What often catches our eyes when we first look at any product is its outward appearance. There is little wonder then that this is also the case even in CNC machining. However, for parts produced through machining operations, the outward appearance is primarily defined by the CNC surface finish. In fact, this property is a direct measure of the quality of machined parts. Moreover, surface finish directly affects the performance of mechanical components (due to its impact on friction). In this regard, machinists strive to ensure a good surface finish. This article discusses the techniques, considerations, and tips to help you achieve superior surface finishes in CNC machining.
Table of Contents
Understanding Surface Finish in CNC Machining
The term surface finish refers to the overall characteristic and description of a surface, including any coating applied, dimension accuracy, texture, roughness, flaws, materials, flatness, waviness, and form. Simply put, surface finish defines the external topology (three-dimensional profile) of surfaces. CNC surface finish is so important in manufacturing and machining that it directly measures the quality of produced parts and products.
Naturally, engineering surfaces machined using different processes have varying topographies. For instance, a surface produced from a milling operation is spatially inhomogeneous. On the other hand, a surface on which a grinding operation has been completed may feature troughs and pits, while a honed surface has cross-hatched grooves.
Against this backdrop, an engineer should decide on the type of surface that will best fulfill the intended function of the part. To make this decision, they must consider factors like friction, adhesion, desired luster, and more. Once they make this decision, the next challenge is to pick the appropriate machining operation to achieve the desired surface finish.
Components of Machined Surface Finish
There are two components to machined surface finish:
1. Geometric or Ideal Finish
This is the surface finish that results from the kinematic motions and geometry of the CNC machining tool. This is the most dominant component of the finish in operations whereby the cutting forces and tool wear are low. One example of such an instance is when diamond tools are used to machine aluminum alloys.
2. Natural Finish
Also known as inherent finish, natural finish results from such factors and parameters as vibration and dynamics of the cutting process, tool wear, rapture at low cutting speeds, built-up edge formation (BUE), inhomogeneity, and work material effects like residual stresses. A natural finish is difficult to predict, unlike an ideal finish. It is the predominant component of surface finish in cases where carbide tooling is used to machine steels and other hard materials or when machining inhomogeneous materials like powder metals or cast iron.
Surface Finish in Various CNC Machining Processes
Surface Finish in Turning
Turning is a CNC machining process usually done using a lathe machine. In turning, a cutting tool is fed into a rotating workpiece. The tool works on the outer surface of the workpiece, creating a conical or cylindrical surface depending on the number of axes along which the tool is allowed to move.
The surface finish in turning depends on factors such as the tool nose radius, depth of cut, feed and feed rate, tool wear, inconsistencies in the work material, vibrations, machine motion errors, and discontinuous chip formation. To put this into perspective, a larger tool nose radius, a low feed rate, and a small depth of cut improve the surface finish. Additionally, surface finish increases with an increase in cutting speed (because it reduces the cutting forces), provided it does not cause vibration and excessive tool wear.
Surface Finish in Milling
The CNC surface finish in milling is generally less uniform compared to single-point boring and turning. Put simply, milling results in a surface that has spatial variations. This is typically because of a few factors, including:
- A varying effective feed rate, which changes based on the angle of the cutting edge from the feed direction
- Grinding or setup errors, which cause the cutting edges to be cut at slightly different depths of cuts and feed rates
- Vibration caused by the interrupted nature of the milling process
- Changes in cutter position due to spindle and cutter runout
Surface Finish in Drilling
Drilling generally combines cutting and rubbing actions, each of which independently impacts the CNC surface finish. For instance, the surface finish attributed to the cutting action depends mainly on the feed rate per revolution. Conversely, the finish associated with the rubbing action is dependent on the hardness and ductility of the workpiece, as well as the margin design and land width of the drill.
That said, drilling is regarded as a roughing process. As such, not much can be said about its influence on the CNC surface finish. In practice, machinists rarely monitor the surface roughness of drilled holes. Nonetheless, drilled holes that must have fine finishes are normally machined using surface finishing processes such as honing, burnishing, reaming, or boring – more about these processes in the section below.
Surface Finishing Processes
Most CNC machining operations are not designed to produce smooth surfaces. Therefore, machinists must finish the parts using surface finishing processes to solve this shortcoming. These processes typically remove a very small amount/layer of the material from the surface, measuring a few micrometers or nanometers. This section covers the various processes of achieving the desired CNC surface finish.
