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Half Lap Joint with 3/4" Plywood- Is this a good idea?

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Forum topic by Sailor posted 06-04-2010 02:19 PM 8631 views 0 times favorited 16 replies Add to Favorites Watch
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Sailor

543 posts in 2732 days


06-04-2010 02:19 PM

I am in a situation where I would like to have a strong 90 degree joint with 3/4” plywood on some saw horse legs. I was trying to think which joint would be best for my situation and I figured a half lap would probably be best.

The legs on the following sawhorse project are cut out of one piece of plywood. This creates a little extra waste that I would like to try and avoid. If I make the legs with 3 pieces then I would be saving myself a little extra plywood. The width of the parts are all 6” although I wouldn’t mide making them 4 1/2”. Would the 6” joint be strong enough? How about the 4 1/2” joint?

Click for details

I want a half lap joint because I do not want the leg assembly to be thicker than one sheet of plywood and the half lap joint would allow for this to be possible. If you can recommend another joint that would also be worth considering it would be greatly appreciated.

Thank you all for you great advice!

-- Dothan, Alabama Check out my woodworking blog! http://woodworkingtrip.blogspot.com/ Also my Youtube Channel's Facebook page https://www.facebook.com/SailingAndSuch


16 replies so far

View barryvabeach's profile

barryvabeach

159 posts in 2511 days


#1 posted 06-04-2010 02:34 PM

I think the half lap should be fine if the plywood is quality stuff. The Glue joint shouldn’t fail, and I am thinking about it right, the forces will be primarily down. What could be a problem is if you had forces perpendicular to the plywood face – you would have only 3/8 of wood, which may not be enough . I would think 4 1/2 would be plenty strong enough.

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CharlieM1958

16244 posts in 3686 days


#2 posted 06-04-2010 02:39 PM

I could be totally wrong, but, intuitively, this sounds like a bad idea to me.

-- Charlie M. "Woodworking - patience = firewood"

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Sailor

543 posts in 2732 days


#3 posted 06-04-2010 02:52 PM

barry,
I was thinking about the same thing. If you look closely at this photo you can see where each leg is braced with a 2” runner of plywood that is pocket hole screwed to the inside. I think this should provide adequate support on the perpendicular face of the plywood.

Legs

Charlie,
You could very well be correct. I have my doubts, which is what brought me here.

-- Dothan, Alabama Check out my woodworking blog! http://woodworkingtrip.blogspot.com/ Also my Youtube Channel's Facebook page https://www.facebook.com/SailingAndSuch

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richgreer

4541 posts in 2542 days


#4 posted 06-04-2010 02:53 PM

With a half lap joint there is a place where the strength of the board is only equal to the strength of 1/2 of the board. If you are using 3/4” plywood, there will be a spot, in a critical location, where you will only have the strength of 3/8” plywood.

With the sawhorse design I see in the picture, why not cut the leg assembly from a single piece of plywood?

If you must have a joint I would put in a row of dowels, made of hardwood and going at least 2” into each piece.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

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richgreer

4541 posts in 2542 days


#5 posted 06-04-2010 02:57 PM

Sailor – When I wrote my post a minute ago, I did not see your post with the picture of the runner. That runner will add a lot of strength.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

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Sailor

543 posts in 2732 days


#6 posted 06-04-2010 03:55 PM

rich- Good point, not sure if the runner will really help that much.

I think I will be able to reduce the amount of plywood used easily. The sawhorse is 4’ long in the photo above. If I make one that is about 6” shorter then I will be able to get more use out of the plywood because the actual leg part of legs would be cut out of the same piece so to speak. Come to think of it, 4’ is probably a pretty long sawhorse. Also, making them shorter wouldn’t really effect them much since if a longer horse was needed you simply put in a longer 2×4 on the top.

I think the problem may be solved. Making a half lap joint for all of the legs would be time consuming for just a couple of sawhorses. Their would be 8 joints total to make up, kind of silly.

Thank you guys.

-- Dothan, Alabama Check out my woodworking blog! http://woodworkingtrip.blogspot.com/ Also my Youtube Channel's Facebook page https://www.facebook.com/SailingAndSuch

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richgreer

4541 posts in 2542 days


#7 posted 06-04-2010 06:11 PM

Let me offer you what I think is a good idea for saw horses – - Cut a couple of notches in the top of the saw horses that are 1.5” wide and 1.5” deep. Cut these 2 notches about 24” apart and centered.

