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Activity 002: Abrasive Media Comparison Test

Activity ID: U10M1-ACT-002 Duration: 45 minutes Objective: Compare surface profiles produced by three different abrasive media on identical metal test coupons.

Overview

Students blast identical mild steel test coupons using three different media types (aluminum oxide, glass bead, and walnut shell) and measure the resulting surface profiles. This hands-on comparison builds intuition for media selection in real-world projects.

Materials & Equipment Needed

  • Blast cabinet with suction-feed system
  • 3 mild steel test coupons (2" x 4" x 1/8", degreased)
  • Aluminum oxide media, 80 grit (2 lbs)
  • Glass bead media, 100–170 mesh (2 lbs)
  • Crushed walnut shell media, 12/20 mesh (2 lbs)
  • Testex Press-O-Film replica tape (Coarse grade)
  • Spring micrometer (±0.1 mil accuracy)
  • Permanent marker and labels
  • Safety glasses, nitrile gloves, dust mask (N95 minimum)
  • Student worksheet for recording measurements
  • Magnifying loupe (10x)

Instructions & Procedure

Phase 1: Setup and Safety Check (10 minutes)

  1. Verify blast cabinet dust collector is operational — check filter indicator
  2. Don PPE: safety glasses, nitrile gloves, N95 dust mask
  3. Label three test coupons: "AO-80" (aluminum oxide), "GB-100" (glass bead), "WS-12" (walnut shell)
  4. Load aluminum oxide media into blast cabinet hopper
  5. Set cabinet pressure to 60 PSI
  6. Verify nozzle condition — replace if orifice is worn beyond 1/16" oversize

Phase 2: Blasting Procedure (20 minutes)

  1. Coupon 1 (Aluminum Oxide): Blast at 60 PSI, 6-inch standoff distance, 45° angle, 3 passes across entire surface. Maintain consistent speed (~2 inches/second).
  2. Remove coupon, inspect for uniform coverage. Record observations.
  3. Purge cabinet. Load glass bead media.
  4. Coupon 2 (Glass Bead): Blast at 60 PSI, 6-inch standoff, 90° angle, 3 passes.
  5. Remove coupon, inspect and record.
  6. Purge cabinet. Load walnut shell media.
  7. Coupon 3 (Walnut Shell): Blast at 60 PSI, 6-inch standoff, 45° angle, 5 passes (softer media requires more passes).
  8. Remove coupon, inspect and record.

Phase 3: Measurement and Analysis (15 minutes)

  1. Apply Testex replica tape to each coupon at center point
  2. Burnish tape firmly with burnishing tool (10 firm strokes)
  3. Remove tape and measure with spring micrometer
  4. Subtract tape base thickness (2.0 mils for Coarse grade)
  5. Record profile depth for each media type
  6. Examine each surface with 10x loupe — sketch the profile pattern
  7. Compare results against expected values:
  8. Aluminum oxide 80 grit: 2.0–3.5 mils
  9. Glass bead 100–170: 0.5–1.5 mils
  10. Walnut shell 12/20: 0.2–0.8 mils

Discussion Points

  • Which media produced the deepest profile? Why?
  • How does particle shape (angular vs. spherical) affect the surface texture?
  • Which media would you select for: (a) pre-paint prep on steel? (b) cleaning a brass artifact? (c) removing paint from wood?
  • What happens if you increase pressure or decrease standoff distance?

Expected Outcomes

  • Students produce three distinctly different surface profiles from identical substrates
  • Aluminum oxide creates the deepest, most angular profile
  • Glass bead creates a smooth, peened finish
  • Walnut shell creates minimal profile suitable for delicate cleaning
  • Measurements fall within expected ranges (±0.5 mil)

Assessment Rubric

Criterion Excellent (4) Proficient (3) Developing (2) Beginning (1)
Safety compliance All PPE worn, all checks completed PPE worn, minor check missed PPE partially worn PPE not worn
Blasting technique Consistent speed, angle, distance Minor inconsistencies Uneven coverage Significant technique issues
Measurement accuracy All readings within ±0.3 mil of expected Within ±0.5 mil Within ±1.0 mil Readings unreliable
Analysis quality Thorough comparison with technical reasoning Good comparison Basic comparison Minimal analysis

Safety Considerations

  • Never open blast cabinet during operation
  • Ensure dust collection is running before blasting
  • Wear N95 minimum respiratory protection even with cabinet systems
  • Handle test coupons with gloves — freshly blasted metal has sharp micro-edges
  • Dispose of spent media according to facility waste procedures
  • Wash hands thoroughly after handling any abrasive media

Last Updated: 2026-03-19