Quality measurement of deep drawn parts

[tralala]

Hi there!
I'm lookig for method to do some quality control on aluminium deep drawn parts. Since this is not my specialty I just asked Google, but could only find info about monitoring the deep draw process (through force measurment while forming), but nothing on finished parts. I'm looking for methods to do some quality check by already formed parts. Could you point me in which direction I should look for?

Besides checking the geomety, surface, foldings and/or cracks. Which properties/defects are to be taken in consideration?

Cheers!

TRala

 

[arunmrao]

Ericsson Cupping Test! This will give you an indication of the suitability of material for deep drawing.


If you think education is expensive, try Ignorance.
- Andy McIntyre


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[CoryPad]

Strength via tension testing of final part

Visual inspection after forming to quantify sheet thinning

 

[metengr]

In addition to visual, I would perform Liquid Penetrant examination on a random sample of finished parts just to be sure output is defect free from forming. This examination can be performed in house.

 

[CoryPad]

I meant to add that you need to section the part, then measure wall thinning.

 

[tralala]

Hi, Thank you for your replies!

@arunmrao: I looked for the method and found out that's called erichsen cupping test. Not quite what I was looking for, but it lead me to some non destructive measurement methods. Thanks

@CoryPad: Are there standard (normed) visual measuring methods?

As for non destructive methods, liquid penetrations sound quite good. Thanks for the tip.

Thankx!
TRala

 

[DannyEQS]

Rather than sectioning and then measuring the thickness with calipers, you may want to consider using an ultrasonic thickness gauge, which is non-destructive. [Remember to calibrate the gauge against your starting (flat sheet) thickness.] This is an easy and quick way to find the thinnest areas in your part, as well as determining the thickness in the critical areas of your part.
Once you have established your baseline of finished part thickness measurements across your stamping, you can track these measurements over time. If you see that you are trending thinner, that will let you know that something in your stamping process is changing. [Do it on a percent reduction from the actual initial thickness rather than the minimum ordered thickness to account for coil-to-coil thickness variations.]

Good luck!
-Danny

 

[CoryPad]

Great suggestion from Danny - this method would be best for continuous monitoring. I would recommend cutting at least one part to observe and measure thickness - there really is no substitute for visualizing both large-scale shape change and thickness variation at the same time.

 

[tralala]

Hello!

The suggestion from Danny might work... but...
My parts are way too small to be measures with a ultrasound sensor. Unless you can point me to a sensor with a spot of a couple of thenth of a millimiter. The Parts I want to measure have a diameter of 1-3mm.

I think I could sacrify some parts for quality control, cut them and measure them in the microscope.

For an online measurement (during the prozess) I found a method that measures the forces aplied by the press and compares them to a force-profile of a "good" part. If the measurent deliver a different profile, then the part would be thrown away.

THaks for your replies... they put me in the right path!


Now i have abother question... lets say that the parts were "big enough". Is there a method to automaticaly check them for mistakes, like folds or cracks? maybe some kind of visual check with a camera or with a 3d scanner?

Cheers!

Trala

 

[Neubaten]

For cracks, liquid penetrant is quite suited and commonly used in the industry, for what I've seen. There are different applications with different levels of sensitivity (and cost) to suit any need.

For folds and other dimensional defects, you can try pass-no pass or standard patterns, or the like, to check some statistically significant number of parts.


I have seen some automated dimensional scanning at the workshop, and it resulted a pain in the ass because of wrong parts passing/correct parts discarded, failures in readings because of optical interferences, etc... It never became an implemented inspection, and turned out to be a (costly) failure.

However, some other might tell you just the opposite. Just be very sure about it if you plan to use an automated dimensional inspection.

 

[DannyEQS]

TRala-
Aye- there's the rub. Mighty small parts you have.
OK - a different approach... what's the cost of poor quality?
If these are going into nuclear reactors, then every aspect of every part (dimensions, microcracks, etc.) is likely critical, and therefore that dictates one level of quality control and assurance.
If these are micro-widgets, then you don't need to inspect every one, and you can rely on sampling.
In spite of the concerns that Neubaten expressed, there several non-destructive optical techniques that can be used successfully to assess dimensional compliance, even at the size of your parts. Laser interferometry is one approach that comes to mind, but I know there are others.

But the best way to screen out problems is to avoid them altogether. A part your size is likely very sensitive to incoming material properties. I'm assuming the specification to which you order your sheet aluminum contains minimum property restrictions (min yield strength, min elongation), but you likely receive properties in excess of these minimums. How close can you get to these minimums without compromising successful formability? Without testing each incoming batch, you won't have the data from which you can make this assessment. You can rely on the incoming certs from the metal supplier, but realize that they test only coil ends, which may have different properties than the section of the coil on which you currently are forming parts. Surface profile (peak count and roughness) also contribute to formability, so you may also want to document these parameters as you are building up your database of successful material properties.

You'll also find tooling condition influences formability. Are the punch or cavity radii wearing unevenly? That suggests an alignment issue. Is the radius in one section rougher than the rest? Be sure to clean it up and blend it in with the adjacent sections.

Whichever approach you take, you should scale your actions to the cost of poor quality.

-Danny

 

[TVP]

Mectron Engineering, Retina Systems, and General Inspection are three manufacturers of inspection/sorting equipment that is used for 100% inspection of small parts (usually fasteners). Use the following links for more information:

http://www.mectron.net/

http://www.retinasystems.com/

http://www.geninsp.com/

 

[CoryPad]

Brown & Sharpe makes some systems for optical measuring:

http://brownandsharpe.com/optiv-overview

 

[tralala]

Thank you very much for the info.
I'll have a look and if I do have more questions I'll ask again!

Cheers,
Trala