Leg Vise Chop - Geometry

[Kevin Hampshire]

I’m starting a Moravian Workbench build (ala Will Myers) and plan to place the chop on the leg versus the original Old Salem inspired vise design.

I’ve sketched up a full-scale leg drawing and realized that I don’t know the optimum distance/ratio to use between the top of the bench, center of the screw and the parallel guide at the bottom of the chop.

The leg is sloped at 15 degrees and I’m choosing between (I already have both) an antique wooden vise screw and a Lake Erie Toolworks wooden screw. While I’m not concerned about the screw from Nick Dombrowski, I don’t want to stress the antique screw if I decide to head down that route.


My current plan centers the screw 11 ½” below the top and has the parallel guide at 31’’ below the top which is 7” above the floor. This is all on the centerline of the sloped leg, and those distances are based on the leg and stretcher structure that Will Myers drew. I have a couple of inches of slop to move things up or down.


Has anyone done the math (I’m not an engineer) or calculated the optimum distances for these three points?



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[Mark AJ Allen]

how do you define optimal? the most force? Clamping the widest boards?

If you want to balance your force applied with the widest clamping depth, you will put the screw 1/2 down the chop.

[Lud martinson]

Fine Woodworking's Tools and Shops #230. Jeff Miller's cover story of a revised Roubo workbench worked for me. It should at least help

[Normand Leblanc]

Kevin,
Do I understand correctly that your bench height will be 31" + 7" = 38"
That's kind of high, no? Unless your very tall

[Prashun Patel]

I could be wrong here, but the longer the lever, the more force it exerts on the load.

In a leg vise, I thought the screw is the fulcrum, the load is the piece in the vise, and the lever is the parallel guide.

Therefore, I'd think you'd want the maximum distance between the screw and the parallel guide - in fact, wouldn't it ideally be just barely above the floor?

To exert maximum force you'd want the screw as high as possible, but that interferes with clamping depth, so you want it just below your usual clamping depth.

[Warren Mickley]

I built a Pennsylvania Dutch bench in 1979 that I still use. The front legs and the vise are slanted at 15 or 20 degrees. The Moravians moved to Wachovia (Old Salem) from Pennsylvania in the 1750's. A slanted vise is much to be preferred. My screw is centered 7 inches below the top and the guide is about 6 1/2 from the bottom. I can recommend these figures.

A screw lower down would have more free space above the screw (probably more helpful for a vertical leg vise), but a bit less holding.

[Prashun Patel]

You'll get more seasoned responses than mine, but I also should add that a leg vise with a lower screw (as is mine) has great depth of holding. But you're forever stooping over to tighten and loosen the screw. For 95% of my work @ the leg vise I would appreciate having a higher screw. I do find it fatiguing and a little awkward for heavier things where I need to position a piece while tightening the screw.

This will give you considerably more force.

My bench (as yours) has a slanted leg and chop. This means I can hold tall things on the top corner of the vise, so they hang next to the screw. This keeps the work close to the top of the bench, so there's less vibration, and keeps the bottom out of the path of the vise so the lever acts on the top, not the bottom of the piece, so you get more force with less effort @ the screw. It of course means you might have to deal with racking horizontally, but the work is usually close enough to the screw that it's not a big deal.


For clamping long panels with a dead man, the high screw does not interfere because the piece will hang next to the screw while being pinched in the top corner of the slanted vise. The weight would of course need to be supported by leg dogs or your deadman.

All this is to say, if I could do it again, I'd keep the parallel guide super low, and the screw at a comfortable turning height a little higher than I have it now..

[Kevin Hampshire]

Thanks for the replies, they actually helped me get my head around what I might want out of this vise.

Mark, that's a very good short answer and it will help. How often will I edge plane 12" wide or wider boards? In my case, I do like to do panels so I'll look at how I'll hold them.

Lud, I'll have to check if I have that Jeff Miller article.

Normand, good catch. I'd worried that I hadn't made it clear that all those measurements were along the 15 degree sloped line. I'm 6'4" and shooting for a 36" bench height but I've been keeping an eye on the structure to allow for some room so I can remove some height from the bottom of the legs to shorten the bench. I have a 5'7" nephew that I may want to accommodate in the future.

Prashun, you actually touched on a couple of my concerns. Bending over to adjust the vise at the parallel guide but also the screw. I'd like to avoid any bending over but the original Moravian design doesn't work as well with the criss-cross so I'm going to try the parallel guide. I'll take a closer look at the height of the screw and see if it should be raised higher.

Warren, interestingly my antique wooden vise screw and chop are all I have of what I think was a Pennsylvania German bench. The vise screw on it is much higher than what I'd drawn for the Moravian and that's what got me thinking more about the distances and leverage. If I use the antique screw, I think I'll want to minimize the strain on it but I plan on incorporating some counter sinks that will allow a quick mod to a the Lake Erie screw and nut.

[Warren Mickley]

Kevin, with a slanted leg vise, you can have the screw quite a bit higher because a wide board can be held securely with just the upper right corner of the vise. A 16 inch wide board for example can be butted up against the screw and held. On a vertical leg vise. the vise tends to rack unless the board is above the screw.

[Warren Mickley]

For a board to be held securely, parts of that board ought to be on both sides of the line running from the guide through the center of the screw. If you have a board held vertically in a 15 degree slanted vise and a 2 1/4 inch screw, the screw needs to be over 8 inches from the top of the bench for this to occur. Twelve inches is adequate.

[Phil Mueller]

Good info already. Just another measurement to toss your way; my leg vise screw is 9" from the top, and the parallel guide is 1/2" from the floor.

[Kevin Hampshire]

Full-scale drawing of leg and vise. I don't know if you can see the pencil on this red builders paper. Maybe if you open and zoom the graphite lines will show. Excuse the 2 x 4...it's showing the 3 1/2" thick top.

That's the antique chop that I can't use with the Moravian bench because of the stretchers between the legs.

I haven't measured, but I must have around 8" vertically from bench top to center of vise screw.

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[Kevin Hampshire]

Okay. , I took a closer look at my full-scale drawing of the Moravian workbench leg and feel that the dimensions seem to be okay as I originally drew them.

- Bench Height - 36" exactly (measured vertically) with maybe 4" of slop to adjust height. I currently have a 34" bench and 35" to 35 1/2" is looking like a good height

- Vise Clearance - 9 1/4" above the vise screw to the top of bench (measured vertically). I may adjust the top stretcher to see if I can raise the screw another inch higher

- Parallel Guide - 6" above the floor (measured vertically) which gives me 19" from the screw along the 15 degree line


I took another shot with the screws and nuts. The Lake Erie nut fits exactly between the two shorter leg stretchers.
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[Curt Putnam]

Suggest you "talk" to Jim Ritter (Ancora Yacht Service) about his chain leg vise and the measurements he's made with pressure gauges about screw and parallel guide placements. Jim is a member here but it is spring boat season so his time is scarce ATM.

[Jim Ritter]

I'm here Curt, thanks. I try and keep an eye on the bench and vise threads but sometimes I can't say anything due to the forum rules about self promotion. I'm glad to answer questions however.
The leg vise can generate tremendous force. I measured mine made with a LN screw(no longer available) and 6" above the screw was well over 1000 pounds per square inch. If any one wants I can post the photos. The lower the parallel beam the less force on the beam. It is a class three lever the beam is the fulcrum and the screw is the force.
Jim