1. Grinding
Grinding produces parts with tight tolerances and fine surface finishes. It uses a rotating abrasive wheel to remove material from the surface of an object. This grinding wheel has small particles of aluminum oxide or silicon carbide, which act as abrasives and are bonded together by a suitable material. The particles can vary in size, with this variation creating a wide range of grinding wheels. Silicon carbide wheels are ideal for grinding materials with low tensile strength, while aluminum oxide wheels are better suited for materials with higher tensile strength.
It is worth pointing out that grinding removes very little material from the surface. This process achieves a good surface finish and gives high dimensional accuracy to parts already machined by other CNC machining methods. It is worth pointing out that grinding as a surface finishing method is typically used to machine materials that are too hard to machine using other processes.
Typically, the ground surface finish depends on several factors, namely:
- Wheel type and properties, including the wheel grit size and spacing, hardness, and effective diameter; for context, a wheel with high effective diameter and hardness as well as fine grains achieves smoother surface finishes
- Wheel dressing method, e.g., rotary diamond dressing and single point dressing, which determine the topography of the wheel surface
- Grinding conditions, e.g., cylindrical plunge grinding, straight surface grinding, creep feed grinding
- Vibration/chatter: vibration adversely affects the ground surface finish
2. Reaming
Reaming combines cutting and rubbing actions to produce a surface. Like drilling, its kinematics involve both linear and rotational movements, with the former occurring along the axis of rotation. The reaming process is done using a reamer.
There are three types of reamers:
- Single-blade reamer: these reaming tools have one cutting edge.
- Adjustable multi-flute reamer: These tools have multiple cutting edges. The ‘adjustable’ tag stems from the fact that they can expand, increasing the reaming diameter.
- Nonadjustable multi-flute reamer: These are reaming tools with multiple cutting edges. However, they cannot expand, meaning the reaming diameter remains fixed throughout the finishing operation.
3. Burnishing
In burnishing, a hard, smooth roller or ball is pressed against the part’s surface. It generates the desired surface finish through plastic deformation. In addition to improving the CNC surface finish, burnishing increases the surface hardness, enhances the fatigue life, and controls tolerance.
4. Finish Boring
To understand what finish boring is, let’s first discuss what boring is. In boring operations, a cutting tool is fed into a rotating workpiece to generate conical or cylindrical internal surfaces along the axis of rotation. Generally, boring is a roughing operation. Finish boring, therefore, is a type of boring operation that primarily achieves dimensional and surface finish tolerances. It is used to finish drilled holes or the internal surfaces of castings.
5. Lapping
Lapping is a process in which the surface of a workpiece is rubbed with a rotating tool called a lap. The lap is made of soft, porous material (e.g., copper or cast iron); it is embedded with an abrasive paste/slurry made by mixing oil or water with fine particles of abrasive materials such as aluminum oxide, silicon carbide, emery, or diamond.
This surface finishing method is preferred in cases where the machinist intends to produce a finish for two contact surfaces that are meant to fit snugly. This is because the process achieves high dimensional accuracy and fine surface finish. In addition, it results in geometrically true and very flat surfaces.
6. Honing
Honing uses abrasive stones or sticks to finish the external or internal surfaces of cylindrical workpieces. Usually, the abrasives are fixed around a metal cylinder called a mandrel, creating a honing tool.
7. Hydrohoning
The hydrohoning surface finishing process uses a stream of abrasives-carrying liquid to smoothen surfaces. This process is usually deployed to remove burrs and marks in metal molds.
8. Superfinishing
The superfinishing process uses large, bonded abrasive stones that move in a reciprocating motion and lightly press on the workpiece. On its part, the workpiece is reciprocated or rotated depending on its shape. Superfinishing produces an extremely high-quality surface finish. Superfinishing is also known as microhoning.
This process is commonly used in the automotive and bearing industries. It is preferred to other surface finishing technologies because it improves the surface finish of the parts.
9. Polishing
Polishing is a surface finishing process that removes small defects and scratches from the surface. It uses a rotating wheel onto which fine abrasive particles (that form a slurry) are applied. During polishing, these particles are responsible for removing the defects through cutting action. Usually, the rotating wheel, which is made of felt, leather, or cloth, is rotated against a workpiece that is held in place.
10. Buffing
Buffing is a refined type of polishing. It produces a mirror finish otherwise unobtainable using the polishing method. In buffing, very fine abrasives are applied to a wheel that is rotating at high speeds. The wheel is itself made of soft material such as a sewn layer of cloth or felt.
11. Shot or Grit Blasting
This process is used to remove burrs, rust, scales, etc. It relies on particles of abrasives, which, moving at high speeds, strike the surface of a material. Shot or grit blasting achieves a matte finish.
