Some bench design questions

[Joe Fabbri]

Hi everyone,

I'm trying to figure out a design for my workbench. Up until a week ago or so, I was pretty set on a traditional European style cabinetmaker's bench, with a trestle of sled foot base, and a separate top. I have two 6ft 4x6 pressure treated beams that are very old, heavy, and straight. There's some slight checking, but nothing too bad, I don't think. I disasembled an old model train table, which had some 4x4s, and I thought I'd use them for the top and bottom trestle, with the 4x6's as legs. Unfortunately, the 4x4's are twisted, and I'm not sure if I want to bother trying to square them. I wanted to use them originally because they are old, and dried out.

So, my first question is, would it be smarter to buy new 4x4's, ones which aren't cut from the center with pith (which the old ones are), and let them shrink onto the dry tenons of the 4x6s? If I find new 4x4's without the center and straight, can I rely on them staying that way, you think? Will the weight of the bench help keep it from moving? I don't want to have a wobble develop in the future, if they twist as they dry.

Also, I was thinking that without great wood laying around for a trestle design, maybe I should go the Roubo route, and just mortise/dovetail the legs into the top. This seems like a decent option, considering the 4x6s should be big enough (and are probably overkill for a trestle base design). However, I don't think I'll come across someone giving away a huge wood slab, so I'd have to laminate the top.

So, I guess my second question is, is this a good thing to do with a Roubo style bench? Roubo designed the bench with a one-piece top in mind. Would it be wise to laminate the top without a trestle for supporting the width of the top? If I modified the design and added a trestle to the top of the legs, would the strength of the tenon/dovetail into the bench top be compromised? By the way, if it helps to know, I was thinking of doug-fir for the top, or pine, depending on what they have at HD or Lowes.

[Simon Frez-Albrecht]

I used two 6x6 pressure treated for the top of my Roubo inspired bench, a really well mated glue joint reinforced with a couple of 10" timberlocks. No issues after a year, with at least a month's worth of heavy use.

(picture at the bottom of this page: http://luv2sharpen.blogspot.com/p/gallery.html)

[Joe Fabbri]

Hi Simon,

Thanks for the reply. That's a nice looking bench. Did you use 4x6's for the bench legs?

If I made a Roubo, I was thinking of the square leg version, not the splayed back legs style. The advantage of yours is each part of the lamination is individually supported by the legs. I'm still curious if having the width relying only on the glue joints is okay. Maybe it could be reinforced with a few 1/2 threaded rods, and hide them on the last lamination--if I used 2x material.

[Jim Koepke]

Maybe it could be reinforced with a few 1/2 threaded rods, and hide them on the last lamination--if I used 2x material.

Or put plugs over a counter sunk hole. That way if you ever needed access to the bolt head it would be easy.

jtk

[Andrae Covington]

...I'm still curious if having the width relying only on the glue joints is okay...

At 3+ inches thick, the laminated top is more than stiff enough to span unsupported between the front legs and back legs. It's not very far when you start thinking about it... say a 24" deep top, sitting on 4" legs is only 16" clear span, 6" legs would be 12" span, etc.

...If I modified the design and added a trestle to the top of the legs, would the strength of the tenon/dovetail into the bench top be compromised?...

Belt and suspenders overkill, not that there's anything wrong with that.

[Ron Brese]

Design questions aside, I would be really cautious about working pressure treated timbers in a way that creates a lot of airborne particles in a confined space. From a health standpoint I think it would be wiser to laminate your top from 2x yellow pine or other untreated material. I have a friend that has recently undergone some major surgery for cancer in sinus passages. A good portion of his face has been removed and now he's looking at reconstructive surgery. The doctors attribute it to exposure to pressure treated pine dust and particles. Also keep in mind that the old version of pressure treating chemicals contained arsenic. Personally I don't think it would be worth the risk just to save a few dollars on materials.

Many of the big box stores carry untreated Douglas fir 4 x 4s that are at least construction grade dry (12%) so it would not take long to complete the drying process if stored indoors especially if allowed to dry thru the dry winter months.

Ron

[Joe Fabbri]

Hey guys,

Thanks for the replies.

Jim, yes, that's also a fairly easy solution to hiding the bolts, if I were to put them in. The threaded rods, lets say three of them, would also aid in clamping up and alignment, aside from avoiding any deflection.

