Converting Common Surface Texture Measurements - Engine Builder Magazine

Converting Common Surface Texture Measurements

The irregularity of a machined surface is the result of the machining
process including the choice of tool, feed and speed of the tool,
machine geometry and environmental conditions. This irregularity
consists of high and low spots machined into a surface by the tool bit
or a grinding wheel. These peaks and valleys can be measured and used
to define the condition and sometimes the performance of the surface.
There are more than 100 ways to measure a surface and analyze the
results, but the most common measurement of the mark made by the tool
or the surface texture is the roughness measurement.

There are several different methods of roughness measurement in use
today. The method used on any given part depends largely on where in
the world the part is manufactured and the measurement parameters the
manufacturer and the customer prefer to use. It is not uncommon for
different parties involved in the production to use different methods
for roughness measurement. In this article we will talk about only two
of the many methods of roughness measurement, how to convert between
these two methods and how to avoid the problems caused by the
inevitable use of more than one roughness measurement.


In North America, the most common parameter for surface texture is
Average Roughness (Ra). Ra is calculated by an algorithm that measures
the average length between the peaks and valleys and the deviation from
the mean line on the entire surface within the sampling length. Ra
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. It’s a simple and effective
method for monitoring surface texture and ensuring consistency in
measurement of multiple surfaces.


In Europe, the more common parameter for roughness is Mean Roughness
depth (Rz). Rz is calculated by measuring the vertical distance from
the highest peak to the lowest valley within five sampling lengths,
then averaging these distances. Rz averages only the five highest peaks
and the five deepest valleys, therefore, extremes have a much greater
influence on the final value. Over the years, the method of calculating
Rz has changed but the symbol Rz has not. As a result, there are three
different Rz calculations still in use and it is very important to know
which calculation is being defined before making the measurement.

There are rules
of thumb that can help clear up the confusion and convert Ra to Rz or
Rz to Ra. If the manufacturer specifies and accepts the Rz parameter
but the customer uses the Ra parameter, using a ratio range for
Rz-to-Ra = 4-to-1 to 7-to-1 is a safe conversion. However, if Ra is
used as an acceptance criteria by the manufacturer but the customer
accepts Rz to evaluate the part, then the conversion ratio would be
much higher than 7-to-1, possibly as high as 20-to-1. Keep in mind that
the actual shape of the part’s profile will have a significant impact
on these ratios.


The approximate and sometimes questionable comparisons can be avoided
by developing an understanding of exactly what a parameter means, and how the various parties involved in the production plan to
check the surface. The best way for those involved in the production to be
in agreement on the parameters for measurement, is to have capable
evaluation equipment in both the manufacturer’s and customer’s facility
making the same check using the same method. If the manufacturer or the
customer uses conversion ratios, then both parties should be aware of
the use of the ratio and be comfortable with the ramifications.


– Tech Tip courtesy of Mahr Federal Inc. Rz = Ra; Rmax

You May Also Like

Shop Solutions May 2024

Those who submit Shop Solutions that are published are awarded a prepaid $100 Visa gift card. Submit your Shop Solution at [email protected].

Engine Builder and Engine Pro present Shop Solutions in each issue of Engine Builder Magazine and at to provide machine shop owners and engine technicians the opportunity to share their knowledge to benefit the entire industry and their own shops. Those who submit Shop Solutions that are published are awarded a prepaid $100 Visa gift card. Submit your Shop Solution at [email protected]. You must include your name, shop name, shop address and shop telephone number. Submitted Shop Solutions not published will be kept on file and reevaluated for each month’s new entries.

Shop Solutions April 2024

Shop Solutions provide machine shop owners and engine technicians the opportunity to share their knowledge to benefit the entire industry and their own shops.

A Different Dyno Design

The dyno is a valuable tool, so it’s nice when an engine builder feels confident in the setup of it. Enter the shipping container engine dyno design.

Properties of Pistons

Pistons are perhaps one of the more sophisticated chunks of metal in the picture. Here’s what you should know.

A New Take on the Rotary Engine

What if we could design a new rotary engine that addresses certain limitations without violating the laws of physics? This is what LiquidPiston has been working at for over a decade.

Other Posts

Perfecting Ring Seal Soup

Using modern honing machines, surface finishes, crosshatch angles, ring materials, and coatings all combine to create a more efficient engine.

Connecting Rod Stress

Connecting rods are subject to constant stress through extreme tensile and compressive loads, each one tied to a different aspect of operation.

Balancing, A State of Equilibrium

The balance of a rotating assembly is critical in every aspect and for every engine.

Factors of Crankshaft Selection

From the high-performance powerplants propelling Top Fuel dragsters to the subdued engines found in family sedans and grocery getters, each crank must be tailored to, and appropriate for, its specific application.