Will sag be an issue?

[Alan Lightstone]

I'm building a 72" wide, 26" deep cabinet to sit next to my table saw and hold lots of jigs, equipment, blades, etc...

There will be 3 vertical dividers that will be connected between the top and bottom, and have multiple drawers held between them. I'm sure the drawer weight will be considerable with about 10 drawers.

Top, bottom, and vertical supports will be made of 3/4" Baltic Birch plywood.

I'm planning on having the table supported on four casters from below.

Now, obviously, 72" is a long span on the bottom. Will the vertical supports prevent the table from sagging in the middle? I'd like to use it as both an assembly table, and support for large sheets goods sliding across the table saw when being crosscut.

I'd prefer not to go to the work of making a torsion box below the bottom. I would consider angle iron attached to the bottom, if the consensus is that sag will be a real issue.

[Steve Eure]

On a span that long, I would suggest putting some casters in the middle also. I've done that on a 60" that I built. I actually put two sets in-between and it helped tremendously.

[Ron Selzer]

a strong back that is well fastened will help a lot.
I would mount the casters about 14-16 inches in from each side, definitely not as far out as they will mount

[Sam Murdoch]

a strong back that is well fastened will help a lot.
I would mount the casters about 14-16 inches in from each side, definitely not as far out as they will mount

^ I agree AND double thick the bottom. No sag that way - I'm pretty certain. Adding a center caster or 2 could create issues with balance or moving around if your floor is not flat. If you are using locking casters on the outside corners - the kind that lift up a bit to move - then I might consider simply adding a couple of post feet at the center which would sit on the floor when the casters are down but lift off the floor when the casters are engaged to be wheeled. My 2¢.

Sam

[andy bessette]

...I'd prefer not to go to the work of making a torsion box below the bottom. I would consider angle iron attached to the bottom, if the consensus is that sag will be a real issue.

You don't need a torsion box--just a substantial bottom crossmember.

[Alan Lightstone]

Adding a center caster or 2 could create issues with balance or moving around if your floor is not flat. [snip]...

Sam
I've had that happen before. And with the seams on the raised access floor tiles, I'd really like to avoid using center casters.

[Lee Schierer]

a strong back that is well fastened will help a lot.
I would mount the casters about 14-16 inches in from each side, definitely not as far out as they will mount


^ I agree AND double thick the bottom. No sag that way - I'm pretty certain. Adding a center caster or 2 could create issues with balance or moving around if your floor is not flat. If you are using locking casters on the outside corners - the kind that lift up a bit to move - then I might consider simply adding a couple of post feet at the center which would sit on the floor when the casters are down but lift off the floor when the casters are engaged to be wheeled. My 2¢.

Sam

I agree with both of these designs. I would avoid having 5 or more casters under the cabinet. I would through bolt the casters as that will be stronger than wood screws into the plywood bottom.

[Alan Lightstone]

Thanks, guys.

I eventually took the easy route and bought two 48" steel angle irons. I'm amazed how much easier these were to drill with my new Voyager DVR Drill Press compared to my old Sears one.

Unfortunately, even though I don't want to, I have to use wood screws to attach the casters. The design makes bolts a non-starter. Hopefully they'll hold up. I usually use bolts for these.

There will be a 3/4" Baltic Birch plywood back, securely fastened, so that should help, as has been pointed out.

[andy bessette]

...bought two 48" steel angle irons...

Angle iron is very poor as a beam.

[Dan Hahr]

Bad plan...you will have some issues for sure if your front isn't as stiff as the back. A very substantial face frame, with a very beefed up bottom framework should work, but otherwise, the flexing will probably cause severe issues with the drawers. Not a big deal if it were just regular cabinets on the floor, but rolling around on an uneven surface....flex city.

Dan

[Thomas McCurnin]

Many plywood suppliers carry 1 1/4 inch flooring plywood. It is ridiculously strong, made for spanning long areas without flexing. Usually found in high end homes under ceramic tile.

[Rick Potter]

As Thomas said, I have used 1 1/8" flooring underlayment (plywood, not OSB) for bottoms in the shop and shed. It is really solid.

I have also used center wheels on 6' shelving units, with no problems. My floors are pretty flat.

I actually set one of those 6' wide, chrome wire shelving units on a 3/4 ply base with 6 wheels, and it has worked well. I have even rolled it loaded with paint supplies down a sidewalk. I drilled holes for the legs in 1x4's and screwed it to the plywood bottom so nothing would shift. I use inexpensive steel wheels from HD, 3" size I think. They won't go flat.

I also have one of those metal storage cabinets, about 6' high, 3' wide, that sits on a plywood base with the steel wheels.

[johnny means]

Angle iron is very poor as a beam.
But it's great as angle iron.

