Another Leg vise question

[Paul Erickson]

I am in the process of building the base for my new bench which will include a leg vise with chain adjuster like the one Jim and Derek have demonstrated elsewhere. I am wondering what considerations people used when they determined how far down the leg vise to place the screw. Obviously the longer the chop below the screw the greater power potential, and you don't want it too low so that you are bending over too far to utilize it. Too close to the top would limit it's functionality.. I am thinking between 8-12" below the top of the bench. Any other reflections would be appreciated.

cheers, Paul

[Jim Matthews]

I'm using a 6 inch diameter cast iron wheel on my leg vise.

If you employ a vise with a traditional handle (like the big wooden vise)
you still want to keep the center of rotation high enough that you can
spin it without stooping.

From a standing position, I reached down with the fingers of my left hand extended.
That's the external limit of the wheel. I placed the wheel such that the rim extends to that point
and used the wheel to mark the center. I'm 5' 6" and that center falls 10 inches below my bench top.

If I had used a larger wheel, I could lower the centerpoint, but it would be one more thing
to run into in my shop. It's a compromise between mechanical advantage and clearance.

[Jim Ritter]

Hi Paul, let me try and clear up some misconceptions, it has been raised before but i never addressed it. Let me give it a go. The longer bottom to the chop does not make a more powerful vise. It is a simple lever. The screw is the force, and in our case the pin is the fulcrum. The reason to have a longer bottom on the chop is to lower the forces on the pin, or in my case and yours with a chain that is a very good thing. The closer the screw is to the top of the vise the more powerful it is. If the clamping force at 6" above the screw is 1000# and the beam is 18" from the screw, the force on the pin is 1/3 or 333# because it is further away. If the beam is only 12" away the force goes up to 500#. Likewise if the pin was only 6" away the force would be equal or 1000#. These are measurements I took from my vise before I sent Derek his set to install because his beam was much closer than mine and I was concerned about the rising forces.

This is the force about 6" above the screw on my bench with the chain.
http://i1135.photobucket.com/albums/m628/boatman53/04003778.jpg

This is the reading just above the beam about 18" below the screw.
http://i1135.photobucket.com/albums/m628/boatman53/08084edf.jpg

This is the reading 12" below the beam where Derek's chain would be.
http://i1135.photobucket.com/albums/m628/boatman53/33462096.jpg

This I another prototype chain vise but the jaw is shorter at only 5". The beam is at 18" on this one also.
http://i1135.photobucket.com/albums/m628/boatman53/e5a75977.jpg

With the chain adjuster there is no need to bend down and move a pin so plan for a lower beam and lower the forces on the chain.

[Paul Erickson]

I'm using a 6 inch diameter cast iron wheel on my leg vise.

If you employ a vise with a traditional handle (like the big wooden vise)
you still want to keep the center of rotation high enough that you can
spin it without stooping.

From a standing position, I reached down with the fingers of my left hand extended.
That's the external limit of the wheel. I placed the wheel such that the rim extends to that point
and used the wheel to mark the center. I'm 5' 6" and that center falls 10 inches below my bench top.

If I had used a larger wheel, I could lower the centerpoint, but it would be one more thing
to run into in my shop. It's a compromise between mechanical advantage and clearance.

Hi Jim,

Where did you get the cast iron wheel? I am looking for something similar to use with a couple of threaded rods I have for a Maxon vise.

cheers, Paul