Options for Machining Dadoes
Here's an extensive collection of methods and tips for cutting dadoes in panels. November 30, 2009
Question
I am trying to gather ideas on the best/fastest/most accurate way to cut dadoes for cabinet construction. We have been using the traditional method for years - adjustable dado set in a table saw. However, I am not always happy with the accuracy of this method. Our sliding table saw isn't dado capable. Another thing I am up against is that the interior material we use is not always consistent thickness, so sometimes the dado width has to be adjusted. If we shim the dado set too much, then sometimes it will leave material behind in the dado. Also, if the piece is slightly warped, accuracy is lost, unless you can maintain heavy down pressure on the piece when cutting the dado.
Do dado compatible sliders work alright? What about CNC? Can you adjust your dado width easily? Somewhere I saw a floating router for making grooves. Is that a viable option? Any input would be appreciated - I am ready for a change!
Forum Responses
(CNC Forum)
From contributor C:
Safety Speed Cut makes a dedicated dado routing machine. It cuts from the top side and uses a router. I don't have one but at times I wish I did.
From contributor F:
Her-Saf makes a vertical panel router.
One method of dadoing for dealing with material thickness that varies is to cut the width of the dado 1/8" narrower than the nominal panel thickness. The ends of the panels that mate with the dadoes are then run through a rabbeting setup that leaves the desired fit for the dado. With this setup, panel thickness can vary a great deal and yet fit tightly in the plow.
From the original questioner:
I really like that idea! Any suggestions on how to rabbet the pieces that fit into the dado?
From contributor R:
The only drawback to cutting dadoes on a table saw is, as you pointed out, a varying depth of cut due to the fact that a typical panel will never lay perfectly flat and sufficient downward pressure is hard to achieve on wider panels. There is a simple and quick solution to this. A Stanley router plane (hag's tooth) set to the correct depth of the dado will quickly clean up any variation - the vertical walls will act as a guide for the tool.
From contributor I:
Hands down - HerSaf panel router. Consistent depth, widths available in .010 increments, reverse helix or insert cutters available, dust collectable, easy, fast, repeatable, takes very little floor space, and cheap on the used market.
From contributor O:
When we used to do our boxes manually we did the dados on the table saw to a shallower than final depth and then we would clean the bottom of the dado with a top bearing dado clean up bit from Amana. The bit has a 1/2" diameter, and only a 1/8" height of cut which allows the bearing to ride in the groove and clean up the bottom. We always left 3/8" of the sheet which allowed us to calibrate the width of the boxes accurately regardless of sheet thickness. We typically made the grooves a little loose and the assemblers would wedge the deck against the desirable side and glue and fasten. The CNC does all this for us now and it does use a stepped dado which utilizes a rabbet cut on the mating male part which does exactly as contributor F suggests. Eliminates the variable of the thickness and always fits.
From contributor F:
Any setup that measures the same as the dado width between the cutters' outside edge and a stationary fence will do the trick. Unfortunately this typically means holding the panel vertical on a table saw or a router table. For a different approach, read on...
After I started working wood with digital calipers, it took the guess work out of a lot of processes including dado work. Here is what I do with a standard table saw and an Amana stack dado set.
I installed the various combinations of outside blades and chippers on the saw arbor one by one and ran test passes with each one. After making a test with each combination of chippers and outside blades I carefully measured the width of the dado it cut with digital calipers and wrote that decimal size down in a permanent location for future reference.
I also made a set of paper shims from a thick paper (card) and a thin paper and carefully measured the decimal amount a single shim of each thickness changed the width of the dado after the set was tightened on the arbor (makes a difference).
Now when I need to run dados for a batch of parts of the same sheet material, I first cut the parts and then, using my digital calipers, I take several random measurements from the ends of various parts in the run and come up with an average thickness in decimal form. Now I refer to my chart that shows the actual size dado that each combination of outside blades and chippers makes.
Keeping in mind that experience has taught me that a dado .010" wider than the part thickness is the fit I am after, I select the dado blade combination that is closest to that measurement without being wider than .010" more than the part thickness. Once in a while I get lucky and the parts fit an unshimmed dado setup, but usually a shim or two is required to hit my target width of .010" larger than the part thickness. Experience has taught me that I can cheat about .004" on my target dado width.
Using this system, with a bit of practice you can nail your dado setup first time, every time. Put the set on the saw once, dado your run, take it off and store it till next time. Buy your sheet goods from the same unit and they should be within .005" tolerance on any part end you measure from the batch of cut parts.
From contributor N:
I was at a vendor fair recently and got a sample of this stuff from True North Plywood. It's got a calibrated core that is supposed to be consistent from batch to batch. That could solve the problem, but so could eliminating the dado completely.
From contributor K:
On most of my jobs, dados are done on the CNC when I have my parts outsourced. When I need to do my own dados and the table saw is not practical or available, I use a router and a shop-made jig. The jig is simple. It is an 8" wide by 4' piece of masonite with a 2" strip of ply or hardwood attached down the center as a fence. The first time I run the router along the fence it trims the masonite and gives me a parallel edge on the masonite. I can then lay that edge along my layout line and cut my dado with my plunge router. The exposed masonite on the opposite side of the fence gives me a surface to clamp the jig to the material. I'll use this method until I can justify/afford the speed-cut router table.
From the original questioner:
Thanks for all the input! Lots of new ideas
From contributor J:
A friend of mine uses a simple system on his face frame table. It's not perfect, but it does dead on clean dadoes, fairly fast, and quite messy!
Using a piece of 1/4" plywood with a second piece glued on (what the router runs up to) to that as a straightedge, you can place your cabinet part on your face frame table, clamping down the straightedge over the panel (the 1/4" plywood) and running your router down along the straightedge. Using a system of blocks for spacers to locate where you place the straightedge or measuring it with a tape, you can get a perfect dado. The face frame clamps hold everything down very well and the straightedge never slips. The 1/4" shop made straightedge is loose, so when you're done, set it aside and you have your face frame machine back.
Then there's the Ultimate face frame machine built by one of the Oregon members here, if you do a search you may find a picture of it. It's by far the best solution ever made and never marketed.
If I was made of money I would buy a small Shopbot Buddy and make it my cabinet end dado and hole machine.
From contributor Y:
I did make a dado clamp like contributor J said, but donated it to the local high school when I picked up my Busellato.
From contributor B:
We use a Safety Speed Cut panel router, like the HerSaf one. Perfectly square-to-the-panel-edge dados. Easy to stop dado - we notch the deck of a cabinet, for instance, to fit in the stop dado in the ends to resemble a butt joint on the front of the cabinet. Stretchers front and back below drawers locate the drawer slides perfectly square to the front of the box. Notches are made with a dedicated router table with the fence set for 3/16" cut. We adjust the router cut depth to match the notch whenever we change bits. HerSaf makes router bits in many different sizes - like .765" wide for melamine (3/4" plus .015). Fast, easy, perfect depth of cuts, etc.
From contributor A:
We have a dedicated router table for dados and the fence never gets moved. Over the years we've cut plywood spacers that go between the fence and cutter to give the offset for all the standard dado spacings we use. These are all stored in an old barrel behind the router table. It takes longer to find the correct spacer than it does to change the cutting distance. I shamelessly stole this idea from another shop so I won't claim it, but it's the fastest method I've ever seen for doing dados short of letting a CNC worry about it.
From contributor M:
If you can get it, the multicore plywood is well worth the money. It is spot on 3/4" thickness.
From contributor L:
With all the fooling around done to use dado construction it makes modern methods look really good. Seems that trying to accommodate all the problems with plywood represents at least half of the problems in the making.
