How to Convert Surface Finish From Metric to English

The surface texture, or surface finish, of a product manufactured using a repetitive process such as an assembly line is of interest to both manufacturers and consumers. A manufacturer may wish to make products that meet the specifications of the ordering entity, whereas the ordering entity may want to evaluate the process based on how consumers are using and responding to the product regardless of its adherence to specifications.

In the United States, metric units are in common enough use for quantifying things, such as 5-kilometer road runs and people monitoring their food "macros" in grams. But these units are still not the everyday standard, including units of length and area. In the U.S., people are still accustomed to working primarily with English units, including feet, inches and derivations thereof; in the metric system, centimeters and meters are the standard.

You may find yourself needing to convert from metric to English units when examining surface finish data, and you may need to do this either precisely or, no pun intended, roughly. What follows should help you chart a smooth path toward this goal.

What Is Surface Roughness?

For purposes of machining, the texture of a manufactured part can be broken down into three elements: Roughness, waviness and form.

  • Roughness is a function of micro-damage made as a result of the manufacturing process itself. The number of these marks may be predictable for a given machine and process over time, but not over short time frames.
  • Waviness is a function of the distance between the cutting tool and the piece being cut throughout the process, which is small but non-zero.
  • Form or rather deviations from perfect form, are the result of tools that are not ideally flat or straight.

Engineers can analyze each of these components independently to locate and correct sources of error. When this is done with roughness, a number of common measures emerge.

Measures of Roughness

The most common unit in use for specifying roughness is Ra, or roughness average. This parameter is determined from a computer calculation, with the computer looking at the number and height of the deviations from an ideal, perfectly flat manufactured surface over a certain length. The most common units are micro-inches and micrometers, or 10-6 in and 10-6 m (μm) respectively.

Ra essentially measures "peaks and valleys," and is used to ensure that a process is not degrading over time, which would be evident in a higher Ra.

A similar measure to Ra in surface roughness is RMS, or root mean square. The calculation done to determine RMS is similar, but its value tends to emphasize large individual errors more and de-emphasize smaller, more systematic errors. That is, with RMS, statistical outliers matter more.

Conversions Between Surface Finish Units

In reality, a meter is equal to 3.28084 feet or 39.37 inches. A meter is divided into 100 centimeters, while a foot is divided into 12 inches; the resulting conversion gives 2.54 centimeters per inch.

The prefix "micro" has the informal meaning of "very small," but in the metric system it means "one millionth," or 10-6. The fact that surface roughness is measured in such small units reflects the precision demanded in the modern machining and manufacturing worlds.

Surface Finish Conversion Chart

When you look at a surface finish conversion chart like the one in the Resources, you will see that in general, RMS is about 1.1 times Ra, and values for micro-inches are often listed as being exactly 40 times greater than the corresponding value for micro-meters. This reflects the margin of error of the evaluation process itself, as more exact conversions between metric and English units in this setting would not add useful information.