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Reader Jon Pile writes about the parallel guide bar in a leg vise: “The parallel guide bar is, in my humble opinion, a cruel joke perpretrated by some historical prankster. I have been using an angled leg vise for a couple years now, and removing the guide bar was the first modification.
Instead, grab a piece of maple, cut it down to 1″ x 2″ x 3″, and let it dangle on the bench leg with a foot of butcher’s twine. When you open the jaws of the vise, just turn the block so the appropriate side acts as a standoff.
Wider stuff? Find a wider piece of scrap. Skinny stuff? Swing the block out of the way altogether.
Infinitely variable. Simple and bulletproof , and I can reach down and adjust my “parallel guide,” left-handed, without looking at it.
Please, I’m begging, don’t let the swiss-cheese-looking parallel guide insanity continue any further!
As for thickness (of the leg vise’s jaw), I started with a 2×6 of white oak and found that it still flexed too much! I doubled up on thickness , it’s 3-1/4″ thick now , and finally I’m happy. With this setup, I have applied enough torque to shear off the 3/4″ maple pin that held my bench top to the legs. Also easily repaired and improved.”
I tried the Pile Block (patent applied for) in my leg vise today. I made it exactly as he described and tied it through a holdfast hole in the leg that I use when dovetailing.
The block works as advertised, which is no surprise. Levers and physics work as advertised. But I’m going to need to develop some muscle memory with the block before I rip out my parallel guide. The nice thing about the parallel guide is that I work in stock that is 5/8″, 3/4″ and 7/8″ thick.
So for the most part, I never move the pin in my parallel guide. It stays in the first hole and can clamp the usual stuff. (By the way, that sweet-spot hole in my guide is Ã?½” from the inside of the vise jaw).
A few other details: I wondered if the block would be handy with an angled leg vise (which is what is on my new English Workbench). With that bench, the parallel guide prevents the jaw from spinning when you crank the vise’s handle. Jon responded to that by saying that his leg vise was angled, and that he merely had the foot of the jaw resting on the floor, which kept it from spinning.
I think the real sexy solution to the parallel guide is St. Peter’s Cross, a French invention that (apparently) revolutionized the leg vise. Even the British raved about St. Peter.
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Eric,
Thanks for asking the question and I would also like to see what a St. Peter’s cross is. I did a search for it on the web but could find nothing that shows it in use on a work bench or vise. Most references are to religious objects.
Eric,
Indeed it is. I’ll post a photo in a future entry. I have some good illustrations of it from the 19th century.
Hi Chris,
What exactly is a St. Peter’s Cross? Is it that scissoring mechanism, that no one seems to sell commercially?
-Eric
Aargh. It’s a cube, not a fourth power. So cutting the length of the jaw’s unsupported section to 80% would reduce the deflection to half; reducing the length to half would result in 12.5% of the original deflection, and the effect of doubling the jaw thickness would be equal to the effect of halving the unsupported section length.
Sorry — I first used these equations for designing bookshelves, and the fourth power relationship is for beams with constant load per unit length like a filled shelf. There, the load increases as the shelf gets longer — a 20" shelf might need to support 20 lbs of books, a 30" shelf 30 lbs, etc. From my experience, typical hardcovers weigh about 1 lb per inch, although textbooks, dictionaries and other works with thin or glossy paper tend to be denser as well as taller and wider.
I now return you to your regularly-scheduled topic.
If you want to make the beam stiffer, you might want to try a L, T or I shaped cross-section — the stiffness increases as the cube of the beam thickness (IIRC), so you can get the same effect with less wood. Even a thin jaw joined to a 1×4 on edge should work better than doubling the jaw thickness. (Use a creep-resistant glue like epoxy if you plan to leave workpieces in the vise for a long time since white glue will tend to "slip" when it’s stressed for a long time).
The other effect that’s huge is the unsupported length of the jaw (from the guide to the screw or the screw to the jaw’s working area). Once again, if I recall correctly,this is a fourth-power relationship, so shortening the distance between the screw and the end of jaw by 20% will reduce the deflection to ~40%. Cut it in half, and the deflection will be smaller than if you doubled the jaw thickness.
Of course, that’ll affect the functionality of the vise, but it’s a really good idea to keep the unsupported portions of the jaw as short as possible.
Nice low tech solutions from John. It’s good to be reminded sometimes that there is more than one way to succeed, and some of them are quite simple.