Counter Boring for Holdfasts?

[JohnM Martin]

A year or so ago, I bought a couple of hold fasts from Black Bear Forge. They are awesome and worked great on my temporary bench top (solid core door). However, now I have built a Roubo with a much thicker top ~4.75" thick. Now, these holdfasts aren't holding very well. If I get them to hold at all, I have to hit it just right and even then it isn't a great hold. I reached out to John at Black Bear Forge and he suggested counter boring from the underside to make the effective thickness less. The question I have is what is a good way to go about this? Any suggestions?

[Hasin Haroon]

Hey Jon,

I plan on building a Roubo with a 6" thick top later this year (I know it's overkill, but I got a great deal on an already laminated slab on Kijiji). My plan is to use a 1.25" forstner to counterbore the bottom to a depth of about 2 inches for each hole.

[J. Greg Jones]

Benchcrafted posted an article and video awhile back that explains one way to do this.

http://benchcrafted.blogspot.com/2015/06/enlarge-your-holes-with-cheap-tools.html

[JohnM Martin]

Thank you both!

[Warren Mickley]

Roubo shows a holdfast in a thick bench, five or six inches, with just a straight hole. His holdfasts however are very flexible, tres elastique,and taper on the arm down to under 3/16 of an inch.

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[Joe A Faulkner]

Have you tried abrading the shaft of the holdfast? That trick often helps. Take some 80 grit sand paper and wrap the shaft of the holdfast and give it several good turns and see if that doesn't do it.

Do you own an electric plunge router? If so, you might use that to counter bore - probably easiest if you can flip your bench over or remove the top and flip it.

[Robert Engel]

Your bench is too thick. :)

[Wojciech Tryc]

Have you tried abrading the shaft of the holdfast? That trick often helps. Take some 80 grit sand paper and wrap the shaft of the holdfast and give it several good turns and see if that doesn't do it.
.
This is exactly what i had to do with my Gramercy holdfasts. My bench top is just shy of 4" thick. Now they hold perfectly

[Randy Chesnut]

The Black Bear Forge holdfasts are great right from the smithy. They are rough forged over the shaft, so don't need roughing up. I have used mine on pretty thick benches but don't think over four inches. I suspect that you simply can't develop the rotational force over that long a hole. I think the money is is reaming it down a bit. I would take it slow and test as you go. I don't think you will need more than about an inch of relief.

[Warren Mickley]

What you need in a holdfast is spring. A rigid holdfast depends altogether on the the wedging qualities of the hole, but a flexible holdfast can use the spring in the arm also. An arm that is thick or cast iron just does not have much to work with. The Gramercy holdfasts, as an example have no taper at all in the arm an are improved greatly if it is ground thinner and more flexible.,