[andy bessette]

But it's great as angle iron.

Yes, but what he needs there is a beam to restrict deflection.

[Alan Lightstone]

But won’t the vertical dividers work to restrict deflection? They’ll be 1-1/4” plywood securely fastened to the top and bottom. Doesn’t that tie the top into the bottom somewhat like a torsion box?

I have several pieces of furniture that are 6’ long with drawers, built by furniture makers likely far crappier than we’re discussing, and they don’t seem to be sagging,

[Peter Kuhlman]

I built a 60” wide low cabinet for my audio video equipment. Loaded it is incredibly heavy. Started with just 4 outside corner legs securely fastened. Cabinet is 3/4” plywood edged with 2-1/2”oak on all faces with solid oak top and rigid back. Still sagged over time - took about a year to fully sag. Ended up adding 2 casters to middle for support and easier mobility and that cured it. Can not believe a cabinet even wider would not have serious issues.

[Sam Murdoch]

But won’t the vertical dividers work to restrict deflection? They’ll be 1-1/4” plywood securely fastened to the top and bottom. Doesn’t that tie the top into the bottom somewhat like a torsion box?

I have several pieces of furniture that are 6’ long with drawers, built by furniture makers likely far crappier than we’re discussing, and they don’t seem to be sagging,

Shop cabinet with 10 drawers filled that will "hold lots of jigs, equipment, blades, etc..." - likely much more weight than household furniture.
I think the answer to your question is, yes and maybe no. Worth a bit of extra precaution, which I think you've covered.

Sam

[Alan Lightstone]

Would a hardwood apron prevent the sagging? I could do about 1" x 3" for the length of the front. Is it necessary for the back also, considering the 3/4" plywood back.

[Alan Lightstone]

Angle iron is very poor as a beam.

Hmmm. Why is that? I certainly can't get it to deflect when I tried to. Seemed rock solid. I can see that angle iron is not an I-beam, but why is it poor at preventing deflection - acting like a beam?

I just tried plugging in some values into an online angle iron deflection calculator. Now I'm just guessing at the moment of inertia values for that size angle iron (but tried a variety in the ranges they were using) and it tells me that the angle iron will only deflect 0.0032 inches over that span - in other words, bubkis.

Now I really don't believe that is true, so what am I missing here?

[andy bessette]

...an online angle iron deflection calculator...tells me that the angle iron will only deflect 0.0032 inches over that span...

Come on! You certainly don't believe that!


But won’t the vertical dividers work to restrict deflection?..,

They only transfer the load into the bottom, which does not have the thickness to act as much of a beam.

In beam loading, it is the extreme fibers (the material at the very top and bottom edges, like the flanges of an I-beam) that does most of the work. An angle has nearly 1/2 its material at only the top or bottom, and the tiniest fraction of its material at its opposite extreme.

A torsion box has very deep vertical, longitudinal members.

[Alan Lightstone]

Come on! You certainly don't believe that!



No I don't, but it is interesting that I think we're all thinking of a sag of the order of magnitude of 1/4" - 1/2" (WAG), but that deflection calculator produced a number that was 2 orders of magnitude less.

[andy bessette]

...the angle iron will only deflect 0.0032 inches over that span...

Preposterous.

[Gordon Stump]

Angle iron is/was a very good solution.

[Gary Ragatz]

Might have missed it, but I didn't see where you mentioned the dimensions of the angle iron (other than 48" long). I don't understand how anyone here can estimate the deflection without that information.

[Alan Lightstone]

Might have missed it, but I didn't see where you mentioned the dimensions of the angle iron (other than 48" long). I don't understand how anyone here can estimate the deflection without that information.

I have to double check, but I think it was 1-1/2" x 1-1/2" x 1/8" thick. And 48" Long

[Ellen Benkin]

Google sagulator and enter the specs for the answer.

[Jerry Lowetz]

Yes, if you're not familiar with it, The Sagulator is a great tool for these kinds of things.

https://www.woodbin.com/calcs/sagulator/

also, square tube had a lot of dimensional rigidity against sag and is straight whereas angle iron tends to curve during the cooling process.

[Tom Bender]

The back will be fine. You can stiffen the front in several ways. If you raise the bottom a few inches you can put a substantial member under it. A 4x6 laid flat should do. Or something stronger, like Oak or Ash etc, maybe 3 x 3. If you want to use the angle iron it can work. To make it into a beam you will need to add an ordinary 2 x 2 under the bottom at the front and reinforce it with a piece of angle iron on the bottom and back of the 2 x 2. Wood screws every 6" or so thru both legs of the angle iron into the 2 x 2. Also wood screws thru the plywood bottom into the 2 x 2 every 6". This will be quite stiff. If you raise the bottom another 2" and use a 2 x 4 it will be much stiffer still.

