Hand Screw / Parallel Clamp

[DTR]

In an effort to use up some ash offcuts on something useful, I made a hand screw. Otherwise known as a parallel clamp (I think?), or in metalworker's parlance, a toolmaker's clamp. This is to replace a vintage one that was riddled with worm holes.

The jaws are just sticks of ash. One jaw has two holes tapped M12, the other has a one clearance hole and one blind hole for the screws.

The screws are made from M12 studding. I'm not much of a turner so I opted for octagonal handles instead. I expect I'll be making octagonal wotsits again in the future, so I made a spar gauge to simplify the marking out:

(that's another offcut gone)

I planed up a square section of ash and marked out the octagonal edges with the spar gauge. Then I just planed down to the lines. I used a homemade vee-block to hold the stock at the appropriate angle up against the planing stop:

The vee block was originally intended for crosscutting round stock on the bandsaw, hence why it has a metal guide on the bottom.

The octagonal stick was cut into two to make the handles. Each one was drilled and tapped to accept the M12 studding. I managed to drill one of the holes a bit skewed, so the rear handle is a bit on the wonk. It doesn't affect the operation though. Finally I cross-drilled the handles to take a roll pin that fixes the studding (sorry, no pics).

These things are often more useful in pairs, so one day I might make another one. I'm not sure how well the threads cut in the ash are going to survive in use. If they fail I'll make up some metal inserts to use instead. In hindsight the M12 thread is a bit too slow in use for a clamp of this size, but other than making my own wooden screws I can't think of a decent alternative.

Thanks for looking!

 

[ColeyS1]

That clamp looks superb! I've got a few really tight metal ones that I thought would come in handy but there so stiff to adjust they hardly get used. I much prefer your version, especially for holding wood.

The only thing I'm a bit baffled by is the spar gauge . I've seen them before with only one pencil so it marks the centre line, how does the two pencil thing work ? I've spent a good few minutes looking at it and still can't see what you used it for.

Coley

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[ScaredyCat]

the other has a one clearance hole and one blind hole for the screws.

Can you explain this again for those of us that are a little slow?

 

[k4wils]

I remember seeing a drawing of a parallel clamp, one thread was reversed if I remember correctly, allowing you to spin the clamp around the centre point to adjust. Have you cut a reverse thread or used threaded rod and accepted that you have to turn each handle separately?
It looks nice, and great for applying just the correct amount of pressure. It’s so easy to overtighten a G cramp.

 

[DTR]

Thanks chaps

The only thing I'm a bit baffled by is the spar gauge . I've seen them before with only one pencil so it marks the centre line, how does the two pencil thing work ? I've spent a good few minutes looking at it and still can't see what you used it for.

The pencils are positioned relative to the dowels using geometry (no numbers required). Because we're working with proportions, the spar gauge will work on any size stock that will fit between the dowels. Like the centre gauge you mention, the spar gauge is skewed across the work so that the two dowels are in contact with the sides of the work:

The pencil lines are gauged on all four sides of the work. Then the pencil lines are continued onto the end grain. When they are joined up, they form an octagon:

Then it's just a case of planing down to the lines.

Hope that helps?

 

[DTR]

the other has a one clearance hole and one blind hole for the screws.

Can you explain this again for those of us that are a little slow?

In the photo above, the jaw on the right has two holes straight through, both tapped M12. The jaw on the left has the clearance hole (the upper hole in the photo). This hole simply allows the screw to pass straight through. The lower hole on the left-hand jaw is the blind hole, and it goes about 5/8" deep (the depth isn't important). It's only function is to capture the end of the screw to keep the jaws parallel.

Now see the photo below. In use, the upper screw is tightened up against the work. The jaws will want to close up together at the back end of the clamp, but this is prevented by the lower screw. If the lower screw is now tightened, it will have the opposite effect; i.e. it will force the jaws together at the front end, tightening up against the work.

I remember seeing a drawing of a parallel clamp, one thread was reversed if I remember correctly, allowing you to spin the clamp around the centre point to adjust. Have you cut a reverse thread or used threaded rod and accepted that you have to turn each handle separately?
It looks nice, and great for applying just the correct amount of pressure. It’s so easy to overtighten a G cramp.

