Shotblasting - Surface Measurement Terms

Shotblasting of steel plate, profiles, construction, and fabrications is a process to clean, descale, provide a specified surface profile and edge break as a surface preparation.   This process takes place prior to a coating or paint application to maximise the adherence potential and corrosion control.

This is a document made up of five parts  on  “How Do You Make Your Coatings Stick Better?”

Section 1 - Specified Consistency

Section 3 - How to Measure

Section 4 - Cleanliness and Surface Profile

Section 5 - Recognising Steel Processes (for hardness of steel)

Section 2 - Surface Measurement Terms

Rmax The largest peak to valley measurement.

The distance from the highest peak to the lowest valley within each sampling segment is measured.

Ra (The most common stated measurement of “average roughness”). The average roughness is the area between the roughness profile and its mean line, or the absolute value of the roughness profile height over the evaluation length.

Rt The maximum peak to lowest valley measurement in the evaluation length.

Unlike Rmax, when measuring Rt, it is not necessary for the highest peak and the lowest valley to lie in the same sampling segment.

Mean Line A line half way between the highest peak and the lowest valley in the evaluation length and centered between the two lines defining the dead band.

Deadband  The distance above and below the mean line that a continuous trace line must cross in both directions (up and down) to count as a single peak. The dead band ignores small, spurious peaks. Typically, the dead band width is usually adjusted to fall in the range from 1.0 to 1.25 μm.

Peak Count (Pc) The number of peak/valley pairs per unit distance extending outside a “dead band” centered on the mean line. The width of a peak/valley pair is defined by the distance between crossings of the dead band region. Because the dead band width is so small compared to the size of the peaks and valleys encountered in coatings work, the dead band region is essentially the mean line. For all practical purposes, a peak would be registered if a continuous trace starts below the mean line, goes above it, and then below it.

Acknowledgements are made to the following documents

1. Definitions are taken from a draft ASTM document “Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument.” Sampling length is defined as “Traversing Length” in ASME B46.1-2002.
2. Taken from a draft ASTM document “Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument.”
3. The five sampling segments within the evaluation length are defined as “Sampling Lengths” in ASME B46.1-2002.
4. Taken from a draft ASTM document “Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument.” Rmax is also called “Maximum roughness Depth” in ASME B46.1-2002.
5. Taken from a draft ASTM document “Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument.” Rt is also called “Maximum Height of the Profile” in ASME B46.1-2002.
6. Taken from a draft ASTM document “Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument.”
7. Taken from a draft ASTM document “Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument.” Pc is also called “Peak Density” in ASME B46.1-2002 and “Peaks Per Inch Count” in SAE J911.