Surface Roughness Chart: The Complete Guide

Surface finish is an important variable in any sheet metal fabrication process.  Whether you want a smooth, rough or shinny surface, the knowledge on surface roughness chart is vital. In this guide, you will find all information about surface roughness chart.

What Is Surface Roughness Chart?

Surface roughness chart is a description that is used to explain the amount of deviation of a given surface from flatness. Additionally, surface roughness charts can enable designers to determine factors like the level of friction between surfaces and part quality.

Importance Of Surface Finish In Metal Fabrication

Precision
Precision

In metal fabrication, parts are produced for specific applications. For these parts to function properly, they must be precise which is achieved through surface finishes.

Durability
Durability

Metal parts usually undergo friction leading to wearing and reducing the lifespan of a given part. Surface finishes can reduce friction between parts increasing durability.

Improved Corrosion Resistance
Improved Corrosion Resistance

Metal parts that are exposed to corrosive substances like chemicals are prone to corrosion. However, certain surface finishes can improve the corrosion resistance of parts.

Enhanced Aesthetics
Enhanced Aesthetics

Various applications for instance in the medical industry, require parts that have improved aesthetic appearance. Surface finishes like polishing can achieve this kind of appearance.

Surface Roughness Chart Symbols

Ra (average surface roughness) – Ra is described as the average roughness taken between the mean line and roughness profile.

Rmax (vertical distance between peak to valley) – Rmax parameter is important when working with anomalies like scratches or burrs and it offers high sensitivity to them.

Rz (average maximum profile height) – Rz is used to give the average measurements of the first five highest differences taken between the valleys and peaks.

Surface Roughness Chart Symbols

RA Vs. RMS (Machining Surface Finish Chart)

RA Vs. RMS (Machining Surface
RA Vs. RMS (Machining Surface

Ra is described as the average value of a calculation done for absolute values of given surface troughs and peaks. Whereas RMS is a calculated root mean square of the surface valleys and peaks for the same surface. Both RMS and Ra are representations of how rough a surface is but their calculations are done differently.

Formula

Calculate Ra as follows:

Ra = (|Z1|+|Z2|+…+|ZN|)/N

Ra is obtained from getting the average of arithmetic values achieved from measuring the microscopic troughs and peaks on a surface.

Calculate RMS as follows

Rq = [(Z12+Z22+…+Z2N)/N] 1/2

RMS is achieved by getting the root square of values obtained by measuring the microscopic troughs and peaks on surfaces

Instrument Used For Measurement

Both RMS and Ra are measured and calculated using an equipment known as profilometer.

However, this equipment is designed to calculate Ra differently from RMS.

Effects Of Big Peaks And Valleys

RMS is highly sensitive to large peaks or valleys on the microscopic surface affecting its value.

Ra on the other, hand is less sensitive to such large peaks or valleys and its value is not affected.

For this reason, Ra is the most preferred parameter for measuring surface roughness.

Units Of Measurement

Initially, RMS was expressed in inches during the time when measurements were done in inches.

However, currently, both Ra and RMS are expressed in inch or metric.

Ra is simple and more accurate compared to RMS.

Converting RaTto RMS And Vice Vasa

A reliable way of converting these two values does not exist, however, an estimated formular can be used for the same.

(Ra/.0254) X 1.11 (Micrometer)= RMS (Microinch)

Below is a table showing the machining surface finish chart comparing RMS and Ra.

RMS (micro inch) Ra (micro inch) Ra (um) Finish method (machined)
2200 2000 50.0 Rough casting or coarse machining
1100 1000 25.0 Planning, boring, drilling or rough turning
550 500 12.5 Drilling, milling, rough turning or planning
352 320 8.00 Boring, normal turning, grinding, drilling or planning
275 250 6.30
220 200 5.00 Planning, boring, drilling, grinding, controlled turning.
176 160 4.00
137.5 125 3.20
110 100 2.50 Drilling, planning, boring and controlled turning
88 80 2.00
69.3 63 1.60
55 50 1.25 Boring, grinding, drilling and controlled turning
44 40 1.00
35.2 32 0.80
27.5 25 0.63 Rolling, boring, reaming and grinding
22 20 0.50
17.6 16 0.40
13.75 12.5 0.20 Super machining and grinding
11 10 0.20
8.8 8 0.20
4.4 4 0.10 Super machining

How To Measure Surface Roughness

How To Measure Surface Roughness
How To Measure Surface Roughness

Surface roughness is the result of calculating relative smoothness of a given profile’s surface. Normally, surface roughness charts are used to show the average of surface heights arithmetically.

