CNC Surface Finish Guide: Ra Values, Causes, and How to Achieve Each Level

Surface finish quality is one of the most visible indicators of machining quality as covered in our [Feeds and Speeds Guide]( G90 G54 G00 X0 Y0 G01 **Z-0.1** F10 G01 X1.0 F20 M30 /posts/feeds-and-speeds-beginners/). A good surface finish looks professional and indicates that the cutting parameters are correct. A poor surface finish suggests dull tools, incorrect feeds and speeds, or machine problems.

Surface finish quality is measured in Ra which stands for roughness average value. Ra is the arithmetic average of the surface profile height deviations from the mean line. Lower Ra values indicate smoother machined surfaces.

This guide covers Ra values, typical finishes for different machining operations, how to achieve specific finish quality levels, and troubleshooting poor surface finish.

Ra Values Reference

The following Ra values represent typical surface finishes achievable with standard CNC machining operations:

• Ra 0.1 to 0.2 micrometers is a mirror finish achieved by grinding or lapping
• Ra 0.4 to 0.8 micrometers is a fine finish achieved by precision turning or milling
• Ra 1.6 micrometers is the standard finish for general machining
• Ra 3.2 micrometers is a visible machined finish suitable for non-critical surfaces
• Ra 6.3 micrometers is a rough finish with visible tool marks
• Ra 12.5 micrometers is a very rough finish from heavy roughing

Most engineering drawings specify a surface finish requirement. The most common surface finish specification for machined parts is Ra 1.6 or 3.2 micrometers. These finishes are readily achievable with standard CNC machining practices.

How Surface Finish Is Measured

Surface finish is measured with a profilometer instrument which drags a diamond stylus across the part surface. The diamond stylus follows the surface profile and the instrument calculates the Ra value from the readings. The measurement is taken over a sampling length that is long enough to capture the surface characteristics.

Contact type profilometers are the most common measurement method for Ra values. The diamond stylus is drawn across the machined surface at a constant speed and the vertical displacement is recorded. Modern digital profilometers display the calculated Ra value and the surface profile graph.

Non-contact measurement methods use laser or white light interferometry to measure surface finish quality without touching the part. These non-contact methods are faster than contact methods and can measure soft surfaces that would be damaged by a stylus.

Surface finish should be measured in multiple different locations on the part. The machined finish can vary across different surfaces depending on the tool path, cutting direction, and tool condition. Measure at least three locations and report the average.

Factors Affecting Surface Finish

Feed rate is the most直接影响 factor on surface finish. Higher feed rates produce a rougher finish because the tool advances further between cutting edges. Lower feed rates produce a smoother finish but increase machining time.

The theoretical surface finish from a milling operation is determined by the feed per tooth and the tool geometry. The formula for theoretical Ra is feed per tooth squared divided by 32 times the tool radius. A lighter feed produces a smoother theoretical finish.

Tool condition significantly affects surface finish quality. A dull tool rubs instead of cutting which leaves a rough burnished surface finish. Replace cutting tools when the surface finish degrades. A sharp tool with a sharp cutting edge produces the best surface finish.

Tool runout causes one flute to cut more material than the other flutes which creates a regular pattern on the surface. The pattern appears as evenly spaced marks on the surface at the feed per revolution spacing. Reduce tool runout by using a precision collet and cleaning the spindle taper.

Machine rigidity affects surface finish quality through vibration and chatter. A rigid machine with a heavy cast frame produces better surface finish than a lightweight machine. Reduce tool stickout and ensure rigid workholding to minimize vibration during cutting.

Achieving Specific Surface Finishes

For Ra 1.6 micrometers which is standard general machining finish use a feed rate of 0.004 to 0.006 inches per revolution for turning. For milling use a feed per tooth of 0.002 to 0.003 inches with a 0.5 inch diameter tool. Use a sharp tool with adequate coolant.

For Ra 0.8 micrometers which is a precision finish reduce the feed rate. For turning use 0.002 to 0.004 inches per revolution. For milling use a feed per tooth of 0.001 to 0.002 inches. Use a wiper insert or a finishing insert with a small nose radius.

For Ra 0.4 micrometers which is a fine finish requires a finishing pass with very light feed. For turning use 0.001 to 0.002 inches per revolution with a wiper insert. For milling use a ball end mill with a small stepover and light feed. Multiple finishing passes may be needed.

