Parallel Leg Vise With St. Andrew's Cross





Introduction: 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.

Step 1: St. Andrew's Cross

  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.

Step 2: You Will Need:


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


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

Step 3: Making the Cross

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.

Step 4: Prepping the Chop

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.  

Step 5: Prepping the Bench

  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. 

Step 6: Assembly

  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.



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26 Discussions

Back in the first decades of the 20th century, this is the sort of project that would appear in Popular Mechanics. In fact, they featured a similar build that used flat stock instead of the angle iron. I think the angle iron is a better choice.

I you'd like to find some really awesome DIY woodshop projects, you could do worse than to go back to the those old issue (via google books) and peruse the freaking awesome tips and projects.

Another excellent source of shop projects are the old Fine Woodworking magazines. They had DIY projects such as a shop-built 8-inch jointer. FWW doesn't have many of those sorts of articles anymore. For that you need to pick up ShopNotes magazine.

Shop-Built machiery is not for the faint of heart, nor is it the path for every wood worker. But man, if it trips you trigger to be able to show off a DIY bench tool, then ShopNotes is your guide.

A note about it went to my Blog:

A note about it went to my Blog:


2 years ago

Very interesting mechanism. I must admit I don't really understand why it works, and doesn't fold/collapse when you apply pressure... I instinctively think the friction at the loose ends at the bottom of the cross is important - I guess it wouldn't work with rollers there? Does it work inverted, with the pivots at the bottom instead - it seems like that would take up any slack in the joints and assembly? I guess I have to make one for myself to see what's happening here...

1 reply

Mate the force on the top of the legs from the screw holds the bottom out

Hi I know this is a older post about the leg vise. I have a question, my bench has aprons, Paul Seller type. Would this system still work having a gap between the leg and chop as the the apron thickness will be the differnce?
I was thinking I could build the leg up to match if it was necessary.


2 years ago

OK, suddenly realized that the cross just restricts the twisting/bending of the clamp/chop, it doesn't act anything like a solid support. Friction at the cross' ends has nothing to do with how it works.


2 years ago

Hmm... I can't see my comment I just made a few seconds ago. I just wanted to add a link with some nice comments about the forces involved:

It doesn't answer my questions, but it's still interesting. It was a long time ago I draw any free body diagrams as mentioned in one post, but maybe this would be a good opportunity to refresh that knowledge.

I have a question about Step #5:
Why does the dust bother the hole saw when drilling?

2 replies

It will clog the teeth, a hole saw doesn't have a way to remove the wasted wood so it can clog, bind, and burn the wood. If you can't drill relief holes then you need to make shallow pecks with the saw and back-out regularly to allow the sawdust to clear.

So that's why it was hard to drill...

Thank you so much!

Thanks for the posting, great advice! But how does one give "...a patina of iron oxide..."? Vinegar and salt? Is there a link that will explain the details?

would this work if mounted up side down to support the bottom preventing pinchining?

if you also have the bar in the bottom with all the holes in, it can support the chop and allow it to slide more freely. that probbly diddent make much sense so heres a picture of what i mean

When i was getting this picture i found that benchcrafted also make a cross vice now.

1 reply

after i look at project cant back up to main paige?

Great idea.
But I guess I'm dumb as I don't see how the two bottom ends of the cross slide in their respective channel ? Or the top assembly is enough to keep them in line and in each channel ? I would have thought the whole thing would be somewhat wobbly if not attached to a rail they could slide in.
Nice inst' anyway : we learn a lot both about this system and tricks about making holes etc …
Better than going to the movies on a rainy day, and shorter too ;))
Thanks !

This has been a really well done instrucables series.

Just as a side note the small sliding board does more than just stabilize the rotation of the vise it works to use the screw as a fulcrum and add clamping strength. Without it there it won't clamp quite as tight.

I made a traditional woodworking bench a couple years ago and can attest to the benefit of the extra strength.

Also is reaching down to adjust the pin location really that difficult compared to saw, drilling, planing etc...

Just my two cents.

I'll probably make this in a month or so after I finish my beside tables and start work on the bench. Have you encountered any problems or improvements that you can think of? I may add bearings in the angle iron holes. Again, this is perfect. The Benchcrafted stuff is nice but over priced by about 30%.