I need to develop (better yet - purchase) a device that will quickly break a stretched piece of stainless wire (0.017"). The idea is to first stretch the wire near its breaking point (reduced to about 0.016") and then apply a current to one section of the wire causing it to break at that point. This results in the wire being tapered and not burred on the end. The wire needs to get very hot very fast. We simulated the ideal condition using a small kitchen torch to the get the wire red hot. We are thinking of using a DPST switch that will create a short between two points on the wire.

 

So, you're trying to melt the wire at the specific point that is being strained.

I guess this really depends on what you want to happen. Do you want the wire to be molten, then the applied tension will pull it apart. Or, do you want it to be brought to a temperature where it starts to weaken and the applied tension can now overcome this.

You should place the wire between two electrodes. Then, energize the circuit. If you energize / engage prior, you will have sparking and eventuallly the failure of the contacts.

The amount of power required is a specific heat calculation. You need to find the volume enclosed by the electrodes. Then, use the specific heat of the stainless you are using to find out how many watts will create the desired temperature rise.

I would consider useage of a capacitor bank to ease your power supply requirements.

Steve

 

Stainless steel has about 53x the resistance of copper wire. Your SS wire will have roughly 0.16 Ohms per inch, or 2 Ohms per foot resistance.

If the wire were gripped by say, a pair of copper V-blocks in two places about an inch apart, a slight tension placed on the wire (perhaps 1-2 lbs) and then current passed throught the wire via connection to the V-blocks, the SS wire would begin to heat. Since the copper V-blocks act as a heat sink, the greatest concentration of heat would occur halfway between the V-blocks. If the tension were maintained, eventually the wire would reach the plastic stage first at the halfway point; the wire would then stretch, and rapidly grow hotter as the cross section diminished, until it melted and separated.

This will take some experimentation to perfect. Not enough current would make the process very slow. Too much current would cause the wire to vaporize across the entire span, causing balls of molten SS to fly all over.

You might experiment using a PWM circuit to control the average current in conjunction with an ATX form factor power supply scavenged from an obsolete PC. You can get about 25A from the 5v side of a 250W supply, which will likely be enough for your purpose. Google "ATX Bench Supply" for lots of conversion ideas.

 

Great ideas! My biggest concern is avoiding as little physical contact with the wire as possible to reduce contamination. The amount of debris must be kept to a minimum and no sharp edges.

 

You might perform some Google searches for such things as "Automatic wire cutter" and "automatic wire straightener" to get ideas on how commercial automatic wire cutters/straighteners are made.

As the wire comes off the spool, it will need to be straightened in order to feed properly through the rest of the device. This might be something like a series of rollers with variable geometry to bend the wire in the opposite direction of its' "set". As the spool is consumed, the "set" bend will likely require more and more of a bend in the opposite direction to remove the "set". A small amount of heating under tension might do the same thing, but too much heat will also weaken the wire.

You're going to have to do a fair bit of experimentation to find the right combinations of bending, tension and current.

If the wire can be sufficiently straightened, automatic feeding would be made much easier.

 

but too much heat will also weaken the wire.

Agreed. Heating up to the melting point and cooling slowly is effectively annealing the stainless. If the wires are being used as support, or whatever, where strength matters, you could be introducing a weak link.

Steve

 

For this application, the wire is installed manually on the stretching apparatus. Once the wire is broken, it is discarded. So there is no problem with damaging the wire. I need to have the process be very quick as to not damage the other components of the product.

 

We don't do complete designs here, if that's what you're expecting.

You will have to explore the ideas presented on your own. You've reaped the benefit of the experience of an old codger and a young and upcoming engineer; you decide who is who.

However, once you perform and document some experiments, come back and share your results.

 

Great ideas! My biggest concern is avoiding as little physical contact with the wire as possible to reduce contamination. The amount of debris must be kept to a minimum and no sharp edges.

If contamination is of concern, I suggest enclosing the thing in an argon atmosphere. The oxygen and CO2 in room atmosphere react quite readily and quickly with steel when it heats to incandescence.

As the spool is consumed, the "set" bend will likely require more and more of a bend in the opposite direction to remove the "set". A small amount of heating under tension might do the same thing, but too much heat will also weaken the wire.

Heating up to the melting point and cooling slowly is effectively annealing the stainless.

While true of many common steels, the above statements do not apply to all stainless steels. A small change in composition can make a huge change in how the steel reacts to cold working, heat treating, and many other things. Austenitic stainless steels can't be annealed in this manner, and won't be changed much. Martensitic stainless steels would be hardened by this, gaining strength and becoming quite brittle. Metallurgy is full of counter-intuitive surprises.

 

Hi Thingmaker,

I have read that most stainless (austenitic and martensitic), can be softened by annealing, but not always hardened by rapidly bringing down the temperature after treatment.

http://www.azom.com/details.asp?articleid=1141

Stephen

 

1) Can you use induction heating? Localized and can be very fast. Can also work in an inert atmosphere with a shielding gas.

2) Heating and melting stainless will likely change its properties, particularly near the melt zone, as already noted. Have you ever noticed how stainless tends to rust just next to a weld bead?

John

 

1) Can you use induction heating? Localized and can be very fast. Can also work in an inert atmosphere with a shielding gas.

Induction heating is the way to go here; localisation and controllability of the B-field being of central importance to such an application. In essence the OP needs to half of an induction-weld process, i.e. the pre-heat without the join.

The physical dimensions of the wire preclude significant considerations of the skin effect.

Dave