Forming fine spring wire 1

[BretMan]

Hello,

I need to understand what's involved for forming fine 304 stainless steel spring steel wire with about a .0066 dia. or less. It comes in a spool which when unwound from it, it has taken the curled shape of the spool.

What I need to do is first get cut lengths of the wire perfectly straight. I think to do this is it needs to be pulled or put in tension, then heated - but I'm not sure. Secondly, I then need to bend the straight wire to have a couple right angles bent into it. When the shape I need is achieved, the resiliency of the spring action needs to be maintained.

The result needs to be as precise as possible. If I'm correct about the tension and heat, what amount of each do I apply? Any guides or manuals on doing this would also be good to read if you know of any.

Many thanks.

Bret

Miami,FL USA

 

[EdStainless]

No tension and no heat.
You form springs directly from the spool. The cast (coil size) of the wire was deliberately set to allow you to do this.
Springs are formed by pushing the wire against a fist tool face and allowing the wire to curl to the correct size. Then the ends are formed.
After all of that the spring is stress relived.

Even if you are trying to wind springs you don't need to straighten the wire, just make sure that you feed it the same way all of the time. You also need to use pinch rolls to provide tension, don't pull against the wire on the spool.

If you cut a piece of wire just long enough to form a little more than a natural circle you will see that if you lay it on a smooth flat surface it forms a circle. The size of this is the cast. If you pick up the loop at its mid-point you will see that the free ends are offset from each other. This is the helix.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Compositepro]

You can buy straight wire. Here,for example:

http://www.mcmaster.com/#stainless-steel-wire/=g7eifx

Stretching past the yield point will make wire straight. It will also neck the wire to a smaller diameter. It may be a tricky process stretch the wire uniformly (or maybe not).

There are also wire straighteners that use rollers that will straighten continuous wire. These are not cheap and are tricky to adjust. They work by bending the wire one way first to give a uniform bend that removes the original bend, and then bending the wire straight.

 

[BretMan]

Thanks to you both for your help. Still, I'm a bit confused now.

To clarify, the wire isn't "bend and stay" like say a copper wire. It's spring steel so it's already resilient so if it's bent and released, it returns back to the original position. I need to keep this behavior after it's formed how I want it.

That said, "no tension, no heat" is good to hear but then you say "You also need to use pinch rolls to provide tension, don't pull against the wire on the spool."

What does the term "yield" mean here?

I think what you're explaining in order to straighten it is that the wire needs to be run between two or more opposing sets of rollers, without pulling it through with a lot of pulling force or the rollers pressing too hard to flatten the wire to change it's diameter. Is that right?

Then to bend it 90o where needed, it's just a matter of pressing a male into a female 90o metal corner shape (die) into the straight length. Is this right? Won't it spring back to being straight when released?



Bret

Miami,FL USA

 

[EdStainless]

You need to look up some info on spring making.
In order to straighten the wire you run it between sets of small rolls and bend the wire back and forth slightly in a controlled way.

I don't understand why you won't use the wire as is. Just make sure that you always feed it the same way so that it will form the same way each time.
The tension that I was talking about is just enough to assure even forming. If you pull too hard you will stretch the wire and change it size and properties.
When you bend the ends you will have to over form them because the wire will spring back. It will take some trial to get it right.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[BretMan]

I wish I could use it at is but it's going into a plastic mold to be insert molded and the exposed wire has to be in an exact position.

I'll check with wire straightener roller makers and see what they recommend.

Thank for you help. I feel a lot more oriented now.

Bret

Miami,FL USA

 

[MikeHalloran]

>>> ... isn't "bend and stay" like say a copper wire. <<<
Correct; but you _can_ bend it; that's how you form it. ... and it fights back, hard.

>>, don't pull against the wire on the spool." << means you need tooling to grip the wire itself, without applying torque to the spool.<<

>>What does the term "yield" mean here?<< See 'elastic yield point' and similar terms.

>> Then to bend it 90o where needed, it's just a matter of pressing a male into a female 90o metal corner shape (die) into the straight length. << No; it doesn't behave like annealed sheet metal. It fights back, hard, so you have to grip it, hard, and overbend it, hard.

For working with small spring wire, you will probably want to make a small pin bender. You can work the stuff with pliers, but it's going to rap your knuckles. Wear eye protection; it likes to jump around. Did I mention that it fights back, hard?


Mike Halloran
Pembroke Pines, FL, USA

 

[BretMan]

That's very good to know too. I looked up pin bender and found this:

http://home.earthlink.net/~bazillion/tooling.html

So over bending with trial and error, will eventually tell me where to bend it to get the resulting form I need. I do see now this is going to be tricky. I hope I can get the repeated consistency as these will eventually need to be made in bulk.

Here's another idea - out of ignorance. Is it possible to use regular 304 SS wire (relaxed spring steel), shape it how I want it, then turn it into string steel while it stays in the shape I gave it? I know spring steel has an alloy formula to it but isn't the spring quality a final treatment of some kind and could I apply that treatment to it without a major investment?

