Machining Surface Finish: Chart & Guide

Machining surface finish influences the quality, durability and performance of manufactured parts and components.

For instance, when manufactured products possess smooth surfaces, they resist wear and tear better than finished parts with rougher surfaces because friction levels remain lower.

Now depending on the application being manufactured, certain roughness specifications must be met. By better understanding the different machined surface finishes, the varying roughness levels and how surface finishes impact function, you can ensure the quality and performance of manufactured applications.

If you’re interested in learning more about machining surface finishes, then you’re in the right place.

What is Machining Surface Finish?

Surface finish involves the process of altering a metal’s surface through reshaping, removing or adding, and is used to measure the overall texture of a surface characterized by:

• Lay – The direction of the predominant surface pattern (often determined by the manufacturing process).
• Waviness – Pertains to fine detail imperfections or coarser irregularities, such as surfaces that are warped or deflected from specifications.
• Surface Roughness – A measure of finely spaced surface irregularities. Generally, surface roughness is what machinists refer to as “surface finish” while the usage of “surface texture” is common when relating to all three characteristics.

The Importance of Machining Surface Finishes

Surface finishing plays an essential role in determining a product’s performance and varies significantly depending on the type of manufacturing process used.

Without the benefits of surface finishes, manufactured goods are more susceptible to corrosion and lose quality.

Additional ways machined surface finishes aids in manufacturing include:

• Eliminating surface defects
• Aiding in the cohesion of coatings and paints
• Improving the visual appearance of manufactured goods
• Being essential for corrosion and chemical-resistant effects
• Adding surface electrical conductions and improving conductivity
• Increasing strength against wear while minimizing friction effects

How to Measure Machining Surface Finish Roughness

Surface roughness calculates the relative smoothness of a surface’s profile in the numerical parameter Ra. With a number of post-processing methods available, improving the surface roughness of manufactured goods also improves its visual properties and wear resistance.

The finishes available in machining include:

Finishing Methods	Machining Surface Finish
As Machined	Minorly visible tool marks with a standard surface roughness (Ra) of 3.2μm. Surface finishes may be increased to 1.6, 0.8, to 0.4μm.
Bead Blasting	Primarily used for visual purposes, bead-blasted parts reflect a matte finish with a light texture.
Powder Coating	Adds a wear and corrosion finish to the surface of parts to provide higher impact resistance which can be applied to any metal and is available in many colors.
Anodizing Type II(Clear or Color)	Adds a corrosion-resistant, ceramic layer to the surface of an aluminum or titanium part which may be dyed to different colors.
Anodizing Type III(Hardcoat)	Adds a stronger corrosion-resistant, ceramic layer than Anodizing Type II surface finishes and may also be dyed in different colors.

Pro Tip: To improve the surface finish of injection molding, combine fast injection speeds with higher melt or mold temperatures to improve overall gloss and smoothness.

Machining Surface Finish by Industry

Machined surface finishes are widely used across aerospace, transportation, pharmaceuticals, home hardware, electronics, defense and other industry sectors. Without proper surface finishes, machined parts are susceptible to greater wear and tear and may not perform as effectively.

Machining Surface Finish Chart, Symbols & Abbreviations

Post-processing a machined part is easier when you understand the surface finish requirement and specifications needed. However, given how there are many units of measurement, different symbols and many abbreviations to know it can be challenging.

Among the most crucial machine surface finish symbols and abbreviations to know include:

Ra – (Average Surface Roughness)

Ra, also known as Center Line Average or Arithmetic Average, is the average roughness between a roughness profile and mean line. The Ra surface is one of the most widely used surface finish parameters and is often used for absolute values.

Rz – (Average Maximum Height of the Profile)

Contrary to Ra, Rz measures the average values of the five largest differences between peaks and valleys by using five sampling lengths and is less sensitive to extremes than Ra.

Rmax  – (Vertical Distance from Peaks to Valleys)

Rmax is a roughness parameter that is often used for machining anomalies like scratches and burrs although it may not be as obvious with the Ra surface finish chart.

Additional Abbreviations to Know

• N = New ISO (Grade) Scale Numbers
• Ra = Roughness Average
• Rt = Roughness Total
• CLA = Center Line Average
• RMS = Root Mean Square

Machine Surface Finish Chart

Surface Finish Chart for Conversions
Ra(micrometers)	Ra(microinches)	RMS(microinches)	CLA(N)	RT(microns)
0.025	1	1.1	1	0.3
0.05	2	2.2	2	0.5
0.1	4	4.4	4	0.8
0.2	8	8.8	8	1.2
0.4	16	17.6	16	2.0
0.8	32	32.5	32	4.0
1.6	63	64.3	63	8.0
3.2	125	137.5	125	13
6.3	250	275	250	25
12.5	500	550	500	50
25.0	1000	1100	1000	100
50.0	2000	2200	2000	200

Steel Castings & Iron Metal Surface Finish Chart

ChinaFinish Degree	ChinaRa (um)	ChinaRz (um)	USARa (um)	USA Ra(micro inch)	USA RMS(micro inch)	Machining Finish Method
1	50	200	50.0	2000	2200	Good rough casting surfaces or the most coarse machining.
2	25	100	25.0	1000	1100	Rough turning, boring, drilling and planning for very obvious machining marks.
3	12.5	50	12.5	500	550	Rough drilling, turning, planning and milling for obvious machine marks.
4	6.3	25	8.006.30	320250	352275	Normal tuning, grinding, planning, drilling and boring for visible machining marks.
5	3.2	12.5	5.004.003.20	200160125	220176137.5	Normal boring, drilling, grinding, tuning and planning for semi-visible marks.
6	1.6	6.3	2.502.001.60	1008063	1108869.3	Number controlled tuning, drilling, planning, grinding and boring for visible but blurred machining marks.
7	0.8	6.3	1.251.000.80	504032	554435.2	CNC machining including tuning, drilling, planning, grinding and boring for visible and direction-blurred tool marks.
8	0.4	3.2	0.630.500.40	252016	27.52217.6	Reaming, rolling, boring and grinding where machining marks direction blur.
9	0.2	1.6	0.20	12.5108	13.75118.8	Super machining and grinding where machining marks directions are invisible.
10	0.1	0.8	0.10	4	4.4	Super machining for surface dark gloss.

Precision Machining Surface Finishes Every Time 

Machining surface finishes are greatly influenced by the manufacturing process used.

From the coolant used to the width and depth of a cut, every process of machining needs to be calculated with pinpoint accuracy to ensure the optimal quality of manufactured goods. With the guide of a machining surface finish chart or the use of precision machinists, high-quality parts can be machined to desired specifications.

For high-quality precision machined parts on time, every time, consider Mosey’s. For more information regarding our manufacturing processes and surface finishes, connect with one of our precision machinists today.