12. Shot Peening
Shot peening is a surface finishing process that strengthens and hardens the surface. It uses steel balls moving at high velocity. The balls strike the surface, making it fatigue resistant and work hardened.
13. Barrel Finishing
In barrel finishing, parts are placed inside a barrel containing abrasive materials and a suitable liquid. The barrel is rotated for a given period, promoting contact between the parts and abrasives. It is this impact that removes surface imperfections.
Influence of Material and Machining Parameters on CNC Surface Finish
There are several factors that impact the CNC surface finish. They can arise from cutting conditions, CNC machine operations, or material properties.
Machining Parameters
A number of machining parameters that can be set well in advance of any cutting or machining operation affect the surface roughness. Given that surface roughness is an indicator of the quality of the surface, tweaking these parameters beforehand can lead to a better surface finish. To help you understand what to do, here are the machining parameters you should consider:
- Cutting/machining forces: A larger magnitude of cutting forces during conventional machining can unintentionally deflect the cutting tool, leading to unstable cutting. This can contribute to poor surface finish.
- Cutting speed: Generally, an increase in cutting speed (measured in m/min) improves the surface finish by decreasing the surface roughness. This is because a high cutting speed reduces the cutting forces and chatter. However, increasing the speed during some machining processes generates a different result. For instance, an increase in cutting speed during helical milling increases the surface roughness. The high surface roughness is attributable to the fact that a high speed often leads to chatter and unstable cutting. This means that a high cutting speed reduces the quality of the surface, leading to a poor CNC surface finish.
- Feed rate: An increase in the feed rate results in a poor CNC surface finish because of increased surface roughness. Conversely, a lower feed rate produces a better surface finish.
- Depth of cut: There are two types of depths of cut: the radial and axial depth of cut. A lower value of the radial depth of cut leads to a better CNC surface finish. The inverse is also true. On the other hand, the higher the axial depth of cut, the better the surface quality.
- Nose radius: A high nose radius leads to lower surface roughness, translating to a better CNC surface finish.
- Tool geometry: Tool geometry is defined by the rake, lead, helix, and relief angles, tool overhang, and radial and axial runout. Briefly, a positive rake angle reduces the surface roughness. A larger lead angle (smaller entering angle) is associated with a better surface finish, while the relief angle indirectly impacts surface roughness through its direct influence on the rate of tool wear. Additionally, the tool overhang increases surface roughness. For a more comprehensive discussion on how these cutting tool factors impact surface roughness, check out this 2024 study.
- Tool wear: Tool wear negatively affects the quality of the surface. Specifically, a worn-out tool increases the surface roughness. In addition, it causes the surface to develop many partially developed cracks.
- Cutting fluids (coolants and lubricants): As discussed extensively later in this article, lubricants and coolants improve the surface finish.
Material Considerations
The type of material affects the surface finish. Typically, different materials have varying degrees of chip formation and hardness. Hard materials increase the rate of tool wear, which, as we have described above, causes a poor CNC surface finish. At the same time, a low harness value may cause BUE formation, which reduces the surface finish.
Illustration Showing Built-up Edge (BUE) Formation and Its Impact on Surface Roughness (source)
In certain cases, the workpiece may be heated to make it softer and easier to cut. It has been shown that elevated workpiece temperature reduces the cutting forces. This results in better surface roughness, translating to a good CNC surface finish. Given the various properties of materials and their respective impacts on machining conditions, surface roughness and finish vary from material to material.
Other Considerations for Superior CNC Surface Finish
- Vibration/Chatter: Vibration negatively affects the surface finish.
- Cutting Strategies: Computer-aided manufacturing (CAM) software recommends many complex cutting strategies that can be easily incorporated into modern machining operations. These strategies include plunge, zigzag, zig, follow-part, trochoidal, etc. Studies have shown that some of these strategies, particularly the trochoidal and follow-part strategies, result in superior CNC surface finish.
Role of Coolants and Lubricants in CNC Machining
CNC machining processes like grinding, milling, turning, and drilling result in significant thermal stresses that are experienced by both the workpieces and cutting tools. In addition, they produce swarf, which affects the performance of the machining operation as well as the quality of the surface. Thus, coolants and lubricants prevent the negative implications of friction and heat.