[Larry Edgerton]

My most successful bench is built on a 2x2 1/4" wall steel framework, no sag and I have put thousands of pounds on it. Different animal as it is 7'6" x 4' and has a welded steel stiffback with drawers on both sides, but my point is, "When in doubt, build it stout!" I count on this bench for door assembly.

[Jeff Mackay]

I recently built a mobile assembly "cabinet" that's actually a set of 8 17x16x26 cabinets screwed together with a homemade MFT top. It currently houses 20+ systainers filled with tools:

410557 410558410559

I built each cabinet separately out of 3/4" ply and screwed them together on top of another sheet of 3/4" ply. I used 6 sets of casters--one at each corner, and one on each side in the center. I've been using it now for about 6 or 8 months, and haven't had any noticeable sag.

[andrew whicker]

That angle iron would do a lot if you install it 'upside down', that is with the vertical web downward. 1.5" x 1/8" of metal over 5 ft is a hell of thing to bend in the vertical position.

(I do metal working as well as woodworking. Real world experience tells me this isn't a bad solution. Would still have a beefy bottom (3/4" ply))


Maybe do 4 pieces.

[Alan Lightstone]

That angle iron would do a lot if you install it 'upside down', that is with the vertical web downward. 1.5" x 1/8" of metal over 5 ft is a hell of thing to bend in the vertical position.

(I do metal working as well as woodworking. Real world experience tells me this isn't a bad solution. Would still have a beefy bottom (3/4" ply))



Maybe do 4 pieces.

I did it that way, but 2 pieces. Thinking of putting roller ball on 4x4 wood piece at bottom front. That should prevent any sag, but I’ll have to screw it in from top (not a big deal). Also, shouldn’t prevent easy movement like an extra caster wood.

[andy bessette]

...1.5" x 1/8" of metal over 5 ft is a hell of thing to bend in the vertical position...

Actually it isn't, because the extreme fibers are only 1/8" wide.

[andrew whicker]

I made a few videos:

#1: https://drive.google.com/open?id=1-7el9KHl1MAaSjRSgx5UYGNLp3TKBeh- (angle iron is 1" x 1/8")

#2: https://drive.google.com/open?id=1-8zHfVFNUqZkQJH3-b_TfJAtliWlPAbR

Hope they work, I made the link share-able.

[Mike Kees]

Question, do they not sell 4x8 sheets of one inch thick Fir plywood in the States ?

[Alan Lightstone]

Question, do they not sell 4x8 sheets of one inch thick Fir plywood in the States ?
I’ve never seen any. FWIW. The 3/4” Baltic Birch plywood sheets were heavy enough. Can’t imagine what 1” sheets would weigh.

[Alan Lightstone]

Seriously thinking of adding a 4x4 small post with one of these wheels mounted to the bottom in the front center of the cabinet. This has got to eliminate sagging, even without one of these in the back. Right?

410640

[Bob Grier]

I built a counter top spanning over the top of washer and dryer. Span is 5' 2" and it is supported on both sides and back. I used 1 1/2" x 1 1/2" x 1/4" angle iron to span the front that is bolted to the front edge of 2 sheets of 3/4" MDF glued together. The front face of the angle iron has a piece of wood attached with screws. I used laminate on top and front face of the approximately 2" finished thickness of the counter. It deflects when I put my 190 lbs on the front edge. Not much but more than I would want on a cabinet.

I also built two 5' cabinets and bolted them together. They are permanently mounted and sitting on a ledger along the back and screwed to the wall studs. I supported the front with adjustable legs at the corners of each cabinet and in the center front of each cabinet. My cabinets did deflect in the center and I used the adjustable legs to keep everything level. If you use adjustable legs like I did for the center, be careful to screw them up so they do not hit anything while moving and make sure cabinet is locked in place before extending them. They are not made for lateral force.

I attached picture of each.

If the cabinet is only occasionally moved then the center supports could be a little short and then shimmed to level after moving. Could also use castors and then shim as necessary after moving.

410641410642

[lowell holmes]

Or you could make a tee by bolting two angles together and install it as a tee.

[Derek Meyer]

I’ve never seen any. FWIW. The 3/4” Baltic Birch plywood sheets were heavy enough. Can’t imagine what 1” sheets would weigh.

I have a 4x8 sheet of 1" MDF leftover from years ago when I built a subwoofer box for my old vehicle. I had to move it recently while organizing my garage and had forgotten how heavy that thing is. I can barely lift it off the ground. I think I ended up sliding it on the floor (smooth concrete, fortunately) and then standing it up into place.

I need to think of a project to use it, but then I'd have to move it again.....