Machining causing 316 SS to anneal??? 5

[daschell]

I have been tasked to machine .009Ø pins 2" long for a customer. Using a CNC screw machine we are succesful at turning the parts from 1/8 bar. Here is the rub. My customer now says the machining is "annealing" the 316 CRES. Is this possible? If so what are any ideas to mitigate, Re solution H/T etc?

Thanks

 

[lgearhart]

Would you know on what basis the annealing claim is being made? Is it that the pins are distorting, or that they were full hard and now are less magnetic?

I ask since it's really unusual for the small, localized heating from cutting to have any effect on the structure of the 316.

 

[daschell]

The customer states that the pins as recieved are not "springing back" like they used to. I am alittle suspect as to the "annealing" from cutting as based on my 35 years of experience the 300 seris stainless work harden when machined. Although we have noticed that these parts seem to be extruding some as they are machined. ie: when we cut them to .050 diameter to 2.28 long they hold length perfect per the program. However when we cut them at .0092 diameter the length growsby .040. Ie they are now 2.32 long. we adjust the program but parts must be extruding to get longer.

 

[lgearhart]

If they're used to getting pins made from half to full hard 300 series wire, and now they're getting pins made from 1/8 stock, it could be that the pins won't spring back like they once did. The yield strength isn't high enough. Most of the 316 I deal with is already annealed (we commonly use it wanting a lack of ferromagnetism) so it's pretty soft to start with: 70 ksi tensile and 30 ksi yield are common spec values. You might have the material certs for the starting stock, so you can check to see if it was already annealed. Then check with the customer to see what they really want, as opposed to what they asked for.

My first attempt at explaining the 'extrusion' is that the outside surface of the bar was in tension from the manufacturing process- however the steel mill made it. The center would have balancing compressive stresses on it. Once you machined off the outer layer, the inner core reacted to no longer being compressed, and grew.

Good luck! Oh, my hat's off to you, by the way- I find it amazing you can turn 9 mil pins from 125 mil bar!

 

[EdStainless]

even if your bar was cold worked to some higher strength level when you draw bar the center is often softer than the outer surface (unless you draw it really hard). You may have thought that you were using higher strength material but then you machined away the strongest part.
He needs to specify what properties he requires. Make it springy is not a spec.

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Plymouth Tube

 

[dgallup]

Like Ed said, the hardness is not uniform all the way through. The simplest thing would be to start with stock a lot closer to your final size. It would save you time & money too, you're machining off 99% of your material! You should be able to buy spring wire already drawn to size and just cut it too length.

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[btrueblood]

Have to agree with Ed and Dgallup. Buy rolled 1/2-hard or harder 300 series spring wire in .012/.015 diameter (or thereabouts), straighten it and centerless grind to size.

 

[daschell]

Thanks for the replies. They pretty much verified what I was thinking. We started with .0469 material. It was not strong enough to push through the guide bushing. Funny thing is I purchased 316 1/8 bar from MSC and ran a few hundred parts. They were perfect. I can not be sure but I'll bet it was full hard material. I do not have the equipment to make the parts from spring stock in conjunction with the fact that they want a full spherical radius on one end only. I am amazed that the screw machine holds the tolerance ±.0002 myself. I will try to specify full hard material and see if that gets me there. The customer understands that his specification is a little lacking.As EdStainless points out the customers statement "I need it to be ""SPRINGY"" is not really a specification I can be held to. Once again thanks for the help. Dave

 

[EdStainless]

Why not see if Gibbs draws 0.0090" 316 spring wire?
They probably draw +/-0.0002" at this size.
I looked in the basement and I don't have any that size laying around.

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Plymouth Tube

 

[prdave00]

I've been able to source 316 drawn wire in the spring temper condition from a variety of places. Some even have it waiting on the shelf (although .010 was the smallest I came across in a quick search). Most of these places can also do secondary operations like straightening, centerless grinding, and radiusing the tip. Try Tegra Medical (

http://www.tegramedical.com/capabilities/quick_wire_inventory.html),

Fort Wayne Metals, or Precision Wire Components. They all deal with drawn wire for the medical industry like guide wires for catheters. The downside is that, unless you're ok with being a middleman, you might lose your customer to them.