Shotblasting - How to measure your surface profile

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 2 - Measurement Terms

Section 4 - Cleanliness and Surface Profile

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

Section 3 - How To Measure

The above schematic and illustration on blast cleaned steel shows a “dead band” and “mean” to peak.The distance from the highest peak to the lowest valley in the first segment of the evaluation length is R1. The distance from the highest peak to the lowest valley in the second segment is R2, and so on. The largest of R1 to R5 is defined as Rmax.

The average value of R1 to R5 is defined as Rz.

Rt is the distance from the top of the highest peak in the evaluation length to the lowest valley in the evaluation length. The highest peak and the lowest valley do not have to lie in the same sampling segment.

The peak count, Pc, expressed as peaks per inch (peaks per centimetre), is computed from the number of peaks counted in the evaluation length (five evaluation segments). The “peak” to the left of peak #2 is not counted as a peak since it does not cross the dead band.

When measuring Rmax, Rz, and Rt, “distance” is measured perpendicular to the mean line as shown in the figure.

The mean line is half way between the highest peak and the lowest valley in the evaluation length and is centered between the two lines defining the dead band. To remind us, the Deadband allows us to ignore minor scratches that do not contribute to the measurement detail required for this purpose.

The Ra, average roughness is by far the most commonly used parameter in surface finish measurement. The earliest analog roughness measuring instruments measured only Ra by drawing a stylus over a surface and integrating (finding the average) electronically.

However, Ra does not tell the whole story about a surface. For example, here are three surfaces that all have the same Ra, but you only need to use your eyes to know that they are quite different surfaces.  In some applications they will perform very differently as well. These three surfaces all have the same Ra, even though your eye immediately distinguishes their different general shapes.

[caption id="attachment_2012" align="aligncenter" width="300"] These three graphs above, show surfaces that differ in the shape of the profile - the first has sharp peaks, the second deep valleys and the third has neither. Even if two profiles have similar shapes they may have a different spacing between features[/caption]

[caption id="attachment_2014" align="aligncenter" width="300"] All three of the above surfaces “also” have the same Ra profile measurement![/caption]

To distinguish between surfaces that differ in shape or spacing, it is necessary to calculate other parameters for a surface that measure peaks and valleys and profile shape and spacing.  The more complicated the shape and the more critical the function of the surface, the more sophisticated we need to be in measuring parameters above and beyond solely the Ra.

This subject leads on, but for the purposes of this blog item we only require to be aware of these facts.

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