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

Maybe the same guys who made my "cobalt drills"

At least one snapped like a piece of glass

Couple of my home made ones

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.

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.

What Tony says!

Neil

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

Russell

Remember when I was at Tech College we had to make various tools, toolmakers clamp, punch, scriber, cold chisel all come to mind although that was 60 years ago, time flies. Dave W

It's stronger full stop, EN8 normalised has UTS 35 tsi, heat treated can be as high as 55 tsi. The yield stress also increases. These figures from a United Steel Co handbook which is about 50 years old.

Case hardening won't change the strength much at all as it's s only skin deep (sorry)