How to measure and specify surface roughness after shot blasting using Ra, Rz, profile comparators, replica tape, and acceptance criteria.
Ra, Rz, and Anchor Profile
Ra is the arithmetic average roughness. It smooths the profile into one average value and is useful for many machined and finished surfaces. Rz is a peak-to-valley style measurement and often better describes the anchor profile produced by shot blasting for coatings.
For coating preparation, the goal is not maximum roughness. The goal is the correct profile for adhesion, coating thickness, and corrosion performance. A profile that is too low may reduce adhesion. A profile that is too high can create peak exposure or require excessive coating.
Measurement Methods
Common methods include stylus roughness gauges, replica tape, surface profile comparators, and optical measurement systems. The selected method should match the project specification and be practical for production. For structural steel, replica tape and comparators are common. For precision components, stylus or optical measurement may be required.
Sampling locations matter. Measure flat areas, edges, weld zones, shadow-prone regions, and representative part orientations. Record media type, wheel settings, cycle time, and readings together so the process can be repeated.
| Method | Best Use | Limitation |
|---|---|---|
| Replica tape | Coating profile on steel | Requires correct grade and burnishing |
| Stylus gauge | Ra/Rz numeric readings | Can be slow on complex parts |
| Comparator | Fast field check | More operator dependent |
| Optical system | Detailed profile mapping | Higher cost and setup |