Chris Schwarz's Blog

St. Peter’s Cross and the Ultimate Leg Vise

If a leg vise has a disadvantage – and I’m not quite willing to admit that it does – it would be its parallel guide.

The parallel guide is a strip of wood at the bottom of the leg vise that prevents the vise’s chop from spinning like a propeller. It also creates the vise’s leverage and makes it pinch your work right at the top of the chop. However, to do its job the parallel guide requires that you move its pin when you want to hold really thin stock or really thick.

And moving that pin is something that would-be bench builders don’t like. They always say something like: I want a leg vise, but I don’t want to stoop all day moving that pin.

If you are one of those anti-stooping skeptics, then you should take a close look at the photos, drawings and video in this blog post. The St. Peter’s Cross (sometimes called “Croix de St. Pierre”), eliminates the parallel guide and the pin and the stooping. I first wrote about this bench accessory in my 2007 book “Workbenches: From Design and Theory to Construction and Use” and showed a drawing of one from the old book “The Amateur Carpenter and Builder” (Ward, Lock & Co.).

Lots of readers have asked about the cross in the last five years. Some asked: How does it work? Does it really work? It doesn’t really work, does it? Why would you discuss something that doesn’t work? But most readers asked: Where can I get one?

Here are the answers: It works like scissors. Yes. No, it works. Because it works. Benchcrafted.

Yup, soon the bench geniuses at Benchcrafted will be offering a cast St. Peter’s Cross that you can install on your workbench and eliminate the parallel guide and pin and stooping. Jameel Abraham and his brother, Father John, brought an oversized prototype of the hardware to the Lie-Nielsen Hand Tool Event in Chicago to show off. They had it paired with their Glide leg vise hardware, and the action was nothing short of magnificent.

Jameel says they are working with a patternmaker now to make the patterns for  castings and they hope to begin offering it by the end of the year. No firm word on pricing. But it will be easy to install on any leg vise, either new or as a retrofit.

I ordered one so I can install it on the Roubo bench I built in 2005.

There are a lot of interesting details to discuss about this hardware. How is it best attached? Where did its name come from? Is it a French innovation? Can you make one from wood or lawnmower blades? But I have to get to the second day of the show right now.

So stay tuned this week for more entries on St. Peter’s Cross.

— Christopher Schwarz

42 thoughts on “St. Peter’s Cross and the Ultimate Leg Vise

  1. Zach

    Oddly-named cross. It reminds me more of St. Andrew’s cross than St. Peter’s. St Andrew’s cross (also called a Saltire) is x-shaped, is the flag of Scotland, and makes up the white on blue cross in the Union Jack.

    Bit of pointless trivia, I’m sure :)

  2. Paul Moldo

    I made a St. Peters’ Cross for my Roubo leg vice and it doesn’t stay parallel (which is its main purpose), when engaging the workpiece. the bottom part of the vice continues to move inward. Full details are on my blog site. Any suggestions will be appreciated.

    http://www.youtube.com/watch?v=uyTRxNYE874 A short video on YouTube. The video is sideways so look to the right of the picture to see the bottom going out of parallel.

    http://awalkinthewoods-woodworking.blogspot.com/

  3. randyk

    I noticed that the chop in the first picture of this vise had something attached to it. Is it to prevent slippage and what is it? The reason I ask is that I have built a leg vise based on the LVL bench by Christopher Schwarz and have had some slipping problems. The chop is made from maple and it butts against maple on the bench and it slips more than it should.

    1. Steve_OH

      The vise jaws are faced with leather. It makes a big difference.

      Benchcrafted uses suede; my leather supplier recommends full-grain cowhide, with the outer smooth surface glued to the wood and the inner fuzzy surface as the actual clamping surface.

      -Steve

  4. McDara

    Forgive my dim-ness, but what is the advantage of a leg vise like this over a basic face vise? I assume it is the distance from the screw to the top of the jaw, or is there some extra leverage applied by going down to the floor?

  5. JayWC

    The boys of Wood Talk Online (Marc, Matt and Shannon) discussed this vise in Episode 96. I believe it was Shannon that mentioned there may be a trade off with this vise in that you can’t put stock under the screw in the way you could with the traditional vise. Other than that, I think they covered it very well and were complimentary to both vises.

  6. Danny H.

    I have their sliding tail vise installed on my personal bench and I love it, but wasn’t able to use their leg vise since the Nodin adjustable leg hardware makes difficult if not impossible to incorporate. I’m thinking though that with this scissor design I may be able to make it work, possibly by making the bottom attachment a quick release bolt for when I want to change the bench height from other than it’s normal height position. What think Ye ?

  7. cbf123

    It may not make much difference, but you’re going to lose some clamping power compared to the parallel guide.

    With the parallel guide the pin at the bottom gives you a long lever arm, for maybe a 2:1 mechanical advantage. (10lbs of pressure at the screw gives 20lbs of pressure at the jaws)

    With the cross, the effective lever arm is the distance to the pivot point of the cross, giving a reduced mechanical advantage (and interestingly, the amount of reduction varies depending on how wide the vise is opened.

    In practice it may not matter much though.

    1. JayWC

      CBF- I understand what you’re referring to, but that’s not clamping force. That’s torque and that’s not what we’re looking for in a “clamp”. In this case, the longer lever arm of the tradition vise will generate more torque, but that’s measured in ft/lbs not psi.

      To be more clear, clamping power would be measured in psi. In both clamps the compressive strength of the vise is generated by the screw. If both clamps had the same screw it would generate the same amount of force regardless if it’s the traditional pin or the crossing design.

      1. JayWC

        Not to beat it to death, but the new one will actually close flatter (jaws parallel automatically) which should also hold the work piece better.

      2. Steve_OH

        Since the horse isn’t quite dead yet…

        To clarify, it’s actually the bending forces (combined compressive and tensile) induced in the clamp jaw that provide the clamping force. This is easy to demonstrate: Replace the jaw with one that’s identical, except that it’s only 1/8″ thick. Now, you can’t generate much clamping force, no matter how strong any other parts of the vise are, and no matter how much you tighten the screw, because the jaw just bends around whatever it is you’re trying to clamp. There’s just no way to generate enough bending stress in a 1/8″-thick jaw. (It will break first.)

        And, as you say, what’s going on below the screw is essentially irrelevant, at least in practical configurations. In principle, however, you can generate more (parallel) force at the clamp jaw if you “preload” the vise by moving the bottom end of the jaw further out and cranking down so hard on the screw that the vise jaw is visibly bent. Of course, in a real vise, something is going to break if you try that.

        The amount of bending force induced in the clamp jaw is a function of both the cross sectional profile of the jaw and the length of the lever arm from the point at which the moment is applied to the point of clamping (i.e., the distance from the screw to the top edge of the jaw.

        Working out the detailed equations is left as an exercise for the reader…

        -Steve

      3. okrejci

        Actually leg vises are class three levers, like tweezers, and doomed to a less than 1:1 mechanical advantage because the resistance to fulcrum distance is always greater than the effort to fulcrum distance; hence, 10 pounds of force at the axis of the screw will yield about 8 pounds at the jaw tips.
        As for pressure, pounds per square inch depends on the surface area: a tiny C clamp can apply 30 pounds of force, may be, and create over 270 psi on its 3/8″ diameter pads, enough put a decent ding into most woods, a bar clamp, with 3″ by 3″ pads, would have to generate over a ton of force to achieve 270 psi.

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