in Vol. 23 - September Issue - Year 2022
Drawing Requirement Issues
A common source of confusion is the drawing. The drawing is considered the master document when specifying shot peening control requirements, and most drawings will simply refer to the specification to which the part is to be peened and show detail as to which areas must be peened, which are optional, and those that must be protected from the process. The intensity is most often stated on the drawing or allied documents. Through regular and committee-driven reviews, the clarity of the shot peening specifications is constantly improving to where significant variation in interpretation was impossible 30 years ago; consequently, there is little today that can be misinterpreted.
The drawings, which as stated, carry significant elements of information, although the depth of information can and does vary somewhat from drawing to drawing. Some companies specify exactly what information is to appear on the drawing and how the peened/optional/unpeened areas are to be identified on the drawing. The presentation of this information varies considerably from OEM to OEM, presenting a challenge for peening shops. The use of flag notes is a widely utilised method for identifying the areas to be processed or protected, which on a complex drawing, can easily result in a flag not being noticed and an area erroneously shot-peened.
The most common method used to identify a featured diameter for shot peening involves a right-angle line that identifies the extent of the area to be shot-peened. This can be clearly identified, with a flag note describing the correct process and specification. One would consider this to be perfectly clear; however, there are often unanswered questions. Does the indicated area adequately identify the associated face or shoulder and require it to be shot peened? It is not possible to tell from a line that is parallel with the face. This results in many calls to the OEM to confirm the intent of the drawing, or sometimes an area being shot-peened when it shouldn’t or missed when it should.
This issue can be remedied very easily by indicating the area with an angled line that clearly identifies the extent of the shot peening, and whether the shoulder requires peening or not. This brings to question the viability of international specifications including a section on an agreed methodology for indicating shot-peen boundaries on the drawing. This method of bringing a common language to the process designer and the peening shop is now made far more viable by the continued migration from standalone OEM shot-peening specifications, to those peening specifications now referring directly to AMS2430 (Automated Shot Peening) or AMS2432 (Computer Controlled Shot Peening). This would therefore automatically introduce an ‘agreed’ method of drawing notation for shot peening to those who produce the drawings - all without restricting the OEM from designing their own shot-peening paraments and thus maintaining autonomy.
Another associated issue regarding specified tolerances of diameters is the growth of the diameter after shot peening. The dimensions of a component will often change after shot peening, thus causing an out-of-tolerance condition when measured on the CMM and a subsequent rejection. The shot-peening shop has no control over this situation since they are duty bound to apply the process to closely measure parameters. An experienced draughtsperson will know that dimensions, especially close tolerance dimensions, will be adversely affected and firstly take into consideration that there will be a change, and secondly, adjust the specified diameter so that the process brings the diameter into size. The drawing should state ‘diameter before peening’, thus highlighting to inspection that a change is to be expected and is acceptable.
Again, this is an area that is not generally recognised and would benefit greatly from being covered in more detail by international specifications. An approach could be to recommend that sample parts be manufactured and measured before and after shot peening, so that the correct pre-process dimension is specified on the drawing. It should be noted though that shot-peened surfaces are allowed to be modified by machining or honing for example, if the material removed does not exceed a percentage of the Almen arc height. However, in some cases this may not be acceptable or possible.
Components of any significant length will grow during shot peening at a rate proportional to the length, and this also must be taken into consideration when designing a component that is to be shot-peened. This normally cannot be remedied by material removal as previously stated. This characteristic is not often mentioned in the shot peening specifications however, and can lead to further rejections after peening when not fully taken into consideration during the component design phase.
In conclusion, although shot-peening specifications are becoming more detailed, there are still areas that can be introduced to remove the potential for ambiguity and unexpected dimensional changes.
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MFN Contributing Editor
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