To overcome this, I developed this interesting workholding jig that plugs into the tail vise, in my case a twin screw vise, and serves to extend the operation of the vise over the workbench surface. Twin screw vises or end vises when used as tail vises, introduce a large gap between the moving jaw and the workbench surface. Another consequence is that the workpiece is hanging over the end of the workbench, not an ideal orientation for working with hand planes. The sliding tail vise emulates a real tail vise in its operation. Racking is also eliminated when clamping narrow boards through the four point system.
The sliding tail vise transfers clamping to the main workbench top. Inherent in its design ,it attaches through four points. Two fixed points in the outer jaw and two sliding points within slots. I can now quickly clamp a workpiece to the workbench surface through a wide bearing surface and if necessary lock it in place through two knurled bolts.
The sliding tail vise is also designed to be portable. I can easily use it on either side of my workbenches depending on the tool being used ( left or right hand orientation). I have also updated the design to work better with plow planes and the depth adjuster on this type of hand plane. This can be seen in the last image. So what began as a project to allow me to use the twin-screw vise to clamp on either side of a workbench, has turned out to be a really versatile clamping aid for an end vise or twin-screw vise regardless if you are left or right-handed.
Instructions and CAD diagrams are now available on how to create this sliding tail vise along with detailed plans. The plans are specific to Veritas workbenches, but easily adaptable to similar workbenches with either an end vise or twin screw vise. Plan available through www.woodskills.com