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Dado Depth?,, added new question???

If you look at the right hand side of the picture behind the table saw is a mobile table that's the same height as my table saw. I just roll it to where ever I need it whether it's on the side or the infeed to the saw for extra support.
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Bob Gibson said:
If you think about it you are making a cut in wood then filling up the same cut with wood and glue. I don't think that you are compromising the strength of the wood at all by going halfway with a dado.
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Sorry, I have to disagree at least a little (if only with the absolute "at all").. :D A problem with dados in general is that you have at least some end grain->side grain glue joins. This is especially true with lumber but somewhat kinda true with plywood depending on how you cut the dados.

For example lets take two pieces of plywood with a dado cut in them and another piece stuck in the middle.

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I think we can agree that the first one is probably better than the second one, but its not just the depth, its also the two side grain glue spots on the top and bottom. I would agree in this case that the upright is not _substantially_ weaker than the original piece, but you have lost about 50% of the strength from the part that was cut out (discounting any structural gain from the finished product being a unistructure which is non trivial).

Consider also a piece of lumber with a dado and a piece stuck in it:

| | | | | |
| | | | | |
| | | | =======
| | | | | |
| | | | | |

In this case there is only end grain <-> side grain contact so its going to be a poor glue joint no matter what (and is realistically likely to fail eventually). The upright is also compromised to be only ~roughly as strong as the remaining solid piece (4 |'s in this case) at least for bending to the outside (bending to the inside it has to break the outside fibers or compress the horizontal piece so its close to as strong as the original).

Obviously if the upright had the grain running horizontally the glue joint would be strong but the panel would also be more subject to buckling so it would be a net loss.

@Larry: I'd be tempted to use a router and a straight edge instead of the table saw, but an outfeed support can work pretty well as well... a table is good or a roller stand. If you use a roller stand use one with the "roll any which way" balls instead of the linear rollers (like this: http://www.amazon.com/Rockler-Heavy-Duty-Fliptop-Roller-Stand/dp/B001DSY630 not like this http://www.amazon.com/Shop-Fox-D2054-Roller-Stand/dp/B00004TQEO). The linear one is pretty much impossible to align and will "drift" and pull your work to the side which is not good.
 
well i used a router and it could have been better,, i used a porter cable 310 and i am gonna change it out to a larger 690, the table might have worked but i was trying to get this done,,, i have decided to rethink my methods.. for the rest of them.. and what is the go to bit for dado's in your opinion.. i used a top bearing template bit but am looking at a up cut spiral with a bushing next time or a large template bit????
 
Some aint gonna like it but My Festool makes quick work of them. It's all in knowing your tools and how to set them up right.:thumb::thumb:
I'm staying with the half the thickness. :thumb::thumb: So far the best argument is the easy math but I can add 3/8 just as easy as I can add 1/2 And while they say an end grain glue joint is not as strong a bigger weak (achieved by a deeper dado) glue joint is better than a small weak one.:thumb:
 
Chuck Thoits said:
Some aint gonna like it but My Festool makes quick work of them. It's all in knowing your tools and how to set them up right.:thumb::thumb:
I'm staying with the half the thickness. :thumb::thumb: So far the best argument is the easy math but I can add 3/8 just as easy as I can add 1/2 And while they say an end grain glue joint is not as strong a bigger weak (achieved by a deeper dado) glue joint is better than a small weak one.:thumb:
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The festool rail plus router does work crazy well for this sort of stuff. :thumb:

I'd reckon you are undoubtedly correct on the rest, if you over build it like you ought to it won't matter much one way or the other :D

Half the fun is in over thinking the problem :huh:
 
Well, you coulda brung it down here and used my saw tables. ;)

On the router approach, I've used a straightedge and that's worked fine. I've also used an upcut spiral bit, but have ordered downcut for the next dadoes I'll be doing - should make a cleaner cut.