Why – This creates the perfect “cradle” for a couple of 2×4s to be set on edge. 2 saw horses and 2 2×4s make a great support for sheet goods. With the 2×4s on edge they are stronger and you don’t have to worry about them moving around. If you cut a little into the 2×4s with your circular saw – no big deal. It’s much better than cutting up the tops of your saw horses.

When I look at your design, this seems very doable.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View a1Jim's profile

a1Jim

115206 posts in 3044 days


#8 posted 06-04-2010 06:54 PM

The thing that worries me about the design is that is the eye hook string that keeps it from spreading
apart it doesnt look like it would hold much weight.

-- http://artisticwoodstudio.com Custom furniture

View Sailor's profile

Sailor

543 posts in 2732 days


#9 posted 06-04-2010 08:11 PM

rich-
Yes! I have seen that done a few times on the net and that is one of the reason’s I opted for a removable 2×4 on the top. So that when that one gets worn out it can be replaced and you can also put 2×4’s in it that are designed for specific purposes, one being to cut sheet good, I know I need that.

Jim-
Your concern is also my concern. I could have bought a few and through bolted them, but the screw in eye hooks are easy to put in and I figured I would give them a try. I have stood on the horse and hopped up and down a bit and they have held so far. Any suggestions on how to avoid that problem easily?

I originally planned to just drill holes in each side and have knots at the ends of the rope. I found out that the knots don’t hold well enough to keep the distance between the legs consistant over time.

-- Dothan, Alabama Check out my woodworking blog! http://woodworkingtrip.blogspot.com/ Also my Youtube Channel's Facebook page https://www.facebook.com/SailingAndSuch

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KnotWright

252 posts in 2955 days


#10 posted 06-04-2010 08:25 PM

How about a short piece of chain and a couple of carrige bolts, that way the distance when spread should always be the same. Size the carrige bolts to the chain and use a washer on the inside of the horse to hold the chain in place.

-- James

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Radu

324 posts in 2511 days


#11 posted 06-04-2010 08:41 PM

You did a great job on those saw horses. It’s a matter of personal preference but overlapping the joints might be a quicker solution. That rope is not going to break. I used a short piece of chain on mine, as James mentioned. It does not have to be heavy duty. Just open up a link (if you can) and get it into the eyebolt or use bitch links (or get the ends from a tie-strap).

View a1Jim's profile

a1Jim

115206 posts in 3044 days


#12 posted 06-04-2010 08:49 PM

Chain is better than string . I’ve seen were some guys use a fold in section that is hinged in the middle and on both sides, it makes it a lot more involved to build. If your not going to put a whole unit of 4×4 s on it the chain should do the job.

-- http://artisticwoodstudio.com Custom furniture

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richgreer

4541 posts in 2542 days


#13 posted 06-04-2010 10:51 PM

The eyebolts would be subjected to less stress if they were lower on the legs,

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View TopamaxSurvivor's profile

TopamaxSurvivor

17677 posts in 3143 days


#14 posted 06-05-2010 07:38 AM

Why don’t you put a splice on the surface inside with the legs brace? It would be stronger than half lap and out of the way inside.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View Sailor's profile

Sailor

543 posts in 2732 days


#15 posted 06-05-2010 03:38 PM

Great advice everyone, thank you!

I like the chain idea very much Knot, I will probable have to try that. Carrige bolts would work great, but I would like to keep the hardware hidden if at all possible.

What if I drilled holes in each brace on the back of the legs and tied the string there? There should be no question that it should hold and I could tie the correct knot like I have been.

Jim, I have seen that to, your right it makes the build a little more complicated. I am trying to keep simplicity down a little bit so I think the chain/rope will be the easiest at the moment.

I will have to reconsider the joint on the legs again. I go back to work with my cousin doing construction Monday and I told him last night I was building him a set of folding horses. So I will have to get in the shop tomorrow and get two more put out.

-- Dothan, Alabama Check out my woodworking blog! http://woodworkingtrip.blogspot.com/ Also my Youtube Channel's Facebook page https://www.facebook.com/SailingAndSuch

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