Parallel Leg Vise With St. Andrew's Cross

  In woodworking and fabricating, a leg vise can be enormously useful.  They offer a large, flat clamping surface on which you can impart a surprising amount of clamping force.  A properly aligned leg vise can hold large workpieces such as doors or 6x6 beams.  But few companies make proper woodworking leg vises or even the hardware for them.  The issue with making a leg vise is keeping the chop (the part that moves) parallel to the bench leg when under tension.  This Instructable features a solution.  

  I make all my leg vises at TechShop.

  On many traditional workbenches the chop is held parallel with a sliding wooden guide which is shot full of holes.  The worker must place a pin in the hole closest to the bench when the vise bottoms-out.  This procedure can get tiresome after a while. 
  Another option is to outfit the chop with a steel scissor-action mechanism which will keep it parallel at an infinitely divisible distance.  This mechanism is often identified as St. Peter’s Cross, however this is a misnomer, as St. Peter’s cross is actually an inverted perpendicular cross.  The X-shaped intersection we see here looks more like St. Andrew’s Cross, which is called a saltire and is featured on the national flag of Scotland.  So I’m calling this St. Andrew’s Cross. 
  You can learn more about Scotland from the books at your local library.

Materials:

Vise screw and nut
2" x 8" x 32" hardwood (for the chop)
36" of angle iron (8 gauge) or thicker
12" of 5/16" metal bar
5/16" bolt
Various nuts, washers, and spacers to fit

Tools:

Basic woodworking tools (saw, router, chisels, drill)
Basic metalworking tools (metal saw, angle grinder, drill)

I want to make the bars of the cross as long as possible so I'm mounting them just below the screw and having them extend almost to the floor.  On my 32" chop this make the bars 17" long.  I will be using angle irons for this to prevent flexing in any direction.  I milled one side of each down to 1" wide then using a metal bandsaw I cut the other sides to be wider in the middle (1 1/2") and 1" at either end.  Then the holes were drilled, each 5/16".  The bars will be hung from the top and bolted together at the center, the bottoms will float up as the vise is opened and down as it closes.  
  The bolt has to hold the two bars together but not under tension.  I am using a stainless steel collar bolt with just the right thickness of washers to hold the bars in place without squeezing them.  
  I gave the bars a patina of iron oxide and a clear-coat to prevent rust.  I will lubricate them with sewing machine oil.

A channel must be dug in the chop (and the bench leg) to accommodate the cross.  Since the bars are an inch wide, the channels must be 2" wide and 1" deep.  Since the bars will cross each other, there is still space for the extra height in the middle.  I used a router to hog out most of the channel then cleaned it up with a chisel.  
  Next the 5/16" through-hole is drilled for the pivot.  I used a Forstner bit and a drill press for this to keep the hole as straight and perpendicular as possible.  

  Now cut the same hole and channel in the bench leg. The nut will be secured on the inside surface of the leg.  Since I don't have a 2 1/4" Forstner bit, I will be cutting with a 2 1/4" hole saw and hogging-out the waste once it bottoms-out, then repeating the process until I get through the leg.  
  It is important that the pins holding each of the bars are at the exact same height.  Here they are placed 12 1/8" on center from the top of the bench. 

  Insert the pins through the leg and the inside bar.  The pins should be tight but not permanent.  The spacers will keep the bars from sliding along the pins.  Make sure it has the necessary freedom of movement in the channel.  The last part to secure should be the bolt at the intersection of the two bars.  At this point the vise is functionally complete. 
  In fig. 2  you can see it holding a large block of wood at the top edge while remaining parallel.  
 
  If your chop experiences any raking, despite all parts being true, you can place a strip of sheet metal in the bottom of one or both of the channels.  This will widen the bottom half of the assembly and should compensate for raking that occurs under pressure.  It's okay for the chop to be out of parallel when not clamping, but slight raking while being clamped will cause a noticeable decrease in holding power as there will be only an edge of contact with the work piece on one side.