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Reinforcing a Dado

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Forum topic by jmos posted 03-04-2017 09:42 PM 532 views 0 times favorited 8 replies Add to Favorites Watch
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jmos

796 posts in 2203 days


03-04-2017 09:42 PM

I’m working out the design of a case piece for a friend (about 60” long, 34” high, and 20” deep.) I’m thinking about how to attach the bottom. I would normally dovetail the case on all four corners and add some legs, but they want the sides to go to the ground and form the legs. I’m concerned that if I just dado the bottom into the sides, even with the back to reinforce it, the joint will be prone to fail. The case will not have a face frame.

I know I could use some screws from the outside to reinforce, but I’d rather not have plugs showing. I guess I could use some dowels along with the dado, but I wouldn’t get much penetration nor much glue surface. I could use a dowel joint I suppose, without the dado.

At 20” deep I figure it’s probably too long for a sliding dovetail. A tapered sliding dovetail is really the perfect joint, but I’ve never done them before, and I’m not sure I want to try to figure it out on this piece, with someone else’s lumber.

My best alternate idea so far is to use some long grain blocks, maybe an 1” to 1.5” long, and glue them to the sides and bottom, where they meet. I’m thinking they would be short enough that the wood movement might not tear them off, and they would get some decent glue surface to reinforce the joint. They would also be quick and easy, and wouldn’t show.

So, am I worried about nothing, and the dado is strong enough? Anyone know a better method of joining the bottom, or strengthening the dado? How would you tackle this?

-- John


8 replies so far

View Bobmedic's profile

Bobmedic

375 posts in 2635 days


#1 posted 03-04-2017 11:46 PM

What about Dominos or pocket screws? Don’t make the dado so deep. It really only has to be about an ⅛” deep. That will give you plenty to screw into or Domino and the stability from racking will be supplied by the back panel.

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bondogaposis

4475 posts in 2185 days


#2 posted 03-05-2017 12:05 AM

Are you using plywood or solid wood? I actually think that this would be a good place for pocket screws on the underside of course. Try to talk them into letting you use a plinth to set it on and then you could dovetail the carcase like normal.

-- Bondo Gaposis

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JBrow

1271 posts in 754 days


#3 posted 03-05-2017 05:03 PM

Jmos,

I appreciate your concern over the strength of the dado joint in solid wood. No matter where the joint is glued there will be long grain meeting end grain. Glue does not seem to take very well to end grain.

The short glue blocks would work ok, but I got lost when you spoke of applying the glue blocks to both the side and bottom. I would think applying the glue blocks to either the bottom or the side would be enough. Keeping the blocks short, perhaps no longer than 2”, could avoid problems with wood movement. Locating the glue blocks exactly where they need to be would become difficult. But a dado into which the bottom is glued could be done first. Once the glue has cured, the glue blocks could be added under the bottom.

I really like the idea of the sliding dovetail joint. But as you say, once glue is applied, trying to position the bottom could become a nightmare. But if the sliding dovetail joint were was a half sliding dovetailed joint, perhaps it could be fine-tuned and go together easier, provide a little better glue purchase than the dado joint and offer some mechanical strength. The half sliding dovetailed joint would be similar to the dado joint but the bottom face of the tongue and dado would be dovetailed.

Since I have not done this joint, I can only say how I would likely approach the joint. Forming this joint on the bottom would be easy enough by forming the dovetail along one face of the bottom. The formation of the mating dovetailed dado in the side would be more difficult. The dovetail router bit could route a dovetail slot in the side. Then a precisely positioned straight bit could remove on side of the dovetail. I suspect a table saw could also be used on the side.

If you have never formed this type of joint before, using some scrape to perfect the making of the joint would be a good idea.

Here is an image of the half sliding dovetail joint of which I am speaking…

https://www.google.com/search?q=half+sliding+dovetail&tbm=isch&imgil=fUFtTiM8_jDMAM%253A%253Bdlob83ysLaY9oM%253Bhttp%25253A%25252F%25252Fwww.craftsmanspace.com%25252Fknowledge%25252Fdovetail-woodworking-joints.html&source=iu&pf=m&fir=fUFtTiM8_jDMAM%253A%252Cdlob83ysLaY9oM%252C_&usg=__5fkFbEAsXmVBqfkQF5PO3Ru2TTY%3D&biw=1366&bih=620&ved=0ahUKEwjA5KTg5L_SAhVmrlQKHRX6DzEQyjcIMA&ei=JT28WIC-Lebc0gKV9L-IAw#imgrc=fUFtTiM8_jDMAM:

Here is a method for hand cutting the half sliding dovetail joint. I am sure it can be done with power tools…
https://www.youtube.com/watch?v=6mVcDonB52I

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TheFridge

8287 posts in 1320 days


#4 posted 03-05-2017 05:47 PM

The back will keep that side locked in.

Maybe a 2-3” sliding dovetail at the front that transitions into a dado the rest of the way. That would lock it in.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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jmos

796 posts in 2203 days


#5 posted 03-05-2017 06:42 PM

Thanks all for the replies.

Yes, solid wood; red oak.

JBrow: You correctly interpreted my weak description of the glue blocks – I would glue the dado, then, after it dried, glue in some long grain reinforcing blocks under the dado, gluing the side to bottom. The half-dovetail is an interesting idea; haven’t seen that one before.

Fridge: I’m liking the idea of only doing a couple of inches of sliding dovetail in the front. I would need to do a stopped dado/dovetail and notch the bottom, since I don’t have a face frame to cover the dovetail. But, that positive connection in the front, along with the screwed in back (probably going to go frame with shiplapped panels) would lock in the bottom pretty well. Thanks.

I wouldn’t have to be terribly precise in the transition from dovetail way to dado either. Nice.

Pocket screws are a good idea too, but I don’t have a pocket screw jig. No Domino either.

-- John

View higtron's profile

higtron

232 posts in 2511 days


#6 posted 03-05-2017 06:55 PM

tongue and dado joint

you can make this with table saw. http://www.woodsmithtips.com/2010/08/26/tongue-and-dado-joint/

-- A friend will help you move, a good friend will help you move a body

View Loren's profile (online now)

Loren

9606 posts in 3481 days


#7 posted 03-05-2017 06:59 PM

I think glued-in in corner blocks underneath
is what I’ve seen in furniture pieces with this
general design feature, extending the “shelf”
of the dado out.

Another way to do it that won’t pull apart
is a sliding half-dovetail. A sliding full
dovetail will work as well but the half
dovetail can be adjusted to fit with a
standard bench plane.

Personally, I’d probably just dowel a butt
joint. It looks clean, modern and pocket
screws can be added to penetrate pretty
far into the other piece.

Multiple mortise and tenon is a variant.
Both joints can be cut with common power
tools and from the edge the work is hidden
so a clean appearance is easier to make than
with a skill-tester like a sliding dovetail. Still,
the m&t joints are a little challenging to
make if you’re looking for a challenge.

View jmos's profile

jmos

796 posts in 2203 days


#8 posted 03-06-2017 12:43 AM

Thanks Loren. I was trying to think why the half-dovetail would be better than the full dovetail – excellent explanation. I did think about the mortise and tenon option; through mortises would have been my preference, but don’t really match the design of the piece, and it’s not for me.

I think I’ll try either the half-dovetail or the sliding dovetail in the front of the piece.

Thanks for the ideas Gentlemen!

-- John

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