Drawer box question

[BuckeyeDennis]

Hey guys, I'm getting ready to build a whole bunch of new drawers for a mobile base that my CNC router will live on. Thirteen, to be exact. And I'm trying to decide on appropriate materials for the drawer boxes.

After looking at how I use the drawers in my mobile tool chests, I decided to make almost all the new drawers quite shallow -- just two to three inches deep. My existing deeper tool drawers all had stuff piled two and three layers deep in them, which is not exactly conducive to finding things easily. So shallow drawers it is for the new base, with just one double-height (6" deep) drawer for larger items.

The drawers will all be mounted on full-extension slides, in a Rockler "Rock Steady" steel cabinet frame that's 56" wide by 28" deep. The frame has center braces front and rear, so each drawer will be roughly 24" wide by 28" deep.

I think I've narrowed my drawer-box material choices down to two design options.

• Heavy duty: 1/2" hardwood drawer frames with 12 mm Baltic Birch (or TigerPLY) bottoms.

• Light duty: 12 mm Baltic Birch drawer frames with 6 mm Baltic Birch (or TigerPLY) bottoms.

Material costs for the "heavy duty" option total about $200 more than for the "light duty" option, or roughly double. In either case, I plan to use rabbet and dado drawer box construction, and to glue in the bottoms. I'll also make make cosmetic drawer faces from some 3/4" red oak boards that I already have on hand.

For a 3" deep drawer, the available storage volume is only about one cubic foot, so the weight of the contents really can't be all that great. One cubic foot is about 40 lb. of solid hardwood, for example, or 62 pounds of water.

So the big question is, are 6 mm bottoms thick enough for drawers that size?

[GetterDone]

The drawers will all be mounted on full-extension slides, in a Rockler "Rock Steady" steel cabinet frame that's 56" wide by 28" deep. The frame has center braces front and rear, so each drawer will be roughly 24" wide by 28" deep.



So the big question is, are 6 mm bottoms thick enough for drawers that size?

I think you will not regret building those with the 12mm bottoms.

If they was not 24" wide you might get by with the 6mm bottoms.

Is an opening with no drawers in the center (And a possible shelf or two) an option?

[twistsol]

I've seen too many drawers come apart in my lifetime and now I build all of my drawers (over 200 to date) the same way regardless of size or intended use. 5/8" (15mm) poplar sides with 1/2" (12mm) bottoms. How I intend a drawer to be used when I build it doesn't necessarily stand the test of time.

[BuckeyeDennis]

Thanks guys, there appears to be a concensus developing.

Twistol, that's a whale of a lot of drawers! What kind of stuff did you build them for?

Dwane, I went with a steel frame assembly for this project because I thought it would save time vs. a fully custom cabinet build. But in hindsight, I don't think it did. It was a compromise on size, which made mounting the torsion-box (CNC base) mounting more complex, and it also limited my options on storage underneath. Not to mention that the zillion tiny machine screws and nuts needed to mount all the drawer brackets and slides were huge time suckers. Sixteen screws plus 16 nyloc nuts per drawer x 13 drawers = 416 fasteners! And that doesn't even count fastening the inner slides to the drawer boxes.

Did I mention that this base has a lot of drawers? The lower ones open to the front of the machine. The upper ones open to the rear, to stay clear of some CNC stuff on the upper front side.

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[edflorence]

Since the unit weight of water and hardwoods got mentioned, I thought I would toss this into the mix: unit weight of steel is approx 490 pcf. Assume that tools fill the drawer volume only as little as 25%, the weight on the bottom is going to be in the neighborhood of 120 plus pounds. I wondered if 12 mm might not even be thick enough. So, I asked chat GPT what it thought. Here is what it said:

"The bending strength of the plywood will be the primary factor in determining how much weight it can hold before it starts to bow. Plywood behaves similarly to a beam when weight is placed in the middle, with the weight being supported by the edges.

Using an approximation for bending failure (beam deflection), a 1/2-inch thick plywood sheet of that size could potentially hold between 50-100 lbs before noticeable bending begins to occur. This would be a conservative estimate, assuming no additional support like a center divider.

If the drawer is fully loaded with evenly distributed weight, this would likely hold up to 100 lbs before you start to see sagging or failure. With additional reinforcement, the load capacity could be higher."

When I refined the question by telling it the dimensions of the drawer box it said:

"Without reinforcement or additional factors, your 24-inch by 28-inch drawer with 1/2-inch plywood for the bottom and wooden sides will likely hold around 50-75 pounds. If you add crossbars or extra support, this number could increase. However, I'd recommend staying cautious and not exceeding this range unless you're reinforcing it further."

[twistsol]

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Here are 20 drawers in my recent office build and there are 32 total drawers in my shop. I've also done a number of kitchens, bathrooms and closet built ins.

Below is about half the kitchen drawers for the remodel of a house my dad and I built in 1978. They are just dry fit so no bottoms in them yet.

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[BuckeyeDennis]

That's a gorgeous cabinet, Twistol! What wood and finish did you use for the drawer faces?

I also love your dovetailed drawer boxes, but I just don't want to spend the time on that for mere shop furniture.

[BuckeyeDennis]

I wondered if 12 mm might not even be thick enough. So, I asked chat GPT what it thought.

One detail that I failed to mention earlier -- the front drawer-slide pairs are rated for 130lb., and the rear ones are rated for 100 lb. So I'm not going to worry about loads in excess of those numbers.

I've personally found ChatGPT to be unreliable for hardcore engineering calculations. In this particular instance, I'd expect a squarish flat plate that's supported on all sides to be at least twice as stiff as a beam that that's supported on only two ends. I have a hunch that ChatGPT simply extrapolates from natural-languge discussions of similar problems.

But you got me curious about how much sag I might really get, especially as I have to allow vertical drawer-bottom clearance for at least some sag. I was able to find an exact formula and calculator online for flat-plate sag, but I don't know all the material properties required to use them, and don't really want to go down that rabbet hole.

But I do have similar size pieces of both 12 mm and 6 mm Baltic Birch plywood on hand, so I think I'll do an actual deflection test on them, and report back on what I learn.

[edflorence]

I agree Chat GPT is to be used cautiously. This is the first time I asked it an engineering sort of question, so no experience with how reliable it might be. I understand that any of the LLM AI's are prone to just making up stuff. In this case, the max loads Chat GPT came up with seemed reasonable, though. It will be interesting to learn what your deflection tests show.

[twistsol]

That's a gorgeous cabinet, Twistol! What wood and finish did you use for the drawer faces?

I also love your dovetailed drawer boxes, but I just don't want to spend the time on that for mere shop furniture.

Thank you, the drawer fronts are single poplar boards that span three drawers horizontally each. I finished the cabinets with Varathane Cognac stain and sprayed three coats of satin poly. There is a bit of trickery here as well 24 handles on 20 drawers. the bottom drawers of the wider cabinets are for lateral files .

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Dovetails are time consuming. If I remember correctly, that stack of drawers was a full day of work, but that included planing the material down to 15mm for the drawers for the kitchen, 4 bathrooms, and drawers for closet builtins in three bedrooms and the two master closets, both of which were larger than my bedroom growing up. The base cabinets in that kitchen only had drawers except where the sink and stove were for a total of 38.