A cylinder that fails post-engraving because the base coating was not verified represents sunk cost at every stage. Machine time. Material. Scheduling.
The same applies upstream on substrates. Surface roughness outside specification on incoming materials introduces variability that compounds through production, and is rarely diagnosed correctly at source. By the time it shows up as print instability or adhesion failure, the damage is done.
The fix is simple. Measure surface roughness Ra and Rz properly, at the right point in the process, with equipment accurate enough to catch what matters.
Why Ra Alone Is Not Enough
Most production environments default to Ra. It is familiar, widely specified and easy to report. The problem is that Ra was designed to average, and averaging hides outliers.
A nickel coating with one deep isolated pit on an otherwise smooth profile can produce an acceptable Ra value. Rz will catch it. Ra will not. That distinction is minor on a low-stakes surface. On a cylinder about to go to laser engraving, it is the difference between a good job and a scrapped one.
Why Rz Earns Its Place
Rz divides the measurement length into five sections and averages the peak-to-valley height across each. Because it focuses on the worst event in each section rather than the overall profile, it is far more sensitive to isolated defects. Surface anomalies that Ra smooths away, Rz surfaces.
For a typical machined surface, Rz runs roughly four to seven times Ra. A ratio significantly outside that range is a signal worth investigating before committing to the next production stage. Catching this at measurement will save a business time and money.
The Conversion Problem
Conversion ratios between Ra and Rz circulate widely, typically quoted anywhere between 4:1 and 7:1. The variation exists because the ratio depends on the specific manufacturing process and surface characteristics of the component. Applying a generic ratio to a critical specification is unreliable. If the application demands precision, measure both directly.
What Else the Surface Is Telling You
Ra and Rz together give a solid baseline. They do not give the full picture. Rsk, skewness, tells you whether the surface is peak-dominant or valley-dominant. A negatively skewed surface retains more material in its valleys, which matters for coating adhesion and ink transfer consistency. Rku, kurtosis, measures the sharpness of those peaks. A high kurtosis value indicates an abrasive surface that will cause rapid initial wear in any mating component.
A surface that passes surface roughness Ra and Rz checks but shows poor Rsk values will still underperform in application. These parameters are not unusual to measure. They are the difference between a surface that looks acceptable on paper and one that performs consistently in production.
The Stylus Problem
Contact profilometry is still widely used. Accurate to around 5 micrometres, adequate for legacy specifications. For modern, tighter tolerances, this often falls short. Critically for any surface where marking is a concern, physical contact is a risk in itself.
Optical 3D scanning microscopes such as the AniCAM HD™ Plus measure the same surface roughness parameters without contact, at accuracies within +/- 1 micron. The scan captures the full surface rather than a single trace line. Ra, Rz, RPc, Rsk and Rku in a single pass.
No stylus wear, no operator variability, no surface marking. Six times more accurate. Faster. Available to view in true 3D visualisation. And the data is immediately comparable across time, operators and sites.
The Long-Term ROI Case
Routine surface roughness measurement at the right production stage is not a quality overhead. It is the cheapest intervention available. Problems found at measurement cost time. Problems found in production cost time, material, rescheduling and customer confidence. Across a year of production, the cost of a single avoidable cylinder failure or substrate rejection dwarfs the investment in proper measurement equipment many times over.
Ra is the more widely used parameter in North American markets. Rz is the preferred standard internationally and across European supply chains. For operations working across both, measuring and reporting both removes specification ambiguity and simplifies supplier conversations.
Troika’s Ra-Rz surface roughness software uses optical 3D scanning to measure Ra, Rz, RPc, Rsk and Rku in a single pass, within 1 micrometre. Filter settings are adjustable from 1 to 100µm depending on sample requirements. Any point on the scan is selectable for instant measurement values, with individual and average results for direct comparison.
If you can’t measure it, you can’t control it. Request a demo to see what your surfaces are actually telling you.
