Paul is on the right track. Most dogs are made with about a 5* angle on the face. This exerts downward pressure on thinner stock (say 3/4" or thinner). For thicker stock I resolve this behavior by raising the dog up so that it contacts the board higher up on the edge. I generally try to just clear the top surface with my dog height. I placed spring-loaded ball catches on my shop made dogs to allow me to set them at different heights easily. There are a number of ways to address this tension requirement; springs, o-rings, growing a third hand ;-)
Any sufficiently advanced technology is indistinguishable from magic.
- Arthur C. Clarke