Achieving the Desired Surface Finish in CNC Machining(manufacturing drawings Dana)

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In CNC machining, the surface finish of a part is a critical quality characteristic that can impact functionality, aesthetics, and manufacturing costs. The rms (root mean square) surface roughness is one of the key metrics used to specify and measure surface finish. By selecting appropriate machining techniques and parameters, manufacturers can achieve the desired rms surface roughness for parts produced on CNC machine tools.
What is RMS Surface Roughness?
The rms surface roughness, usually abbreviated as Rq or Ra, quantifies the textures and irregularities present on a machined surface. It represents the root mean square average of the roughness profile ordinates, measured from the mean line. In simpler terms, Rq gives a numerical assessment of the overall variation in surface heights over a given sample length.
Typical rms surface roughness values for machined parts range from 16 to 250 micro-inches (0.4 - 6 μm). Finer surface finishes below 16 μin (0.4 μm) may be specified for parts that require tighter tolerances, better wear resistance, or enhanced aesthetics. On the other hand, rougher finishes above 250 μin (6 μm) are often acceptable for unseen internal surfaces or non-critical features.
How Surface Finish Relates to Machining Process
In CNC machining, the attainable surface finish is largely determined by the machine tool, cutting tool, machining parameters, and capabilities of the CNC program. Each machining process has a certain minimum Rq that can be obtained based on these factors.
Processes like grinding and honing allow for extremely fine surface finishes below 10 μin (0.25 μm) Ra. CNC milling with small tools permits finishing to around 16-32 μin (0.4 - 0.8 μm). Turning operations generally fall between 32-125 μin (0.8 - 3 μm) Rq. Processes like CNC drilling and tapping result in coarser finishes over 125 μin (3 μm).
The tool’s nose radius also influences surface finish. Using micro-grain carbide tools with small nose radii can improve finish over standard end mills. However, reduced tool life is a tradeoff.
Increasing feed rates and cutting speeds usually degrades surface finish in CNC operations. Lower feeds and speeds allow each tool pass to overlap more, averaging out irregularities. The CNC program should utilize appropriate feed rates for finishing passes.
Strategies for Achieving Target Surface Finish
Here are some key strategies manufacturers use to obtain the required rms surface roughness on CNC machined parts:
- Optimize tool paths and feeds/speeds for finish cutting. The program should progress from roughing to semi-finishing to light finishing passes with decreasing stepovers. For milling, stepovers between 5-15% of tool diameter are typical for final passes.
- Employ smaller stepovers. Taking finer cuts and smaller axial/radial steps with each tool pass provides more overlap and averaging. This enhances finish but impacts cycle time.
- Utilize 5-axis machining. The extra rotary axes allow the tool to approach surfaces from different orientations. This improves access and finish for complex, contoured geometries.
- Incorporate min corner radius into the tool path. Rather than machining sharp corners, programming a small radius reduces tool marks and maintains edge integrity.
- Apply specialty tool coatings like diamond or CBN. The ultra-hard crystalline coatings minimize tool wear to maintain sharp cutting edges and fine finishes.
- Use new, sharp tools. Worn or chipped cutting tools produce poor surface finish due to rubbing, ploughing, and increased vibration. Proper tool condition monitoring helps identify issues.
- Provide rigid setups. Deflection between the tool, workpiece, and fixturing can deteriorate finish. Heavier cutting requires increased rigidity from machine components, vises, and clamps.
- Control vibration/chatter through tool path, speeds, and stability. Chatter marks negatively impact surface finish. This can be minimized through trial testing, spindle speed variation, and stepover/DOC reduction.
Validate and Measure Surface Finishes
After programming and machining, it’s critical to validate that the attained rms surface roughness meets specifications. There are several methods for measuring surface finish on CNC parts:
- Benchtop contact profilometers. A diamond-tipped stylus traces along the surface to produce a roughness profile for calculating Ra, Rq, etc.
- Non-contact laser/optical profilers. These scan surface heights without contact to generate 3D maps and roughness statistics.
- Atomic force microscopy (AFM). An extremely fine probe maps surface variations down to the nanometer scale. Used for ultra-precise measurement.
- Surface finish testers. These portable instruments provide quick in-process roughness spot checks using electronic traces or visual scales.
- Visual comparison. Plastic replica comparators with known Rq values can be visually compared to machined surfaces.
By leveraging the appropriate techniques and inspection methods, manufacturers can consistently hold tight tolerances for rms surface roughness specifications in CNC machining. Producing the target finish requires an integrated approach of selecting optimal processes, tools, programs, parameters, and quality control procedures. CNC Milling CNC Machining