Bret

Miami,FL USA

 

[MikeHalloran]

Formed wire parts are usually made in bulk by something called a four-slide machine. You can contract to buy formed wire parts; there are specialists who have or can adapt the machinery already, and there are specialists who will adapt a machine that you buy.

Start by making a drawing of exactly what you need and sending it out for quotes.




Mike Halloran
Pembroke Pines, FL, USA

 

[metman]

So over bending with trial and error, will eventually tell me where to bend it to get the resulting form I need. I do see now this is going to be tricky. I hope I can get the repeated consistency as these will eventually need to be made in bulk.

BretMan
Yes over bending by trial and error and when you get the shape needed, you must mark the line of angular rotation of the movable portion of your forming tool so that you can repeat it with accuracy for the next springs.
For bulk, if it is large enough quantity, then MikeHalloran's suggestion of 4-slide is appropriate unless it is only a few thousand pieces. In that case, because you only have two right angle bends, some every simple tooling can be made and used with a pneumatic forming press using a vee shaped forming block and punch with less than 90 degree included angle to allow for over bend (spring back).

Here's another idea - out of ignorance. Is it possible to use regular 304 SS wire (relaxed spring steel), shape it how I want it, then turn it into string steel while it stays in the shape I gave it? I know spring steel has an alloy formula to it but isn't the spring quality a final treatment of some kind and could I apply that treatment to it without a major investment?

NO -- you do not want relaxed (annealed 304 SS). This will still act like a spring up to its yield point because it will have the same stiffness as work hardened 304 SS wire but the yield strength will be less than work hardened wire so that it cannot be bent as far before it permanently deforms. You need to understand the difference between stiffness and strength when designing springs. Heat treatment does not change the stiffness but it does change the strength.
Also you cannot heat treat 304 SS (except to soften it by annealing it). The only way to strengthen (increase the yield strength of) 304 SS is by cold working it but this does not change the stiffness (springiness).

And yes if you can buy straight 304 SS wire in 1/4 hard or harder (work hardened condition) this will save you a gob of trouble. Try McMaster-Carr for a source.

Design for RELIABILITY, manufacturability, and maintainability

 

[BretMan]

Ok. So "annealing" is heat treating the wire which relaxes the wire and reduces the stiffness. Correct? I read somewhere that, a compression spring, is made by winding the wire around a rod, then heating it, then dipping it in oil. What kind of oil? But this may be a different process since it's not the same as what you said to NOT to heat it. My spring's function is like that of a torsion spring - like a sway bar in an car suspension.

"Stiffness" = springiness = "spring rating". Correct? The spring wire samples I've received so far are what was on the shelf and for my .0066 dia. wire, the spool has many thousands of feet of it. They must each have specific detailed specs about the alloy, the stiffness and how they were made. I need to ask for that information - is there an industry term other than "spec sheet" for this information? Also, I may find an ideal stiffness that's different than from the stock wire. Would it be a major cost increase to have a custom wire made?

Bret

Miami,FL USA

 

[swall]

No, stiffness or more correctly Young's modulus (aka modulus of elasticity), will remain unchanged with heat treatment as others have pointed out. My SMI book shows 28 million psi for Youngs modulus in austentic stainlesses like 302/304. Carbon and low alloy steel wire is 30 million psi. Young's modulus effects spring rate, as does spring geometry and wire size.

 

[btrueblood]

"I read somewhere that, a compression spring, is made by winding the wire around a rod, then heating it, then dipping it in oil"

Some types of steel wire (e.g. high carbon steel) will harden when subjected to heating above a phase transition point (roughly 1600F or higher) and then quenching (rapidly cooling) to "freeze" the structure in the high-temperature phase condition.

Stainless steels in the 30x series won't harden this way. There are other "stainless steel" wires than can be hardened, using a different, heat "aging" technique.

Trying to uncurl spooled spring-hard wire without the proper tools and fixuring is a fool's errand. Buy straight wire.

BretMan, more importantly, the naivete of your questions so far make one wonder if you have taken a course in mechanics of materials? Your handle says "mechanical engineer" but I'd guess you really aren't one, or you have forgotten a lot of pretty basic stuff. I'd strongly urge you to find some experts (i.e. a spring-winding house) to engineer your part for you, rather than try to learn several semester's worth of materials science and structural mechanics via a web forum.

 

[BretMan]

Thanks again for your clarifications. You've saved me a great deal of time on a subject I've not needed specialized information about for several years.

Bret

Miami,FL USA

 

[EdStainless]

The alloy and wire dimensions determine the spring constant.
The strength determines how far you can flex it before it fails to spring back.
The 300 stainless grades only achieve high strength though cold drawing. You have to start with the high strength wire to get a good spring.
Higher strength gives more deflection and higher fatigue life.
Even when a pro stets this up on a four-slide they will have to run a few hundred springs to get the dimensions all correct.
coil OD, coil pitch, end angle, ends relative to each other, over all length, correct spring rate.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube