Achieving the Right Surface Finish in CNC Machining(face mills Gill)

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Surface finish, often measured as rms surface roughness, is a critical specification for machined parts. The surface finish impacts appearance, performance, friction, wear, light reflection, ability to hold lubricant, and more. With CNC machining, there are various techniques to control and achieve the desired surface finish for the application.
What is RMS Surface Roughness?
Rms or root mean square surface roughness is a measurement of the texture of a surface. It represents the standard deviation of surface heights from the mean height. Rms values are generally given in microinches or micrometers (μm). The lower the rms value, the smoother the surface is.
Typical rms surface roughness values:
- Ground surfaces: 16-32 μin (0.4-0.8 μm)
- Machined surfaces: 32-128 μin (0.8-3.2 μm)
- Highly polished surfaces: <16 μin (0.4 μm)
Rms is more commonly used than Ra (average roughness) as it better captures peaks and valleys in the surface profile. Specifying an rms value ensures the extremes stay within range.
Factors Affecting Surface Finish in Machining
There are several factors that impact the resulting surface finish in CNC machining processes, including:
- Tooling: The shape, sharpness, material and coating of the cutting tool affects surface finish. Worn or damaged tools produce poorer finishes.
- Speeds/feeds: Appropriate speeds and feeds for the material, tooling, and operation are required to achieve the desired finish. Too fast can cause vibration and chatter. Too slow can rub and burn the surface.
- Rigidity: Stable and rigid fixturing prevents workpiece movement that degrades finish. The stiffness of the machine components and cutting tool is also important.
- Tool path: The programmer can manipulate the tool path to change finish, for example with contouring versus transversal moves. Cut direction relative to part features also impacts finish.
- Depth of cut: Lighter depths of cut and final finishing passes improve surface finish. Multiple step-downs are often used.
- Coolant: The proper coolant helps flush away chips and prevents built-up edge on tooling. This allows smoother cutting action.
- Material: Certain materials are more prone to rubbing, tearing, welding, and chip build-up during machining which degrades finishes. Material microstructure impacts results.
Machining Techniques for Surface Finish
Different CNC machining processes lend themselves to certain surface finishes. Here are some of the most common processes and the typical finishes they achieve:
- Milling: Standard end milling can produce 32-63 μin rms. However, using faster spindle speeds, smaller step-overs, specific toolpath techniques, and ball end mills can improve finish to under 16 μin rms.
- Turning: Turning generally produces better finishes than milling. With proper technique, turning can achieve 8-16 μin rms, sometimes lower for finer precision work. Slow speeds with light cuts are preferred.
- Drilling: Standard twist drills are not designed for fine surface finishes. However, when hole finish is critical, special end mills or boring bars can drill holes to 16-32 μin rms.
- Boring: Boring can produce hole finishes to 16-32 μin rms. Boring allows better finish than drilling due to increased tool control and rigidity when generating the hole.
- Reaming: Finishing reamers are exceptional for hole surface finish. They can achieve 8-16 μin rms finishes by removing a very light amount of material on the final hole size.
- Grinding/Honing: Grinding operations are used for very fine surface finishes below 8 μin rms. While these operations require special equipment, they are commonly used to finish automotive engine components among other applications requiring extreme precision. Honing improves hole finishes from reaming or boring to under 8 μin rms.
- Polishing/Lapping: Polishing and lapping use fine abrasives to produce mirror-like finishes under 1 μin rms. While not a machining process, this may be used as a post-process if absolute surface perfection is required.
Programming Techniques for Surface Finish
Here are some best practices CNC programmers use to optimize programs for surface finish:
- Specify appropriate feeds/speeds for the material, tooling, and operations performed. Follow tooling manufacturer recommendations.
- Include multiple step-downs in milling operations, taking lighter depths of cut on the final passes.
- Utilize contouring rather than transversal movements. Contouring moves or arcs leave a nicer finish. Straight moves across the surface make step lines.
- Directionally orient operations to cut with or perpendicular to part features. Consider which direction marks or roughness will be most visible.
- Minimize stop/start movements which can leave tool marks. Optimize toolpath movement with steady state cutting.
- Leave adequate material for finishing allowance. Rough operations target leaving at least 0.010-0.020” extra material for finish passes.
- Use appropriate tooling designed for finishing in the final operations. Ball end mills, specialty end mills, and inserts all enhance finish.
- Program slower spindle speeds for finishing passes, maximizing rigidity and minimizing vibration.
- Include backoff moves for drilling/boring to cleanly exit the hole. Dwelling or withdrawing too quickly mars hole edges.
Achieving the desired surface finish for a machined part requires planning in both programming and process execution. With the right technique, rms values under 8 μin are possible for most common machining operations. Precision grinding/honing can reach extremely fine finishes under 1 μin rms for applications demanding perfect surfaces. CNC Milling CNC Machining