Stainless - changing thickness and measuring hardness

[antknee]

How would I best change the thickness of stainless steel? For example, if I bought some with a thickness of 0.25mm how would I best change this to 0.23mm. I would like to test 0.21mm, 0.22mm, 0.23mm and 0.24mm but it is only available in 0.20mm or 0.25mm.

How do I test the hardness of stainless steel? For example if I bought some that was 0.25mm, how would I know it was half hard, full hard etc. with more accuracy than bending it by hand.

Ultimately the purpose of this is to make a stainless steel disc with a resonant frequency of 50KHz for a piezo buzzer. Changing the diameter is relatively easy but i'm constrained with the diameter so I must alter the thickness and hardness of the disc. I may need for example a thickness of 0.22mm and a hardness of "xyz". The stainless grade is 304/316.

 

[EdStainless]

Hardness on this thin of material is usually measured using a low load superficial Rockwell scale. One of the 15kg load scales should work.

For thinning samples the only thing that comes to mind is lapping them. You have to keep them flat and minimize cold work. This should work.

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Plymouth Tube

 

[antknee]

What is lapping? Is it putting the steel in a press? For some reason I thought I could roll them more but now I've realised this would make them less hard.

Antknee.

 

[MikeHalloran]

Rolling, like any other forming process, typically work-hardens stainless steel.

Lapping involves rubbing the work article against a reference surface with loose abrasive grains interposed. Sort of like grinding, without the heat, and without the speed. It's possible to do it by hand, but it's very time-consuming, for which reason lapping machines exist.



Mike Halloran
Pembroke Pines, FL, USA

 

[antknee]

I see, so lapping will reduce thickness through grinding and presumably reduce the stiffness of the remaining part.

Meanwhile cold rolling will reduce thickness and increase hardness.

Pressing will at a guess not be effective although it'll make the part flatter.

 

[MikeHalloran]

Because of Poisson's ratio, rolling also increases the length and width of a strip.

Pressing can reduce the thickness also, but it requires a huge and stiff press to achieve the necessary pressures, that are achieved with much less force in a rolling mill. I.e. stressing a line to yield is much easier than stressing an area to yield.


Mike Halloran
Pembroke Pines, FL, USA

 

[MiketheEngineer]

Lapping is like "sanding" the part to the final desired thickness. Except it is done under very controlled conditions to maintain exact thickness and parallelism. Also - it is typically a rather slow process and no real heat is generated.

 

[antknee]

Thanks for the information. Antknee.

 

[dgallup]

I agree with the lapping comments, this would be very easy with parts small enough for a piezo transducer. You can control the thickness to a very small tolerance. We lap millions of parts to +/-1 micron thickness.

I don't think hardness will have any effect on your resonant frequency. It will affect strength.

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[EdStainless]

By lapping and making the parts thinner you will make them less stiff. I have also seen this done by machining concentric groves into them, but this is very delicate.

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Plymouth Tube

 

[antknee]

I tested some piezo buzzers I made up today, each stainless steel disc had a diameter of 15mm but some had a thickness of 0.2mm and some 0.3mm. They all had an almost identical resonant frequency, but the thicker ones required more power to get working. It has me stumped and I wasn't expecting this.

The thing that occurs to me is that the critical factor effecting the resonant frequency is the stiffness of the stainless, the 0.2mm stainless might be as stiff as the 0.3mm despite being thinner - I have no accuarate way of checking. It appears I need the stainless rolled stronger or harder but I don't know the difference between hardness and strength.

Does that sound sensible?

What is the difference between hardness and strength?

Regards,

Antknee.

 

[EdStainless]

No, the modulus should be identical unless they were cold worked to very high strengths.
How are these mounted?
Is the edge tightly clamped?
It might be that the diaphragm stiffness outweighs the change in thickness.
You may need to try and figure out how to cut some shallow concentric grooves into these to make them more flexible.

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Plymouth Tube

 

[antknee]

I didn't measure the frequencies correctly first time around. I measured the acoustic resonance which was 47KHz and is irrespective of thickness. My setup is complicated and hard to explain but I need the buzzer to mechanically resonate at 50KHz.

The pzt resonates at 40KHz. Attached to the 0.2mm stainless the combined mechanical resonant frequency is 45KHz. So I can guess the stainless on its own would resonate mechanically at say 55KHz. So I need the stainless to be stiffer and pull the combined frequency up a bit more to 50KHz.

PZT = 40KHz
Stainless = 55KHz?
PZT + Stainless = 45KHz

How do I order stainless with a different stiffness? I mean do I ask for some stainless 0.2mm at 3/4 hard and full hard? Or do I ask for specific shore hardness values?

Is it possible to buy a cheap hardness tester? I don't need huge accuracy and couldn't justify the large cost for an industrial one.

I might be able to get away with some off the shelf stainless for this purpose but the next buzzer is 100KHz which I'd say will have to be cold rolled much stiffer/stronger

 

[EdStainless]

You need it thicker to make it stiffer. There is no way to change the modulus without cold working to very high levels.
There are some SS alloys that are a little stiffer, such as 439.
This might be enough to help you.

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Plymouth Tube

 

[antknee]

The thing I can't work out is this - I have 2 types of stainless at 0.2mm thickness, they were bought from different suppliers at different times. One is noticeably stiffer than the other, I've used the stiffer one, but surely if I tested the other the mechanical resonance would be lower? It would be something like below?

PZT = 40KHz
Stainless = 30KHz?
PZT + stainless = 35KHz

 

[MikeHalloran]

The stiffness of a beam, and of a diaphragm, is extraordinarily sensitive to the beam depth, in this case the thickness. I.e., 0.20mm is different from 0.21mm thickness. You really need to measure with a resolution well under 0.1mm, possibly under 0.01mm, for the stated problem.


Mike Halloran
Pembroke Pines, FL, USA