For Ra 3.2 micrometers which is a standard rough finish use standard feeds and speeds. For turning use 0.008 to 0.012 inches per revolution with a standard insert. For milling use a feed per tooth of 0.004 to 0.006 inches with a standard end mill. This finish is acceptable for non-critical surfaces.

Surface Finish in Milling

In milling the surface finish is affected by the stepover between passes. A larger stepover distance leaves taller scallops between tool passes which increases the Ra value. The scallop height is determined by the tool radius and the stepover distance.

For a given stepover distance the scallop height equals the stepover squared divided by 8 times the tool radius. This relationship shows that increasing stepover has a quadratic effect on scallop height. To reduce the scallop height by half reduce the stepover by approximately 30 percent. Smaller stepovers improve finish but increase machining time.

Climb milling produces better surface finish than conventional milling for most materials. In climb milling the cutting edge engages the material at the maximum chip thickness and exits at zero. This shears the material cleanly and leaves a better surface.

Use a wiper insert tool for the best surface finish in milling operations. Wiper inserts have a secondary cutting edge that flattens the scallop peaks. A wiper insert can produce a surface finish equivalent to a feed rate reduction of 50 percent or more.

Surface Finish in Turning

In turning the surface finish is determined by the feed rate and the tool nose radius. A larger nose radius produces a smoother finish at the same feed rate. The theoretical Ra equals the feed rate squared divided by 32 times the nose radius.

Use the largest nose radius that the part geometry allows. A 0.032 inch nose radius produces a smoother finish than a 0.016 inch radius at the same feed rate. The larger radius also provides better heat dissipation and longer tool life.

Wiper inserts are available for turning as well as milling. A wiper turning insert has a modified cutting edge that flattens the feed marks. Wiper inserts can improve surface finish by one Ra grade at the same feed rate.

Surface Finish in Drilling

Drilling surface finish is affected by the drill geometry and the feed rate. A sharp drill with proper point geometry produces better hole finish than a dull drill. Use a split point drill for better hole finish in most materials.

Peck drilling improves surface finish in deep holes by breaking chips and reducing heat buildup. The peck action clears chips from the flutes and prevents them from scratching the finished surface.

Reaming produces the best surface finish for drilled holes. A reamer removes a small amount of material typically 0.005 to 0.015 inches and produces a smooth finish with Ra 0.8 to 1.6 micrometers.

Troubleshooting Poor Surface Finish

Chatter marks on the surface are caused by vibration between the tool and workpiece. Reduce tool stickout, increase rigidity, or adjust spindle speed by 5 to 10 percent to shift out of the resonant frequency. Variable helix end mills reduce chatter.

Feed marks that are too visible indicate the feed rate is too high for the required finish. Reduce the feed rate or use a tool with a larger nose radius. Wiper inserts can also reduce visible feed marks.

Built-up edge marks appear as rough patches on the surface where material has welded to the tool and then broken off. Increase cutting speed to reduce cutting pressure or use a coated tool to reduce friction. Ensure adequate coolant reaches the cutting edge.

Vibration marks appear as a regular pattern of lines on the surface perpendicular to the cutting direction. Check the spindle for excessive runout and check the tool holder for damage. Reduce tool stickout and ensure the workpiece is rigidly supported.

Burnished or shiny patches on the surface indicate rubbing rather than cutting. The tool is dull or the feed rate is too low. Increase the feed rate to ensure the tool is cutting and replace or resharpen the tool.

Surface Texture Parameters

Ra is the most common surface finish parameter but it is not the only one. Rz is the average maximum height of the profile which represents the average of the five highest peaks and five lowest valleys. Rz is useful for applications where seal surface quality is important.

Rmax is the maximum roughness depth which is the vertical distance between the highest peak and the lowest valley in the measurement length. Rmax is used for applications where the deepest scratch or lowest valley must be controlled.

Rp is the maximum peak height above the mean line. Rv is the maximum valley depth below the mean line. These parameters are used when the surface must have a specific bearing ratio or when the peak height is critical for contact applications.

The choice of surface finish parameter depends on the application function. Ra is suitable for general surfaces. Rz is better for sealing surfaces. Rmax is used where deep scratches cannot be tolerated. Specify the correct parameter for your application.

Surface Finish by Material

Aluminum machines to a bright surface finish with sharp tools. The typical finish for finish-machined aluminum is Ra 0.8 to 1.6 micrometers. Aluminum can be machined to a mirror finish with diamond tooling and optimal parameters. The softness of aluminum makes it susceptible to built-up edge which degrades finish.

