During a Hand Tool Chat over at WoodCentral a few weeks ago the topic of clamping was discussed and holdfasts came up in the conversation. We were talking about a proper sized hole and the desired thickness of the workbench for using holdfasts. A 1 ½” thick top is minimum for a holdfast, although I have used mine on 1” thick apron of my bench; and 1/16” larger hole than the diameter of the shank of the holdfast seems to be optimum. Slightly larger holes will work as long as the holdfast can jamb from the top of one side of the hole to the bottom of the other side of the hole.
The holes for holdfasts should be straight up and down and as smooth as possible on the inside. This smooth surface on the inside of the hole gives greater friction than a rough hole. Then someone mentioned that a Celebrity Woodworker had said that to improve the ‘workings’ of the holdfast to roughen the shank with sandpaper.
At that point I remembered one of my three holdfasts had rough scale from the forging process covering the shank and I don’t use that particular one because it is rough and will destroy a hole after some use. I did use it in a pinch but was careful not to drive it too deep. I had just been too lazy to smooth off the shank and my other two have very smooth shanks and I use them all of the time.
I told the participants of the chat that that was probably not a good idea, because that would turn the shaft of the holdfast into a file that would abrade the hole every time it was used. I argued that the smoother the shaft the greater the friction, same as the hole in the bench. You don’t want a rough shank you want a very smooth shank.
So I set about rectifying the situation with my third holdfast. I clamped it in the vise and using a mill bastard file dressed the shank by ‘draw filing’ the surface. The scale came off leaving the shaft bright. I also collected the iron filings for making iron buff and as an ingredient in ‘cultler’s cement’.
Now all of my shanks are smooth and ready for use.