Dado width

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I'm going to cut a dado in 2 pieces of plywood to put a third piece of plywood into on the tool stand I'm building. Is 1/32" wider than the plywood thickness enough.
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Mike
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It really depends on humidity and things like that. I'm think'n you're gonna have some expansion problems.
If you're afraid to jump out of a perfectly good H-19 into a jungle full of gooks running around shooting up the place, you need to be in the Navy. Old Air Commando Saying
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asmurff wrote:

How smooth/flat is the plywood?
Take a piece of scrap, cut a dado the width you think will work. Too sloppy? Too tight? Cut another dado wider or narrower as the case may be.
IOW, use your head fer cryin' out loud! :)
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dadiOH
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Sorry I grouched I was having a lousy day with my arthritis, which can turn me into a crab. I asked the question after I had quit working for the day and was doing the math on a couple of cuts I need to make today and thought I ask here to see if there was a normal procedure used by most. I was planning on cutting a test piece with the info I got here in mind. The plywood is the best grade Home Depot sells with one face pre-sanded and the other face is fairly smooth also. It isn't for a piece of fine furniture, but a Flip-Top Tool Stand, I got the plans from "The Complete Small Shop". I know how to make the piece fit into the dado I just wasn't sure if I needed expansion room and if so how much. If you had left off the "IOW, use your head fer cryin' out loud! :)" I would have said okay I'll try that.
Mike Watch for the bounce. If ya didn't see it, ya didn't feel it. If ya see it, it didn't go off. Old Air Force Munitions Saying IYAAYAS

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asmurff wrote:

You don't need any expansion room.
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dadiOH
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Do you want to be able to slide the piece out or will it be permanent? Want to slide it out, I'd go at least 1/8" wider. Want it to be permanent, a perfect fit will be much stronger.
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Thanx Leon you answered a question for this project and a future one.
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Mike
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wrote:

Plywood varies in thickness, not just from sheet to sheet, but in different places on a single sheet.
Better quality cabinet grade plywood varies less but still varies.
What I have come to do is to not use dadoes that are the full thickness of the ply. I usually make a dado that is in width from a third to a half of the nominal thickness of the ply. Then I cut a tongue in the other piece to match that width.
This takes the variation in thickness out of play.
Let's say that you are making a bookcase out of 3/4" plywood. You make a dado that is about 5/16" wide on the vertical (side) pieces. Then you make a tongue on the bottom of your horizontal (shelf) pieces by setting your tablesaw fence 5/16" away from the nearest edge of the blade. You will wind up with an offset tongue that will fit perfectly.
The offset tongue should be made on the bottom face of the shelf so that the laminations of the shelf ply are loaded in compression, rather than being under tension.
I usually make thelength of the tongue very slightly shorter than the depth of the dado, so that the glue has somewhere to go when you clamp up. By 'very slighty' I mean no more than the thickness of a sheet of paper.
HTH
Tom Watson tjwatson1ATcomcastDOTnet www.home.comcast.net/~tjwatson1
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Okay I know plywood comes in varied thickness this sheet happens to be 45/64 of an inch. I was planning on cutting the dado at 47/64 to give me a tad of room on each side. The piece will be permanent with cleats below, but yes I'd still like it to look good even if it will only be seen in my garage/shop.
DadiOH I do use my head which is why I asked here, I thought there might be a pat answer to the question, if there isn't I'm more than willing to listen to the group but don't insult me.
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asmurff wrote:

1. I gave you a perfectly acceptable answer
2. I wasn't trying to insult you - ergo the smiley - but was pointing out that you could have easily and (most) effectively answered your question for yourself. ___________
Pat Answers Ah, yes...the pat answer...the be all and end all of existence. Unfortunately, there is rarely a pat answer to anything other than "it depends".
In your case, it depended on the type and quality of the plywood, how tight you wanted the dado/tongue to be, how long they were to be and what you meant by "Is 1/32" wider than the plywood thickness enough." Enough for what? To put together? To stand without support? To glue well?
In my case, I want what I think of as a "snick" fit; the tongue goes into the dado easily with moderate pressure, doesn't wobble and when separating the pieces there is sort of a "snick" feel/sound...sort of like popping your cheek with a finger And no, I can't better describe it. Trouble is, a fit that is fine on, say, a 4" dado/tongue probably won't be if the dado/tongue is 30"...it will probably feel too tight. It would probably be OK if I slightly beveled the edges of the tongue - which I regularly do either with a small plane or a four-in-hand file depending on the material - but even then it will likely take considerable pressure to get the tongue into the groove/dado.
Tom Watson gave you a nice way to make a tongue that is reasonably precise in size. Works most of the time; however, it would be very difficult to impossible to cut a tongue in that manner on the four foot end of a sheet of 4x8 foot plywood. Or on the short end a 1x6 foot one either ______________
I think most people that have read newsgroups - ANY newsgroup - for a while would agree that many of the questions could easily and more effectively be answered by the poster if s/he only thought about it for a moment and/or drew on their empirical experience.
That doesn't mean I don't think people *shouldn't* ask questions - many times the asking provides answers that are useful though not necessarily germane - merely that they could often work out a solution for themselves and in so doing they have a solution (and brain exercise) that can be modified for use in other circumstances.
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I would use the same technique on ply up to about four or five feet in length but would use a different one on the longer stuff.
The short edge of the 1x6 is no problem. If you don't like the idea of running it through by hand, a typical sliding fixture with clamps will hold the piece safely.
I've posted some pictures of this joint on ABPW.
Regards,
Tom
Thos.J.Watson - Cabinetmaker tjwatson1ATcomcastDOTnet www.home.comcast.net/~tjwatson1
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That makes good sense, but I would be inclined to make the tongue more like 2/3 to 3/4 of the thickness. I don't see any advantage to making it less.
If it is less than half, the some of the veneers that aren't glued into the dado will be in tension.
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FF