Steel machines to a matte surface finish that varies with hardness. Softer steels produce better finish than hard steels with the same tooling. The typical finish for finish-machined steel is Ra 1.6 to 3.2 micrometers. Harder steels above 40 HRC may produce Ra 3.2 to 6.3 micrometer finishes.

Stainless steel produces a surface finish that is typically Ra 1.6 to 3.2 micrometers with carbide tooling. The work hardening tendency of stainless steel can degrade surface finish if the tool rubs. Maintain adequate chipload to prevent work hardening.

Brass produces the best surface finish of any common machining material. The typical finish for finish-machined brass is Ra 0.4 to 0.8 micrometers. Brass can achieve a mirror finish with sharp tooling and optimal feeds and speeds.

Plastics produce a surface finish that varies significantly between materials. Acrylic can be machined to a transparent finish with single-flute tools. Delrin produces a smooth matte finish. Polycarbonate is prone to stress cracking which degrades surface finish.

Surface Finish Specifications on Drawings

The surface finish callout on an engineering drawing uses a checkmark symbol with the Ra value written above the symbol. For example a 1.6 above the symbol indicates a maximum Ra of 1.6 micrometers. The symbol points to the surface being specified.

The surface finish callout can include additional requirements. The lay direction symbol indicates the direction of the surface texture. An equals sign indicates parallel lay. A perpendicular symbol indicates perpendicular lay. A circle indicates multidirectional lay.

The cutoff length is the sampling length used for the measurement. The cutoff length should be appropriate for the expected surface texture. A longer cutoff length captures more surface variations but may include waviness that is not part of the roughness.

When specifying surface finish on a drawing use standard Ra values. The preferred Ra values are 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.3, and 12.5 micrometers. Using standard values ensures that the finish can be measured with standard profilometer settings.

Cost Impact of Surface Finish

Tighter surface finish requirements increase machining cost significantly. Reducing the Ra from 3.2 to 1.6 micrometers typically increases machining time by 30 to 50 percent because lower feeds and additional finishing passes are required.

Reducing Ra from 1.6 to 0.8 micrometers can double the machining time. The finer finish requires significantly lower feed rates and often requires a separate finishing operation with a different tool. The cost of surface finish inspection also increases because finer finishes require more careful measurement with higher resolution profilometers and more measurement locations. For production parts specify the loosest surface finish that meets the functional requirements to minimize manufacturing cost.

Surface Finish Comparison Table

Ra Value	Appearance	Typical Process	Applications
0.1-0.2	Mirror	Grinding, lapping	Gauge surfaces, seals
0.4-0.8	Fine matte	Precision turning, milling	Bearing surfaces, shafts
1.6	Standard machined	General machining	Most engineering parts
3.2	Visible tool marks	Standard roughing	Non-critical surfaces
6.3	Rough	Heavy roughing	Hidden surfaces, clearance
12.5	Very rough	Sawing, flame cutting	Structural, non-machined

Improving Surface Finish

To improve surface finish by one Ra grade the most effective changes are reducing feed rate, using a tool with a larger nose radius, and taking a light finishing pass. Reduce feed rate by approximately 30 percent to improve Ra by one grade.

Using a wiper insert can improve finish by one to two Ra grades without reducing feed rate. Wiper inserts cost more than standard inserts but the productivity improvement often justifies the cost.

A finishing pass with a light radial engagement of 3 to 8 percent of tool diameter removes the scallop marks from the roughing pass and produces a better surface finish. The final finishing pass should remove 0.005 to 0.020 inches of material from the surface.

For production parts specify the loosest surface finish that meets the functional requirements. Do not specify a finer finish than necessary because the cost impact is significant. A Ra 3.2 finish is adequate for most non-critical surfaces.

Achieving the correct surface finish on machined parts is a balance of feeds, speeds, tooling, and machine condition. Understanding the factors that affect surface finish quality allows you to dial in the right parameters for any job. Start with conservative parameters and adjust based on the measured surface finish results from your parts. Surface finish is a critical quality parameter for any machined part. Understanding how to measure, specify, and achieve the required finish level is essential for CNC machinists. With the right tools feeds speeds and parameters you can consistently produce parts that meet surface finish specifications. For more machining guides see our Feeds and Speeds Guide and our CNC End Mill Selection Guide.