My apologies to anyone opening this post expecting to read about a stunning antique or news of my latest furniture-making exploits. No, this post is about a persistent technical topic that came to a head this morning with the arrival, in my in-box, of eight similarly headed emails.
There seems to be a conundrum that perplexes a great many woodworkers, who, while in the planning or building stages of a new workbench, cannot determine what thickness of bench top to opt for, or if they have settled on a thickness, cannot determine whether their holdfasts will grip in it satisfactorily.
For the popular Gramercy holdfasts (of which I have a couple), those offered by North American blacksmith, Peter Ross – and possibly others, the optimal hole diameter for 3″ – 4″ thick bench tops is 3/4″.
In 3/4″ diameter holes, the holdfasts won’t grip reliably in timber less than 1-1/2″ thick (my softwood bench’s aprons are 1-1/2″ thick and the holdfasts work admirably) and they tend to bounce out of timber much thicker than 4″.
If your bench is one of those increasingly popular mass-of-railway-sleepers-on-legs creations, then it’s simply a case of boring slightly larger diameter holes in the top to get the holdfasts to grip. Dismally, I’ve heard of people retrospectively counterboring the undersides of 5″ – 6″ thick tops, a couple of inches deep, in order for the holdfasts to gain traction!
With such extreme bench tops I would suggest trying a holdfast in a 13/16″ diameter hole in a bench top off-cut before attacking the bench proper with the Jennings.
Jack Plane
Pegs and 'Tails 
Holdfasts aside, is that a dustboard on the bench?
LikeLike
It is indeed; a circa 1740 dustboard in the making.
JP
LikeLike
I work off a bench that I built 30 years ago. The top is 1 1/2″ thick and the holes are 7/8″ because back then I used 7/8″ doweling for bench dogs. Gramercy hold fasts work just fine in these holes, especially after scoring their circumference with 60 grit sandpaper.
LikeLike
No apology is necessary, this post is as valuable as any of the others. Thanks.
LikeLike
If I remember well, C. Schawrtz made some experiments and the best grip was obtained when the stem of the holdfast was making an angle of about 7° with the perpendicular to the bench top. If this is correct, the thicker the bench is, the larger the hole diameter must be. For me it makes sense , it works with the same principle as the tube clamp where the tube is askew in the lamellae (or the other way around as the holdfast is older than the pipe clamp).
Sylvain
LikeLike