Sliding dovetails with racking loads...

[moofie]

Hey gang! I'm thinking about building some ludicrously-overengineered pedestals for the washer and dryer. I want to experiment with router-cut sliding dovetail joints for the carcase.

I plan to make a four-sided box: Top, two sides, and a plywood back in a rabbet in the back of the case. I'm planning on using 3/4" ply. (Beefy.)

I'm curious as to which way I should cut the dovetails to best resist racking loads. In the picture below, will the left side or the right side better resist racking/vibration from the washer/dryer? I haven't decided if I'm going to make one double-width pedestal, or two identical pedestals for the washer and dryer.

The back is really going to carry most of the racking loads, but I was curious if there was an optimal method to make the strongest case. If I was also going to put on a front, I'd be convinced it didn't matter a bit. I'm leaving the fronts open for possible future drawers or slide-out platforms (on which one could place a laundry basket, if one were so inclined).

[rmac]

As far as racking goes, my gut says that both ways are about the same. With either arrangement, a racking load would try to rip off the little bit of wood on the side of the socket nearest the edge of the board (A and B in my picture).

I would be more concerned with supporting the weight of the appliances (the washer in particular, especially when it's full of water). Considering that, I like the joint on the right much better because the weight is carried by the beefy tail of the dovetail. With the joint on the left, the weight of the washer is trying to rip off the thin strip of wood at C, and that would bother me.

Two other thoughts:

1. You could use a face frame (essentially a front with a hole cut in it) to resist racking on the front and still give you a way to have your drawer or platform.

2. If you really want to overengineer this thing, you might think about joints that are more suited to the task and save the dovetails for a different project. Or make it out of titanium.

-- Russ

[moofie]

Well, as it turns out, my wife has contacts at a company that does large aerospace castings in titanium, so when I suggested that idea she thought it was a ) awesome and b ) absurdly expensive. So I think I'll stick with ply. : )

I'd considered face frames, but I never really thought of them as a way to increase strength. When I think of face frames, I think pocket screws, and when I think pocket screws, I don't really think good bending strength. I'm sure my horizons could be expanded!

RE: vertical loads, yeah, a washer is pretty heavy, but 3/4" ply seems pretty strong. I suppose if I were feeling paranoid, I could double up the walls, or (even better!) put a vertical web left-to-right half way through the depth of the pedestal.

Could you say more about joint selection? I've seen platforms like this made with dadoes and glue, and I figured the sliding dovetails would be at least as strong as that. Or are sliding dovetails in plywood the issue?

Thank you for your insight!

[jimmykx250]

I may be on the conservative side here but im not sure i would do either. I look at my front loader shaking when it's in the spin cycle and think thats alot of racking and weight. It's more than likely strong enough but thats an expensive way to find out for sure. Let us know what you decide to do and how it works out!

[Beechwood Chip]

I don't think your design is ludicrously over-engineered enough. I'm with Russ - sliding dovetails are not the right joint for this. I'd use a butt joint (so the top is supported by the entire thickness of the side), with dowels, dominos, biscuits, or screws to hold it in place, and L brackets or angle iron to keep it from racking. I like the idea of a face frame to prevent racking in the front, in addition to brackets.

[rmac]

Could you say more about joint selection?

Sure! My first thought was butt joints assembled using glue and screws. This isn't (I don't think) fine furniture territory, and (as Beech says) that way you get the support of the full thickness of the top sitting on the sides. If you put a front or a face frame on the platform, then I think the L brackets would be overkill. Without the front, then the brackets would help with the racking problem in a way that would still let you have your drawer or platform for the laundry basket.

One step fancier than butt joints would be to use some very shallow rabbets (maybe 1/8", or even less) as I have shown in the picture. They wouldn't contribute to the strength at all, but they would make it easier to get everything aligned during assembly.

-- Russ

[Beechwood Chip]

Yeah, face frame and brackets are overkill, but you definitely need at least one. You're talking about lots of weight and lots of vibration, and a messy and expensive accident if the thing fails. I'd go for "ludicrously over-engineered". The brackets don't cost much in time or money.

[Pete Bennett]

I'm still trying to understand why you need to place a washing machine on a plinth to begin with Are American washers different in height to European ones they are normally around 870mm which just fits under a European standard worktop height?

[iSawitFirst]

Short sliding dovetails are enough to drive you crazy getting a good fit. I can't imagine trying to do it twice on these long joints.

I agree that there are better joints for this application. Your top will be spanning at least 6' so top sag should be a consideration (unless you don't mind it resting on your appliances). I'd consider using a spline joint or lots of tenons. I've used hollow core and solid core doors for tops and they work much like a torsion table resisting sag.