Fixing Surface Finish with the ACT Equation
Surface finish is a common challenge in grinding, but the solution doesn’t have to be complicated. When Ra values drift out of spec, the result is wasted material, extra dressing, and costly downtime. In Episode 12 of The Grinding Chronicles, CDT Application Engineer Blu Conrad explained how tracking both profile and surface finish helps you identify which factor is limiting your process. Now, in Episode 13, CDT Application Engineer Harrison Sheldon takes the next step by showing how the Average Chip Thickness (ACT) equation provides a clear, data-driven way to predict surface finish and improve part quality when grinding with superabrasive wheels.
The Grinding Chronicles - Episode 13
Surface Finish Matters
A part can pass dimensional checks but still fail inspection if surface finish is out of tolerance. That’s because Ra—the measure of a surface’s micro-level peaks and valleys—directly affects performance, durability, and consistency. Left unchecked, poor surface finish leads to higher scrap rates, shorter tool life, and inefficiency on the shop floor.
To understand and control surface finish, you need more than trial and error. That’s where the ACT equation comes in.
Understanding the ACT Equation
Every time a grit on the diamond grinding wheel removes material, it leaves behind a tiny chip and a corresponding valley on the workpiece surface. Smaller chips equal smoother finishes. Average Chip Thickness (ACT) captures this relationship in a measurable way.
The ACT equation, developed by Dr. Richard Lindsay, calculates chip thickness using common grinding parameters:
In the equation’s numerator, we multiply exponents of:
grain diameter (d)
specific material removal rate (Q′)
In the equation’s denominator, we multiply exponents of:
workpiece speed (Vw)
equivalent wheel diameter (De)
wheel speed (Vs)
abrasive concentration (f)
Once ACT is calculated, two simple equations let us predict surface finish Ra:
For fine finishes (1–10 µin Ra): Ra = 6 × ACT^0.3
For finishes in the 10–100 µin range: Ra = 2.2 × ACT^0.72
These equations, based on inch values, let you estimate surface finish outcomes for cylindrical grinding with superabrasive wheels before wasting time and material. For Metric values, see our downloadable document.
download ACT & Surface Finish Equations
Sign up for our email list and get instant access to the equations (English & Metric) for Average Chip Thickness (ACT) and Surface Finish (Ra). You’ll also get CDT’s quarterly updates. Your info stays private, and you can unsubscribe anytime.
Which Parameters Matter Most?
Not all grinding parameters influence surface finish equally. The ACT equation makes that clear:
Wheel speed (Vs) carries the greatest weight because it appears in the denominator raised to a relatively large power. This means even a modest increase in wheel speed can cause a significant reduction in ACT, which directly translates to a smoother surface finish. In practical terms, wheel speed is one of the most powerful levers you have for improving Ra without overhauling your entire setup.
Workpiece rotational velocity (Vw), on the other hand, has a much smaller exponent in the equation. Adjustments here produce only minor changes to ACT, so increasing or decreasing work speed alone won’t dramatically impact your finish.
By understanding these relationships, you can focus your process adjustments where they’ll make the most difference, rather than wasting time chasing variables with little effect.
Of course, external factors still play a role. Coolant contamination, wheel runout, and machine vibration can all interfere with finish quality. But ACT provides a reliable framework to prioritize changes, isolate issues, and refine your process with confidence.
Take Control of Your Finish
Combining surface finish tracking with ACT calculations gives you the power to:
Predict Ra values with accuracy
Optimize dressing intervals
Reduce scrap and downtime
Maximize parts per dress
💡 Ready to put this into practice? Sign up to download the English and Metric equations for free and make your own data-driven adjustments.
Need additional help running the numbers or troubleshooting your process? Contact CDT’s Application Engineers at TheGrindingChronicles@cdtusa.net.
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