Knockdown Roubo Bench question

[Tony Shea]

I have another question about the design of my bench. I am making a knockdown Roubo and want a leg vise (big wooden screw style). I plan on glueing together my base so I have to end assemblies with a lower and upper strectcher. And the long front and back aprons will be attached using bench bolts.

My concern now is how to attach the top to the base so that the leg vise is not going to push the top out of alignment with the front plane of the base. I originally was thinking about small bullets (large dowels) in the tops of the legs that lined up with holes in the bottom of the top. Is this strong enough? I also thought about lags through the top stretchers of the end assemblies into the top and am concerned with the strength of this. My last idea is to just make large tenons on the top of the legs to fit matching mortises under the bench, but will this actually come apart reliably when I need it to? Am I over thinking this?

[Jim Ritter]

I would and will be using a mortice and tenon on the next bench. The top will be removable. I'm thinking a stout tenon half the leg in width but only about an inch or less in height. It can be a little smaller than the mortice, but the only critical dimention is the edge to mortice spacing so the bench front lines up with the leg.

[Jessica Pierce-LaRose]

The benchcrafted plan, if I understand correctly, (and I haven't purchased the plan, just looked at the descriptions and photos they've provided online at the BC website and their blog, as well as other folks building descriptions) uses lag bolts (Spax brand, natch) to do the majority of the holding of the top to the upper stretchers, and then uses smallish (1" high) tenons (which appear to be size other wise pretty much like you've plan a tenon of this size) to resist racking motions forward and aft. Since the motion you're most concerned about is really only in one direction, you could probably get by making these a little bit looser than you might for other tenoned construction - you're probably not concerned about the top lifting off the base into the air, (and even if you were, the lags would take care of that) and even if there's a little play forward and back, you're mostly concerned about the bench not moving any further back - which the side of the tenon and the edge of the mortise should resolve.

I'd like something along those lines much more than a bullet, it just seems more substantial, and just because it seems to be worth the extra effort. If you want to skimp, I guess you could make just the one mortise-tenon combo, putting it on the leg that holds the vise.

[Paul Saffold]

When I built mine I had 1.5" tendon the full width of the leg (5") and a lag bolt thru an elongated hole in the top stretcher with a fender washer. I haven't been using it very long but haven't had any problems yet either. If you look closely at the top of the leg you can see the tendon. Since the tendon is 5 inches wide by 2.5" thick by 1.5 inches long I expect the top won't move. The purpose of the lag bolt is to hold the top down on the stretcher, but allow side-to-side movement with humidity changes.
Good luck with your build.
Paul



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[Andrae Covington]

Am I over thinking this?

Let me tell you all about overthinking a knockdown roubo bench.

I tried a variety of solutions. Ultimately, I think mortise and tenon is the best choice, just don't use glue so you can take it apart. But dowels should work.

To make things extra complicated and frustrating, my top is not all one piece, I glued up four separate sections so I could manage moving each one around. Should have just glued it all together and tricked a friend into coming over to help.

I first had a 3/4" dowel ("bullet") in the top of each front leg going into the top. As far as resisting the force of the vise, that worked just fine. To hold the other sections of the top against the first section, I had a combination of wedges in the back. My nicely flattened top soon looked like tectonic plates as the sections twisted and buckled.
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So I added stopped sliding dovetails and removed the dowels and wedges. That was better, but still had problems keeping the sections planar on top.
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So I bought some long lag screws and screwed each section to the one in front of it. Now the top stays flat (well, as much as any benchtop would).
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With the bullet dowels removed, I had problems with the top shifting when I clamped pieces with the leg vise. So I drove a couple lag screws through the upper side stretcher into the top. That works most of the time, but looks and feels like a kludge.

Were I to build another bench, I would try using unglued mortise and tenon joints to hold the top to the legs.

[Mike Holbrook]

If I am reading Bob Lang's plans for his 21st Century Split Top Workbench correctly, he simply screws the pieces of the top to the carcass using lag screws. The tray supports on the under side of the top will provide some help in holding the top pieces in place as well. If lag screws and or joist hangers with nails can hold the floor for the room I just built in place I think they can handle the pressure from a vise. As a matter of fact I lifted the entire 60 x 40 ' building with a large bottle jack when I replaced the metal posts holding it up with treated wood ones.

