My Gramercy holdfasts from Tools For Working Wood arrived yesterday. I had already build a tiny workbench surface for testing. It is 2 7/8" thick with a round hole and two square holes. One of the square holes is 45deg to the other - I am curious if the angle has any effect on holding power on the square holes. The holes are as close to 0.75" as I could get - a micrometer is needed to measure the error.
I followed TFWW's recommended break-in for the holdfasts (de-grease and sandpaper), dropped my test bench into my tail vice, grabbed a few scraps and started pounding away. In this very informal test, I found that the holdfast held very well in both square holes. Much better, in fact, than the round hole - I had to hammer the holdfast multiple times to get it to hold in the round hole.
This is probably all the evidence I need that square holes are fine, but I am still considering getting a scale and doing a more scientific test with varying bench thicknesses.
I followed TFWW's recommended break-in for the holdfasts (de-grease and sandpaper), dropped my test bench into my tail vice, grabbed a few scraps and started pounding away. In this very informal test, I found that the holdfast held very well in both square holes. Much better, in fact, than the round hole - I had to hammer the holdfast multiple times to get it to hold in the round hole.
This is probably all the evidence I need that square holes are fine, but I am still considering getting a scale and doing a more scientific test with varying bench thicknesses.