Introduction( milling fly cutter Frank)

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Surface roughness is an important measure of the quality of a machined surface. It indicates how smoothly a surface has been machined, with lower values indicating a smoother surface finish. Controlling surface roughness is critical in many manufacturing applications, as a rougher surface can negatively impact the performance, durability, and aesthetics of a part.
In CNC machining, there are various techniques that can be utilized to obtain a desired surface roughness. The main factors that influence surface roughness in CNC machining are: tool selection, cutting parameters, machine rigidity, vibrations, and finishing processes. By optimizing these factors, CNC machinists can achieve the target surface roughness specified in the component drawings.
Understanding RMS Surface Roughness
The most common parameter used to quantify surface roughness is Ra or RMS (root mean square) surface roughness. It represents the arithmetic average of the absolute values of the profile deviations from the mean line, measured within the sampling length. In simpler terms, Ra gives a measure of the overall roughness of a surface. It is expressed in units of micrometers (μm) or microinches (μin).
Some key points about Ra surface roughness:
- A lower Ra value indicates a smoother surface. A higher Ra indicates a rougher surface.
- Ra only gives a general indication of roughness. It does not differentiate between periodic and random roughness features.
- Surface profiles with the same Ra value can appear significantly different visually. Supplementary parameters like Rz are sometimes used along with Ra to better characterize surface textures.
- Typical Ra values for machined surfaces range from 0.05 μm for polished/ground surfaces to over 12 μm for coarse-finished surfaces.
- The desired Ra value depends on the function and performance requirements of the component. Critical mating surfaces demand lower Ra in the 0.05-0.3 μm range. Non-critical surfaces can have higher Ra up to 6.3 μm.
Machining Techniques for Controlling Surface Roughness
Tool Selection
Choosing suitable cutting tools is the first step in controlling surface roughness in CNC machining. Some guidelines for tool selection:
- Finishing operations demand sharp tools with fine finishes on the cutting edges. Polished edges can produce burnished surfaces with Ra in the 0.2-0.4 μm range.
- For roughing, tools with special coatings and geometries are used. They effectively break up chips and withstand high cutting forces.
- Positive rake cutting tools like reamers produce lower Ra compared to neutral rake tools. Higher rake angles shear the material cleanly with less ploughing.
- Harder tool materials like carbides and ceramics are preferred for fine finishing as they resist wear and maintain sharp cutting edges.
- Multi-point cutters generate lower Ra values owing to overlapping cutting paths that break up peaks and valleys.
Cutting Parameters
The cutting speed, feed rate, and depth of cut determine the degree of roughness left by the cutting tool. Following general guidelines helps obtain lower Ra:
- Higher cutting speeds promote shearing and minimize roughness somewhat. But speeds should be controlled to avoid tool chatter.
- Lower feed rates and depth of cut values ensure lighter cuts. This allows each cutting edge to finely shave off material.
- Last pass feed rates between 0.05 and 0.2 mm/rev at 90-95% of full depth produce good surface finishes.
- Coolants wash away chips and thermally stabilize the cut zone. This results in favorable cutting conditions.
Machine Rigidity
Insufficient rigidity in the machine tool can detrimentally affect roughness. Some ways to enhance rigidity:
- Machines with box way construction provide the highest stiffness for precision finishing requirements.
- Avoid extended reaches by properly fixturing the workpiece close to the spindle.
- Heavier machines with superior damping characteristics help maintain workpiece-tool stability.
- Ensure all joints, especially in the tool mounting, workholding, and rotary axes, are firmly preloaded.
Vibration Control
Relative vibrations between the workpiece and cutting tool can leave waviness and chatter marks on the machined surfaces. Recommended ways to minimize vibrations:
- Balancing cutting forces by radial depth of cut adjustment or using vibration damped tool holders.
- Tuning spindle speeds to avoid excitation of resonant frequencies.
- Using dynamic vibration absorbers and damped boring bars that actively suppress chatter.
- Isolating the machine from ambient vibrations using shock pads under the bed.
Finishing Processes
Secondary finishing processes are frequently employed after primary CNC machining to further reduce surface roughness. Some approaches include:
- Abrasive finishing like honing, lapping, and superfinishing. They use bonded abrasives to produce mirror-like surface finishes.
- Fine brushing and roller burnishing plastically deform asperities to attain low Ra.
- Diamond turning uses a precision diamond tool on rigid machines for nanometer-level surface roughness.
- Chem-polishing and electroplating techniques give metal surfaces with Ra less than 0.1 μm.
- Coatings like PVD, CVD and plating provide protective layers without altering substrate roughness.
This article has provided an overview of the important factors involved in controlling surface roughness in CNC machined components. With proper tool selection, cutting parameters, machine setup, vibration control and finishing processes, machinists can consistently meet the specified Ra requirements as per the component drawings. Mastering surface roughness leads to high-quality machined parts that perform reliably in service. This ensures customer satisfaction and continued success for production shops in today's competitive manufacturing markets. CNC Milling