[Wilbur Pan]

My last idea is to just make large tenons on the top of the legs to fit matching mortises under the bench, but will this actually come apart reliably when I need it to?

Yes. If the M/T joint isn't glued or pegged, it will come apart. In a pinch, you can rig up a jack to help get the top off the base.

My Roubo bench was not designed to be a knockdown, and I used double M/T joints at each leg to attach the top to the base. I had planned on drawboring those joints, but then realized that gravity would keep everything in place. Even with 8 M/T joints, I can get the top off the base if I wanted to. Of course, you don't need to do double M/T joints for this. If you use a regular large M/T joint to attach the legs to the top, it will be easier to get the top off.

https://lh3.googleusercontent.com/-o1ZNysEAxPM/S6aJAzHFs6I/AAAAAAAABAo/c16MfLCZPD4/s640/IMG_4149.jpg

[Jim Matthews]

If your top must be removed at some later date (the knock down feature, if I gather correctly?) Why not drill a couple through hole pegs in the top stretcher, in line with the leg vise?

If you drive dowels in, up from the bottom and leave them long (so that there is some protruding) you can work them back out with vise grips.
Set them in tight, with a coating of wax to facilitate removal. I would install these after you have the top set as you like it.

When you assemble the top, it may be useful to apply a strip of double sided tape to keep things from roaming around.

jim
wpt, ma

[Jessica Pierce-LaRose]

If I am reading Bob Lang's plans for his 21st Century Split Top Workbench correctly, he simply screws the pieces of the top to the carcass using lag screws. The tray supports on the under side of the top will provide some help in holding the top pieces in place as well. If lag screws and or joist hangers with nails can hold the floor for the room I just built in place I think they can handle the pressure from a vise. As a matter of fact I lifted the entire 60 x 40 ' building with a large bottle jack when I replaced the metal posts holding it up with treated wood ones.

Bob's bench uses a twin screw vise though - so it's not going to be applying the pressure against the top from the base of the bench they way a leg vise would.

[Stuart Tierney]

I used threaded inserts into the short planks I used for the top (2" x 12" x 3') and 1/2" bolts through the top stretcher.

When I do eventually get around to making the leg vise, it'll likely end up being a floating assembly attached directly to the bench top and the bottom stretcher.

I used some really ugly hardware on mine, but it'll knock down into something that'll fit into our small car within 5 minutes and can be reassembled just as quickly. I need to be able to get that vise off the bench in a reasonable time as well, and locking it into a leg adds complication that I'm not prepared to deal with. The ultimate cost of the bench was also a significant factor, and I don't think I have even $200 in it, including all hardware and current iron vises.

Also, making that big, powerful leg vise free floating means I can put it anywhere I decide is best at any given time.

But that's what I'll do, when I get to it. The bench is done and has a modest iron vise in the usual place and a clamp on vise on one end and that's enough for now.


Stu.

[Jim Matthews]

Your neighbors probably wonder why you need a vise, at all...

[Stuart Tierney]

Your neighbors probably wonder why you need a vise, at all...

My neighbours need to be told what it is (a bench) and what it's used for (making stuff).

But, the vises are made in Japan, and very good. Not expensive (but not cheap) and I wish I'd known about them years ago before I actually made a vise.


Stu.

[Paul Saffold]

Please tell us more about these vises.

[QUOTE=Stuart Tierney;1904071]
But, the vises are made in Japan, and very good. Not expensive (but not cheap) and I wish I'd known about them years ago before I actually made a vise.


Thanks, Paul

[Stuart Tierney]

http://www.nabeya.co.jp/search.php?action=Detail&Key=1376

I got the smaller one. Price has gone up some since I got mine I think.

It doesn't have guide rods, but does have a box section guide the screw runs inside. Doesn't rack very much, fairly smooth, works well.

I also picked up 2 of these;

http://www.nabeya.co.jp/search.php?action=Detail&Key=1377

at the local homecentre's multi-annual junk dump for about $20 a piece. I would have grabbed more, but 2 is enough for me and I can't really do anything with more of them.

FWIW, shipping would be a killer and by the time you account for that, and the fact these things are over $100 each, it's difficult to justify the outright cost vs. the quality.

They're good, but they're not $150-200 worth of 'good'.

Stu.