Centering the dado

[Eric Roberge]

We have all seen Norm do this several times. He runs a board against the fence (vertically) through the dado, flips it around and runs it through again to “center” the dado.
Now, I haven’t done this yet (haven’t needed too), but if you just cut a ¼ dado in the stock and flip it and it were to shave some more off to “center” the dado. Wouldn’t your dado be larger than you want?
I may have just answered my own question, but it should be all relative to how you have your fence set, correct? In other words, if your fence is set to the exact half width of the stock to the center of the blade it shouldn’t matter.

Am I thinking about this correctly? Food for thought

[Chris Padilla]

Yes, you're thinking correctly. The dado could be made wider by doing the flip and run it again to center it move.

[Chris Szczygiel]

You are correct. Your dado will be enlarged by the amount you are off center. If you are really careful in your setup it should effect the joint fit much.

What you don't see is Norm running test pieces and making adjustments. By the time we see it he is almost dead center taking off a hair when he re-runs the piece.

[Paul Demetropoulos]

A 1/4" dado blade will indeed result in a dado larger than 1/4" if the board is flipped and passed a second time unless the first cut was perfectly centered, an unlikely event.

[Peter Quinn]

This technique (run/flip/rerun) can be used to verify a dead center set up by running in just a little on a test piece to see or measure how much you are taking off on the second pass, or it can be used in situations where the dado being centered is more important than the absolute width of the dado. If you must have both a 1/4" dado and a perfectly centered dado I don't recommend you run your material twice on actual work pieces as it is highly unlikely you're final dado width will be exactly 1/4" even in the best of circumstances. In fact simply running a dado twice without flipping rarely yields a dado of consistent width.

As a corollary, this run/flip/run and check technique is also a quick way to re-saw a board in two pieces for a book match and insure that your BS is set to cut the dead center of the stock.

[Ed Hazel]

If you want a centered dado (all pieces the same on both sides) and you want it 1/4" then set your dado blade to 3/16 and then do the flip and turn so that you can take a little off on the second pass. the nice thing about this is you don't have to have your dado set at an exact width you can just adjust the fence to get your exact dodo width and each piece is the same you don't have to keep track of front and backs.

[David DeCristoforo]

Also the "two pass flipping" technique is completely dependent on all of the pieces being exactly the same width. Otherwise, not only could the dados be wider but they could vary as well...

[Eric Roberge]

Thanks! That's what I was thinking. (BTW, the whole 1/4 width measurement was just an example)

What lead me to this question was I will be starting my blanket chest this week and will be using tongue and groove jointing.

I assume that most of you just run it through once and done?

[Jules Dominguez]

The reason for using the two-pass method is if the minimum width cut by a single pass of your dado blade is wider than the board which is to fit in the dado.
The method is to use a less thick blade and "sneak up" on the desired width by moving the fence and repeating the two-pass cuts until the width is just right. That gives you both a snug fit and and a centered dado.

[Rod Sheridan]

Hi Eric, if your wood isn't perfectly straight, when you run it over the dado cutter it will probably move in relation to the fence.

This will give you a groove that's not centered.

If you then flip it, you repeat the process with the not flat stock and you get a variable width groove.

I make tongue and groove pieces on the shaper using a power feeder. The feeder holds the material down on the table with far more force than I can by hand. This results in consistent dimensions since the feeder can actually bend material that isn't perfectly flat.

(I always use flat material however).

For your application I would make sure that all the pieces are flat and straight, and that you mark the top surface so that you always run the pieces in the same orientation.

Make your groove first, then cut the tongue to match. It doesn't matter if the groove is exactly in the center, since you're keeping track of the good face.

Any other method requires that all pieces be exactly the same thickness, and perfectly flat, something that isn't always possible.

Regards, Rod.

[Cliff Rohrabacher]

I assume that most of you just run it through once and done?

Yup. Norm centers his dado in a two pass operation for reasons known only to Norm. I center my dado at the time of set up and run it just the once.

You might send him an e-mail and ask why he doesn't just take the extra ten seconds to get it right the first time? Ya never know, there might be a reason or on the flip side it might have just been a little trick he felt like sharing that day.

[Jules Dominguez]

Also the "two pass flipping" technique is completely dependent on all of the pieces being exactly the same width. Otherwise, not only could the dados be wider but they could vary as well...

David, assuming that you thickness plane all of the stock before cutting the dados, what's the problem with it all being the same thickness? I've had problems with thickness variation of S2S lumber from lumber yards, but not from lumber that I've thickness planed in my shop.

(I agree that if you can get the desired width of dado with one pass, that's the way to do it, but I think the purpose of the two-pass method is to match the exact width of a mating board, and the fact that that also centers the dado is a bonus.)

[David DeCristoforo]

"...assuming...."

Well if we are assuming, lets assume that there would be no problem, assuming everything was thicknessed perfectly....