Shot Peening & Robotic Abrasive Blasting Vector Prepress Guide
Shot peening introduces deep compressive residual stress that prevents fatigue failure in aerospace landing gear, turbine roots, and automotive drive axles. Master Almen saturation curve verification, robotic CAM raster toolpaths, and precision masking prepress rules.
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1. Peening Mechanics, Almen Arc Height & Saturation Curves
Shot peening is a cold-working process where spherical metallic, ceramic, or glass media impact a metal surface at high velocity (30 to 90 m/s). Each impact creates a localized hemispherical plastic indentation, yielding the surface grains and inducing a sub-surface compressive residual stress layer that extends 0.1 mm to 0.6 mm deep.
Core process control parameters:
- Almen Strip Verification (SAE J442): Intensity is measured using standardized spring-steel Almen strips (Type 'A' for standard peening, 'N' for low intensity/thin sheets, 'C' for heavy shot). Deflection arc height is checked on an Almen gauge.
- Saturation Definition (AMS 2430): The saturation point ($T_{\text{sat}}$) on the Almen curve is reached when doubling the exposure time ($2T$) produces an arc height increase of no more than 10%.
- Coverage Saturation: 100% visual coverage occurs when dimple indentations completely obliterate the unworked surface; 200% coverage represents twice the exposure time required for 100% saturation.
Engineering Standards & Process Parameter Reference
| Target Alloy | Shot Media | Almen Intensity Range | Typical Coverage | Compressive Depth |
|---|---|---|---|---|
| 300M / 4340 Landing Gear | Cast Steel S230 / CW14 | 0.012 - 0.016 A | 100% - 150% | 0.20 - 0.35 mm (-950 MPa) |
| Ti-6Al-4V Turbine Blisk | Ceramic Z400 / Z600 | 0.008 - 0.012 A | 125% - 200% | 0.15 - 0.25 mm (-650 MPa) |
| 7075-T6 Aerospace Spars | Glass Bead GP105 / CW10 | 0.006 - 0.010 N | 100% - 125% | 0.08 - 0.15 mm (-320 MPa) |
| Inconel 718 Disc Roots | Cast Steel S170 / CW14 | 0.010 - 0.014 A | 150% - 200% | 0.18 - 0.28 mm (-850 MPa) |
| Automotive Transmission Gears | Cast Steel S280 / S330 | 0.014 - 0.018 A | 100% - 150% | 0.25 - 0.45 mm (-1100 MPa) |
2. Vector CAD/CAM Rules for Robotic Peening Rasters & Shadow Masking
- Generate robotic 6-axis toolpath vectors with continuous bi-directional serpentine rasters and 50% nozzle spray pattern overlap to ensure uniform dimple distribution.
- Maintain normal blast nozzle impingement angles between 75° and 90°; angles below 45° induce undesirable surface shearing and micro-grooving rather than pure plastic dimpling.
- Laser-cut rigid polyurethane or hardened steel shadow masks with 30° chamfered edge bevels to deflect ricocheting media away from protected ground bearing journals.
- Program corner slowdown ramps with proportional pressure throttling to prevent over-peening and stress concentration risers at acute geometric transitions.
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