Strong method for bottom of a box

Question

I want to make a box for a sewing machine. I can make dovetails or more likely simple box joints for the four vertical corners, but what's a good sturdy design for fitting a bottom so the weight of the contents doesn't mean it stays behind when you pick it up?

I'm considering routing a rebate around all four sides, so the bottom fits in. It'll probably just be glued in. It'll leave a very shallow "cavity" under the box but that's no problem. I'm not sure how far up from the bottom edges of the sides the rebate should be, though.

Answer 1

I think the most sturdy way to put a bottom in a box is to capture it with grooves on the sides. With a rabbet the glue joint is holding the bottom on, while in a groove the structure of the wood itself is holding the bottom in place.

For added strength I would make the bottom out of plywood (so it doesn't change size) and glue it into the grooves. This will prevent the bottom from sagging and then pulling out of the grooves. (Note that this is assuming the grain of the sides is running horizontally around the box, not vertically. This is how you would need to build for the dovetail or box joints you mentioned. If your grain runs vertically you cannot glue the bottom in place because you will have movement issues.)

Answer 2

As SaSSafraS1232 mentioned making a box for the purpose you stated is much like making a drawer. 1. Groove all sides, front and back of the box/drawer, then glue the sides to either the front or back piece than slip the bottom piece of plywood into the grooves, (no glue necessary) and glue on the other end. This captures the bottom completely. 2. If you wish to be able to someday replace the bottom, then make the back piece only as high as from the top of box to top of grooves so you can slide out the back for replacement. You can secure the bottom from slipping out with a couple of small nails.

General rules. A groove should be at least as deep as the bottom is thick and up to as deep as half the thickness of the sides. Allow 1/16" for expansion of the bottom panel during humidity increases in the environment. Make the groove as far from the bottom of the sides and ends as the bottom panel is thick (minimum). Preferably as far from the bottom as half the thickness of the sides and ends.

If you are using the box for the sewing contents, I would use a 1/4" panel, if it is for the sewing machine, I would use 3/8". For my shop drawers, expecting up to 100lbs., I use 1/2" plywood for bottom panels and 3/4" for sides and ends.

To learn proper box and drawer making, there are some very good tutorials on Youtube. If you want to build a box style, search "make a heavy duty box", if you want to build a usually more flexible and stronger drawers style, search "make heavy duty shop drawers".

Answer 3

Your making a box to hold a sewing machine? They are pretty heavy items. I don't think a rebate would be sufficient enough. If you have 3/4 material, then the groove is usually 3/8 deep - more then sufficient for most needs, but a sewing machine?

I would go with a dado for aesthetics or just glue and screw the bottom panel on. If you're using solid wood (not so much plywood), you will get a lot of long grain glue surface and with the addition of some mechanical fasteners, your box will certainly be sturdy.

Add some nice looking box corner protectors and you're laughing.

Answer 4

Wood glues (PVA) are fairly strong in tension (pulling apart) but weaker in shear, (sliding surfaces in opposite directions. Although a rabbet joint should be strong enough, I like belts and suspenders when possible. I would recommend creating a dado offset to use the wood shear strength in addition to the glue. Locate the dado approx. 1/2" from the edge so that the wood shear strength can be used in addition to the glue strength.