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View Full Version : Loose tenons for holding benchtop in place



Mike Holbrook
10-05-2017, 10:49 AM
I am building a Roubo style bench in one place and then moving it to another place where I may end up moving it around, as I expand or change the shop space. Therefore I am considering making a bench that can be broken into pieces for moving. The small bench I have been using is made to break down. The top on the bench I have been using is held in place with small "standard" dowels which has always worried me.

One of my design issues is whether or not to run structural support, between the legs, width only. Running structural support lengthwise may get in the way when clamping to the top, whereas support between the legs on each end may help hold the legs in place without getting in the way of clamps etc. I should add that I expect the top of my bench to be 3 1/2- 4" thick x 7' x 20-24". Making structurally independent end "frames" that can be removed from the top and stretchers is a common design tactic for benches that may need to be moved. I have Benchcrafted bolts designed for holding the legs to the stretchers.

Holding the top in place on the frame is an issue. I am currently considering making or buying: much larger, hardwood, straight grain, loose tenons, probably round (dowels) to reduce the chance of splitting. Loose tenons could be removed and adjusted if the top expands or contracts. The mortise holes in the top could also be adjusted if loose tenons are used.

Eric Schubert
10-05-2017, 11:56 AM
I'm currently building a split-top Roubo-style bench, myself. My plan to hold the top slabs in place is to use double tenons on each leg into the slabs. I also have short stretchers directly underneath the top slabs on each end to help support them. I just plan to drill through the stretchers into the slabs and connect them with lag screws. This way, I can just unscrew the slabs from the short stretchers and pull them off of the tenons to remove them. Long stretchers will only be used near the floor between the legs, none underneath the slabs. These will be connected with barrel nuts so I can also break the base down after removing the top slabs.

Hopefully that all makes sense...

Oskar Sedell
10-05-2017, 12:05 PM
Mike,

I've just started to build a "roubo" bench, very much similar to yours. 20'' wide, 7.5' long, 4'' thick top and of a knock-down type. I decided on independent, drawbored and glued end frames with lower and upper stretchers, where the upper ones are placed just so that the won't interfere with holdfasts through the top. The long, length-wise stretchers will connect to and through the legs with tusk tenons.

Can you describe closer what you mean by loose tenons, and what their purpose are? I plan to make ordinary tenons on top of the legs, going at least half way through the top, and leave them unglued. Also, I will make the mortises at the back side of the top snug lengthwise, but extra wide in the cross direction to allow for a shrinking top.

Robert Engel
10-05-2017, 2:07 PM
If I'm understanding correctly, I think it will either be loose or eventually get loose and you will be disappointed.

I would look into bolting the top down rather than dowels.

Mike Holbrook
10-05-2017, 2:22 PM
Thanks for the input Eric and Oskar. Should have thought to define "loose tenons". Dominos, dowels, biscuits are examples of loose tenons. Instead of making a mortise in one piece and a tenon in the other, a hole/mortise is made in both pieces. The "tenon" is a separate piece of wood that can be glued in both or neither mortise.

I bought a copy of Schwarz and Myers's (the Moravian bench guy) latest video on making a Roubo bench. They show how to make a double through mortise and tenon. To be fair much of the wood they use is green or not fully dried. They wound up having some issues with the wood changing shape during the build. They end up using wedges to tighten some of the joints. I like to build with green wood too, although I have dry wood to build my first bench from. I am still interested in "green" wood techniques as I have plans to build a good many things from wood that is "wet" or partially dry.

I actually took a class from Schwarz at Highland Woodworking, which involved making "sawbenches" with green wood. I have also taken Windsor and Welsh stick chair classes too. Thoose classes use "staked" chair legs, made by inserting tapered/mortised chair legs into tapered holes in chair seats. These too might fall into the realm of loose tenons.

I am also thinking that a technique used in chair making could be used to lock joinery down later on if the wood moves. I could remove the tenons so the tenon or tenon holes could be modified. I could start with stopped tenons, that do not go all the way through the top. If I have trouble with those, I could also remove them and saw a kerf through the middle of the top of a longer tenon, wedging the tenons in place, with a wood wedge, once I am comfortable the wood is stable.

Daniel O'Connell
10-05-2017, 2:41 PM
FWIW, I have had pretty good luck using loose 1 1/4" birch dowels and gravity to hold my workbench top on. In the worst case I've had a tiny amount of shift when changing planning direction as the seasons change, but generally that is a one time per change thing and everything else holds up pretty well.

That said, thats totally anecdotal and my experience is not something anyone should rely on :)

Mike Holbrook
10-05-2017, 10:36 PM
Thanks Daniel, good to know it can work. I guess the movement you have seen has not been enough to motivate you to improve the fit? Certainly a loose fit may be a larger challenge to fix, although a wedge, oil, paint, WATCO, hammering the end to widden it, hide glue...might work.

glenn bradley
10-05-2017, 11:32 PM
Similar to others, I used large dowels. In my case I rounded them off and drove them up from below. They penetrate about 1-1/4" into the 3" thick top. Gravity is my friend.

