weight carrying capacity of plywood cabinets

[Jim Barstow]

I built a bunch of 24" wide by 24" deep cabinets for my garage out of high quality 3/4" plywood. They are screwed together. The tops are also 3/4" ply. The bench on top of the cabinets is 2 layers of 3/4" ply.

I'm looking for a new metal lathe that could weigh 400 lbs. The weight would be spread over at least 2 cabinets. The load on the ply would be compressive.

My gut feeling is that the cabinets can handle this load but I want to make sure I'm not being overly optimistic. A failure would be very unpleasant.

[James Pallas]

Jim, think about the floor in an average house. Fully loaded fridge sitting on 3/4" ply floor supported on 16" centers.
i would think you may get some odd vibrations from it that you won't like but doubtful it would crash thru to the floor.
Jim

[Mike Cutler]

Compressive load only? you could park a truck on top of them.Well maybe not quite a truck, but a large fish tank easily, and lot's of those are made out out of 3/4" plywood. So maybe a little truck.:D
Properly supported, 3/4" plywood in the vertical plane is very strong. The design of your cabinets, being basically cubes, is very strong by itself. 400lbs. is nothing.
i wouldn't worry as long as it can't rock side to side and front to back. The lathe spinning with an unbalanced load would be the issue here.I don't imagine that will be much of an issue though with a metal lathe.
You might want to ensure that the bottoms are tied together mechanically. The load will be transferred, and shared by the bottom. The top is in compression. The bottom is in tension. Again though, you only have 400lbs static load. I would tie the bottoms only because your load will become dynamic with the lathe in operation.

[Lee Schierer]

If two boxes are joined together, then you could have up to four pieces of 3/4" ply in compression. If the double plywood top is is a continuous 48" long sheet you essentially have four supports carrying the load, depending on where the feet of the lathe are placed. Worst case you would have 200 pounds of down force on only two legs with a 3/4" x 24" area which equates to ~11 pounds per square inch which is a light load on plywood at the ends of the box.

[Bill Dufour]

I did frame and panel for my mill/drill table to span a 40"tool cabinet. The top is welded 2x4 laid flat but the front/back members are right under the mills feet. It is around 600 pounds + at least 50 pounds of tools on the top. It feels fine when I added my weight and bounced a bit. I had the machine just slack from the hoist in case anything let go.
But the front back members land on top of vertical 2x6's in back and 2x10 support the front cross members so all loads go vertical through 2x lumber. The plywood is only 1/4" but glued in to reduce flex.
If you are worried about the load I would add 2x4 or angle iron front to back under the feet.
Bill D

[johnny means]

Years ago I built my first two work benches from plywood. Each was built with four 4" plywood Ls for legs. It wasn's unusual for each of these benches to have well over a thousand pounds of MDF parts loaded on to them. I would imagine that your bench could literally support a small truck, if placed gingerly.

[Robert Engel]

You should be fine. Think about a granite countertop......

[Tom Bender]

Plenty of strength there but a metal lathe is a messy beast. Add a sheet of steel on top of the plywood. Get a sheetmetal shop to make it for you, with the edges bent down to cover. Sure it's going to cost a few bucks but you will never regret it.

[Peter Christensen]

You would actually want that pan to have the edges bent up so the swarf (metal chips) cutting oil or coolant stay on top to clean up rather than drip down the sides. Most lathes come with a drip pan and splash guard. If yours is "vintage" and doesn't you'll want to incorporate that into the metal pan.

Something else to keep in mind is that you may need to be able to shim the corners of the lathe base because if there is any twist in the bench top it will twist the bed of the lathe causing it to cut a taper or put a drill bit off centre to the chuck.

[Tom Bender]

Thanks Peter, both good points.