While I'm at it, an option I've used on some dado joints is making the dado narrower than the mating piece. For instance, when mating a 3/4" shelf to a cabinet side, I'll use a 1/2" router bit to make the dado. Then rabbet the shelf so the end fits nicely into the dado.
 
well it was still in the rough stage at the time bill :) i almost did the stepped dado like you mention but opted against it i will just fit the shelfs to my dado's with a hand plane kinda like your idea but lees of a step.. and the up cut is much better than the trim router approach,, more power is good:) just need to use two routers this time will have another setup before the next round.. one more time where carols book came to a good use:)
sure glad i got to meet that little preacher lady ("router lady") :thumb:
 
Roger Tulk said:
Does this have an advantage over a full-width dado? :huh:
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Two advantages I can think of:
1) you use a standard width bit (1/2") and make the dado regardless of whether the plywood is 3/4" or 23/32" or ???, then rabbet the ply to retain the 1/2" to match the dado.

2) the edge of the shelf conceals the edge of the dado, so it can hide any tear-out. A variation is to rabbet both sides of the shelf end to conceal both edges of the dado.
 
Richard McComas said:
Never found a compelling reason to go more that 1/8 deep.
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Assembly of cabinets with multiple decks are way easier when the dado is holding the part. I worked at a shop that used 1/8 dados, and it was a massive pain compared to a 3/8 dado.

Say I've got a base cabinet with three partitions. I'd put a horse under each dado, put glue in the dado, then drop the partition in. After each of the partitions are set in place I pop a nail into it from the underside towards the front, then tip the whole mess at an angle and finish nailing the partitions in. With 1/8" dados, there just isn't enough there to grip the part before I get them nailed. Plus, once in a while, a part needs to be tapped out of its dado a little bit because the deck is a twinge short. Usually not much, but if its a 1/16" on an 1/8" dado, that doesn't leave much.
 
The only compelling reason I can come up with is to go and do a little experiment.

Take a 12" X 12" 3/4" board well call it the wall run an 1/8th deep dado in it. Now turn it around and run a 1/2" deep one in the other end.
Now take another 12 x 12 3/4 well call it the shelf and insert it into the 1/8 dado. At this point stand the wall board up with the shelf paralleling your bench. Can you let go of it with out it falling out of the wall board? Now try the same thing with the 1/2" deep one. Will it stay in place?
The answers I came up with are no and yes There for telling me a 1/2 deep dado is stronger than one that is an 1/8" deep.:thumb:
 
I machine dadoes in ¾" stock ¼" deep. I like to leave ½". I don't use a TS, but rather a handheld router, and a jig you can easily make.

It's faster than doing them on a TS.
It leaves your TS available.
You aren't machining face down subjecting the stock to tearout.
You can see what you're doing.
The dadoes are clean with flat bottoms.
You don't have to force press sheet stock down on the TS table. This can be a strain with large sheets.

See the jig here.


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mike marvel said:
I machine dadoes in ¾" stock ¼" deep. I like to leave ½". I don't use a TS, but rather a handheld router, and a jig you can easily make.

It's faster than doing them on a TS.
It leaves your TS available.
You aren't machining face down subjecting the stock to tearout.
You can see what you're doing.
The dadoes are clean with flat bottoms.
You don't have to force press sheet stock down on the TS table. This can be a strain with large sheets.

See the jig here.


MikesMasterLogo2.gif

movingplane1.gif

.
Click to expand...
well guys and mike specifically, i fully agree with you on the full sheets and trying to hold it down.. i used a router on this project last month:) worked much easier than TS and the only complaint was getting a tight fitting joint. which i did get on all but one small section. how do you maintain the tightness mike for said shelf thickness?
 
larry merlau said:
well guys and mike specifically, i fully agree with you on the full sheets and trying to hold it down.. i used a router on this project last month:) worked much easier than TS and the only complaint was getting a tight fitting joint. which i did get on all but one small section. how do you maintain the tightness mike for said shelf thickness?
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You can use an undersized bit. Plywoods can vary from the same stack. Or, you can start with a smaller bit and use shim stock against the guide for a correct fit.


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