Joel OF

Uneven twist. Why?

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Anyone got any clues as to why this 30mm x 30mm (1 3/16" x 1 3/16") bar has twisted so unevenly and got a horrible straight bit down the middle? This is a 8 inch long half turn twist.

 

I deliberately concentrated the heat into the middle of the twist area to avoid this problem. There was an even heat all sides and the bar had been in the fire long enough for the heat to have soaked into the middle. This was bar number 14 of 14 however this problem started at about number 9.

 

The first few were perfect and behaved as they should. Unlike this one in the picture I twisted them with an even heat all along the twist area (and even a bit beyond so that the heat wouldn't drain out in the time it took to lock the vice and put on the twisting wrench), but as this problem started to creep in I began concentrating the heat into the middle of the twist area to counteract it, but it obviously didn't work.

 

Any clues to this headscratcher would be greatly appreciated. Cheers

post-26685-0-30161900-1397049887_thumb.j

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Greetings Joel,

 

By the looks of your twist it does not align for a 1/2 twist...  One thing you can try is to take a longer heat so that the vise does not have time to heat transfer and slow the twist..  Normally you can use the torch and local heat for a correction..  I have reversed twisted many and torch corrected with success.   Just keep in mind the metal will always yield to the point of the most heat..  Keep it even and it should solve your problem..  I hope this helps.

 

forge on and make beautiful things

Jim

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Hi Jim, if I unserstand you correctly you're saying it's not straight. In the flesh it's straight but in the picture it looks kinked because of the wonky camera angle.

To clarify, I'm trying to figure out why the twist isn't tighter in the middle.

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Joel,

 

I think  is see what you're getting at. The start of the twist at each end is more acute than the center of the bar.  It occurs to me that structural steel is performance based rather than chemically defined.  It's possible that some of the bars you're using have a slightly different makeup causing them to react differently to the heat.

 

I wonder if a longer soak at temperature would solve the problem.  You could quench the ends at different distances to center till you found the right answer.

 

It may be unlikely but I know that I've made this mistake before.  Did you count the same number of rotations?  I've been a quarter rotation off before and had similar results.  Those last few degrees tend to twist the center more because the ends have cooled.

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Not being able to hold it I'd have to say it's reasonably even for such a short twist.

 

To speed things and maintain heat a few dedicated tools are handy. I'm a guy who likes things simple and while I have an old monkey wrench with another handle for twisting it's generally too slow for light stock AND it really draws heat. Not as bad as the vise but it really draws heat.

 

Sooooo, for twisting light stock I like putting a piece of square tubing in the vise with an ID that matches the OD of the stock. The wrench is a piece of fat bar with another piece of square tubing welded through the center that matches the stock. All you do is mark the stock, heat it, pull it from the fire slip it onto the wrench, slip it into the vise socket and twist it. Fast simple and the much lighter weight square tubing draws heat very slowly. Being as the stock is held in tubing it isn't marked like wrenches or the vise mark hot stock.

 

It ain't a universal tool, nothing always works but this one does just fine.

 

Frosty The Lucky.

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Simple; if it is mild steel heat the bar really well and dip each end into the slack tub..twist the bar and it will be ,,like said above moved in the place it is hot. Do a practice run before the heat to get the moves correct.

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Another thouight..is there a chance you are holding the bar in the middle with tongs while you stick one end in vise?

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I think  is see what you're getting at. The start of the twist at each end is more acute than the center of the bar.

Yeah that's it. It's bizarre, steel moves most where it's hottest but these weren't doing that!!!

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shot in the dark here, are you running these one after another in a gasser?  if you are placing stock that large sequentially into your forge at roughly the same place and immediately replacing them with the next as soon as the previous one comes up (or close to it) it is conceivable that the floor of your forge is developing a cold spot and while you may be allowing the bars to soak for what appears to be long enough, there is only fresh heat radiating onto it form 3 sides.  if the twist seems lopsided or the straight lines you are observing seem to be more prominent on only one face and not the others that could be the case.

 

try rotating the stock as it heats, or shim them up on a scrap of kiln shelving inside the forge or elevate them somehow off the floor.

 

of course if you are already accounting for any of this please disregard :) that was the only thing I could think of that would cause the center to resist twisting more than the edges barring variance in alloy (which seems unlikely that it would show up in the same configuration in 5 different spots sequentially, that's too much coincidence) 

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worth a shot =/ good luck!  that is quite a conundrum...

could there have been a clinker sucking up the heat in the forge?

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Anyone got any clues as to why this 30mm x 30mm (1 3/16" x 1 3/16") bar has twisted so unevenly and got a horrible straight bit down the middle? This is a 8 inch long half turn twist.

 

I deliberately concentrated the heat into the middle of the twist area to avoid this problem. There was an even heat all sides and the bar had been in the fire long enough for the heat to have soaked into the middle. This was bar number 14 of 14 however this problem started at about number 9.

 

The first few were perfect and behaved as they should. Unlike this one in the picture I twisted them with an even heat all along the twist area (and even a bit beyond so that the heat wouldn't drain out in the time it took to lock the vice and put on the twisting wrench), but as this problem started to creep in I began concentrating the heat into the middle of the twist area to counteract it, but it obviously didn't work.