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On Sat, 24 May 2008 11:10:52 -0700 (PDT), Fred the Red Shirt

Then you would only have 3/16" - 1/4" of meat left beyond the groove when using this joint at the top or the bottom of a carcase. Your theory works fine for a shelf or divider in the middle of the panel but would require different joinery for the corners, which is not optimal.

The tongue is formed on the bottom part of the thickness of the board. The shearing load is compressive throughout the thickness of the board.
Only at the bottom of a carcase, where the tongue would, of necessity, be formed on the top part of the thickness of the board, would there be tension on the glue lines of the ply. The reason that you can get away with that is because you can have a frame below the carcase at this point.
Regards,
Tom
Thos.J.Watson - Cabinetmaker tjwatson1ATcomcastDOTnet www.home.comcast.net/~tjwatson1
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Ah! that makes sense. I was only thinking of shelves.

I don't follow what you mean by 'shearing load is compressive'. Shear, compression, and tension are three distinct orientations of stress.
For a beam with a symmetrical cross-section, the sheer due to bending is maximum midway between the upper and lower surfaces. It is zero at the upper and lower surfaces. From that midpoint up the stress is compressive, and maximum at the upper surface. From the midpoint down the stress is tensile and maximum at the lower surface.
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FF

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On Sun, 25 May 2008 19:11:03 -0700 (PDT), Fred the Red Shirt

If you look at the joint in the photos, is it more likely to delaminate under load on the shelf with the tongue at the bottom, or with the tongue at the top?
Regards,
Tom
Thos.J.Watson - Cabinetmaker tjwatson1ATcomcastDOTnet www.home.comcast.net/~tjwatson1
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At the top, of course.
My point is that making the tenon less than half the thickness of the shelf also makes it more likely to delaminate than if the tenon is at least half the thickness.
Nice work, BTW.
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FF

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Thanks for the links to the images!
When I do it, i cut the top and bottom of the "shelf" edges about an eight or so to make the tongue. But your approach seems to leave as nice a line as mine with less work!
Thanks, again for the links to the images.
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I don't think that takes the variation out of play, it just hides it somewhat. Generally you'll see plywood that is a bit wavy in thickness, but rarely does it taper. Cutting a tongue like you suggest will remove the waviness from one side of the board and the wavy side, being the tongue on the bottom, is generally more hidden. That technique doesn't address the waviness of the plywood sides. The perfectly straight rabbet cut on the shelf will be on the top and you'll still see the waviness that's in the uprights.
R
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On Sun, 25 May 2008 08:14:53 -0700 (PDT), RicodJour

The point about variation in thickness was that, if you cut a " width dado to receive shelves, and some of the shelf material is greater or less than " thick, you will be too tight on some shelves and too loose on others.
Machining a tongue using the method described allows you to create a tongue that is of consistent thickness throughout its length. You will then have a tongue of consistent thickness and length mating to a groove of consistent width and depth. This is required for a good joint that has a consistent glue line.
The waviness that you describe, which I take to be a combination of variation in thickness and the cup, or even snakiness (alternating concavity and convexity relative to a straight line) of the material can be moderated by good clamping and gluing technique.
I've made a test joint using construction grade plywood, which was cupped and varied in thickness throughout by as much as .031". I tried to find some CDX in the shop but the closest I could come was some leftover decking ply, still pretty much a worst case scenario when we are really talking about making cabinets, typically out of much better material.
I've photographed both the top and bottom of the 'shelf' where it joins the 'side'. You can judge whether or not it is a good joint and whether or not there is any 'waviness'.
The photos are on ABPW.
Regards,
Tom
Thos.J.Watson - Cabinetmaker tjwatson1ATcomcastDOTnet www.home.comcast.net/~tjwatson1
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Tom,
The photos that you posted in a.b.p.w illustrate this very well. You have added a technique to my bag of tricks. I can see how this can be used to advantage.
I would have thought that the tongue would be thicker - 1/2 to 2/3 the width of the shelving.

That makes sense. Thanks for the rationale behind that part of the technique.
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