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Stretching 316 1/8 hard wire.

rcoope

Stainless
Joined
Sep 25, 2010
Location
Vancouver Canada
I have a need to make some ~6" pieces of 316 wire rod which is say 0.064" for an inch and then around .06" for the next five. The thought was to take .064" wire from McMaster and stretch it on a 10,000N (2000lb) Instron machine which we have down to .061 or .060. It should yield at around 500lbs and only needs to stretch about 11% to get to the desired cross section. My main questions are: can we expect it to stretch evenly and what would be the best way to hold the section of wire you don't want stretched without distorting it. We were pondering making some hardened mini V-jaws. A little bit of distortion is probably OK, or at least if I could have sections of wire of highly controlled diameter to experiment with I could sort out the double diameter part later (e.g. we could manufacture these by centreless grinding perhaps)

I've searched the PM archives and there was quite a, shall we say, lively discussion about straightening music wire back in 2006 but nothing obvious about stretching wire.
 
Several problems. One as you have pointed out is how hold it. Second is the reduction in size. I would advise taking a piece of wire, stretching it and then measure the diameters along the length. I would be surprised if it is uniform. The only way I know to reduce the diameter would be through a die. But that doesn't solve the two diameter problem. I strongly suggest you return to the drawing board and come up with something practical. If it isn't easy to make, its a poor design.

If you still must do this, send the wire to a grinding shop and let them do. If they say its impractical, then you problem is solved.

Been there, done it.

Tom
 
I think you are going to have a lot of trouble stretching that wire. First I don't think it will stretch evenly and being hard it just might break. Can you draw it through a well lubricated die? I know that's how most wire is made and it might work better then trying to stretch it.
 
I think you are right they may neck down in one spot and break. I would expect to grind them in quantity but need some test pieces first. I can actually prototype them in two parts and laser weld with a connector but McMaster has .059" and 0.064 but I suspect the sweet spot for me is in between.
 
How about chemical etching/milling? Not sure what etchant you'd use for 316, but a timed dip to required depth should be fairly quick and predictable. I know its done on other materials and someone probably knows what to use for 316.
 
Instron probably has chucks to grab that wire. If you price the chucks I suspect they'll make turning it out of 2 inch round look cheap.

Could you put a drawing die in a lathe chuck and pull the wire with the tailstock or carriage?
 
A small collet chuck at the end you don't want stretched would probably work to keep that section at the original size. Whether the wire would stretch uniformly along its length is anyone's guess, you'd just have to try it. How much of a tolerance have you got?
 
I think you'd be farther ahead to make a box tool rig to turn off the .003". You'd have to machine a neck in the stock anyways for a place for a split die to start.

For clamping, I think you'd need extra stock length that you can cut off later. Machine a split ring clamp with a threaded inside and harden it up to bite into the stock.

Drawing by die might not be dead easy either: from a bit of drawing that I have done with tubing, it seemed like there was always a tendency for the stock to bow a little bit. It could be that the angular position of the hole in the die with respect to the pulling ram is extremely sensitive. I couldn't seem to correct it, so I had to straighten the parts.
 
How about a rotary swaging it? You would probably have to get a die made but I would check with swaging companies.


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Thanks for the thoughts. The main new fact I discovered today is that Mcmaster has some other sizes including a 0.59 if you can go to 304 which is sort of OK here and I love that I can get like $3.50 worth. Then I just have to machine little collars to laser weld them to some .064" sections. I think I will try the instron test just for your collective entertainment but probably after Christmas and I've started to agree with those above that it's likely to be highly uneven. We have some wire anchors that were made earlier in the project which I should be able to adapt them for the instron so it won't be a death march to test. Without going into details of the project, even if I can get 20mm sections that are reasonably even at say .062" and .061" that would shed light on what to do next. I'm pretty sure the long term answer is pro-grade wire straightening and centerless grinding but I'm trying to get to the stage of demonstrating a convincing prototype without specialized tooling or sending anything out anywhere. More on this in due course but for now, McMaster here I come!
 
If you've got tension grips, holding it for a test isn't the issue. If you want the gripped portion to be a sharp line between stretched and non stretched, it doesn't work quite that way and it'll stretch down into the jaws.
 
They are only 6" long pieces? How many do you have to make? Depending on volume why not sub it out to a Swiss shop? If you had to make thousands that would be a perfect job for an Escomatic D-2.
 
I have to admit I'm very curious why anyone would need a wire that swells from .060 to .064. It doesn't seem like a useful shape.
 
If you can find someone with a dual rotary head wire edm you could hold an eight inch long piece in a collet on both ends you could even start with a larger diameter wire
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I suppose the best way to go would be determined by quantity. Unless I am reading something wrong it looks like an easy Swiss job. All the screwing around trying to even make a few prototypes using poor methods could take quite a long time.
 
I have to admit I'm very curious why anyone would need a wire that swells from .060 to .064. It doesn't seem like a useful shape.

It's a device prototype and now you mention it I should clarify that this is really a prototyping exercise and I seriously need like four of these pieces of wire. Maybe eight at the most. The reason for the change in width is it allows me to do one function with the thick end that happens to fit the existing (very complex and not made here) parts perfectly at 0.064" but then it needs to be thinner in the following section. If we can demonstrate the principle then the other parts can change so we could use say 0.061" all the way along. But I have flexibility in the prototyping and just found the .059" 304 wire that my former engineer thoughtfully purchased back in 2013. On that note, you have to love McMaster which says, "you previously ordered this on XX/XX/XXXX" and then you go to the shop and find it in a tote bin. So I'm going to laser weld the .059 to .064 with a tiny collar and that should allow me to demonstrate the thing I need to demonstrate. But I still want to try the Instron thing and we're using it again after Christmas so we'll see what happens.

Sorry I have to be so vague but it's for the usual reasons.
 








 
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