Surface Roughness in CNC Machining

Surface Roughness is an important indicator for measuring the microscopic unevenness of a part's surface, directly affecting the part's wear resistance, fit properties, fatigue strength, sealing performance and appearance. In numerical control machining, precisely controlling surface roughness is the key to measuring machining quality.

I. Core Concepts: Ra and Rz

You will most often encounter two parameters:

· Ra (Arithmetic Mean Deviation) : The most commonly used indicator. It is the arithmetic mean of the absolute values of the contour offset within the sampling length. The smaller the Ra value, the smoother the surface.

· Intuitive understanding: Ra reflects the "average" height difference of the surface. For example:

· Ra 0.8 μm: Visible processing marks, but smooth to the touch (achievable through precision milling and precision turning).

· Ra 1.6 μm: Slightly visible processing marks (general standard requirements for fine processing).

· Ra 3.2 μm: Visible processing marks (common value for semi-finishing).

· Ra 6.3 μm: Clearly visible tool marks (in the state after rough machining).

· Rz (Maximum Height) : The distance between the peak line of the contour and the bottom line of the valley within the sampling length. It pays more attention to the extreme peaks and valleys on the surface, which are more important for performance such as sealing.

Simply put: Ra looks at the "average level", while Rz looks at the "extreme situation". In general mechanical processing, Ra is more commonly used.

Ii. Four Key Factors Affecting Surface Roughness

Controlling surface roughness essentially involves controlling the interaction between the cutting tool and the workpiece. The main influencing factors can be summarized into four points:

1. Cutting parameters (CNC program core

· Cutting speed (Vc) : Increasing the rotational speed/linear speed is usually the most effective way to improve the finish. Higher speeds can reduce the formation of built-up edge (BUE), making the cutting process smoother and thus achieving a smoother surface.

· Feed rate (F) : This is one of the parameters that have the greatest impact on roughness. Reducing the feed rate can directly decrease the height of the residual tool marks on the machined surface (theoretical roughness is proportional to the square of the feed rate).

· Cutting depth (Ap) : Usually, its influence on roughness is indirect. Excessive cutting depth can cause vibration and tool deformation, thereby deteriorating the surface quality.

2. Tool selection

· Tip arc radius (Re) : Increasing the tip arc radius is another powerful tool for improving surface roughness. A larger arc radius can make the tool marks smoother and significantly reduce the theoretical roughness value.

· Tool material and coating: Coatings with good wear resistance (such as TiAlN) and harder substrates (such as cemented carbide) can keep the cutting edge sharp and reduce surface deterioration caused by wear.

· Tool wear: Worn tools are the biggest killer of surface quality. A blunted blade will squeeze rather than cut the material, causing burrs, tears and a sharp increase in surface roughness. Cutting tools must be inspected and replaced regularly.

3. Machine tools and Clamping

· Machine tool rigidity/condition: Old machine tools with clearance in the main shaft bearings are prone to vibration (flutter), which will leave obvious vibration marks on the surface and seriously damage the roughness.

· Workpiece clamping rigidity: The workpiece must be firmly clamped. Any slight shake will be replicated onto the machined surface.

· Tool clamping accuracy: Using high-quality tool holders (such as heat shrink tool holders or hydraulic tool holders) can reduce tool runout, ensure that all cutting edges participate in the cutting evenly, and achieve a consistent surface finish.

4. Material properties

· Material hardness and toughness: Viscous materials (such as aluminum and stainless steel) are prone to built-up edge, and the chips will adhere to the cutting edge and tear the machined surface. Brittle materials (such as cast iron) are more likely to achieve a better surface finish.

· Cooling and lubrication: The correct use of coolant is of vital importance.

For materials such as aluminium alloy, sufficient coolant can wash away chips, prevent scratches on the machined surface, and lower the temperature.

For stainless steel, titanium alloy and other materials, the use of oil-based coolant or high-pressure coolant can effectively lubricate, reduce cutting force and suppress built-up edge.

Iii. Practical Skills and Solutions

Problem phenomena, possible causes, solutions

There are regular vibration marks on the surface of the machine tool/workpiece/tool vibration (flutter). 1. Check and enhance the clamping rigidity. 2. Adjust the cutting parameters (fine-tune the rotational speed or feed rate) to change the vibration frequency. 3. Shorten the overhang length of the cutting tool and use a thicker one instead.

There are torn and burr materials adhering to the tool on the surface, and the built-up edge is severe. 1. Increase the cutting speed. 2. Use sharp tools with polishing grooves. 3. Increase the coolant concentration and flow rate, or switch to a coolant with better lubricity.

Uneven surface roughness and tool wear: 1. Inspect and replace the worn tools. 2. Optimize the processing path to avoid the tool staying at the corner for a long time.

The target Ra value cannot be reached. The parameter combination is unreasonable. 1. Try to reduce the feed rate (F) first. Secondly, try to increase the spindle speed (S). 3. Replace the tool with one that has a larger tip radius (Re).

Problem of programming tool path for roughness difference in specific areas: 1. Use a smaller step distance (step) during finish machining (usually less than 10% of the tool diameter). 2. When finishing the contour, use arcs for entry and exit to avoid direct penetration into the workpiece.

Iv. Classic Parameter Adjustment Strategies (Taking Fine Machining as an Example)

To obtain a smoother surface (such as Ra 0.8), please make adjustments in the following priority order:

First choice: Reduce the feed rate (F). This is the most direct and effective method.

2. Second choice: Increase the cutting speed (Vc/S). However, the rated speed of the cutting tool and the capacity of the machine tool need to be taken into consideration.

3. Again: Increase the radius of the arc at the tip of the knife (Re). If the tool and program permit.

4. Final resort: Use a polishing blade tool. Some modern cutting tools are equipped with specially polished cutting edges, which can achieve extremely low surface roughness even at higher feed rates.

Finally, don't forget:

· Measurement: Objectively evaluate the results using a surface roughness meter rather than relying solely on the naked eye and touch.

· Communication with suppliers: Tool suppliers can provide parameter suggestions for specific materials and processing types.

· Experiment and Record: Conduct simple trial cuts, record successful parameter combinations, and establish your own processing parameter library.

By systematically understanding the above factors and flexibly applying these techniques, you will be able to effectively predict and control the surface roughness of CNC machining, and produce higher-quality products.