Standards Forum

in Vol. 18 - September Issue - Year 2017
Information is not knowledge

Paul Huyton

A quotation attributed to Albert Einstein is:
"Information is not knowledge. The only source of knowledge is experience."
Using the experience of others is a good way to improve our knowledge of specifications and this includes their experience of non-conformances. The following are the most common non-conformances to shot peening specification requirements in the aerospace sector in recent times.

1. Almen gauge inspection and calibration. The usual failure modes were not inspecting the strip contact points for wear and not calibrating using the minimum five extension points as defined in SAE J442: 2013.

2. Media shape inspection. Specifications require that the sample is representative of media impacting the part and the common failure was to sieve the sample for the size analysis before inspecting it for broken shot, etc. The sieving could remove broken pieces of shot from the sample and so invalidate the shape inspection.
Another failure was to not completely fill the sample area with media, so the sample size was below that required. Specifications require that there would be a continuous layer of shot over the sample area.
Also, not inspecting media with the correct magnification according to the required specification. If the magnification is not adequate, then defective pieces of shot may not be identified.
Media shape inspection is a vital process quality control and is defined in specifications such as AMS 2431 Peening Media, General Requirements for new media and AMS 2430 Shot Peening, Automatic for “in-process” checks.

3. Automatic shut-down of the process. Specifications such as AMS 2432 Shot Peening, Computer Monitored, require that the machine will shut down when process control parameters are not adhered to. Air pressure is a primary example for most types of peening but for computer-monitored peening, other parameters, such as the speed and travel distance of the nozzle, must also be monitored. Mechanical movement of the nozzle or part must conform to the approved technique or the equipment must automatically shut down.

4. Process parameters missing from the technique sheet.
The technique sheet must include all the parameters, which the operator can adjust or verify as correct during the process. Process specifications may include a list of required parameters, such as in AMS 2430T, Table 4. Nadcap checklists belonging to the audit criteria AC 7117 also indicate the parameters required for inclusion in production documents.

5. Process steps missing from the technique sheet.
Process specifications, whether they are customer’s proprietary specifications or the industry specifications such as AMS 2430, all define the various steps necessary to achieve the desired result. Process documentation must also include all the steps necessary for a successful outcome. Checking for the completeness of production documentation is included in the Nadcap audit criteria AC 7117.
The required process parameters and steps will normally have been defined in the first article inspection report (FAIR) which should conform to AS9102 Aerospace First Article Inspection or to the customer’s specified requirements.

6. Failure of post peening inspection.
Inspection for the required coverage and for the absence of damage such as edge bulging or rollover is vital in order to verify a conforming product. Coverage inspection requires the use of magnification, which is normally in the x10 to x30 range. This is defined in SAE J2277 Shot Peening Coverage, but may also be defined in the customer’s proprietary specification.

7. Saturation point not correctly identified as SAE J443.
Determination of the intensity value is a key step in the development of a shot peening technique. The intensity of the process cannot be non-destructively checked by part inspection and so must be accurately determined during the development phase. Nearly all process specifications require the identification of the saturation point as defined in SAE J443. Any other methods can lead to serious errors, major non-conformances during audits, and possible non-conforming product.
Understanding the saturation curve process is vital and should be included in a well-defined training programme, such as the MFN courses. The use of software to identify saturation and intensity is generally encouraged to provide consistency between users and repeatability of results.  The software must conform to SAE J2597 Computer Generated Shot Peening Saturation Curves.

A large amount of information is provided in the form of specifications. Reviewing all this information takes resources and identifying the key and critical points is not easy. Usable knowledge comes from experience and so consulting those who have gathered experience and reviewing the common non-conformities to specifications is an efficient way to improve a shot peening process.

For questions contact paul@mfn.li

Standards Forum
by Paul Huyton,
MFN Course Director World Wide
more information at www.mfn.li/trainers
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