Unlock the Secrets of Surface Finish Symbols [Must-Read Guide!]

Last modified: February 11, 2024

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Definition of Surface Finish

Before we get on with Surface Finish Symbols, let's understand how Surface Finish is defined. Engineering prints call out a great many things in their attempt to make sure the part that gets made matches the designer's intent. Aside from dimensions and tolerances, another important callout is Surface Finish.

Surface Finish is a measure of the overall texture of a surface that is characterized by the lay, surface roughness, and waviness of the surface. Surface Finish when it is intended to include all three characteristics is often called Surface Texture to avoid confusion, since machinists often refer to Surface Roughness as Surface Finish. Another term, analagous to Surface Texture, is Surface Topology.

This diagram gives an idea of how to think of the relationship of Waviness, Lay, and Roughness:

Machined part surface finish with lay, waviness, roughness, and profile characteristics.

The relationship of Waviness, Lay, and Roughness...

Lay

The Lay is the direction of the predominant surface pattern, and is usually determined by the production method used to process the surface. Lay is the first component of Surface Texture.

Here are some typical Lay patterns:

Surface finish lay patterns: vertical, horizontal, radial, cross-hatched, circular, and isotropic designs.

Surface Finish Lay Patterns...

Surface Roughness

Surface roughness is a measure of finely spaced surface irregularities. Surface roughness is the second component of Surface Texture.  Surface roughness is usually what machinists refer to when talking about "surface finish." When talking about all three characteristics, they may use the term "Surface Texture" more properly.

Waviness

Surface Roughness speaks to fine detail imperfections, but there may also be much coarser irregularities. For example, a surface may be warped or deflected from the ideal.  Waviness is the third component of Surface Texture.

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Surface Finish and Product Function

How do product designers decide what surface finishes to require, and how should CNC machinists think about different surface finishes?

There are a variety of considerations, and they may even vary for different stages in the manufacture of a particular part. For example, we may have a surface finish requirement on a casting that ensures the finish (think of it as the deviations of the surface of the casting from an ideal) is good enough that the allowance made for extra material that will be machined off in a future step is sufficent. If the casting is too imperfect, the trough of an imperfection may be below the expected machined surface of that future step.

Another important consideration is friction. Reducing surface roughness typically reduces friction which can be critical to reducing wear and increasing efficiency of sliding parts.

The desired surface finish is very much determined by the function and use of the component. Ship's propellors start out with fairly high surface finish standards but in actual use they erode pretty quickly. Optical and especially components used with X-Rays have some of the finest surface finish requirements achievable.

The goal of product designers is to specify surface finishes that are as coarse as possible but will still function within the part's desired operating parameters.

The goal of CNC machinists is to achieve surface finishes on parts that are as good as those required by the designer, but not better as that results in the cheapest to manufacture parts.  The machinist must choose a manufacturing process and cutting tool that will achieve the desired results as cheaply as possible.  Those are the best results possible.

It's important for designer and manufacturer to agree on exactly which parameters (Ra, Rz, etc..) are to be used for inspecting and parts acceptance. While there are commonly accepted conversions between these, actually using the same parameters for the inspection that the designer used with the design always yields the best results.

Surface Finishes for Various Manufacturing Processes

Surface Finishes vary tremendously by the manufacturing process used to achieve them. A flame cut plate edge has a radically different surface finish than a ground surface, for example. Choosing a manufacturing process that's capable of achieving the desired surface finish is the first step in figuring out how to manufacture a part that requires a particular finish. Sometimes, more than one manufacturing process must be overlaid to achieve the desired result cost-effectively.  For example, a machined surface may need to transition to a ground surface to meet the surface finish requirement.

In addition to choice of manufacturing process, there may also be a choice of cutting tool.

Here's a chart table showing the relative surface finish roughness of various manufacturing processes:

Machining professionals chart with surface finish roughness by process.

Relative Surface Finish Roughness of Manufacturing Processes...

For more on manufacturing processes and the cost of maintaining tight tolerances and surface finishes, see our article:

[ The high cost of tight tolerances ]

Measuring and Inspecting Surface Finish

Measurement implies characterizing something as a number-something that's very important to CNC'ers.

There are a number of different characteristics we might measure with respect to Surface Finish. Two of the most important are the height of the roughness peaks and their separation, often referred to as "Ra" and "D":

Black-and-white graph illustrating Ra vs D with waviness trendline.

