Broken toolmakers clamp.

Recently I has holding a kitchen cabinet hinge in a vice via a toolmakers clamp. Modifying the hinge as one of ours broke and its an unusual design that I could not find on the Internet. On my second adjustment I managed to be a bit hamfisted and broke the clamp across the thread hole. Actually I didn't think I was being that brutal with it, but after tapping their is not much metal left creating a weak point. I made a replacement part. I did wonder if hardening the jaws does make them weaker?
But it occurred to me that it is a bit of a design flaw, they should be a bit wider around the tapped area.
It occurred to me that I could have used a larger strong one with a thinner piece that would fit inside the hinge part, but it would be fiddly keeping it all aligned. So why not have a thinner piece that can be screwed to the inside of the jaw or a series of screws that can be used in the gap. Not done so yet but intend to make a couple from 12mm sq mild steel, one that will take a narrow insert on one, perhaps both jaws and one that is tapped for 2 or 3 screws.
The previous ones I have made are cruder than commercial or neater homemade ones. Simply 2 pieces of threaded rod inserted in one jaw and 2 nuts that adjust and tighten the other one. I thought my crude design was unique to me untill I saw an identical one in a photo in ME. The one I broke was a commercial one that I bought some time ago.
Has anyone tried a similar modification?

I made this pair from stuff in the scrap box, definitely not hardened. Plenty of meat around the holes and I don't really see how you could apply enough force by hand to break them. They are after all designed to be tightened by using the knurled knob.

Or are we talking about a different design?

John

My two Eclipse clamps and two newer ones all have holes for a tommy bar of some kind.

Funnily enough in my last job and after 49 years in mainly precision engineering I broke my first 'Toolmakers' clamp, freshly supplied by RS Industrial solutions, obviously from China or similar and they seemed to be through hardened and snapped like a carrot after a moderate clamping force was applied. Never had any problems like that before and the clamps I made back in the day are still going strong, from memory just case hardened as they should be. BTW all my clamps have tommy bar holes.

Tony

Maybe the same guys who made my "cobalt drills"

At least one snapped like a piece of glass

Glass hard, so useful.

Couple of my home made ones

The easiest option would be to use rectangular stock. So instead of 3/8" or 10mm square use 3/8" x 1/2" or 10 x 12. Put the holes through the shorter distance then there will be more metal at the sides of the holes for added strength. Then to save the jaws being too wide taper on three sides not just the outer one.

I've snapped a couple of cheap no brand ones.

I was using a tommy bar (small nail) but didn't apply much force. I did think of Jason's idea.but have a couple of others I am going to try for future use. The piece of steel I was holding is Ushaped so one jaw has to be thin to fit into the U, hence my idea of a fixed insert. But might try the jaws being a fixed width apart with screws to give the grip.

Finally got in the workshop today. i made a simple modification to one of my 12mm sq jaws. Fixed a short length of 20mm T section mild steel to the top jaw using a couple of 4BA screws. Works well.
I like Neil's idea of using 2 screws instead of my threaded rod and nuts. Next time I make one I will try this.
I didn't use Jason's idea as the rear nut prevents the two jaws meeting, although a cutaway where the rear threaded rod enters the jaw would solve this, allowing the nut to drop flush with the jaw.
I made these cruder versions of Harold Halls alternative some time ago. His look nicer.

There are a lot of ‘modern’ hobby targeted toolmakers clamps around of various levels of quality. Generally they all have course threads on the adjustment screws. This nearly always requires the application of a Tommy bar to achieve a reasonable level of grip which tends to lead to over tightening as anything to hand gets used as the bar. The old style clamps have fine threads which as well as leaving more meat on the clamp jaws allow a decent grip by finger tightening. In addition over tightening using a bar tends to strip the thread on the screw rather than breaking the clamp. Making a new screw is straight forward and I would guess that spares would have at one time been available.

regards Martin

PS I too have broken a cheap machinist clamp but never a second hand old school version.

Interestingly, my 40-50 year old Eclipse clamps have M8 coarse threads, just as the modern unbranded counterparts I own have.

Given the only very slightly higher TPI of 5/16” BSF (which I assume is what older British made clamps of this size would have had) compared with M8 coarse, I suspect that the durability of these clamps has more to with build quality and manner of use than with the fineness of the thread pitch. The design of these clamps means that if the jaws are not kept at least close to parallel in use, the threads and clamp bodies can be subject to significant undesirable strain.

Personally, I couldn’t do without tommy bar holes on these sorts of knurled thumbscrew clamps, whatever the thread pitch. The same can be said for tap wrench handles.

Edited By Bill Phinn on 23/09/2023 01:01:37

It's a fairly common misconception that hardening steel makes it somehow stronger or more resistant to bending, it doesn't. For the same cross sectional area, pretty much all of the more common steel alloys will have the same amount of bending strength hardened or unhardened. Or so close to each other it makes no real measurable difference outside a laboratory test facility. Google Young's Modulus for the lengthy details. As others have mentioned, incorrect levels of hardening would make the steel brittle. I suspect that's what was the cause with your toolmakers clamp File Handle. It's impossible to get very high through hardness levels without it also becoming brittle at the same time. Surface or case hardening does what the terms indicate, the surface layer can be very hard for durability and wear resistance, but the core is much softer and still ductile. Or you can heat treat to a lower level on the hardness scale, a bit less durability and less brittle, but everything is a trade off in engineering and materials. For what we do, toolmaker clamps made from unhardened and ordinary mild steel would be just as strong as something from even Starrett. They just won't be as wear resistant. But I doubt many of us could completely wear out one we made ourselves in a lifetime anyway.

On the better tool maker clamps, then no doubt the drilling and tapping is done before hardening, That hole and the threads would allow the same hardening to happen across and through areas that could be detrimental to the jaw strength and just how brittle it becomes. However they do it during the heat treating process, then no doubt that's something they address so it can't happen. On the much cheaper one's, that's most likely ignored and the steel quality, heat treating etc might well be much more poorly controlled. So in general, there's almost always valid reasons for the higher cost for something that to the eye appears to be exactly the same. But even the best manufacturers can still make the odd quality control mistake. The difference with that and buying new, it would be immediately replaced without any questions.

I have a pair of toolmakers clamps made when I was an apprentice 45 years ago. They are still working well (touch wood). The clamps were made from scrap shanks for lathe tools. Our factory must have gone through a fair amount of tooling as they managed to provide our apprentice training school with around 200 pieces of steel a year.

The origin of the steel made it difficult to drill and tap because it was harder than the mild steel that we had used for our previous tool making projects - a few drills were blunted and a few more taps were broken during the manufacturing process.

Looking at Harold Hall's site the thread used is half the thickness of the jaws. This was not the case on the one I broke, 3/16th thread and jaws substantially under 3/8th. This would contribute to the failure. As an experiment I have made one were I went the other way, 4BA thread and 8mm jaw. But I also like Neil's idea of thinning the end of the jaws, so a future plan is to use 12mm square and reduce the jaw ends to 8mm.
HH design uses metric threaded rod, so course, but he does also suggest using ME threads for a finer thread.
Does the taper on the end of the jaws have a purpose, or is it just to look better, or how they have always been made.

File Handle, in my opinion the taper jaws look better and also practically they give better access to 'things' in confined spaces and offer a better view of what you are clamping.

Tony

What Tony says!

Neil

Hardening steel makes it stronger, but not stiffer. The two are often confused especially by journalists

I find this style are often more useful in confined spaces:

Russell

I made a couple from key steel, seems a little tougher than mild steel but still machinable