More About the VX 20

VX 20

There will be some very minor changes to the VX 20 between the vise shown and the production unit. The mounting will change from four ¼” lag screws to four #10 pan head wood screws. For this application the lag screws are overkill and the #10 screws will package better and be easier to install. I am considering changing the color of the housing to black instead of the gold and adding the Hovarter Custom Vise logo to the housing in white letters. If you have an opinion on the housing color I would like to hear it.

Here are some additional specifications on the VX 20:

The mounting base is 3” wide by 4” long and the housing is approximately 2-1/2” high. The maximum jaw opening is 11-3/16” when used with a 1-3/4” thick jaw and a 3” thick leg. The carburized and hardened steel clamp shaft is designed to be fully retracted flush with a minimum 3” thick bench leg. The 20-1/4” long clamp shaft may be lubricated if desired. A coating of wax will help prevent corrosion and improve the sliding action. The clamp shaft is retained by the housing and can’t be removed without dis-assembly of the housing. The vise mechanism provides clamping action similar to a 4 thread per inch screw and will provide high clamping forces for a minimal force input to the handle.

The vise is virtually maintenance free. The totally enclosed housing prevents dust and dirt from entering. The aircraft grade anodized aluminum housing will not corrode and the machine screws which hold the housing together are stainless steel to prevent corrosion. All internal wear parts are constructed from steel for long life. If any maintenance or cleaning is ever required the housing can be simply removed by un-screwing the four Phillips head machine screws.

Introducing the VX 20

The VX 20 is the culmination of years of development work to produce a robust, economical quick action leg vise that is also simple to install. This versatile vise mechanism can also be made into a quick action face vise or be used just as you would use a conventional vise screw and nut. The VX 20 is constructed from aircraft grade anodized aluminum, steel and stainless steel to provide a lifetime of service.

See Video Here

To install the VX 20 simply drill holes through the bench leg and vise jaw and screw the housing assembly to the back of the leg. A Delrin bearing is screwed into a counter-bore in the front of the leg. The vise handle is attached to the shaft with a quick release pin which allows the jaw to be removed entirely in seconds. The pin system also allows you to select either a wooden hub and handle or a metal hand wheel to customize your vise. Other options are in development and will be introduced at a later date.

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VX 20 Housing

Perhaps the greatest feature of this vise will be the price. The VX 20 vise assembly will start at $140.00! The hand wheel or wooden hub will be sold separately to allow the consumer to fully customize the vise. Please check back often to see the latest developments and look for a pre – order discount!

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.

Leg Vise Forces Capture

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.

 

Use the (Vise) Force Luke

My youngest son Luke never tires of my use of phrases from Star Wars. “Use the force Luke” is my favorite. I sometimes modify the phrases a little bit to suit the shop environment like, “Use the clamp force Luke.” or “Use the vise force Luke.” Luke doesn’t think a lot about forces generated by the vises in the shop. Do you ever wonder how much force is exerted by a screw operated vise on a work piece? My guess is that you, like my son, probably don’t. So that’s why we have engineers, to help answer all these inane little questions.

Drawing1

 

In your typical vise the screw acts as a force multiplier. A simple way to look at a screw is like a ramp wrapped around a shaft. The steeper the ramp is the quicker you get where you want to go but it is a harder climb. The other part of the force multiplication comes from the handle which is really just a lever. A longer lever means lower forces need to be applied but you have to turn it a much longer distance. As with most things in life tradeoffs need to be made. In the case of a vise the tradeoff is quickness of movement verses applied force to the handle to clamp your part. Ideally we want the vise to move quickly as we turn a small handle and apply lots of clamping force with only a small hand applied force to the handle. We know we can never get to this ideal vise so we try to balance things out to a level that is acceptable. So let’s look at how we can figure out the clamping force for a screw operated vise and see the tradeoff’s for a few example vises.  As it turns out there is a very simple formula to let you figure out the vise clamping force.

CLAMPING FORCE = 6.3  X  HANDLE FORCE  X  HANDLE RADIUS  X  SCREW TPI

The handle force is just the amount of force you apply to the handle with your hand. The handle radius is the distance from the center of the screw or hub to where you are applying the force and screw TPI is the number of threads per inch of the screw.

Let’s do an example of the Record 52-1/2 ED vise:

The Record has an 8” radius handle and a 4 TPI screw. Assume you apply 10 pounds of force to clamp a work piece, the clamping force works out to:

Clamping force = 6.3 X 10 X 8 X 4 =  2016 pounds of clamping force.

Most large wooden screws have a 2 TPI screw and they are desired for the quick movement when turned to clamp. But this faster movement comes at a price; higher force is required on the handle for clamping. To overcome this, wooden screws typically come with an extra-long handle of about 15”. Let’s look at a wooden screw example with a 3-1/2” hub, 15” handle and 2 TPI thread with the same 10 pounds applied to the handle:

The handle radius is actually 15 – (3.5 / 2) = 13.25     (you have to subtract half of the hub diameter to get the radius to the center of the screw.)

Clamping force = 6.3  X  10  X  13.25  X  2 = 1670 pounds of clamping force.

You can see that if you apply the same amount of force to the extra-long clamp handle you still come up short…..about 17 % less clamping force than the Record vise for the same amount of applied handle force.

Let’s look at one last example, a vise with 4 TPI screw and an 8” hand wheel. The handle radius in this case is going to be 4”. So for 10 pounds of force applied to the handle you get:

Clamping force = 6.3  X  10  X  4  X  4 = 1008 pounds of clamping force.

This is half the force you get with the Record vise. Or another way to look at it is that you would have to apply twice as much force to the hand wheel to get the same clamping as the Record vise.

You might be asking why I care about screw vises when I don’t even sell them. This little exercise points out the beauty of the quick action vise. You get quick action when moving the jaw up to the work and you get lower handle force with more clamping power. I also design my vises to mimic the action of vise screws so this formula is useful. I typically design my vises to work like a 4 TPI screw because that gives the best balance between low applied handle forces and good clamping force. May the vise force be with you……