Ra and D are two important surface finish parameters...

The Surface Finish Units we would use for parameters like Ra would be either micro-inches (English or Imperial) or micrometers (Metric). Our chart of surface finishes by manufacturing process (see above) gives both.

When we try to measure a surface finish, the methods fall into three categories:

Let's consider the ultrasonic scattering area technique.  Basically, surface roughness degrades the reflection of ultrasound.  The degree to which the signal is degraded is how this area technique determines surface roughness.  The optical scattering area technique operates on very similar principles.

Instrumenets used to measure Surface Roughness using these various techniques are called Profilometers.

Digital surface roughness tester with built-in display and measuring stylus.

Pocket Profilometer...

Factors Affecting Surface Finish

There are many factors that affect surface finish, with the biggest being the manufacturing process (see table above). For machining processes, such as milling, turning, and grinding, factors such as cutting tool selection, machine tool condition, toolpath parameters, feeds, speeds, tool deflection, cut width (stepover), cut depth, coolant, and vibration are just a few of the many important consideration.

Various Techniques:

Feeds and Speeds for Best Surface Finish

In general, adjust feeds and speeds for a lower chip load at the same rpms for a better finish. Take a finish pass that's relatively light as well. Make sure as you're doing this to avoid rubbing, which is very hard on tool life.  Proper settings for good surface finish should produce good tool life as well.

Our G-Wizard Calculator software has a handy "Tortoise-Hare" slider that makes it easy to dial in proper feeds and speeds for a finishing pass:

Torture Hare machining surface finishes comparison.

G-Wizard's Tortoise-Hare slider for dialing in your finish...

The further towards the Tortoise you go, the finer finish you will get.  G-Wizard accomplishes this by reducing the speed relative to the chip load.

G-Wizard will also warn you if rubbing is a risk, which will help you avoid that problem.

Ballnose Scallops and 3D Profiling Surface Finish

If you're 3D Profiling, each pass of the ballnosed cutter leaves a "scallop" in the surface of the material. Our G-Wizard Calculator software has a special Mini-Calc that will help you adjust for a scallop height that delivers the required surface finish:

Ballnose end mill surface finish calculator with input fields.

Ballnose Surface Finish Calculator...

Enter the maximum scallop height and G-Wizard will tell you the resulting RA and RMS surface finish as well as calculate the stepover. BTW, if you want to know how to pick optimal stepovers and tool diameters for 3D Profiling, we've got an article on that too!

Surface Finish While Turning

Turning is very similar to 3D profiling in that the stepover, in this case determined by the feedrate while turning, results in scallops that match the shape of the turning insert. We use this to good effect when turning threads, but if we want a smooth shaft, we'll have a surface finish requirement that must be met. That will determine feedrate and insert nose radius we can use for the job. G-Wizard has a nice calculator for that too:

CNC Surface Finish Turning Calculator tool screenshot from CNCCookbook.com.

Surface Finish Turning Calculator...

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Surface Finish Units From RA To RZ

A Surface Finish Requirement is specified in terms of some unit or other.

Ra - Average Roughness

Ra is also known as Arithmetic Average (AA) or Center Line Average (CLA). It is the average roughness in the area between the roughness profile and its mean line.

Mean Line is the reference line about which the roughness profile deviations are measured. The mean line of the roughness profile is usually established by analog or digital filters with the selected cutoff corresponding to the roughness sampling length. Profile Peak is the point of maximum height on a portion of a roughness profile.  This mean line is why RA is also known as Center Line Average.

Graphically, Ra is the area between the roughness profile and its centerline divided by the evaluation length. The evaluation length is normally five sample lengths where each sample length is equal to one cutoff length.  Hence the term "Arithmetic Average."

Ra (average roughness) is by far the most commonly used Surface Finish parameter. One reason it is so common is that it is fairly easy to take the absolute value of a signal and integrate the signal using analog electronics, so Ra (average roughness) could be measured by instruments that contain no digital circuits.

Ra, while common, is not sufficient to completely characterize the roughness of a surface. Depending on the application, surfaces with the same Ra can perform quite differently. Here are 4 surfaces with the same Ra (average roughness) and quite different shapes:

Rough surface finishes of various shapes.

All four surfaces have the same Ra but quite different shapes...

To distinguish these differences, more parameters are needed.

