In my previous article, which appeared last November, we started speaking about process control and we underlined its importance for a reliable and consistent process. This is valid for all industrial sectors but has even greater importance in the automotive field. About ten years ago I had the opportunity of seeing a shot peening line in the plant of a well-known French car manufacturer. The plant had a very high productivity level, approximately 600 pieces per hour. Try to imagine what would happen if process control were lost for only a few hours: several thousand pieces would need to be scrapped! Let us now try to understand, by making a few simple considerations, in what possible ways we can achieve a reliable and consistent process. First of all, we must ensure that the line is correctly set up and that it can guarantee consistency during the entire production phase.

Concerning the set up, as usual, we must start with intensity. We have already examined the Almen method which consists of drawing a saturation curve with a minimum of four Almen strips. Each measurement will be the sum of the gage error plus the intrinsic Almen strip error. It is quite clear that using very accurate gages is of the maximum importance but, even if could sound strange, the real measurement instrument is the Almen strip, not the gage. The Almen strip is the sensitive and deformable element, and for this reason it has to be very accurate and of the highest possible quality. We could also point out that usually the Almen saturation curve is drawn manually, whereas the curve’s accuracy and consistency could be furthermore increased by computer-aided calculation methods. Nowadays it’s possible to buy software that can calculate Almen intensity. I designed my own Almen saturation curve software. Using historical data, I can detect even the slightest anomalies and, by correcting them in advance, I can achieve more consistent set ups. Unfortunately most shot peening specifications accept the Almen strip as the only reliable source of data and in fact limit the use of specific software, thereby limiting also the advantages which undoubtedly derive.

Let us now turn our attention to shot peening media. The Almen strip arc height is the average result of the plastic deformation left on the surface by the peening dimples. Being an average value, it can’t take into account the effect of any incorrect dimples caused, for instance, by broken and deformed shot or by shot with incorrect hardness. The real component could be deeply affected by a single incorrect dimple because a fatigue fracture always nucleates from a single specific point and not as a result of an “average” effect, as indicated by the arc height. It is my personal opinion that present grain size distributions and hardness ranges suggested by shot peening specifications can be in some cases too wide to ensure the right performance and consistency of the process. Dimensional problems can be solved by a correct use of the sieves but regarding the hardness we have to choose, once again, a source of very high quality.

At the beginning of the 1990’s I had the opportunity of participating in a conference where Ford Motor Company presented its experimental campaign to decrease the spread of fatigue performance in the gears of their cars. They made a very complex experimental plan to assess and weigh each single shot peening parameter. Keeping each of these under strict control, they decreased the data spread, thereby achieving the highest global reliability of their transmissions. Once more we must say that the greater costs of high quality processes, if well managed, can enable better performance and reliability and, as a consequence, reduce the costs deriving from poor quality.
In other words, the best processes allow us to achieve overall cost saving.

Shot Peening in the Automotive Industry
by Michele Bandini (Ph.D.)
Contributing Editor MFN
Teaches at Mechanical Engineering University of Bologna & Milan
E-mail: michele@mfn.li