Why We Prefer a Tight Fit & an Untapered Shaft
Posted on 20 September 2016
The Crucible Iron Holdfast is patterned after a French holdfast that a friend of ours, Brian Anderson, discovered on a dung heap in a barn. Like many French holdfasts from the 20th century, this one had a shaft that wasn’t tapered. Additionally, the shaft was just shy of 1” in diameter – about .970” all along its length.
A straight shaft that is just shy of 1” allows this French holdfast – and ours – to work in benchtops from 2” to 8” thick, and to work with the pad up to 8” off the benchtop.
So why aren’t all holdfasts made this way? It’s a good question.
A holdfast with a tapered shaft has a number of advantages. It enters the workbench easily, it requires less raw material and it is easier to manufacture. These tapered-shaft holdfasts work fine in many situations, especially in benchtops thinner than 4”. But they rattle like loose teeth when pressed into service on a 6”-thick benchtop, or when the work you are clamping is more than 2” or 3” thick.
When we decided to make our holdfast with an untapered, .970”-diameter shaft, we knew it would require some trial and error. But everyone involved with the project, from our patternmaker to our foundrymen, were bemused by how much tweaking it took to get the right result, including numerous changes to the original wooden pattern, plus gating and risering to get the ductile iron to flow correctly.
When we finally got a holdfast to come out the correct size, Zack Erhart at Erhart Foundry sent me a message that said: “I think we’ve got it.” I ran over immediately – the foundry is about two miles from our office. I brought the holdfast back to my shop and realized that the only holdfast hole I had was 1-1/4” in diameter. I decided to give it a try.
It cinched down hard.
And that’s when I finally relaxed after months of wondering: Would this really work?
— Christopher Schwarz