Opinions needed on structural strength

[David Wadstrup]

I'm no physicist(and only a novice woodworker), and have a question regarding the structural strength of a leg/mortise design. I'm currently building a Benchrafted Roubo. I've encountered a problem(my own fault) and have come up with a potential work around. It's too long and confusing of a story to go into, so I will simply show you a photo which lays it all out(or so I think). Can you tell me, does this look like it would be a sound construction? Ideally the through mortise(for the leg vise's parallel guide) would be higher. Having it here, though, allows me to solve an earlier mistake. The cross hatched areas represent the mortise cavities(the one on the right is for the rail's tenon). And, the bolt extends through the leg into the end of the rail. It will be tightened and under tension. Make sense? What do you think?

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[Peter Cobb]

Hey Dave,
it depends on the thickness of the leg, if it's 2" stock that's asking for trouble... my guess is 4" plus should be fine (especially as the mortise for the parallel guide actually bears on the pin on the outside of the leg).
Also make sure the bolted tenon doesn't bottom out in the mortise but tightens the shoulders (easier to ensure a square assembly)
Cheers,
Peter

[David Wadstrup]

Thanks Peter. The leg is 3 1/2" thick, and 5 1/2" wide. And it's Ash. In other words, pretty thick and pretty tough.

[Gary Kman]

I'm not seeing the need for the depth of the mortise on the right. With the bolt keeping the rail compressed against the leg the mortise and tenon merely holds the rail in alignment. A 1/4" long tenon would resist motion in all directions. I don't see a mode of failure. If you want to make the other seven a more conventional scale help yourself but I don't see it adding any NEEDED strength. I have been using a bench of this style with pine 4 x 4 legs and 2 x 6 stretchers and a 6" thick top for over 30 years. It looked like a huge mass balanced on small legs that would soon start loosen from planing but I figured I could tighten the bolts as needed. Other than dis-assembly for moving once I have never touched the bolts and the top is rock steady. Enjoy.

[David Wadstrup]

Gary,

You're right. I probably didn't need such a deep mortise for the rail. Unfortunately, though, it's already done. The plans specified ones this deep, because some people opt to drawbore the tenons rather than use the knockdown hardware.

Do you think there will be any chance that the walls of the potential through mortise would get "sucked in" because of the tension of the bolt?

Thanks,

David

[Wilbur Pan]

Given the dimensions of your leg and your use of ash, I would be surprised if any "sucking in" occurred. If the walls of the through mortise do get "sucked in", though, that shouldn't be too hard to fix. Just widen the mortise again until the guide passes through cleanly.

I don't have the Benchcrafted leg vise, but on the leg vise on my Roubo workbench there is a little play in the slot that the parallel guide rides in. I imagine that some play for the Benchcrafted leg vise won't affect its operation.

You should also ask Jameel over at Benchcrafted about this. He's quite approachable, and I'm sure he would be more than willing to help.

[John Coloccia]

Even if it ended up being rickety (doubtful), couldn't you just drill another hole later and add another threaded rod to tighten it all up?

[Gary Kman]

Gary,

You're right. I probably didn't need such a deep mortise for the rail. Unfortunately, though, it's already done. The plans specified ones this deep, because some people opt to drawbore the tenons rather than use the knockdown hardware.

Do you think there will be any chance that the walls of the potential through mortise would get "sucked in" because of the tension of the bolt?

Thanks,

David

Chop your mortise, see what happens. Nothing, I imagine. If you don't like it, glue a plug in the deep mortise. Be the only time in your life you can apply a patch that never shows from anywhere. I was probably sweating some detail like that 30 years ago and forgot it and a thousand more since.

[Jim Matthews]

If it was me (and it isn't), I would just use the side of the installed rail as one side of the mortice for the guide.
The amount of material remaining in the layout is insignificant, and fiddly to maintain.

If strength appears compromised, a corner block on the three remaining sides will add some glue surface.
It might be heresy with the Roubo plans, but some heavier plywood would add loads of security.

[Peter Cobb]

Gary,
Do you think there will be any chance that the walls of the potential through mortise would get "sucked in" because of the tension of the bolt?


The tenon shouldn't bottom out in the mortise. The bolt AFAIK tightens the shoulders against the inside of the leg (right side in your pic). You could actually remove the thin partition between the mortises with no structural compromise: stretcher shoulders butt against right side of leg, the vise parallel guide pin butts against the face of the leg... the bottom of the stretcher's mortise and the wall of the through mortise have no structural significance at all. (Hoping that makes sense)
With the amount of material around it you'll be fine.
Cheers,
Peter

[Trevor Walsh]

I agree with Gary as well, the potential I could see would be for an overly long tenon (compared to the mortise) squeezing the mortise for the parallel guide, just make sure the tenon doesn't bottom out, which is easy. I don't see a reason why it would be a problem, just tight on the layout and maybe an errant chisel blow from having connected morti (mortises?)