Chris Schwarz's Blog

Thanks Be to Mongo

For this Roubo workbench to work, I’ve got 16 joints that have to come together all at once. There is not an option to glue things up in stages and still guarantee success.

As a result, I tried to give myself some wiggle room. I fit the edge cheeks of the tenons a little looser than normal so they could shift around. I made my tenons’ face cheeks a shade thinner than when I build a typical door, for example. And I cut the through-tenons so that I will have to wedge them.

That said, getting this bench together for a dry-fit required Mongo, the 2-pound English mallet. I tried my other wimpier mallets first. I would have been banging all day with the little guys.

But Mongo made it happen.

Each leg had to be pounded about 20 times to get it to seat. Then I clamped the lower stretchers to the legs to see how all the shoulders looked. The shoulders on the front stretcher look good. The side stretchers each have a gap I need to tend to. The rear stretcher is just going to have to learn to be different. (Like the kid in my high school who had a weird affection for live poultry.)

Next up: Fix the shoulders (if the workbench will come apart), start drawboring the joints and select a glue with a two-day open time.

But I’m about to hit a wall. This weekend I’ve got to build a birdhouse. Yup, you read that right.

- Christopher Schwarz

32 thoughts on “Thanks Be to Mongo

  1. Niels Cosman

    Adrian,
    Hxtal is a super high purity epoxy that was developed for archival restoration of glass. In my opinion is the best glue on the market for glass- it’s water clear, strong, and versatile… and very very expensive.

    It’s open time and curing are proportional to the ambient temperature in the room. At 70 degrees It’s reasonable to say that the open time is 24-48 hours, meaning that that’s how long you have before you get out the epoxy solvent (nasty nasty stuff). You can also turbo charge he process by heating the glue with heat(lamps) up to 110 degress, reducing the total cure time to well under 36 hours. When mixed, hxtal is super runny (like water), however let sit for 4-6 hours it becomes thicker than molasses. The ultimate bonding strength is not affected at this point allowing you to customize thickness for the task. when it is so thick that that gravity barely has any effect on it- it’s like glass spackle. neet!

    HOWEVER… I’ve only used hxtal glass to glass. Other glues are recommended for glass-to-other bonds. It’s my understanding that is because hxtal is an extremely rigid glue. That inflexibility can cause to problems (self-destruction) between materials with differing coefficients of expansion. Over large surface areas, this is a problem even between different glasses. Also did i mention the price, it’s pretty expensive.

    wow, sorry for rambling- sort of got carried away there.

    The folks at HIS Glassworks distribute hxtal and are fountains of knowledge when it comes to it’s preparation and application. Call them up and ask if how it works with wood, they are very friendly.

    Cheers,
    Niels

  2. adrian

    It took me 45 minutes to assemble my last project, and I used fish glue, which worked well. (Advertised open time of 60 minutes.)

    But when I mentioned the need for a long open time adhesive someone suggested an epoxy called HXTAL. Since the workbench already makes use of epoxy, perhaps you should consider using this product. The information I’ve encountered is a little vague, but it appears to offer an open time of about 24 hours. One set of instructions suggests letting it thicken for "several hours" after mixing before use, for example. It is seven days to full cure.

  3. greglease

    I’m also interested in the concept of drawboring without glue, especially if the through-tenons are set with wedges. I’m working on my first real workbench (all hard maple) based loosely on Garrett Hack’s recent design. In assembling the 2 ends of the stand the cross braces need to be set and then the double through-tenons of the legs set into the feet and tops and wedged. I already planned to drawbore each joint, as much for the look as for stability, but the concept of leaving them glueless would sure make things less frantic in the final assembly of the ends.

  4. AAAndrew

    This is a serious question, maybe not an intelligent one, but serious.

    What is the possibility of this coming apart just as it is, without drawboring, without glue?

    There is so much tension built into and working on the whole structure. Each piece is holding the other pieces in place because of tension from somewhere else. Think of the amount of force it took to get everything in place, and then think that the same amount of force, or more, will be required to work it all loose, applied in very specific amounts at very specific locations in a fairly specific order.

    I ask this in all seriousness because when I built mine, I didn’t glue the long stretchers, nor did I do anything to the blind mortises into the bench top but get the tenons in. There is so much mass and natural friction that I can’t imagine what I’d have to do to the bench to really make it even begin to work loose.

    I wonder if some of it is because of the shorter size. Mine is only 5′ long and looks similar in scale to yours. Without the great lengths of bench top, I suspect the racking tension and other forces working on the base and base-to-top connections is less.

    Just a thought. Perhaps you’ll try a glue-less bench next time. :-) (ducking and running)

    AAAndrew

COMMENT