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Test - large hole for Gramercy holdfast

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Forum topic by Sylvain posted 02-18-2019 03:48 PM 270 views 0 times favorited 3 replies Add to Favorites Watch
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Sylvain

804 posts in 2797 days


02-18-2019 03:48 PM

Topic tags/keywords: workbench accessory jig test idea

Tools for working wood says about its “Gramercy holdfast” :
“The holdfast is designed for 3/4” hole in a workbench top of a 1 3/4” thick or thicker.”
In the video they say it will work in a benchtop up to 3”1/2 thickness.
They also say: “Maximum clamping 7 1/4” in a 2” thick benchtop.” which means, if I undertand correctly, clamping something very thick which would allow the stem to bend.

My Grammercy holdfast has a diameter of about 0.724” (about 18.4mm). Working with 3/4” hole means a relative tight fit. A tight fit implies that the stem could hardly be canted in the hole.
The ticker the bench, the less canted the stem can be. TFWW recommend reducing the workbench thickness around the hole if one has a very thick benchtop.

Now if you look at the Roubo plate 11 , you will see a holdfast visibly canted in a hole substantially larger than the holdfast stem.
Chris Schwarz once made a blog where various stem inclinations were tested and the conclusion was the best inclination was around 7° (Since he made recommendations for the hole for his “crucible” holdfast, I could not anymore find this blog).

So I decided to test the Gramercy holdfast with a larger hole. I had a chunk of wood about 65mm thick. To have the holdfast canted 7° in the hole, the hole should have a diameter of about 18.4 mm+ 65mm X TAN (7°) = 26.4 mm

I have a bell-saw with a 25 mm diameter, so I used that. After a few test, the hole is ovalised, so I have no precise number now.

Here is the picture: the chunk of wood is in the vise and the holdfast in the hole.

Simply said: it works.
(whack obliquely on the arm in direction of the head)

The question is then why is TFWW recommending precisely a 3/4” hole?
- obviously the holes can be used for bench dogs;
- is the Gramercy holdfast relying on its elasticity?

YMMV, so test for yourself.

—Sylvain, Brussels, Belgium, Europe – The more I learn, the more there is to learn

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn


3 replies so far

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WayneC

14258 posts in 4395 days


#1 posted 02-18-2019 04:00 PM

I think the 3/4 is that it is designed for a standard dog hole. I use mine in a carving fixture that is about 2” deep and have not had any issues with them holding.

-- We must guard our enthusiasm as we would our life - James Krenov

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HokieKen

8630 posts in 1436 days


#2 posted 02-18-2019 04:10 PM

I think it’s dependent on what angle the holdfast is designed to work at. Looking at your picture above, the contact area of the holdfast with the work is in the crook behind the foot. So your holding at a point of tangency on the radius of a round part. Which means you have a single point of contact. Which isn’t very effective workholding. In order for the holdfast to work well at that angle, you would need the foot to end at a shallower angle. I have a 3.25” thick bench with 3/4” holes for my dogs and holdfasts and the Gramercys perform very well for me. YMMV.

-- Kenny, SW VA, Go Hokies!!!

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Sylvain

804 posts in 2797 days


#3 posted 02-18-2019 06:04 PM

I agree that with such an angle, the contact area is not optimum.

The object of the test was to verify if the stem would hold in a bigger hole, not specifically to recommend a bigger hole.

Dieter Schmid fine tools says this:
“We have often been asked if these holddowns and holdfasts will also work in 20 mm holes. I will say here again clearly, no! It is a bad idea to use over-size holes as the contact between the shaft of the clamps and the bench top is much reduced if there is too much play in the holes. This both reduces the available clamping pressure that is possible, and will over time damage the edges of the holes, decreasing still further the holding power of the clamps.”

which is in contradiction with history.
Now is it that a 25mm hole works better than a 20mm hole with this very holdfast?
Old forged holdfast didn’t rely on elasticity and I wander if this makes a difference.

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

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