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This, I figured, is a good thing. It will do well enough for the purpose, and I really don't like oak anyway (though that bit of elegant flake showing on the visible top edge is really very nice).
Design considerations There are four basic parts to the shoulder vise fixture: the box, the arm, the screw, and the jaw. Just one jaw to the fixture, the other jaw is the side of the bench. For a screw I used a 9" veneer press screw, which is about the same diameter and feed as my host vise screw; I figured a real shoulder vise screw would put on so much pressure that it would just bust my poor little built-in even sooner. This was quite long enough for my little sketch, and extra screw length is not good if it can be avoided; but a fixture with a thicker arm and more daylight might very well need a longer screw (they also come in 12" and in an even longer length). The box and the arm could be made as separate pieces, the way Jack made it I think, but it seems more natural to me to have one piece serving as part of the arm and one side of the box. The six inch opening of my host vise is way too small. It means that with a 1-1/16" thick arm (after planing), a 3/4" jaw, and something like an inch to allow for the screw hardware, I don't get much more than 3" daylight in the shoulder fixture - enough for the main thing I intend to use it for (planing the end grain if finishing presses) but not really desirable. I thought the 1-1/16" would be thick enough for the arm, supported as it is by the jaw of the host vise, but this was wrong; it deflects noticeably even over the two or three inches between the host vise and the screw. Traditional designs of Tage Frid and Frank Klausz from FWW have 2-3/4" and 3-1/4" arm thicknesses, and I would say that this should also be the arm thickness for a fixture - but that adds another 2" to the necessary daylight of the host vise. Conclusion: a shoulder vise fixture should be made for a **big** host vise, even if the fixture is not overly large.
Looked at from above there is an absolutely classic physics-textbook force situation. On the right the shoulder vise screw pushes outward; in the center the host screw pulls inward; and on the left, the left side of the box resists inward pull, in effect pushing outward. Beautiful, classic situation demonstrating the way force varies as the square of the distance from the fulcrum (the host screw), with the clear implication that it is desirable to move the left side of the box as far left as possible while retaining full support of the bench and host vise jaw. It is also desirable to move the shoulder vise screw as far left as possible so that its force will not be unduly multiplied and the host vise screw will not be strained more than necessary; to do this while retaining the same jaw width, the right side of the box must be moved as far as possible to the left. In my design I moved the left side of the box to the left edge of the host vise, and the right side of the box to just outside the right guide bar of the host vise. Definitely not symmetrical.
In fact, I think it would have been even better to put the right side of the box directly over the right guide bar, thus moving the whole jaw in another inch. There is about a 4" distance between the right edge of the box and the shoulder vise screw; the arm goes only about 2" beyond this, but the shoulder vise jaw (I used a pine off-cut) is about 8" long with the screw centered on it. Apart from the right-to-left racking that is an inherent danger in the design, there is also the possibility of top-to-bottom racking if the entire box sits above the screw. In my dinky face vise, the screw sits fairly high in the jaws. To keep the host vise from racking I made the box as deep as the jaws (actually deeper, but that is laziness) with deep arch-shaped cut-outs so that the box can be dropped all the way down into the host vise. I have noticed that in a cast-iron Record the screw and guide bars sit entirely below the jaws, with jaws that angle inward to balance top-to-bottom racking, so perhaps the box for a Record host vice won't need these deep arches. Certainly, Jack's original shoulder fixture had only very shallow arches, and his vise looked like a Record.
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