Surface Finish Optimization in Turning

In this article, you will learn the basic theory of surface roughness and how you can optimize it in turning operations.

What is Surface Finish?

Surface finish, also known as surface roughness, is a numerical representation of a surface’s smoothness. It can be problematic as it condenses a visual 3D representation into a single number.

There are 3 main methods to acquire the surface roughness value:

• Mechanical Scan: The most commonly used method to determine surface finish is scanning the surface with a needle that records the micro hills and valleys along a linear section. The measurement process produces a chart that is analyzed using a mathematical formula to yield a surface finish value. (A typical scan output is shown in image #1)
• Visual Comparison: An old traditional method that is rarely used today is the visual comparison of a surface against a standard palette of different surface qualities with the human eye.
• Visual Computerized Scan: A high-definition digital camera connected to a computer performs an optical scan of an area and calculates the surface finish.

Image #1

Surface Finish Units

Several units are used to measure surface finish, with Ra and Rz being the most popular. As there is often confusion between them, let’s break it down.

Ra, called roughness average, is the most commonly used unit. According to ASME B46.1, Ra is the arithmetic average of the absolute values of the profile height deviations from the centerline, recorded within the evaluation length. It is calculated by the formula:

[Formula 1 (ra)]

* The parameters refer to image #1

Rz, called the “Peak to Valley” average, is the second most commonly used unit. Rz is calculated by measuring the vertical distance from the highest peak to the lowest valley within the scanned profile. The highest five peaks and the five deepest valleys are taken, and then averaging these distances. Rz will always yield a higher value than Ra. It is calculated by the formula:

* The parameters refer to image #1

A common question among machinists and engineers is how to convert from Ra to Rz. A precise conversion between Ra and Rz is impossible since they are two different properties.

It’s like asking, “How do I convert height to weight?” You cannot convert height into weight, but you can make a rough estimation based on statistics. For instance, a person who is 6 feet tall will weigh between 140 and 290 pounds with a probability of 99%. It’s improbable that the person will weigh 100 or 400 pounds.

When converting Ra to Rz or vice versa, statistical analysis can be used to confidently forecast the Rz range for a given Ra value. This approach has been proven to yield accurate results with reasonable probability. You can use the conversion charts on image #2 or this online Ra/Rz convertor.

Now, after the basics are sorted out, let’s see how we can improve the surface finish of a turning operation.

The Surface Finish Formula and what we can learn from it

To understand what to do, you should first become familiar with the surface finish formula.

[Formula 3 (surface finish)]

By examining the formula, we can draw three important conclusions:

1.Decreasing the feed improves the surface finish.

2.Increasing the corner radius improves the surface finish.

3.Since the radius is squared in the formula, it is the more significant parameter to “play” with.

You can experiment with this equation using the Surface Finish Calculator.

Clamping

Rigid clamping of the workpiece and cutting tool is crucial for achieving a proper surface finish. If either is done incorrectly, optimizing the feed and corner radius will be ineffective.

If surface finish is a priority, make sure you are using a high-quality power chuck with good jaws. You can take it one step further and grind the jaws to the diameter of the blank. It will increase the contact area between the clamping unit and the workpiece and reduce the runout.

After properly clamping the workpiece, the next step is to stabilize the cutting tool.

1.Use Capto (ISO 26623) shanks or other technological clamping solutions for external tools and avoid traditional square shanks.

2.Use slotted, shrinking sleeves for internal tools and avoid traditional direct screw clamping.

3.When using external and internal tools, setting them up with the shortest possible overhang is critical. According to the bar deflection formula, the deflection is directly proportional to the overhang cubed. Therefore, even a minor reduction in overhang can significantly impact the system’s flexibility.

[Formula 4 – (Deflection)]

Additional Tips

• Opt for a sharper (ground) cutting edge.
• Higher (positive) peripheral and top rake angles will decrease the pressure on the workpiece, thus reducing vibrations and improving the surface finish.
• Avoid built-up edge (BUE) at all costs. When BUE starts forming, all the other steps mentioned above will not help. In most cases, this can be done by decreasing the cutting speed. Other remedies are trying a different carbide grade and using a sharper edge.
• Select a suitable chip breaker so that the chips are directed away from the workpiece (and not scratching it)

Success is never achieved by improving just one factor alone. Achieving the best results requires a systematic approach to improving all the above mentioned factors.