This style of clamp uses common right-handed threads. In practice the jaws can be quickly adjusted to a rough setting before final clamping (it would be quicker if I didn't use such a slow thread!). Imagine holding the handles like the pedals of a bicycle in each hand, then pedalling with your hands! This action winds the jaws together (or apart) whilst keeping them parallel. There is another style of clamp that uses both left and right-handed threads. They look similar but work differently.

 

[ColeyS1]

Thanks chaps

The only thing I'm a bit baffled by is the spar gauge . I've seen them before with only one pencil so it marks the centre line, how does the two pencil thing work ? I've spent a good few minutes looking at it and still can't see what you used it for.

The pencils are positioned relative to the dowels using geometry (no numbers required). Because we're working with proportions, the spar gauge will work on any size stock that will fit between the dowels. Like the centre gauge you mention, the spar gauge is skewed across the work so that the two dowels are in contact with the sides of the work:



The pencil lines are gauged on all four sides of the work. Then the pencil lines are continued onto the end grain. When they are joined up, they form an octagon:



Then it's just a case of planing down to the lines.

Hope that helps?

Ahhhh,now it makes perfect sense. Thanks for the extra pictures.

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[ro]

Awesome idea! I've been looking at buying a hand screw for a while now, but I will be copying your design instead! Thanks

 

[StraightOffTheArk]

I also liked the spar gauge very much and as I need to plane a square piece round, I looked it up to check proportions and found an excellent explanation here:

https://hillbillydaiku.com/2017/01/01/t ... zer-gauge/

Thanks once again,

Carl

 

[DTR]

I don't understand why he used Pythagoras to calculate the offsets, then checked it using the proportional method with a compass? I just set it out with a compass in the first place and sidestepped the numbers! Trying to calculate an accurate 1:1.41:1 ratio sounds like a recipe for disaster to me :lol:

 

[ColeyS1]

I've just re-read this but think I may be barking up the wrong tree. So you can mark out any sized piece of wood (so long as it fits between the dowels) and it'll work correctly so it's an equal octagon ? I thought it would only work on that size section of wood ?

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[DTR]

Yep, it will mark a regular (equal) octagon on any stuff that will between the dowels. When you skew the gauge, you are effectively reducing the distance between the pencils in proportion.

 

[ColeyS1]

Yep, it will mark a regular (equal) octagon on any stuff that will between the dowels. When you skew the gauge, you are effectively reducing the distance between the pencils in proportion.

That's amazing!!!!!!!!!!!!!!! Now I understand why you went to the effort of making one instead of just using a combination square and pencil lol. So how would I go about calculating where the pins and pencil holes go if I wanted to make one myself ? It'd be great for marking octagons for anything on the lathe. Make knocking the corners off much quicker !

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[DTR]

If you need a quick and dirty answer then use the 1:1.4:1 ratio as mentioned in the link that StraightOffTheArk posted.

Otherwise......
1. Knock the two dowels into your stock. I think I put mine about 3" apart, but the exact measurement isn't important.
2. Draw out a square on some paper, board etc. The sides of the square are equal to the length between the inside edges of the two dowels*
3. Draw two lines between the diagonally opposite corners of the square (i.e. a cross through the middle)
4. Set your compass to the distance between one corner of the square and the centre of the cross
5. This distance is how far the pencil hole centres need to be from the inside edges of the dowels

* I took all measurements using the compass alone. Converting into an arbitrary scale (mm, inches, bananas, whatever) is ultimately pointless in this scenario.

I hope that makes sense? If not then I'll take some photos

 

[ColeyS1]

Thanks for the explanation. It seems such a simple thing you'd only ever need to make once. I wonder if every wood turner has one or knows about them ?

Cheers
Coley

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[Bm101]

I remember someone posting about spar gauges before Dave. I was impressed but as mentioned, trigonometry was discussed. Also the distance to the moon and possibly casting of bones.
Many thanks for explaining with clarity to the bears of little brain like me. Legend.
Regards and with thanks.
Chris