Andrae, I guess it's not a large span, especially if I were to use large 6x6 legs. I'd just like to use the 4x6's that I have. It seems most if not all Roubos that I see have the x6 side of the legs parallel to the top, and the 4x width front to back. If I were to switch it and make the x6 go perpendicular to the top, it would support it more. In one of Roubo's pictures, with the upclose diagrams of the bench parts, he shows the legs mounted in this fashion (although maybe the picture is deceiving). I'm not sure if this would compromise the rigidity of the bench side to side, though.

Ron, I appreciate your concern about the pressure treated material, and I'm sorry to hear your friend is suffering now because of it. That's a shame. The only area I'm considering the pressure treated material is for the base, I'm not going to use it for the top. I figured it's safe enough for the base, particularly since the material is very old and was sitting outside for a while. So I'm assuming it 's not too too strong anymore. But I understand your concern.

Going back to one of my earlier questions, do you guys think it would be okay to use new 4x4s for the trestles (if I do that design), provided I find the best grain pattern and they're straight to begin with?

[Jim Koepke]

Going back to one of my earlier questions, do you guys think it would be okay to use new 4x4s for the trestles (if I do that design), provided I find the best grain pattern and they're straight to begin with?

As long as they are dry when you start, they should be Okay.

jtk

[Andrae Covington]

I'd just like to use the 4x6's that I have. It seems most if not all Roubos that I see have the x6 side of the legs parallel to the top, and the 4x width front to back. If I were to switch it and make the x6 go perpendicular to the top, it would support it more. In one of Roubo's pictures, with the upclose diagrams of the bench parts, he shows the legs mounted in this fashion (although maybe the picture is deceiving). I'm not sure if this would compromise the rigidity of the bench side to side, though.

Aesthetically I think orienting the 6" dimension with the longer span across the front of the workbench would look better. Structurally I don't think it matters. Consider that a Douglas-Fir 2x4 used as a floor joist is rated to span 6'-7" with only 1/4" of bearing surface at each end. And that is at 16" on center. A bunch of 2x4's (or 2x6's or whatever) glued together will have a much higher modulus of elasticity, and you'd have 4" or 6" of bearing at each end, plus a tenon or two extending through at each bearing point, all of which reduces the potential deflection of the span even further.

Going back to one of my earlier questions, do you guys think it would be okay to use new 4x4s for the trestles (if I do that design), provided I find the best grain pattern and they're straight to begin with?

As Jim said, as long as they're dry. I think you'd want to let them sit and acclimate to your shop before you start gluing and mortising and all that. One or more could end up twisting significantly in the process, and you'd be better off finding that out beforehand and discarding rather than trying to deal with the aftermath of movement in an assembled bench. Of course, the wood is going to continue to move forever unless the relative humidity of your shop never changes, but the seasonal changes should be minimal and manageable.

Design questions aside, I would be really cautious about working pressure treated timbers in a way that creates a lot of airborne particles in a confined space. From a health standpoint I think it would be wiser to laminate your top from 2x yellow pine or other untreated material. I have a friend that has recently undergone some major surgery for cancer in sinus passages. A good portion of his face has been removed and now he's looking at reconstructive surgery. The doctors attribute it to exposure to pressure treated pine dust and particles. Also keep in mind that the old version of pressure treating chemicals contained arsenic. Personally I don't think it would be worth the risk just to save a few dollars on materials.

Many of the big box stores carry untreated Douglas fir 4 x 4s that are at least construction grade dry (12%) so it would not take long to complete the drying process if stored indoors especially if allowed to dry thru the dry winter months.

Ron

A sad story about your friend. As I recall, in a thread a while back Joe had asked about using the old pressure-treated lumber he has. I posted some cautionary information, but someone else vehemently pooh-poohed the concerns. It can be difficult to find a position between "macho man" and "frightened by rustling leaves". Personally I err to the side of caution with chemicals, but to each his own.

[Joe Fabbri]

Hey Andrae,

Yeah, we'll see about the pressure treated material. The way I'm seeing it now, I might just forget about trying to salvage what I have. I mean I'm making design decisions based on what scrap wood I have laying around--kind of backwards, or business as usual....

Joe