Forces in a Leg Vise
For as long as I can remember I have always read about the remarkable clamping force generated by leg vises. When I began to design leg vises I found out that this is not necessarily the case. If you think I am going to start bad mouthing leg vises you are wrong. I like leg vises but you need to understand the shortcomings and not overlook their faults. I like to understand all the forces involved in the leg vise so I can make it operate at its maximum potential. Let’s take a look at the forces generated in a leg vise and see what is really going on.
A leg vise is typically characterized by a very deep throat (the distance from the very top of the vise jaw to the screw or clamp shaft) of 8” or more. Where you clamp the work piece in the leg vise greatly affects the clamping force generated. If you clamp a board so that the bottom portion of the board rests on the vise screw then nearly all the clamping force will be applied to the work piece. See FIG. 2. If however you clamp a relatively thin board flatwise at the top of the jaws then you will have reduced clamping force. See FIG. 1. This is the worst case clamping scenario for a leg vise. When I design a leg vise this is the clamping configuration that use.
Now let’s go through some simple math and figure out just what’s going on in a leg vise. From my last blog we found out how much force is generated by a vise screw. We will use the last example, a 4 TPI (threads per inch) screw with an 8” diameter hand wheel. This screw will generate approximately 1000 pounds of clamping force when you apply 10 pounds of force to the rim of the wheel. For the vise jaw we can assume a 9” throat and 16” from the screw centerline to the parallel guide pin board. The distance from the screw centerline to the parallel guide pin board is called the fulcrum length. As force is applied to the work piece to clamp it the bottom of the leg vise wants to pivot in as if the jaw was hinged to the work piece so we counteract that pivoting by pinning the lower parallel guide. What we end up with is a screw force acting in towards the bench and two reaction forces acting in the opposite direction. One of the reaction forces occurs at the work piece and is used to clamp and the other occurs at the parallel guide and is wasted on stopping the jaw from rotating. To calculate the forces a simple proportion will give you the reaction force and the parallel guide force. Here is an example for the worst case with a board clamped at the top of the jaws as shown in FIG. 1.
CLAMPING FORCE = SCREW FORCE X (FULCRUM LENGTH / OVERALL LENGTH)
The fulcrum length is the distance from the screw centerline to the parallel guide pin board. The overall length is the throat length + the fulcrum length. For our example this is: 9” + 16” = 25”. So for our 1000 pounds of screw force we get:
CLAMPING FORCE = 1000 X (16/25) = 640 pounds of clamping force.
The parallel guide force is simply 1000 – 640 = 360 pounds. The parallel guide force and the clamping force must equal the force applied by the screw. This 360 pounds of force is wasted on preventing the jaw from rotating.
Now let’s take a larger board that is almost resting right on top of the vise screw as shown in FIG. 2. The throat length is now 1” and the overall length is 17” (1” + 16”). With the same screw force of 1000 pounds we now get:
CLAMPING FORCE = 1000 X (16 / 17) = 941 pounds of clamping force!
Obviously from our calculation it makes a big difference where in the vise jaw you clamp your work piece.
Let’s look at the effects of a longer fulcrum length. In our previous example our fulcrum length was 16”. What happens if we increase it to 24” while keeping everything else the same and clamping at the top of the jaw?
CLAMPING FORCE = 1000 (24 / 33) = 727 pounds of clamping force.
With the 16” fulcrum length the clamping force was 640 pounds. Increasing the distance between the screw and the parallel guide board will definitely increase the clamping force of our theoretical leg vise. We will never get the full 1000 pounds of screw force but to get the most out of your leg vise make the fulcrum length as long as possible.
So let’s summarize what we have discovered and look for ways to optimize our vise design. Leg vises do not generate remarkable clamping forces, in fact they are the least efficient of any vise type because of the wasted force on the parallel guide. There are three ways to compensate for the reduced clamping force; Increase the number of threads per inch (TPI) of the screw. A 4 TPI or greater screw will help reduce the amount of force you have to apply. Increase the length of the vise handle. The increased leverage will reduce the amount of force you have to apply. Two tooth per inch wooden screws typically have a very long vise handle for this very reason. If you have a hand wheel you are just going to have to apply more force by hand.
A shorter throat will generate more clamping force. When possible you can also clamp your work lower in the vise jaws. A longer fulcrum arm will similarly generate more clamping force. Obviously the fulcrum length is limited by bench height and other factors but longer is definitely better.
Leather lined jaws (or anything that will increase friction and offer some compliance) are a requirement on a leg vise for two reasons; the increased friction helps overcome the reduced clamping forces generated. The compliant leather lining helps grip when the jaws are not exactly parallel with the sides of the part being clamped. This is especially true for leg vises which use a parallel guide pin board because it is difficult to get the jaws precisely parallel with the work piece.