Building a display case to hold 250-300 pounds

[Dan Mages]

I want to build a display cabinet for my old cash register. It was used by the family hardware store for about 65 years. The big problem with the register is that it is purely mechanical and weighs 250-300 pounds of pure steel and a bit of wood. I want to build the cabinet with a drawer for the keys and a couple of open shelves on the bottom. The display cabinet will be 36 inches tall, 30 inches wide, and 30 inches deep. The register has a mahogany finsh, so I plan to use the same for the cabinet. Will a 3/4" plywood base on the sides and back be sufficient, or should it be beefed up? How would you design it?

Thanks!

Dan
http://www.sawmillcreek.org/attachment.php?attachmentid=7276

[Hank Walczak]

Dan - With that kind of weight, consider torsion boxes for the top and sides. They will be somewhat thicker but a whole lot stronger. my .02 anyways.

[Jamie Buxton]

Dan --
I'm not clear on your proposed design. If you're proposing 3/4" ply for the sides, they would be structurally sound. 3/4" ply standing on end can support enormous loads. The piece of the structure which might get overloaded is the top -- whatever you have running horizontally just below the register. I'm presuming that the front of the cabinet is open and not supporting the top. If the register is significantly narrower than the 30" of the top, its weight will first bear on the top, before the top carries the load over to the sides. And if the top is only 3/4" ply, too, it might well sag under a 300 pound load. The way to make the top stiffer is to make it thicker. You can make it thicker by putting a structural member across the front of the cabinet, supporting the top. Or you can simply use thicker material -- two layers of 3/4 ply, or 6/4 lumber, or the like.

Maybe you can make a sketch of your proposed design? Don't worry about fancy sketchup stuff. Use paper and pencil, and shoot it with a digital camera to get a jpeg to post.

[Ted Shrader]

Dan -

Structurally the ¾" ply will be plenty strong. If the front of the top has a 2 wide face frame piece, it will keep the top from sagging. My main concern would be racking. Think it will need at least a ½" plywood back set in a rabbet or even a dado in about an inch from the back.

Regards,
Ted

[Dan Mages]

Here is a quick sktech up of the display cabinet. The sides will be 3/4" plywood. I will use a face frame on it. I am thinking of using a 1.5" - 2" for the face frame width. I hope this clarifies things.

Dan

[Jim Becker]

The strength of the 3/4" sheet goods is not an issue, although you'll want to be sure that it's adequately braced to prevent racking as well as sagging on the top surface under the weight. The question you need to ask, Dan, is if the style of the stand is complimentary to the antique register? Would a face frame design look more "comfortable" with that kind of mass on top of the piece? Etc. To my eye, a 3/4" edge isn't broad enough to visually support the register.

[Dan Mages]

Okay Jim. I am open to suggestions. What would you reccomend? How would you design this case?

Dan

[Jim Becker]

The structure is fine. I'm just suggesting you consider using a face frame to make the piece "look" proportionally more supportive of the heavy object it's intended to hold up in the air. If I get some time later, I'll do a quick sketch...unfortunately, I have a customer meeting this afternoon to prepare for and a few conference calls between then and now.

[Dan Mages]

The structure is fine. I'm just suggesting you consider using a face frame to make the piece "look" proportionally more supportive of the heavy object it's intended to hold up in the air. If I get some time later, I'll do a quick sketch...unfortunately, I have a customer meeting this afternoon to prepare for and a few conference calls between then and now.
Hey. No prob. I am planning on using a face frame in the 1.5" to 2" range (see post 5).

Dan

[Jim Becker]

Hey. No prob. I am planning on using a face frame in the 1.5" to 2" range (see post 5).
My bad...I missed that in the text. Sorry!

[John Stevens]

Dan, there are formulas and data in Ken Horner's book "Woodworkers Essential Facts, Formulas and Short Cuts" that will help you calculate how much strength your cabinet needs to keep from sagging, and how much wood is required, of which types, and in what dimensions, to provide that strength. Another chapter compares relative strengths of joints, in case that's an issue for this project. Outstanding book, covers lots more than this subject area--

http://www.amazon.com/exec/obidos/tg/detail/-/1892836157/qid=1102949295/sr=1-1/ref=sr_1_1/102-0303425-6507352?v=glance&s=books

[Mark Singer]

Dan,

Here is a try...it has the character of the register...M&T joinery

[Dan Mages]

Mark. That looks wonderful! However, I am concerned about the open legged bottom. With 250-300 pounds of weight, I am affraid that it may rack or buckle. Thanks for taking the time to draw out your idea. I will incorporate some of it into the final design.

Dan

[Mark Singer]

Dan.

You can run the plywood skirts lower and end in another molding. I think the key structural thing is to make the base large enough that qverturning is not an issue, since it is an inverted pendulum with the weight at the top. It is in concert with the design details of the register rather than just a plywood cube. If you make the legs fron 8/4 it should be ok.