004 5:17 pm ET by cstanford
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