04-26-2012, 07:40 PM
Ed,
That's pretty strange. I've used both styles over the years and in my experience the wedge type offer the superior feel and repeatability. The reason is this: With the piston type, the force is applied in such a way that the piston must force the toolholder block straight out and equally upon each side. That rarely happens, so the block can be skewed to one side or the other to some degree. Depending upon the flatness of the piston face and it's perpendicularity to the dovetails, it may actually push the blocks to one side. That would actually help repeatability.
With the wedge type, the action always firmly nests the dovetail opposite the wedge into the mating surface, regardless of the block's variation in dovetail width, depth, and angles. This action assures the tool position repeatability. The wedge then engages it's own dovetail side firmly for the rigidity.
My older Dorian CA-size QCTP has a hybrid version of the two. It's "side piston", sort of. The "wedge" projects from inside the dovetail on one side. I'll have to get a photo of it to explain it better, but it works quite well.
That's pretty strange. I've used both styles over the years and in my experience the wedge type offer the superior feel and repeatability. The reason is this: With the piston type, the force is applied in such a way that the piston must force the toolholder block straight out and equally upon each side. That rarely happens, so the block can be skewed to one side or the other to some degree. Depending upon the flatness of the piston face and it's perpendicularity to the dovetails, it may actually push the blocks to one side. That would actually help repeatability.
With the wedge type, the action always firmly nests the dovetail opposite the wedge into the mating surface, regardless of the block's variation in dovetail width, depth, and angles. This action assures the tool position repeatability. The wedge then engages it's own dovetail side firmly for the rigidity.
My older Dorian CA-size QCTP has a hybrid version of the two. It's "side piston", sort of. The "wedge" projects from inside the dovetail on one side. I'll have to get a photo of it to explain it better, but it works quite well.