Almost every woodshop has some Hold Downs (or Holdfasts) because they are essential to secure the workpieces in place for safe operation. Hold Downs have been around for centuries, but most of them still need to be satisfactory. One of the flaws of the regular hold downs is that they are designed in deformable Cantilever Structures. When holding force is applied, the clamping point will produce excessive tilting displacement due to the bending deformation of the structure, which obviously will undermine the holding security and damage the workpiece surface. In addition, because of the overturning bending moment under loading, the hold down pillar shaft continuously damages the workbench dog holes. So it has been Harvey’s aspiration to design a better hold down for woodworkers.
Inspired by the ancient wisdom of Arch Building Structures, Harvey team came up with a completely new structural design for hold downs, called AH-3 Arch Hold Down. With the unique “Arch” shaped frame, the AH-3 is able to firmly hold workpiece down with no tilting deformation.
And it features a balancing leg which provides adaptive full balance to the working leg to eliminate any overturning moment on the pillar shaft. Also its innovative “Quick-Release Pressure Knob” and “Toolless Workbench Installation Mechanism” allow you to clamp your workpiece ideally with joy!
Besides the AH-3 Arch Hold Down, Harvey also created the AH-3T model – the version for T-slots mounting application.
By nature, an ideal Arch Structure is a pure compression mechanical form. When a vertical force is applied on its Crown Point (the highest point), the Arch will resolve the load into compressive stresses, and thereby eliminate tensile stresses. This miraculous property gives the structure a great deal of mechanical advantage over a beam structure (especially cantilever structure) , because the Arch can minimize the deformation.
Stress Analysis of the Arch
Stress Analysis of the Arch
Owing to the unique Arch structure, the AH-3(T) hold downs are able to securely clamp the workpiece in place (with pure vertical pressure) without pressure deformation and overturning moment on the pillar shaft, thus achieving the highest stability.
Applicable for thin workpiece
Because of the balancing leg of the AH-3(T) and the mechanical nature of the Arch, clamping force on the working leg will be always automatically balanced without transferring any bending moment to the pillar shaft, no matter what the thickness of workpiece is.
Applicable for thin workpiece
The pressure applied by the Arch Hold Down can be easily adjusted to the proper amount by simply rotating the pressure knob. The maximum downward force applied by the Arch Hold Down can be as great as 1100N (246 lb) which is sufficient in most cases.
The pressure knob of the AH-3(T) hold downs is equipped with a Quick-Release Button to allow you quickly releasing the clamping force.
By pressing the quick release button, you don’t have to take the time to unscrew the knob when you need to release the hold down force, and the knob will glide to anywhere you like along the pillar shaft.
To eliminate the hassles of hold down installation on a workbench, Harvey developed a unique “Toolless Installation Mechanism” on the bottom of the Pillar Shaft. So when using the hold down, you simply insert the pillar shaft into a dog hole, the latch on the end of the shaft will automatically flip over (by off-center gravity) when it passes the bottom of bench top.
By turning the installation knob, you can easily lock the pillar shaft firmly between the upper and bottom surfaces of the bench top without touching the dog hole, and the pillar shaft is perfectly perpendicular to the bench top surface.
When removing the hold down, what you need is to unscrew the installation knob, the shaft latch will automatically retract (under gravity), and you can easily pull out the pillar shaft to remove the hold down. This smart and tricky mechanism makes the AH-3 a lot of fun to use!
