Surface Roughness

At a microscopic scale, no surface appears perfectly smooth. Surface roughness is determined by calculating R_a and R_z.

• R_a = average surface roughness
• R_z = information for any pore, hole or surface deformities. These can be detrimental to strength.

R_a averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points so that the extreme points have no significant impact on the final results.

R_z is calculated by measuring the vertical distance from the highest peak to the lowest valley within five sampling lengths, then averaging these distances. Therefore, measured extreme points influence the final number.

Therefore, depending on what you are after you need to be careful about what you specify and what you measure,

The following plot shows average surface roughness (R_a) for a number of different manufacturing techniques.

• Fractal Dimension is a measure of roughness. Designed to describe, by a single parameter, surface roughness over many orders of magnitude. For smooth surfaces, the value of fractal dimension is near to 2.0 which gradually increases with an increment in roughness of the surface. For drastically rough surfaces, the fractal dimension comes close to 3.0.

Surface Roughness Measurement

There are many different measurement techniques for measuring surface roughness and these are first broken into 4 main categories:

1. Direct measurement methods
2. Comparison based techniques
3. Non contact methods
4. On-process measurement

Looking at the different measurement methods it is useful to break them into the above categories even though some methods may appear more than once:

• Direct measurement methods
  • mechanical contact stylus instrument
• Comparison based techniques
  • subjective comparison of samples based on tactile and visual senses
• Non contact methods
  • laser scanning
  • atomic force microscope
  • machined vision
  • inductance sensor
  • ultrasonic sensor
• On-process measurement
  • machined vision
  • inductance sensor
  • ultrasonic sensor

References

• Peter Pfeifer, “Fractal dimension as working tool for surface-roughness problems“, Applications of Surface Science, Volume 18, Issues 1–2, May–June 1984, Pages 146-164
• Surface Texture From Ra to Rz, Modern Machine Shop

Mill Stiffness = Top Finish

Stiffness is a fundamental for any milling machine big or small. Increasing stiffness reduces vibration.

I increased the column to base connection stiffness with this mild steel plate.