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Upset by Twist?


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Today I needed some strong hooks with handles to drag my chicken coop with and so forged a couple from 1/2" rebar. I needed the handle hook 90 degrees rotated from the grab hook and ended up doing a twist to get it right. I noticed that the rebar upset slightly after twisting (about 270 degrees twist). It got me to thinking about the possibility of using this effect intentionally. I find upsetting kind of tedious work anyway and then to get it done in the center of a long bar is even more difficult... so maybe this could be a useful tool for say swelling a bar for a slit to mount a tenon or whatever? Do any of you have experience/expertise to share on this type of upsetting?

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Never heerd-tell of such a thing. It must've been a short twist; was the original cross-section used in the twist?
If a guy twists round stock, nothing happens visually. If you twist square stock, the corners of the stock give the visual effect, and in thinking about measuring corner to corner, the finished twist will have a larger diameter than the parent square measured flat to flat. But that's not upsetting. Am I on the right track?

http://www.turleyforge.com Granddaddy of Blacksmith Schools

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If you twist square stock you can "see" the twist because the edges give a visual reference. If you put a line down the round stock you can then "see" the twist in the round stock as the eye has a visual reference to follow.

To straighten the same twist most people place it on the anvil and hit it with the hammer. This is wrong in two ways, first it bends or rounds the edges of the twist. Second most people now have the twist in alignment with one of the flats of the square stock. That is to say one flat side across the bar-twist-bar section with the other 3 sided out of alignment. The proper way to straighten the twist is to use a wooden stump and a wooden mallet and straighten on the diamond or edge of the bar. Then way the edges are sharp and the twist is centered in the bar. The cross section of the metal does not change.

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I don't think that's what I am seeing Frank. I did leave the original deformations in the rebar stock so that there was texture to track the twist by. It seemed to swell by about 1/16" or so though in a twist of about 2" in length. I believe that I have read about the shortening of stock as it is twisted and that must mean that it is getting thicker as it twists. I am working on the theory that even for round stock the same effect will occur. I was surprised by the strength of the effect in this case though. Since I had not intended an upset I forged it back to original size... but my thought wheels were already in motion. Upsetting by pounding on the ends of longer bars is a sort of tough prospect IME... whereas twisting generally is not hard to do and stays pretty straight. Even if the effect is subtle it might be useful in some situations, I think so anyway. I believe that twisting the stock is shortening (by thickening) in the same way that twisting a cable or rope would do. I think that there are probably smiths who are pretty expert in regard to this and could even quantify the shortening with regard to the twist/stock size etcetera. I am hoping for some education from some of them. I am also thinking that a combined technique might be helpful … the twist upsetting a bit while being end hammered adds to the effect and the twist helps to keep the stock straight as it is worked? I’ll likely be doing some experiments along those lines.

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John,
An odd thing..I agree with you...no shrinking, but I did a very tight twist (think threading) on a 3/4" square bar that was 66" long. It lost four inches.
I never quite figured out what was occurring....assuming I measured correctly before and after.

Ric /quote]

This from a post by Richard Furrer. I found this old thread discussing this sort of effect with a little different point of view. It must be obvious that that the edges of the metal must stretch to make a twist... as a line chiseled down the length of a round bar will have to grow much in length to account for it's spiral path after twisting. Not much of a stretch then to see that the same forces that stretch the edges of the bar will be countered by forces that squeeze the center of the bar. As noted in Richard's post above, empirical testing proves this theory out. Richard seems to reject the evidence of his own experience... but I am more accepting. Link to the thread:metal-forming-formulas
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If the stock lost length then it must have thickened. On the other hand I've never seen it happen and never heard of anyone actually having it happen when tested. To test it simply saw it in the twist and measure across/between the flats.

Twisted stock will take more room in cross section as the corners are now in different positions but there's no change in area. In other words it looks fatter and won't fit in the same size hole but actual thickness/wt/in" isn't changed.

Hand forging sq from textured round like rebar leads me to suspect the sq isn't actually sq. for a couple reasons. First it's an eyeball operation on odd shaped stock. Second the ribbing on rebar isn't quite consistent. All this makes me think the bar you twisted was slightly rectangular rather than properly sq. To make "sq" actually reasonably sq. you need a "No Nogo" gage to check or you can just draw it through a draw plate.

Of course I could be wrong so anyone out there who actually knows different, please speak up.

Frosty the Lucky.

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When Re Bar is textured by roll forming You will often see a straight continuous deformation down opposite sides of the bar. In effect the bar is not perfectly round but ovoid, Twist that and what you measure is the diameter of the straight deformations rather than the minor bar diameter. In other words twist a oval and the major axis forms a circular profile.

Check this out .

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The thickening of the twisted stock is NOT an illusion! What kind of myth is this, which persists despite direct and easily tested evidence that disproves it?!? Please READ the thread that I linked to above. The effect is real and quantifiable... the task remaining is to harness it and refine it for useful purposes! To continue to debate it's existence is a waste of time. I will be doing experiments toward this end. I'd love for some of you to also experiment and share your findings in this thread. The thread is from this very forum about a year ago. It deals with this phenomena from a slightly different point of view, but clearly involves the same effects.

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Here is the hypothesis which will be the focus of my first tests: I will guess that the deformations on the surface of rebar might make the surface stiffer and less easily stretched... thereby forcing MORE thickening when twisted than would occur with smooth bar. I will test this by doing duplicate twists on nearly identical lengths of rebar and smooth round bar. Then I will measure them and compare. I will check lengths rather than thicknesses as a more sensitive measure of results (assuming thickening to occur relative to shortening). It might be very interesting to see if UNtwisting the same section would restore original lengths!

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Another way to test for upsetting would be to cut out a given length from both the untwisted and the twisted areas of the bars and weigh them in a gram scale. Maybe an accurate postage scale would be sufficient.

Thanks for bringing this interesting subject to our attention. You might also consider that there may be more than a single process going on here . In any event the weight test will give hard data to consider.

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Another way to test for upsetting would be to cut out a given length from both the untwisted and the twisted areas of the bars and weigh them in a gram scale. Maybe an accurate postage scale would be sufficient.

Thanks for bringing this interesting subject to our attention. You might also consider that there may be more than a single process going on here . In any event the weight test will give hard data to consider.

That's a good idea Dan, Thank You! I agree that the whole range of variables could be more complex than simple. All the more reason to pursue a clearer understanding. I'd like to develop the technique into a useful tool... but first have to know at least most of the important variables that will influence it.
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