holdfast in square vs round holes - help me test?

[merrill77]

A while back I asked about using a holdfast in square holes, since I've seen varying reports. I asked Gramercy, and they said square holes won't work but one or two people here said they work fine. Someone also suggested testing it myself. I initially dismissed that suggestion, but I'd like to give that a shot. This seems to be a woodworking question that has not been adequately answered.

So I've built a tiny workbench top, 3" thick, with both square and round holes to test them side-by-side. I'd like to test how the holding power varies with the benchtop thickness as well, so I plan to test and then plane down the top 1/4" and test again, repeating until...well, until it appears to be pointless to continue, I guess :>

I'd like this to be as accurate as can be reasonably achieved. So far, I haven't determined how I'm going to measure the holding power. I could set the holdfast on a board and give it a yank, and I suppose that would answer the question for me. But it isn't very scientific. Ideally, I would use some sort of a tension gauge and then pull until the clamped board comes loose - to objectively compare the forces required to loosen the board with various holdfasts, holes and bench thicknesses.

Any ideas? Anyone want to assist?

Also, I'd like to try with a variety of holdfasts - so if anyone can lend me one, I'd appreciate it.

TIA!
Chris

 

[bash]

How about attaching a spring scale (like a fish scale) between a hook or hole in your clamped board and your vice. That will allow a controlled pull. You can get high capacity spring scales that measure up to several hundred pounds relatively cheaply (especially compared to your time setting up the rest of your experiment).

Just a thought.

 

[bluedawg76]

I have the gramercy holdfasts and use them often in my square dog holes (~3" bench). I would be interested to see if there's a significant difference between the two. For what it's worth, my dog holes are angled a few degrees toward the tail vise.

How about attaching a spring scale (like a fish scale) between a hook or hole in your clamped board and your vice. That will allow a controlled pull. You can get high capacity spring scales that measure up to several hundred pounds relatively cheaply (especially compared to your time setting up the rest of your experiment).

yeah i was thinking something along those lines as well.

 

[manfre]

Use a ratchet strap to apply force to the board. It will allow you to apply pressure in a gradual and controlled way that should minimize the chances of the board going airborne. If you don't have a spring scale and only care about relative holding strength, you can measure the delta length of the strap from start until holdfast failure, but that may not be too helpful if the holding strength for each test is close. It might also be a good idea to put a pulley in to the mix so that in the low chance that the board takes flight, it hits a wall and not you or something breakable. The ratchet strap should detention as soon as the holdfast lets the board slip, but it's possible that the holdfast could fully release the wood.

This sounds like a fun experiment and I'd like to assist, but I'm not sure if my schedule will allow me to.

 

[eyekode]

Just a data point: I have found my gramercy holdfasts work well in 3" and 1.75" thick material. But they don't work in my 4" thick legs at all for thin material.

Oh, there is another variable. The thickness of the material matters. It changes the effective geometry of the holdfast.

That is bunch of variables. You might be better off playing with the math and geometry of holdfasts. It should help to determine the parameters of a good experiment.

Salem

 

[merrill77]

Use a ratchet strap to apply force to the board. It will allow you to apply pressure in a gradual and controlled way that should minimize the chances of the board going airborne.

Great suggestion! I was wondering how I could increase the force gradually and slowly enough to watch the scale. I plan to tether the board to the workbench so it won't go far if it lets loose :>

I've priced out some scales online and found a 550lb scale for $20. As long as it reads consistently, I don't care too much about the accuracy - I'll be looking primarily at the relative differences. Once I get that, and a holdfast or two, I'll give it a first pass to see how well it works.

 

[merrill77]

How about attaching a spring scale (like a fish scale) between a hook or hole in your clamped board and your vice.

"Spring scale" - that is the search term I needed! Tension meter returned much more expensive devices. The curse of using engineering terms, I suppose.

Thanks!

 

[Alan in Little Washington]

All great thoughts so far but as one or more folks mentioned there are a lot of variables- bench thickness and stock thickness (affect gripping angle of holdfast), hardness of top material, amount of force used to set holdfast (how hard you whap it), area of the stock, roughness of the stock and bench top, roughness of holdfast shaft and dog hole surface, etc.. If you can get those to be the same or test in categories where they are all the same, you may have some useful info.