To be able to determine the machine and method to use for the production of parts, use surface roughness charts. Roughness, lay and waviness are the three basic components of a given surface which affect its overall geometry. Surface roughness can be measured through various measuring systems including;

Direct Systems

In this method, a stylus is used for measuring the roughness of a surface.  An operator draws the stylus perpendicularly to the surface and uses the registered profile to be able to obtain surface roughness parameters.

Non-Contact Systems

Non-contact methods can use either sound or light for measuring the roughness of a given surface. Optical devices like confocal or white light are used instead of stylus and microscope or optical sensors are used.

The first step is to send ultrasonic pulses to the part surface through the instrument. Secondly, the sound is reflected back by the surface to the instrument. Through the waves or the reflected sound, the operator is able to determine the roughness of the surface.

Comparison Systems

Comparison methods use samples that have been determined through visual sensors or tactile for comparing the results.

In-Process Systems

In-process systems use techniques such as inductance which helps in evaluating surface rough through magnetic materials. An electromagnetic energy is used to determine the distance from the surface.  Parametric value obtained is compared with roughness parameters. Surface roughness can be measured through various methods which are divided into three categories:

Profiling Techniques

A high-resolution probe is used to measure the surface for instance using a phonograph needle instead of a typical CNC probe.

Area Techniques

Finite area on the surface is measured to give statistical average of valleys and peaks found on the surface. Optical scattering, ultrasonic scattering and capacitance probes are some of the techniques used in area methods.

Microscopy Techniques

Microscopic techniques rely on comparison systems to provide information on troughs and peaks on a given surface.

Surface Roughness For Different Fabrication Process

25 to 12.5 Ra
25 to 12.5 Ra

This is the expected Ra value range for Hot rolling, sand casting and flame cutting process.

12.5 to 3.2 Ra
12.5 to 3.2 Ra

Forging is the only manufacturing process that can result into this range for Ra values.

25 to 1.6 Ra
25 to 1.6 Ra

Ra measurements within this range can be expected to result from investment cutting, shaping, perm mold casting, chemical milling, sawing and EDM.

6.3 to 0.8 Ra
6.3 to 0.8 Ra

Reaming, milling, die casting, broaching, extruding, drawing, cold rolling, laser cutting and electron beam curing all fall here.

6.3 to 0.4 Ra
6.3 to 0.4 Ra

Manufacturing processes like turning and boring fall under the Ra value range of 6.3 to 0.4.

0.8 to 0.2 Ra
0.8 to 0.2 Ra

Processes that fall under this range include electrolyte grinding, barrel finishing and electro-polishing.

1.6 to 0.1 Ra
1.6 to 0.1 Ra

Honing, electropolishing and grinding processes fall under this range.

0.4 to 0.005 Ra
0.4 to 0.005 Ra

Parts that have undergone lapping have surface roughness of between 0.4 and 0.05.

0.2 to 0.025 Ra
0.2 to 0.025 Ra

Super finishing processes result into surfaces with Ra values around 0.2 to 0.025.

People Also Ask:

What Is RA For Surface Finish?

Ra is described as the calculated arithmetic value of the average of the measured microscopic peaks and troughs on part surface. Ra is used to determine the part surface roughness.

What Does 32 Surface Finish Imply?

32 surface finish is also referred to as number 4 standard satin finish. The kind of surface finish suitable for processing and architectural applications with a satin grain appearance.

What Is Ra 0.8 Surface Finish?

Ra 0.8 surface finish is considered to be high grade and it requires high control and close monitoring to be achieved. Parts that have been exposed to concentrated stress require Ra 0.8 kind of finish

Does Lower RA Imply Smoother Surface?

Lower Ra means smoother surface because the greater the deviation is the rougher the surface becomes.

Is Higher RA Smoother?

An Ra value which is higher shows that the machining process was rough and the surface has become very rough from the operation. Therefore, a higher Ra means a rougher surface.

What Is Surface Finish?

Surface finish, also referred to as surface topography or surface texture, is described as the level of flatness of a surface. Surface finish of a part is defined by three aspects including lay, waviness and roughness of the surface.

Who Uses Surface Roughness Chart

Surface roughness chart has various applications including engineering and other manufacturing industries. Manufacturers use surface roughness charts in order to come up with the best surface finishes.

Why Use Surface Roughness Chart?

Surface roughness chart is an important part of the manufacturing process because it helps in choosing the machining method. Surface roughness chart is also important when choosing the kind of finish to be used for your applications.

More Resources:

Surface Roughness

Surface Finish

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