 

Any clues to this headscratcher would be greatly appreciated. Cheers

attachicon.gifugly twist.jpg

 

Hi Joel, A comparison picture with the others would have been helpful. Not knowing your hearth set up is also an unknown

 

If the problems started at about item number 9 , here's an hypothesis if you are using a side blast with coke, I would consider the amount of clinker in the fire,

 

If the clinker is covering the centre of the heated section, then the appearance may be that of an even heat on the surface, but not fully soaked through.

 

It has been noticeable that with the Monkton coke, as the day goes on the fire seems to be working OK, however, the smaller/shorter sections of material being heated tend not to get heat where it would be expected,

 

What seems to be happening is the clinker has built up and although air is permeating through the mass, the outer area of this clinker base is actually hotter than the centre, so if your bar is left in situ without being passed through, then the ends will be soaked, whilst the central section has the appearance of being at a similar heat, but the heat is not all the way through the bar.

 

Try again with a clean fire and see what happens,

 

Similar lines as to previous post Mr Sells put up as I was posting this.

Edited by John B

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I know of three causes of twist variations: difference in temperature; difference in cross section and difference in composition---with the last one quite uncommon with modern steels; but not totally unknown.

 

With the time factor (9th item)  I vote with the clean the fire crew!

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A half turn over that length is not quite enough to let the tension in the metal even things out, so it really needs to be soft I am guessing you would have got away with it over 6 inches. 30mm holds the heat for quite a while, perhaps you could have gone past half a turn and backed it off, that may have saved it?

 

If you have some left over and some time to spare, I think it would be fun to play with it a bit to see if you could do a half turn one way, then back to straight, half turn the other way or go past to full turn and back it up and see if it is stretched. Just to see what you can do and get a feel for it. perhaps try a long quarter turn.

 

Compared to 25mm you have about 30% more cross section area in 30mm so it is a reasonable step up in heat needed

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A comparison picture with the others would have been helpful.

I couldn't bring myself to take a picture of them side by side, they irritated me so much. Luckily these were for the fabricator I work alongside and he wasn't bothered either way as they were tiny detail in a massive project he's got on.

What seems to be happening is the clinker has built up and although air is permeating through the mass, the outer area of this clinker base is actually hotter than the centre, so if your bar is left in situ without being passed through, then the ends will be soaked, whilst the central section has the appearance of being at a similar heat, but the heat is not all the way through the bar.

I think you're onto something there. I actually have a bottom blast forge and now that you've mentioned this I think an additional factor could be that the firepot itself was red hot and even though I was pushing the bar back and forth + rotating it, the lips on the firepot (that stop it from falling through the table) were red hot too, so I reckon there was some kinda heat transfer going on where the ends of the twist section were touching the lips but the middle was suspended over the clinkered up fire.

 

perhaps you could have gone past half a turn and backed it off, that may have saved it?

Nope, tried that on the last one, didn't work.

 

The one and only thing I didn't try which could have had a chance at rectifying this was to go 1/4 turn with a full heat in the twist zone, then let the heat drain out of the ends towards the middle as it's cooling from the contact with the vice and wrench, then do the remaining 1/4 turn when the heat is specifically in the middle.

 

Oh well, at least I haven't missed some really obvious and even the more experienced folks are saying "it could be..." and not "you wolly brain you didn't do..."

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I recently made a small table with lots of twists.  I did heat my legs in a three burner gas forge for a really really really long time to get an even heat.  We twisted 10" at a time and heated one leg at a time in the forge.  I learned to heat it until I thought I had a really good even heat throughout, and then gave it another 10 mins to soak up more.  So, I would think that the issue has to be with the coke forge as others have suggested.  Good luck.    

 

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I know of three causes of twist variations: difference in temperature; difference in cross section and difference in composition---with the last one quite uncommon with modern steels; but not totally unknown.

 

With the time factor (9th item)  I vote with the clean the fire crew!

Difference in composition uncommon with modern steels?!?!?!? LOL, you must not be getting any of the A-36 I have to suffer with... The composition of this stuff from inch to inch varies DRASTICALLY sometimes... From what I gather, alot of the steel recyclers tend to pour from the bottom of the crucible while feeding them up top... no aggitation at all to homogenize the alloying elements. I've had more than a few projects where after some initial forging, I had to drill at 1 inch or so intervals, to speed the process, I quenched the supposedly mild steel to get it to the drill press faster... Well, it went like butter, butter, butter, smoke 2 cobalt chloride drills, butter, butter, destroy another drill, until I went and normalized the whole thing again. I also see alot of irregular bends in the stuff due to varied composition... how they can spec this stuff for structural steel is beyond me.

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I'm not really seeing a problem with the twist. It looks twisted to me!

If this were my "problem", I would set that length aside and come back to it.

After doing a number of these, you might find your feelings have changed.

 

Ultimately, the job has to be right, but also factor in the fact that it is handmade, and also factor in that if you want to maintain productivity you shouldn't get too caught up in details that are unimportant, as this one (with all due respect for your concern) seems to be.   

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Not hot enough,, or bar was bent to the side while twisting.

What type of wrench are you using, what size is the material, what color are you twisting at?

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