I think it will be hard to measure applied pressure- because of how you whap the holdfast to set it. An electronic compression sensor might be damaged due to the initial impact load which will far exceed the holding compression load. Most (all) of the suggestions here will measure the resistance to slipping which is what you really want to know anyway, not the initial pressure applied.

As to the testing rig- a REALLY HEAVY DUTY spring scale would work as would an electronic tension gauge. You also might want to put a video camera on the gauge and the stock to record the tension and when the stock first starts to slip then goes flying!

As suggested, you'll want some sort safety chain or rope to prevent the stock from flying and doing damage to your shop.

Going back to what you are trying to determine- will a round holdfast in a square hole hold a piece of stock as well as a round holdfast in a square hole. While it might be interesting to measure the vertical holding force, you really want to see if the holdfast keeps the board from moving sideways.

Drill round and mortise square holes in the bench top. Use the same stock for all tests. Place the holdfast in the test round or square hole, with the foot centered over the stock. Lift a large heavy block of wood or a log a measured distance above the bench and drop it to set the holdfast- you might want to do that two or three times for EACH TEST RUN to make sure the holdfast is set firmly and with the same force for each test.

Attach the gauge to the stock, start the camera, and begin to increase the pull

Without testing I think a good holdfast like a Gramercy will hold the same thickness stock with the same force in a round or square hole.

 

[merrill77]

All great thoughts so far but as one or more folks mentioned there are a lot of variables- bench thickness and stock thickness (affect gripping angle of holdfast), hardness of top material, amount of force used to set holdfast (how hard you whap it), area of the stock, roughness of the stock and bench top, roughness of holdfast shaft and dog hole surface, etc.. If you can get those to be the same or test in categories where they are all the same, you may have some useful info.

Yup. I don't intend to test every possible variation. I plan to test using 3/4" stock (since that is most common) and a variety of benchtop thicknesses - by planing down my test surface in iterations and testing each. I'll also test the holdfast at two angles in the square holes - square to the sides and along the diagonals. I can uniformly sand the benchtop and workpiece to ensure consistent roughness. The set force is the one that has me stymied at the moment.

I think it will be hard to measure applied pressure- because of how you whap the holdfast to set it. An electronic compression sensor might be damaged due to the initial impact load which will far exceed the holding compression load. Most (all) of the suggestions here will measure the resistance to slipping which is what you really want to know anyway, not the initial pressure applied.

Agreed.

As to the testing rig- a REALLY HEAVY DUTY spring scale would work as would an electronic tension gauge. You also might want to put a video camera on the gauge and the stock to record the tension and when the stock first starts to slip then goes flying!

I've found a relatively cheap 550lb gauge. If I need more than that, I should be able to rig up pulleys or levers to multiply the force.

Good idea - putting a camera on the gauge. That should allow me to review readings for consistency.

Going back to what you are trying to determine- will a round holdfast in a square hole hold a piece of stock as well as a round holdfast in a square hole. While it might be interesting to measure the vertical holding force, you really want to see if the holdfast keeps the board from moving sideways.

Exactly what I was planning.

Drill round and mortise square holes in the bench top. Use the same stock for all tests. Place the holdfast in the test round or square hole, with the foot centered over the stock. Lift a large heavy block of wood or a log a measured distance above the bench and drop it to set the holdfast- you might want to do that two or three times for EACH TEST RUN to make sure the holdfast is set firmly and with the same force for each test.

Attach the gauge to the stock, start the camera, and begin to increase the pull

Dropping a log from a set height - that might work.

Without testing I think a good holdfast like a Gramercy will hold the same thickness stock with the same force in a round or square hole.

I hope so. However, I asked Gramercy, and they specifically said to NOT use a square hole. I've heard from people whose experience contradicts that advice.

 

[Alan in Little Washington]

Sounds like a plan. I have seen this question asked a number of times, so hopefully if it comes up again somewhere, we can post a link to your results.

I think Gramercy is just trying to cover themselves.

When you think about it, the round holdfast shaft has a diameter smaller than the dog hole so it can easily slide in and rack enough so it wedges into the hole for a three point contact- bottom of the hole, top of the hole, work piece.

Different diameters mean, theoretically the holdfast shank will only touch the sides of the dog hole at small points- top edge and bottom edge. That is also how it will contact the sides of a square hole. The only thing that will change the size of the contact points, where the dog hole conforms more to the curvature of the holdfast shank, is if the dog hole is soft wood. But that will be true for a square hole as well.