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I'm a relative neophyte to woodworking and just sketched the first project I'm looking at approaching

My dual questions are:

  1. are the 1 1/1 inch legs enough to support the structure as drawn? How does a woodworker calculate load bearing for legs, especially
  2. I have read a bit about wood movement due to humidity in an all, whole wood project. Do I need to account for this in my project or is that more restricted to items like tables/will my the components of my piece move in unison. If so, how would I adjust for that so I don't get a split?

Thanks!

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  • I recommend that you include an image of the the project in the question. This way if the link you have dies the picture can live on! – Matt Aug 31 '16 at 22:55
  • Done and done. If someone can illustrate the method described below with the dado for the back panel--so I can visualize it--I'd be eternally grateful. I guess the other option is using a ply backing. – Grant Malcolm Sep 1 '16 at 1:27
  • Cut a dado offset 3/8" +/- from the back face to receive the back panel approx half the depth of the panel thicknesses. Size the back panel to fit inside the dado opening with a gap in dado to panel edges to allow for the expansion of the panel. – Ashlar Sep 3 '16 at 3:12
  • @Ashlar Like this: imgur.com/a/cxztA ? I think I might have been confused by rabbets vs. dados. I assumed cutting them from the other panels to receive the back panel, or maybe I'm cutting from the back panel. You probably want to strangle me and I would certainly understand that. – Grant Malcolm Sep 3 '16 at 5:26
  • Your image is a rabbet and the back panel must be fully glued in the rabbet to stay in place and this must be avoided to prevent attaching side to side with end to end grain. By moving the rabbet recess towards the front of the piece another 3/8" +/- making a dado instead of a rabbet you can have the back panel float free so that opposing wood grains are able to move independently. The back panel length/width should be smaller than the opening so that the panel has room to expand in the recess. Insert the back panel when gluing the rest of the carcass. – Ashlar Sep 3 '16 at 14:49
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are the 1 1/1 inch legs enough to support the structure as drawn?

Yes the should be, but species matters. If the legs are cheap pine for example they won't be nearly as strong as if they were any reasonable pieces of white oak. Just to note that you will need to cut the tenons to fit the mortises very carefully as any play will result in weaker joints*.

You might find this previous Answer worth a look, What is the strongest join for splayed legs?

How does a woodworker calculate load bearing for legs, especially

That's a good question and the answer is they usually don't. This is generally a seat-of-the-pants thing for nearly all furniture work. It's widely acknowledged that a lot of furniture is a little to a lot over-engineered, using material that's thicker than needed and with joints stronger than than strictly necessary. But together they give better assurance that something will hold up well to use and abuse over a long lifespan.

I have read a bit about wood movement due to humidity in an all, whole wood project. Do I need to account for this in my project or is that more restricted to items like tables/will the components of my piece move in unison.

Yes you need to account for movement. Only on quite small projects can you get away with ignoring it completely (if using solid wood).

If so, how would I adjust for that so I don't get a split?

There's nothing in the overall design that needs to be adjusted, but there are a few areas to pay attention to.

Vertical divider
In case it's not obvious you need to orient the grain of this vertically to match the case, so that expansion and contraction occurs back to front and not up and down.

If you oriented the grain on this horizontally at maximum expansion (during the wettest part of the year) the panel would exert enough force that it would probably bow the top and bottom, and if the back panel were fixed in place it could split it or tear it free from the case.

Back panel
I would recommend you make this from plywood or another board material.

If you make it from solid wood the grain must run horizontally and it would be highly advisable to make it up from quarter-sawn wood to keep dimensional changes within reasonable limits.

Leg assembly
This is a little over-engineered by the way. Two leg frames that aren't joined at all would be fine here, but go ahead with it as designed if you want the practice in the joinery. If you would like to simplify it slightly the cross-members can be joined with half laps/halving joints instead of with M&T joints which aren't necessary for strength here.

