Comments on: St. Peter’s Cross and the Ultimate Leg Vise

By: Zach

Zach — Wed, 16 May 2012 12:19:35 +0000

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

By: Paul Moldo

Paul Moldo — Mon, 07 May 2012 10:50:25 +0000

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/

By: wizz

wizz — Wed, 02 May 2012 22:40:16 +0000

Here is vice that I picked up from a want ad, Just needs some new wood and put back to work.
http://www.picturetrail.com/sfx/album/view/24103650

By: Steve_OH

Steve_OH — Wed, 02 May 2012 12:31:31 +0000

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

By: okrejci

okrejci — Wed, 02 May 2012 08:21:52 +0000

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.

By: randyk

randyk — Wed, 02 May 2012 04:21:30 +0000

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.

By: Steve_OH

Steve_OH — Tue, 01 May 2012 19:07:38 +0000

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

By: McDara

McDara — Tue, 01 May 2012 19:06:41 +0000

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?

By: JayWC

JayWC — Tue, 01 May 2012 17:52:40 +0000

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.

By: JayWC

JayWC — Tue, 01 May 2012 17:37:49 +0000

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.