Generally, within the context of CNC machining, which involves cutting operations, coolants and lubricants are collectively known as cutting fluids. These fluids serve the following functions:
- Dissipate heat generated in the contact zones between the workpiece and cutting tools
- Remove chips from the surface of the workpiece
- Reduce friction
- Prevent overheating of the CNC machine
- Protect against corrosion
- Resist the formation of sticky, gummy residue on parts and machine tools
How Coolants and Lubricants Promote CNC Surface Finish
But perhaps the most crucial function, at least within the context of this article, is the fact that using lubricants and coolants improves the surface finish. The heat produced during CNC machining, primarily due to friction, can affect dimensional accuracy, damage the surface and subsurface, and induce residual (internal) stresses. Residual stresses disturb the initial mechanical equilibrium, causing changes in the form/deformations. Thus, by dissipating heat, cutting fluids facilitate better surface quality.
Coolants and lubricants also impact CNC surface finish by removing chips. Naturally, processes like drilling, which involve continuous cutting, produce strain-hardened continuous chips. These chips rub against the newly generated hole surface, leading to unplanned deep grooves and scratches. Thus, the cutting fluids improve the surface finish by flushing out these chips.
Using Coolants and Lubricants Efficiently for Good CNC Surface Finish
When it comes to CNC machining, machinists have to decide on the cooling action. Typically, they are presented with four cooling choices:
1. Dry machining
This option involves coating the cutting tool tip to reduce friction. It relies on the coating deposition technology. It is ideal for machining aluminum and carbon steels, wherein the chips dissipate the heat.
2. Flooding
Flooding is the most common option. In this system of cooling workpieces, a nozzle directs the coolant to the cutting zone, dissipating the heat produced and flushing away the swarf. Flooding is ideal for operations like grinding, drilling, and milling. However, it is a costly method, given the running and maintenance costs of the coolant system.
3. Cryogenic machining
Cryogenic machining uses cryogenic gasses like hydrogen, nitrogen, helium, oxygen, and neon as coolants. These gasses typically have a low temperature of below -150°C. Unfortunately, cryogenic machining is comparatively new and requires specially designed cutting tools, which presents certain difficulties. In addition, this method is ideal for use with certain materials, as it can result in thermal cracking.
4. Minimum quantity lubrication (MQL)
MQL aims to reduce the quantity of coolant required while also lessening tool wear and thermal stress at the point of contact between the tool and the workpiece. In MQL, machinists aim to use no more than 50 ml of the lubricant per machining hour (mL/h). However, the recommended range is between 50 and 500 mL/h. This range is roughly three or four orders of magnitude lower than the volume released during flooding.
Of the four cooling options, MQL enables the efficient use of cutting fluids as well as promotes a good CNC surface finish. According to a 2023 study, MQL achieves good surface roughness. It also facilitates lower cutting temperatures, feed rate, and cutting speed. Moreover, the MQL increases the efficiency of CNC machining processes, leading to better performance than the other cooling choices. Other additional benefits of MQL include a reduction in maintenance costs and the cost of running coolant pumps and other management resources. Put simply, MQL helps you reduce CNC machining costs.
Troubleshooting Common Surface Finish Issues
The CNC surface finish is an excellent indicator of underlying issues that affect the machining process. A poor surface quality signals an issue with the machine, tool, or machining process. For instance, reduced surface quality can indicate progressive tool wear, nonhomogeneous workpiece material, cutting tool vibration, and other underlying issues.
Given that the surface finish is a marker of the quality of the products your CNC machining shop produces, any deviations from the norm warrant an assessment to identify the cause. Here are some of the aspects you should check as part of the troubleshooting excessive:
- Check the machining tools: this helps you identify the level of tool wear or incorrect toolholding, which may, in turn, affect the tool geometry and cause chatter.
- Review the machining parameters: It is clear from the discussion above that machining parameters directly affect CNC surface finish. In this regard, it is crucial to regularly review them to identify deviations from correct settings.
- Monitor levels of cutting fluids: Cutting fluids improve the surface quality. And given that the flow rate of coolants and lubricants also affects the machining performance and efficiency, it is easy to recognize the importance of monitoring their levels.
- Ascertain workpiece and machine stability: Vibration can sometimes be caused by workpieces that are not tightly held in place or machines that are not securely fastened to the ground.
Conclusion
Machinists always aim for quality, which often means striving to achieve a superior surface finish. This is because CNC surface finish is one of the properties that define the desirability and appearance of parts. To achieve a good surface finish, you must use dedicated surface finishing processes like polishing, grinding, burnishing, reaming, barrel finishing, honing, superfinishing, and so on. The necessity of these processes stems from the fact that conventional machining operations, such as drilling, milling, and turning, do not produce features that have good surface quality. And while the surface finishing processes promise good surface finishes, it is vital to consider machining parameters, CNC machine conditions, material considerations, and operating conditions.