How you join this to the case is critical. Since the leg assembly is much like the legs and apron on a table, and the grain on the case is as it would be on a tabletop you should basically attach the two together in the same sort of way. See this previous Answer for two good simple methods.


*If this does occur you don't necessarily need to re-cut the components however. Instead you could glue using epoxy, which is a very good gap-filling adhesive (most common wood glues are not). Fill the epoxy with fine sanding dust to a thicker consistency if necessary, aim for a viscosity anywhere from around thick honey to that of mayonnaise. Use a slow-setting epoxy to give yourself plenty of time to work.

If you want even further security in addition you could peg the tenons through the face of the joint. Glue the tenons in first, wait for the adhesive to set then drill and peg.

  • Last question: Excessive to use finger/box joints on the carcass? I've noticed for this kind of project most people seem to use double rabbets because there doesn't seem to be much lateral force on a piece like this--mostly vertical. – Grant Malcolm Sep 1 '16 at 16:03
  • @GrantMalcolm. Yes they're more than you need for a case like this which isn't expecting dynamic forces. You could by all means go ahead with them anyway for looks, it's an attractive joint. You might go with a single or double rabbet at the top, but for the bottom joint especially a dowelled butt joint would be enough, hiding the end grain if you'd like. These used to be very common for case construction before biscuits and Dominoes. For this type of structure they're all basically the same principle, dowels are cheaper and easier (although a jig is handy for accurate 90° drilling). – Graphus Sep 2 '16 at 7:12
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  1. It is unclear if you intend to use 1" or 1 1/2" square legs. In either case the legs can certainly carry the vertical load. However, the legs are shown on an angle from vertical and this will place a greater deal of strain on the joint between the horizontal and vertical elements of leg assembly. To resist this I would definitely use a deeper profile to maximize the gluing surface between the two elements. I would use a mortise and tenon connection, and if possible a double mortise joint to provide maximum strength against the pressure to have the leg splay out in the direction of the angle. Better yet would be to have a vertical leg to avoid the lateral force.

  2. If the case is built using plywood or MDF the wood movement will be minimal and can be ignored. On the other hand, if the panels are made from solid wood panel glue-ups then there will be significant differential movement across the width of the panels relative to the length which will not change at all. Any place where the woods grains are connected perpendicular to each other should not be fully glued up, but must accommodate the differential movement. This question, amoungst many others, dscusses the basics of planning for wood movement.

  • I had planned on 1 1/2 square legs and the angle was 75 deg. I figured to reduce cost if I could get away with 1 inch legs rather than buy a couple 6/4 board for smaller pieces that would be more efficient. I had planned mortise and tenon joints and a number of crossbars (page 3 of the mock up). I like the aesthetic of angled legs but fully vertical legs would be much easier to make with my skill level. I read the movement piece--how would I would I implement not fully gluing up the case joints? I had planned on finger joints but would another type of joint work better? Thanks – Grant Malcolm Aug 31 '16 at 17:28
  • You can glue all pieces if made with plywood or if solid wood panels, any that have grain running in the same direction. Thus top, side, bottom , side could be glued together but back panel should be installed in a dado and only glued at the center of the panel edges so that the ends can expand and contract in the dado slots. – Ashlar Aug 31 '16 at 19:04
  • Ashlar: If I have finger/box joints for the carcass, how would I install with a dado? Could you elaborate? I've opted to use veneer core 2 sided ply as recommended, but want to inset it, if possible. I've been modeling this and having trouble doing something that's simple... Thanks. – Grant Malcolm Sep 3 '16 at 1:12
  • I am referring to the back panel being placed in a dado cut into the top, bottom , and sides. IF you simply glue the back panel as suggested in your sketch then the back panel will expand vertically (across the width of the grain), but the side panels will not (the grain runs vertical), causing stress and possible failure. Also, the center divider must also be able to expand similar to the side/end panels. – Ashlar Sep 3 '16 at 3:06

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