Help! My Scissors are Bending!
The above patent illustration is from a May 13, 1845 patent by W.H. Taylor and A.P. Norton of Waterville, NY. I guess the guys in the mid Nineteenth century didn’t like to bend over to adjust a pin either. Scissors mechanisms have been around for a very long time in various forms and have found their way into many household items. The application of a scissors mechanism (sometimes referred to as St. Peters Cross) to a leg vise presents some interesting design and engineering challenges. I have viewed some YouTube video’s with home-made scissors parallel guides that did not function well and the above inventors must have had similar problems because their patent utilizes a rack and pawl mechanism to help restrain movement.
When I built my first prototype scissors mechanism I knew there would be some deflection (or more simply bending) of the links in the mechanism and in fact I calculated what it would be. But the scissors mechanism deflected significantly more than what I had calculated. This deflection allows the bottom part of the vise jaw to swing in and can make it feel as if you are compressing a stiff spring in the jaws. If the deflection is severe enough the top of the vise jaw will pivot away from the part being clamped causing looseness. I knew there had to be something else happening, so through much experimentation I have found the factors that cause the excessive deflection. This simple mechanism from the 19th century turned out to be quite complex. So complex that I had to create an excel spreadsheet to do all the calculations and allow me to accurately predict the performance of a scissors mechanism.
In a leg vise the scissors mechanism is loaded at the lower free end of the link sliding against the vise jaw. In engineering speak we call this asymmetrical loading but in simple terms the lower part of the scissors mechanism has a force on it and the upper part does not. This asymmetrical load happens because we are clamping a work piece at the top of the vise, putting screw force somewhere near the middle and trying to stop the jaw from rotating with the scissors mechanism at the bottom. The scissors mechanism resists this force because the legs of the mechanism always want to remain parallel with one another. We know the force applied to the scissors mechanism will be upwards of 300 pounds and we can calculate how much the links will bend under load. If they are properly sized this bending will only be thousandths of an inch. So let’s investigate now what is causing all the additional deflection.
When the scissors mechanism has the asymmetrical load applied to the lower part of the mechanism it will deflect inward slightly. When this happens it also allows the jaw to pivot in with the hinge point being the board that is being clamped. The upper jaw link pivot is located in the jaw that is pivoting and acts as a multiplier for any jaw rotation. So any movement of the upper jaw link pivot point is going to be multiplied and causes the scissors mechanism to further deflect. I call this phenomenon pivot factor for lack of a better name. It is very similar to what happens in an old time pantograph enlarger which is also a form of scissors mechanism. The jaw also deflects slightly at the pivot point when a load is applied and this is multiplied as well. Additionally, when the links of the mechanism deflect, this small deflection is doubled because it occurs at the center pivot pin and displaces the lower contact point of the leg link. So if you have a typical scissors mechanism leg vise and apply 1000 pounds of screw force the scissors links will only deflect 0.006” but the mechanism multiplies this and actually deflects almost 1/8”. In a screw operated vise you just make sure the top of the jaw touches the work piece first and the bottom of the jaw is angled out, and add a few more cranks to take up the deflection. In a quick action vise you don’t have the option of additional cranks and this is just wasted motion and effort anyway. With this in mind I set out to optimize the scissors mechanism to minimize deflection. Deflection can’t be eliminated entirely but there are a few ways to minimize it:
1. Shorten the links.
2. Use a stiffer material for the link construction.
3. Use a thicker jaw.
4. Decrease the mortise size in the jaw.
5. Strengthen the center pivot of the two links.
Shorter links will be significantly stiffer than longer ones and changing the material from cast iron to steel for example will increase the stiffness by 61%.
A thicker jaw will reduce the deflection of the upper pivot. I have actually measured jaw deflection using a straight edge and a feeler gage. If we have an 8” wide by 1-3/4” thick jaw and apply 1000 pounds of load to it, it will bend almost 1/16”. If we increase it to 2-1/2” thick it will only bend 1/64”. And keep in mind that this is a solid jaw for the whole length. The deflection will increase due to the mortise in the jaw and to a lesser extent any reduction in width.
The center pivot should be very solid and close fitting. Any deflection of this pivot point will be doubled at the bottom of the vise jaw.
Let’s look at two hypothetical designs and compare the performance.
Option 1: Cast iron links, 17-3/4” long, 3/4” thick and 2” wide. Centerline of screw to upper pivot is 3”. The jaw is hard maple, 2-1/2” thick and 4” wide with an 8” throat and will be the same in both cases. With 1000 pounds of screw force applied with a 3/4″ jaw opening you will get 720 pounds of clamping force, the jaw will deflect 0.038”, the links will deflect 0.006” and the total deflection of the lower part of the jaw at the link contact point will be 0.089”.
Option 2: Steel links, 14-7/8” long, 1/2” thick and 1-1/2” wide. Centerline of screw to upper pivot is 2-13/16”. The jaw is the same. With 1050 pounds of screw force applied with a 3/4″ jaw opening you will get the same 720 pounds of clamping force, the jaw will deflect 0.031”, the links will deflect 0.005” and the total deflection of the lower part of the jaw at the link contact point will be 0.074” a 16% reduction.
Option 2 will require a 16” long by 1” wide mortise of 20 cubic inch volume. Option 1 will require a 19” long by 1-1/2” wide mortise of 49 cubic inch volume. Option 2 reduces the mortise volume by 59% which will make the jaw stiffer and reduce deflection on a similar sized jaw. The drawbacks to option 2 are that it will require a half pound more force applied to an 8” diameter hand wheel with a 4 thread per inch screw and it has slightly less clamping force at the widest jaw openings. I think that is a good tradeoff for a stiffer and more compact scissors mechanism.
We will put option 2 to the test and see how it actually performs when paired to the VX 20 vise mechanism. Check back later to see the results.