To Dado or not to Dado

[terrydowning]

I'm building shelves to put our drums on for the local high school band.

I will build three of these units.

10324

The material I have to work with is Formica laminated Particle Board (Melamine?) 1" X 73" X 25"

I have constructed a prototype using butt joints basically a table with the span of 63" attached to the verticals using 4 9 X 2 1/2 particle board screws on each side. It seemed to hold the weight OK, but I'm not not sure if using Dadoes for the shelves would be better or not.

I have tried Dadoes in composite materials, and wasn't thrilled with their performance, they seemed to place a lot of stress on the vertical members once weight was applied to the shelf. One even cracked along the dado although I think I cut the dado too deep for that one.

Are their clear advantages to using Dadoes in composites like melamine, particle board and MDF?

There is also some minor lateral instability, any ideas on how to mitigate that? High school students can be hard on shelves like these.

Just looking for opinions

[shipwright]

I'd use shallow(about 1/8" -1/4") dados for a lot of reasons and still use the screws. As for lateral stability, I'd use an 1/8" plywood back.

Paul M

[terrydowning]

I'd use shallow(about 1/8" -1/4") dados for a lot of reasons and still use the screws. As for lateral stability, I'd use an 1/8" plywood back.

Paul M

Care to share the reasons? I trust your experience, but I'd also like to learn.

[boostfan]

I agree with the 1/8" Plywood, because it creates a triangulation, like how adding a piece of sheetrock to a wall makes it strong.

It doesn't sound like appearance is not the most important thing, so I would consider adding a 3/4" x 3/4" cleat on the underside of the sides and back, skipping the dado. That way you do not thin out the side walls, but provide support to the shelves.

[shipwright]

Care to share the reasons? I trust your experience, but I'd also like to learn.


I guess it just seems to me to be the right way to do it. Since you called me on the "lots", let's see:
1) takes the shear force off the screws
2) makes alignment a non-issue
3) increases glue surface
4) looks cleaner

and "lots" of others.:D

Paul M

[charlese]

I completely agree with Paul M.. The dados will add stability the screws will help. The back will also give great resistance to racking. To me it looks like the boards (cleats) you have designed across the shelf fronts will keep the shelves from sagging. Recommend using glue and screws here also. Using the proper glue will also help. I recommend you buy a container of their glue that is specifically designed for joints holding melamine to wood. It is an easy to use white glue that really holds in this type application. I used this on a large three story rat cage and it is holding up very well for several years so far.

http://www.titebond.com/ProductLineTB.asp?prodline=20&prodcat=3

http://www.woodcraft.com/Family/2020012/2020012.aspx

[8iowa]

When we owned a storage complex I used to make a lot of open backed utility shelving out of 1/2" A/C fir plywood. I used rather large cleats made out of 2 x 4's. The 1 1/2" edge was screwed and glued to the sides. The shelves were secured to the 3 1/2" face of the cleats with 2" carriage bolts. This provided a lot of support for the shelves, each of which usually held six heavy cardboard file boxes. With four shelves, these shelving units held a lot of weight, and I never had one fail.

I drilled the holes for the carriage bolts with a 5/16" drill, and used a spacer so that the holes in all the cleats and all the shelves were in the same place, making the shelves interchangeable. I countersunk the bottom of the cleats with a 7/8" brad point drill so that the washer and nut would not protrude below the bottom surface of the cleat. It was easy to assemble or disassemble these units right at the site. I could carry 12 shelving units in the back of my station wagon.

My experience with MDF is limited, but I understand that this material does not hold wood screws very well.

Post note: I bought my Shopsmith 500 in '83 and probably made 100 of these shelving units before I was "told" that you can't cut 4'x8' sheets on your Shopsmith. Ah....ignorance is bliss.

[foxtrapper]

The clearest advantage to dadoing is the spreading of force. Otherwise the shelf is being held by the screws, and all the force is on those locations. That's why shelves mounted that way rip out, right at the screws. The dado doesn't need to be deep in the vertical piece. All you need is a lip for the edge of the shelf to sit on.

Stability can be addressed as previously described, with thin backing board. You can even do it with two pieces of diagonal wire. Though wire localizes stresses and stretches.