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ken hatch
10-06-2017, 1:18 AM
Similar to others, I used large dowels. In my case I rounded them off and drove them up from below. They penetrate about 1-1/4" into the 3" thick top. Gravity is my friend.

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Glenn is correct, gravity is your friend. With a base that is a complete box, in other words has lower and upper stretchers...Add in a 50mm to 100mm thick slab and there is little need for for more than four dowels to keep the top stable. Truth is you do not really need the upper stretchers to make it rigid, just dowels at each corner with the back set having elongated holes to allow movement of the slab will do the job if the lower stretchers' joints are solid.

BTW, I just finished a small bench with a less than 50mm thick slab and only lower stretchers. The slab is located and bench stabilized by four 40mm long 5/8" Oak dowels. The whole shebang is rock solid.

Just "blind peg" the top of each leg or upper stretcher that runs between the legs and glue in a dowel that is long enough to penetrate less than half the thickness of the slab. Drill six holes in the slab, one to match up with each front side leg, two next to each other on the corners of the back side of the slab with the "web" chiseled out to allow slab movement. Done in less time than it takes to write about it and you will have a solid connection between base and slab.

ken

Mike Holbrook
10-06-2017, 8:27 AM
Thanks for the comments Glenn and Ken. I suspect that a 7'+ 20" wide x 4" thick top will be heavy enough to utilize gravity in my favor. If I can lift it ;-)

I like the ideas which allow the bench expansion & contraction space. I think I am going to leave my "slab" 20" wide to help with that issue too. The compact Hammer bench I have been using is 20" including the tool tray and I have adapted to that width.

I have enjoyed Ken's Moravian bench build. I thought about making that bench too, but after using a small light bench for quite a few years I am more interested in a heavier, sturdier, bench that will be used mostly for planing. I will also build a height adjustable bench I have been working on plans for too.

Daniel O'Connell
10-06-2017, 9:55 AM
Thanks Daniel, good to know it can work. I guess the movement you have seen has not been enough to motivate you to improve the fit? Certainly a loose fit may be a larger challenge to fix, although a wedge, oil, paint, WATCO, hammering the end to widden it, hide glue...might work.

Nah, its not worth it, a 1/16" shift every couple of months when I plane hard is not a big deal for me, and its likely due to low quality boring than anything(I was and still am a rank amateur, I made mistakes ;)). My bench is a split top, with each slab something like 9'x16"x3.5", weighing in around 90-100lbs each, and I have not had any real trouble. I'd guess your top is going to be somewhat heavier and should have even less trouble.

Mike Holbrook
10-06-2017, 10:46 AM
I "hear" you Daniel I have been known to make the occasional mistake myself ;-) Schwarz and Myers make a jig (one or two pieces of wood sawn or connected at 90 degrees, making a drilling guide) which they use to try to keep their holes straight when drilling holes with an electric hand drill or brace. I am thinking about just buying the 3/4" and 20mm dog hole bushings Lee Valley sells, $9.50 ea. The bushing is inserted in a piece of 2x? providing a similar, possibly even more accurate guide. If I knew my Wood Owl bits would fit the bushings so I could use the bits I have and like vs having to buy two new $33-$35 bits. The "3/4"" Wood Owl I have is actually 19mm .748", not sure about the LV Wood Owls. Mine were bought from Traditional Tools which is now out of business. Guess I need to call Lee Valley.

Daniel O'Connell
10-06-2017, 11:59 AM
Hah, yeah, hopefully they fit.

I've been making due with a variety of older bits I've collected. The level of consistency is not great ;)

I've considered the bushings myself. I might just do so when it comes time to cut more dog holes. I've been going with the "only bore what you need" approach for the last year.

Mike Holbrook
10-08-2017, 1:52 PM
I missed Robert Engel's suggestion above. I know some people use lags screws through stretchers to hold their top on. I just worry that I might end up buggering wood threads. I have given consideration to using Lee Valley or Benchcrafted bolts that thread into metal barrels to hold the top on. I am having trouble figuring out where to locate the metal threaded barrels where they want be in the way but still afford accessibility.

Brian Holcombe
10-08-2017, 2:07 PM
I have bolts going into my benchtop but lag screws are more ideal since their thread is designed for use in wood. Metal inserts are fine.

Pat Barry
10-08-2017, 6:19 PM
My bench top is just dropped down over the tenons cut on top of the legs into some rather loose mortices in the underside of the top. My top is 2 1/4 inch thick Ash and is 5 ft long by 24 inches wide. I have never noticed it being either loose or moving around during use. I have never felt the need to secure it to the crossbraces with lag bolts.

Joe A Faulkner
10-10-2017, 10:03 PM
Similar to what Pat had done, the legs are mortised into the top, but like Eric, there is a stretcher running front to back and one slotted lag bolt in the middle of the stretcher. In theory, I could remove the lag screws and simply lift the top off. No issues with racking. Bench is 7' long, 23 1/4" wide, 3 1/4 thick top.
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