Monday, May 20, 2019

Ash log Roubo project 2, legs and mortises

It is with a slight embarrassment that I discovered that I forgot to blog further about the build of the Ash log Roubo bench, so I'll try to spread the rest of the pictures and information over a few blog posts to make up for that.

I had read in the Roubo book, that the legs should be rectangular, which puzzled me a bit, since they are square on all the drawings. But I decided to follow the written instructions. So I made the legs 4x6".
I made a little shoulder on the inside of the legs, and then sawed out for the large double tenon with the angled front.

I used the chain mortiser to make the mortises in the top. I had to make a new attachment to get it to work that way, but a bit of plywood and a few small strips of wood was all that was required.
The machine took care of the bulk of the work, and there was just a little bit of cleaning up in the corners of the front mortises that had the corners defined by a saw cut.

Once I had made all the mortises in the top, I remounted the chain mortiser in its stand and made the mortises in the legs for the stretchers.

Making the tenons on the stretchers was easy, basically a bit of sawing and chopping.
Prior to making the tenons though, I had drilled a hole in the two legs destined to become the front legs of the workbench. And I had threaded those holes with my homemade tap.
My plan was to make sure that the bench could be used for boat building (just in case), so I wanted to make two leg vises on the front.

I had decided to use as much ash from the same tree as possible, so the legs and stretchers were all from the same trunk as the top itself. It was far from bone dry, and in the end one of the stretchers had twisted a bit - but not enough to stop me.


Making a double tenon on a leg. Notice the shoulder.

Marked up and ready for sawing.

Portable chain mortiser attachment.

Drawbored stretchers.

4 comments:

  1. The grain in that ash is beautiful. But I can see it might be difficult to work with.

    Did you have any problems with tear out using the chain mortiser? How did you mitigate that?

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    1. Hi Brian
      I didn't have any tear out from using the chain mortiser. I first defined the ends of the mortise with a chisel prior to plunging the mortiser into the top. That made a really nice exit hole.
      Also this mortiser works like the one that I borrowed from Olav with the chain going "with the grain"

      brgds
      Jonas

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  2. We don't know for sure which type of projection was used on the Roubo plates. 'Cabinet projection' or 'cavalier perspective' an which reduction factor?
    Interesting link:
    https://en.wikipedia.org/wiki/Oblique_projection

    Now, on some other Roubo plates, perspective with vanishing points is used (e.g. plate 314).

    Taking measures of the end vise (on my PC screen after zooming) of plate 280
    https://archive.org/stream/gri_33125009321973#page/n809/mode/1up

    I it seems that the reduction factor for the Y axis is about 3/4.

    which would give for the foot rectangle a ratio of about (20 mm X 4/3) /40 mm or 2.7/4 which correspond to your 4 X 6.

    Congratulations

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    Replies
    1. Hi Sylvain

      Thanks for the interesting links!

      I was a bit puzzled because the the plates seem incredibly accurate compared to some other old texts where the plans are wildly off regarding measurements.
      And The Rooubo text suggests 4x6 but the legs seem to be square on the plates.
      Not that it will matter much - I think that 4x6 ash will still be able to hold up the top :-)

      Brgds
      Jonas

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