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

45 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 :)

    1. rjm60

      It is a St. Andrews Cross and I don’t think it’s pointless at all. The correct name for this device would be St. Andrews Cross since it does resemble a saltire. St Peter’s cross is an inverted Latin cross, or a Petrine cross, and looks nothing like the scissor device used in this leg vice. So, if we’re going to define this device by comparing it to a cross, St. Andrews would be the correct one.

  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

        1. rjm60

          I know this post is old but some clarifications are needed.

          If you want to get technical, the correct term is bending moment, not torque or bending force.

          If you (carefully) draw a free body diagram of the chop (and then solve the resulting equilibrium of force and moment equations) several things become apparent.

          Assuming that the components do not bend or twist, for the scissor mechanism, the clamp force is equal to the load applied by the screw, no matter how far the object being clamped is placed from the screw.

          What does change is the magnitude of the reaction forces exerted by the scissors. The closer the object to the screw, the smaller the load on the scissors.

          Scissor loads are also minimized by keeping the top pivot as close to the screw as possible and by making the scissors as long as possible to keep the bottom thrust point as far from the screw as possible.

          The maximum bending moment in the chop stays the same since a longer chop (i.e. lever arm) results in a lower reaction force at the scissor thrust point at the bottom of the chop. Higher screw load (i.e. tightening the vice) increases the maximum bending moment.

          Another interesting thing that happens is that clamping wider objects (bigger jaw opening) also increases the reaction loads since the scissors become “shorter” (i.e. vertical distance between pivot at top and thrust point at bottom of chop).

          With the scissor device, the clamp load is equal to the load created by tightening the screw (screw load); however, with a conventional guide and pin leg vice, the clamp load is reduced by the reaction load exerted by the guide and pin at the bottom.

          Ignoring gravity, a conventional leg vice chop has 3 forces acting on it: the force from the screw, the force from the object being clamped, and the reaction force from the guide at the bottom.

          A scissor leg vice has 4 forces with the guide force being replaced by the 2 contact points of the scissor mechanism. This 4th force cancels the load at the bottom and lets the clamping load be equal to the load applied by the screw.

      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.

        1. rjm60

          This is true for a “conventional” leg vice (with a guide and 3 points of force application) but the reduction is minimized when the clamped object is placed closer to the screw.

          For a scissor style vice (and a chain style as well), this is not the case. The clamp load is equal to the load applied by the screw. This results from a 4th point of force application which cancels the reduction created by the reaction force at the bottom of the chop.

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