Dado depth?

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Bookcases again.
Worst case: a 30" long shelf in a cabinet with all fixed shelves, all glued and dadoed. The bookcase will have a back and a face frame.
My dado jig works nicely so far (I even whipped up a miniature "bookcase" to try out the system http://www.flickr.com/photos/gdguarino/8311879665 / ), but the pattern bit I have can only do 1/4" depth. I can't think of a way that such a joint could easily come apart, but 3/8" "feels" like the right depth for 3/4 ply. Should I spring for a bit with greater cutting depth, or is 1/4" enough?
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"Greg Guarino" wrote:

--------------------------------------------------------------------- Mikey likes 1/4" deep dados for 3/4" thick shelves.
Cut shelves 1/16" short thus allowing 1/32" per side.
SFWIW, the shelves are strictly in shear at the dado end points.
As far as a router bit is concerned, don't you already have a shelf space reserved for that new bit?
Lew
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I'm not sure I get this last bit. Could you flesh it out a little?

You mean, inside the micro-bookshelf? :)
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I wrote:

"Greg Guarino" wrote:
I'm not sure I get this last bit. Could you flesh it out a little? --------------------------------------------------------------- If the inside distance between the verticals is 30", then using 1/4" deep dados, the distance from bottom of dado to bottom of dado is 30-1/2" and the shelf would be 30-7/16" allowing 1/32" per side for slop.
Lew
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Lew Hodgett wrote:

If instead the dados were intended for vertical inserts(structural), would you anticipate some *slop* in forcing the joints square (and thus rely on an additional mechanism, such as screws or dowel-joinery for more strength). or would you not expect this to be an issue, with plywood. If so, would you (really) add 1/32" to a 1/4" dado cut in 3/4" ply to help account for this? My guess is no, but you raised my curiosity about the way that you approach achieving the desired results--exactitude in the desired height, for instance, the sort of perfection that Swingman has preached to me about!
Bill
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On 12/27/2012 2:17 AM, Bill wrote:

That is a holdover from days of yore that is no longer operable in most situations, particularly the one under discussion.
Cut it the exact/proper project dimension and be done with it ... if you don't batch cut, or if you use only hand tools, you will have at least a 1/32 variance in part dimensions, so it is a moot point and you best be fitting each individual part in that case.
If you are working with modern tool and take reasonable care to properly use consistent reference edges and proper machines techniques, and unless there is a specific reason and/or you are simply that damned sloppy, there is generally NO need to build in any "slop" into a dimension in the circumstance under discussion.
*If you don't attempt perfection in every step, you will achieve it in none!*
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Lew Hodgett wrote:

I would do that for shelves with one or two shoulders but definitely not for shelves without shoulders. With shoulders, the distance between them end to end determines the shelf width and you want the tongue fully within the dado; making the tongues a skosh short assures that the shoulders will be touching the uprights and that tells you visually when the tongues are all the way in.
Without shoulders - I rarely do this - you want all the shelves to bottom out so I make the dados slightly sloppy so that the shelves are an easy fit.
--

dadiOH
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That's what I figured you meant, and that's why I'm confused. Whatever length I cut the shelves, won't the shelves of necessity bottom out in the dadoes during glue-up, whether held in place by clamps or screws? Are you suggesting that the glue will take some extra space, or will swell the wood such that the shelves expand a hair (lengthwise)?
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On 12/27/2012 8:55 AM, Greg Guarino wrote:

Lots of "French model" advice on the Internet these days confusing you ... beware, and bawn jyour.
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On 12/27/2012 10:29 AM, Swingman wrote:

Mair Cee Bo Koo.
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On 12/28/2012 9:14 AM, Greg Guarino wrote:

Quick, TradeMark that before some RAP group locks onto it, then used to name a future NFL running back ...
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On 12/28/2012 10:33 AM, Swingman wrote:

I prefer to envision the overseer of a town in Senegal (wait for it) "Mayor Siboku"
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Greg Guarino wrote:

:)
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On 12/28/2012 10:53 AM, Greg Guarino wrote:

LOL ... and, by the time he immigrated here that be spelled "Mercy Buckets".
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On Thu, 27 Dec 2012 06:55:22 -0800 (PST), Greg Guarino

If you ignore Lew you will be better off.
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"Greg Guarino" wrote:
That's what I figured you meant, and that's why I'm confused. Whatever length I cut the shelves, won't the shelves of necessity bottom out in the dadoes during glue-up, whether held in place by clamps or screws? Are you suggesting that the glue will take some extra space, or will swell the wood such that the shelves expand a hair (lengthwise)? -------------------------------------------------------- Not if you factor in my brain fart.<G>
Forgot to include description of "shoulder" on end of shelf.
See dadiOH post for explantion.
Lew
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On 12/27/2012 8:55 AM, Greg Guarino wrote:

Simple question to ask yourself. Would you force a 1/32" glue gap between two board edges when making a wider plank? Then why on a dado?
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On 12/27/2012 3:34 PM, Leon wrote:

I figure myself for a fairly bright guy, and one of the things fairly bright guys do (on those chance occasions when their egos don't get in the way) is consider that other people may know things that we don't. So no, I couldn't think of a reason why one would leave a gap, and in fact, couldn't figure how it might even be accomplished. Hence my puzzlement, and my question.
I try to weigh the advice I get here against my own sense of how things work. I also weigh responses against each other. So fear not, my cabinets will not be 1/16" too narrow. My mistakes are seldom that small. :)
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On 12/28/2012 9:24 AM, Greg Guarino wrote:

Responses are easy to come by ... responses with actual hands-on experience factored in, not necessarily. Some folks even try to make it easy for you to take that into account. :)
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On 12/26/2012 6:51 PM, Greg Guarino wrote:

Suit yourself ... almost all of the dadoes/rabbets I cut in ply are 1/4" in depth, but 3/8" is fine in 3/4" thick ply also.
I usually use 1/4" for one overriding reason ... almost without exception I prefer to cut all dadoes/rabbets with one machine setup, as it fosters consistency, which fosters fit and square, and 1/4 allows that, even with dividers and shelves that are back to back.
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