Rmax - Vertical distance from highest peak to lowest valley

Rmax is particularly sensitive to anomalies such as scratches and burrs that may not be obvious from measures such as Ra that rely on averages.

Rz - Preferred by many Europeans

Rz is often preferred to Ra in Europe and particularly Germany. Instead of measuring from centerline like Ra, Rz measures the average of the 5 largest peak to valley vertical distance within five sampling lengths. While Ra is relatively insensitive to a few extremes, Rz is quite sensitive since it is the extremes it is designed to measure.

Surface Roughness Grades: "N" Numbers

Here is a chart table showing how Roughness Grade Numbers convert to Ra numbers:

Surface Finish Grade Roughness Numbers Chart with RA, Rz, and Rk values and conversion tables between micrometers and inches.

Roughness Grade Numbers and Ra Measures...

Surface Roughness of Abrasive Grits and Sandpaper

One approach to Surface Finish is to use abrasives or sand the surface. This chart converts from abrasive grit to Surface Finish Ra values:

Abrasive grit to surface finish comparison chart.

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Surface Finish Math and Equations

To Calculate

Equation

Notes

Ra

Ra = CLA = (M1 + M2 + M3 + M4) / 4

Where:

M1, M2, ... Mn are measure values

Average Roughness in micro-meters or micro-inches. Ra is the arithmetic mean deviation of the profile

Rc

Mean height of profile irregularities

Rku

Kurtosis of the profile

Rmax

Maximum roughness depth

Rmr

Material Ration of the profile

Rp

Rp = MAX( M1, M2, M3, ... )

Max profile peak height

Rq

Rq = RMS

Root mean square deviation of the profile

Rsk

Skewness of the profile

Rt

Max height of the profile

Rv

Rv = MIN( M1, M2, M3, ... )

Max Profile Valley Depth

Ry

Max height of the profile

Rz

Rz = Ra x 7.2

This is a very ROUGH estimate and not an exact conversion!

Avg Max height of the profile

Rz ISO

Roughness Height

RPM

Average max profile peak height

RMS

RMS = SQRT( (M1^2 + M2^2 + M3^2 + M4^2) / 4 )

Root Mean Square

CLA

CLA = Ra

Center Line Average

Cut-Off Length

Length required for sample

N

N = Ra (um) x 40

New ISO (Grade) Scale Numbers

Surface Finish Symbols, Callouts, and Standards

Surface Finish, like so many other machining-related subjects, is defined by various international organization.  In the United States, surface finish is usually specified using the ASME Y14.36M standard. The rest of the world commonly uses International Organization for Standardization (ISO) 1302.

Callouts and surface finish symbols used for different surface finishes can be slightly different, so we'll look at a couple.

ISO Surface Finish Symbols and Callouts

Technical diagram showing surface finish removal symbols.

ISO surface finish symbols with explanation.

ANSI Surface Finish Symbols and Callouts

Surface texture symbols defined by ANSI Y14.36 standard for engineering drawings.

Surface Texture Symbols used in machining, metal fabrication, and metrology.

Surface Finish Symbols...

Surface Finish Symbols and Callout Example

Technical drawing with surface finish callout symbols for machining and manufacturing.

GD&T Surface Finish

GD&T specifies parameters like Flatness, but these are not really surface finish. To specify surface finish use either the ISO or ANSI surface finish symbols.

Surface Finish Cheat Sheet

A super handy Surface Finish "Cheat Sheet":

Surface finish cheat sheet with micrometer and micron-inch ratings and applications.

Surface Finish Symbols

Surface Roughness Conversion Chart Tables - Metric and Imperial

Cut-off Length

Ra (µm)

Ra (µinches)

RMS

Rt

N

in

mm

0.025

1

1.1

0.3

1

0.003

0.08

0.05

2

2.2

0.5

2

0.01

0.25

0.1

4

4.4

0.8

3

0.01

0.25

0.2

8

8.8

1.2

4

0.01

0.25

0.4

16

17.6

2

5

0.01

0.25

0.8

32

32.5

4

6

0.03

0.8

1.6

63

64.3

8

7

0.03

0.8

3.2

125

137.5

13

8

0.1

2.5

6.3

250

275

25

9

0.1

2.5

12.5

500

550

50

10

0.1

2.5

25

1000

1100

100

11

0.3

8

50

2000

2200

200

12

0.3

8

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