blackleafforge

metal fatigue

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Hi, I recently made some tongs for picking up hot logs / bits of wood. The client described the design an I made it out of mild steel. It was all one pice with the flexible spine made by thinning out the material at the bend. It worked after a fassion but I didn't get a lot of movement and apparently cracks have now started to appear. I looked at similar designs online like the ones below and noticed that some had inserts riveted on to the bend. Am I heat treating the steel wrong, or is it to thick or will mild steel never have the correct flex and memory to be suitable for this type of application?

thanks

 

gimson_fire_tongs_1_000.jpg

02-SAM_1356_ml.JPG

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Don't the dots take forever!

The Gimson designed ones made by Bucknel father and son were mild steel and looked similar. Some are well over 100 years old and still in use.

Where are the cracks developing? Are they from work hardening, or weak points (bruises, knicks) in the spring forging? Why did you do any heat treatment? What heat treatment did you do? How evenly did you get the thickness of the spread? What material did you start with: new, second hand, clean surface, rust pitted? How did you heat it? Did you burn it at any point?

I have to say that robustness notwithstanding, yours are in a different league to the fabricated ones...very elegant.

Alan

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Im sorry to say that the first picture is just an example i found and not one i made. I don't have it anymore but the client told me that they were developing in the flattened bending section. Its made with new mild steel bar, flattened to a fairly uniform thickness on the bend (about 3 mil). I tried to get a blue tempering colour on the flexing section as I was told that equated to a flexible state. I don't think i burnt it. 

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If you are using A36 instead of mild steel---there can be a difference---Then trying to harden them at all is the problem.

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In that case I would say those are more than likely by Bucknell to Gimson's design. Cotswold Arts and Crafts. Norman Bucknell the son was still making them into the 1990s at the age of 80 or so!

See if you can get the client to photograph the spring, so we can see where the cracks are forming to be able to help analyse the cause. 

An Honourable repair would be to rivet in a piece of sheet to your spread area...make a feature of the rivets, and take the opportunity to play with the profile of the spring to make a positive contribution to the piece. The bigger the loop, the longer the spring, the less the stress.

I made a poker to match a similar set of Gimson/Bucknell tongs and the chasing and dot marking octupled the forging time...mind you the draw filling that they did at the time on those meant that when I researched on the Gimson Workbook in Cheltenham Art Museum and Art Gallery collection they took around 121/2 hours for their standard design poker.  I had taken 14 hours...so with modern equipment like a power hammer or two, a belt grinder, drum and angle sander I was not too far out for a one-off, working to my design but another's aesthetic. By way of comparison...my own pokers...forged from 16mm (5/8") square with a well balanced full taper chamfered shaft, an anthropometric handle and with an armour bright finish took about half an hour to forge and half an hour to finish if I did them in a batch of ten or so.

Alan

Alan Evans Poker to compliment Gimson Bucknel tongs.jpgErnest Gimson Daneway workbook.jpg

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16 minutes ago, blackleafforge said:

very impressive. reviewing the design and looking at yours i suspect i may not have thinned the flexing section enough... 

Not mine...as I said, I just made the poker to reflect the style of the client's Gimson/Bucknell tongs. The tongs illustrated are originals.

The poker is an amalgam of my poker balance and diamond point/heat sink ideas and the decorative detailing from the Gimson designs.

Well if the photo has helped you diagnose the problem that's great. I have a few other images of the implements from different angles if you want them...

As I said above you could make a repair using a similar construction to the twisty ones you posted...but with a wider and thinner spring from sheet metal. 

The width of the tong spring gives the positive alignment of the jaws (so they don't parrot/crossbill) and the thinness takes the stress out of it and makes them easily flexible.

The springiness is fairly minimal, you virtually open them as you would a top hinge pair.

By spreading the movement over a fairly long thin spring there should not be anywhere that gets fatigued through overwork.

Alan

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