01-18-2022, 09:42 AM (This post was last modified: 01-18-2022, 09:42 AM by mound.)
Help settle a friendly debate I had with a buddy. Consider a very basic utility box to be screwed to a wall as a cabinet and assume all parts are 3/4" plywood and verified square, all butt joints glued and screwed together. aesthetics and visible edges aren't important.
Is either of these designs inherently better or stronger than the other for any practical reason?
On the left the back panel is inset inside the box and screwed from the outside in.
On the right the back panel is the full outer dimensions of the box and is screwed onto the back.
Obviously these are viewed from the backside and the opposite side would be open.
01-18-2022, 10:08 AM (This post was last modified: 01-18-2022, 10:18 AM by Handplanesandmore.)
Since look isnt a factor, the one on the right is marginally stronger and easier to make. Stronger for holding heavier load inside and easier to measure and fit the back panel.
For look, I'd choose the left design.
Am I on your side or against?
Edit: I missed the screwing to the wall. The cabinet on the left would hold on to the wall stronger/longer than the right one before coming apart!
So which one is stronger would depend on your criterion.
(01-18-2022, 10:08 AM)Handplanesandmore Wrote: Since look isnt a factor, the one on the right is marginally stronger and easier to make. Stronger for holding heavier load inside and easier to measure and fit the back panel.
For look, I'd choose the left design.
Am I on your side or against?
Edit: I missed the screwing to the wall. The cabinet on the left would hold on to the wall stronger/longer than the right one before coming apart!
Simon
My argument was for the one on the right specifically for ease of measuring and installing. The back panel dimension isn't dependent on the actual dimensions of the plywood (ie. a 32nd off) and could be cut ahead of time from a cutlist based on 3/4" plywood. Buddy said the first would guarantee a square box, but I think they're both the same as long as the panel is verified to be square.
I'm pretty sure the one on the left will be stronger screwed to the wall because the forces on the screws holding the back in place are mostly in shear. With the one on the right the forces on the screws are mostly in tension.
(01-18-2022, 11:40 AM)Handplanesandmore Wrote: Cut two identical back panels (dead square of course), and clamp the cabinet in place with the panels inside. Remove the front panel after screwing.
This technique is used by many cabinetmakers who make large mitered carcases. They use mdfs as the squaring cauls.
For practical purposes, there wouldn't be much difference under normal usage. If you want to go to the extremes of testing to see which is the strongest, then you might find different results between the two. For the boxes alone (unmounted to the wall), resistance to racking would be much better in the left one as the snug fitting inset back would provide maximum resistance regardless of the fasteners. Same answer for straight crushing resistance. Once the boxes are mounted to the wall, assuming that they are mounted securely, I think, again, that the box on the left would offer the most resistance to failure. Downward forces would cause both shear and pulling stresses on the inset back. Because the top is "hooked" onto the top edge of the back, it would provide a lot of shear resistance. Pulling forces, greater at the top and diminishing toward the bottom, would be resisted better by the inset back putting both the glue line and the screws into shear. Assuming that the glue itself does not fail, with the left box, the glue is adhering to all of the plies and will not tend to de-laminate just one. The screws may tend to split the plywood, so will probably not offer much resistance. The applied back of the right hand box would suffer more from pulling forces that would tend to pull out the screws and, at the glue line, probably cause mostly de-limination of the plywood as it will be adhering to just the top lamination. I favor the left box for the stated use.
(01-18-2022, 12:07 PM)Willyou Wrote: For practical purposes, there wouldn't be much difference under normal usage. If you want to go to the extremes of testing to see which is the strongest, then you might find different results between the two. For the boxes alone (unmounted to the wall), resistance to racking would be much better in the left one as the snug fitting inset back would provide maximum resistance regardless of the fasteners. Same answer for straight crushing resistance. Once the boxes are mounted to the wall, assuming that they are mounted securely, I think, again, that the box on the left would offer the most resistance to failure. Downward forces would cause both shear and pulling stresses on the inset back. Because the top is "hooked" onto the top edge of the back, it would provide a lot of shear resistance. Pulling forces, greater at the top and diminishing toward the bottom, would be resisted better by the inset back putting both the glue line and the screws into shear. Assuming that the glue itself does not fail, with the left box, the glue is adhering to all of the plies and will not tend to de-laminate just one. The screws may tend to split the plywood, so will probably not offer much resistance. The applied back of the right hand box would suffer more from pulling forces that would tend to pull out the screws and, at the glue line, probably cause mostly de-limination of the plywood as it will be adhering to just the top lamination. I favor the left box for the stated use.
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