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Thread: workbench leg tenons

  1. #1

    workbench leg tenons

    Quick question for the experts:

    I'm in the middle of a Schwarz-inspired workbench build. I've just finished planing enough wood to laminate up the legs. Since one leg contains a Hovarter leg vise, it is on the big size (6x6). I want the others to be as close to 6x6 as I can get. The laminations are out of 4/4 boards finished out between 7/8 and 1" (most are ~15/16). My original plan was to glue up the legs, and cut the (leg-to-top) tenons on with a dado stack on the TS. However, with the size of these legs, that might not be the easiest option. So then I thought about "making tenons out of nothing" as described by Schwarz, basically where you stagger the boards to create the tenon without having to cut it. Again more thinking. Each leg is 6 boards. I can make the tenons four boards thick, leaving 15/16 thick for each shoulder. OR I can make the tenons two boards thick (a bit shy of 2"), leaving nearly 2" thick shoulders. Is there a preference? Also, since the boards are not uniform thickness, each tenon will be a little different thickness, and thus each a different offset. Will this be a problem? Opinions/advice greatly welcomed.

    Other, possibly relevant info:
    Top is 4.5" thick (2.25" tenon length). Size is 30" by 8'. Walnut
    Legs are also walnut
    Other candidate tools at my disposal: Bandsaw and RAS

    Many thanks in advance

  2. #2
    Join Date
    Sep 2008
    Location
    Rochester, NY
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    681
    Hi Jake,

    I can't comment about which tenon-to-shoulder ratio would be better because I have no idea. Instinctively I would guess the latter with the wider shoulders, since they support the weight of the top and your top is pretty large.

    Another method might be to cut the cheeks on the bandsaw.

    Finally, the mismatch in tenon dimensions will not matter if you mark the mortise locations directly from the tenons.

    Mike
    Last edited by Michael Peet; 10-12-2011 at 6:41 PM.

  3. #3
    Join Date
    Aug 2009
    Location
    'over here' - Ireland
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    2,532
    Hi Jake. I'm no expert, but I'm about to start a Roubo too.

    To think aloud about the leg top tenon dimensions. The shoulders basically have only to carry the static weight of the top. The tenons on the other hand have to resist whatever racking forces the bench may experience when in use or being moved (although doweling the joint will transfer some of the resulting forces to the shoulders), and since the legs will act as levers and the top is very heavy these forces could be quite high.

    The classic tenon divides the thickness in thirds, but is I think as much about not thinning the wall of the mortice too much in a joint with an equal thickness rail. Which is not a consideration on a bench top unless it gets too close to the edge of the top.

    So taking this with the above it'd make sense to me to maybe go a 1/3 shoulder at the front, or a bit less if it doesn't thin the wall of the mortice too much - and to definitely widen up the tenon a bit to the rear at the expense of the rear shoulder. (all subject to fit with other details - laying it out on paper is probably a good move)

    Some of the Roubos you see about (e.g. initial Schwarz model, Benchcrafted) use only a short blind tenon. While obviously it works perfectly well I'm not too sure it's structurally the best available option in a situation where the top in effect functions as the upper rail - the sides/edge faces of the tenon are probably not long enough to resist racking of the legs very effectively. The dowels will help, but their function is surely to hold the joint together - they shouldn't have to carry the racking forces.

    Which is possibly why the traditional Roubo and indeed Chris' latest use through tenons. The bit I don't yet understand about the traditional design though is what the function of the second open dovetail tenon to the front of the leg joint is other than maybe that it ensures a flat front surface right to the top of the bench for clamping/vise use - maybe it'll become clear when drawing up my version.

    For the moment though I'm tempted towards a single wedged through/full length single tenon design like that used by this guy in what looks like a very nice bench: http://chesapeakewood.wordpress.com/...ubo-workbench/ His write up and photos give a sense of what size his tenons were too - they look nice. So far as I can tell this design covers the racking resistance issue very well, and as well as that makes it possible with a bit of forethought to form both the tenon and the mortice in the top during glue up - avoiding the risk of making a bags of cutting the mortices in the top and ending up with unsightly gaps at the top. (the wedging will anyway help eliminate this as an issue)

    The possible disadvantage I've so far thought of for the wedged through tenon is that it clearly doesn't lend itself to disassembly should this be required for transport at some point in the future. The lower rails would need to be removable/bolted up if transport is to be a consideration too...
    Last edited by ian maybury; 10-13-2011 at 7:42 AM. Reason: typo

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