I had a sort of opposite problem

Post #41,683 by Andrew Grygus 6/8/02 10:46:11 PM Reply	I had a sort of opposite problem In the Aircraft industry, landing gear hinge pins are considered to be rather critical parts (for some reason). For a particular pin, I read the specification up and down and sideways and wrote a detailed manufacturing plan for inside manufacturing steps and exactly which outside vendors would be used for certain steps we were not equipped for. Every time I had a batch ready to go, the Menasco source inspector tried to reject them because "they didn't look like the parts everyone else was making". That's because ours were the only ones made to spec. Everyone else had to kiss grind after shot peen to hold tolerance. This is a very big no no. Shot peen needs to be the last step before hard chrome, because any touch of grinding negates the positive effects of the shot peen. But my parts "looked different", so every time we had some to ship we had to call in his supervisor to buy them off. [link\|http://www.aaxnet.com\|AAx]
Post #41,688 by dmarker2 6/8/02 11:20:46 PM 6/9/02 12:03:11 AM Reply	Re: I had a sort of opposite problem That is interesting - 'They don't look right" so don't want em. Sheesh Am interested as to what the 'shot peen' process involved. Am guessing that peen means 'hitting' & shot implies using ball bearings to do the hitting ? am also guessing that this was a hardening process after machining. Cheers Doug UPDATE#1 LOOKED IT UP ON THE WEB ... Shot Peen Forming [link\|http://www.curtisswright.com/segments/metal_treatment/shot_peen_forming.asp\|Shot Peen forming] Peen forming is a dieless forming process performed at room temperature. During the process, the surface of the workpiece is impacted upon by small, round steel shot. Every piece of shot impacting the surface acts as a tiny peening hammer, producing elastic stretching of the upper surface and local plastic deformation that manifest itself as a residual compressive stress. The surface force of the residual compressive stress combined with the stretching causing the material to develop a compound, convex curvature on the peened side. Parts formed by peen forming exhibit increased resistance to flexural bending fatigue. Another distinct advantage with peen forming, unlike most other forming methods, it that all surface stresses generated are of a compressive nature. Edited by dmarker2 June 9, 2002, 12:03:11 AM EDT Expand All History
Post #41,696 by Andrew Grygus 6/9/02 12:47:31 AM 6/9/02 12:58:20 AM Reply	Only the last sentence is relative Shot peening of (very hard and not at all formable) critical structural parts is done with very tiny steel shot at high velocity. The objective is to create those compressive stresses on the surface, similar to the stresses induced in tempered glass. Grinding after shot peen tends to relieve these beneficial stresses through removal of material and heat, and introduces microscopic cracks and unwanted grinding stresses. Properly, there should be a heat cycle after grinding operations to remove the stresses produced by that process, but any such heat cycle would also remove the beneficial stresses from shot peening. On the other hand, shot peening can be used to relieve grinding stresses if a heat cycle is not possible due to metalergical (temper) requirements. [link\|http://www.aaxnet.com\|AAx] Edited by Andrew Grygus June 9, 2002, 12:58:20 AM EDT Expand All History

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