There are different CNC machining processes today, each with its specific advantages and use cases. For cylindrical and rotational parts requiring high surface finish and precision, no process outperforms precision CNC turning.
This article covers the entire machining process and applications of precision CNC turning. What’s more, this article examines the factors to consider before making actual purchasing decisions, especially how to identify the best supplier for your machining needs.
What Is Precision CNC Turning?
Precision CNC turning is a subtractive machining process that creates tight-tolerance rotational parts by removing material from a rotating workpiece using a linearly moving cutting tool. CNC lathes and turning centers (either horizontal or vertical orientations) are used for precision CNC turning.
When Do You Need Precision CNC Turning?
Here are the factors that indicate you need precision CNC turning.
Tight Tolerances
Tight Tolerances Standard CNC turning is normally adequate for non-critical parts with tolerances between 0.05 mm and 0.5 mm. However, parts that require much tighter tolerances (<0.005 mm) cannot be reliably machined with standard CNC processes, as tool wear and thermal expansion become significant sources of error.
For such critical parts, precision CNC turning is needed, as it can produce parts with tolerances as tight as 0.001 mm. However, the tradeoff is higher production costs. Therefore, a cost-to-benefit ratio is essential when deciding on the machining type to use. Precision CNC turning should only be used if the potential benefits outweigh the increased cost.
High Surface Finish Requirements
Standard CNC turning can guarantee surface roughness values between Ra 1.6 and 0.8 μm, which is actually good enough for many structural or non-sealing parts. However, some parts (especially those exposed to high stress and surface contact) require Ra values lower than 0.8 μm, sometimes as low as 0.005 μm (mirror finish). These parts need precision CNC turning.
It is worth noting, though, that products that require smoother surfaces and tighter tolerances have longer per-part cycle times than products that don’t, particularly due to more controlled cutting and finishing passes. Therefore, precision CNC turning often takes longer to complete production compared to standard CNC turning.
While the exact difference in cycle time depends on the product being machined and the tool condition, precision CNC turning may take as much as 40% longer to produce finished parts. By implication, engineers may sometimes be better off with standard turning if products do not require as stringent surface requirements and are needed faster.
Consistent Quality for Small to Medium Batch Production
Precision CNC turning has a repeatability range of around ±2 to 5 µm, compared to ±10 to 20 µm for standard turning. Essentially, standard CNC turning may be able to reach the required tight tolerance and dimensional accuracy in certain isolated cylindrical parts.
However, it often falls short when machining products with several parts and across different batches. In contrast, precision CNC turning has a narrow repeatability range and demonstrates consistently high quality levels across different parts and production batches.
CNC Milling vs CNC Turning: Which Process Should You Choose?
CNC milling and turning are both subtractive machining processes that involve rotation to attain desired part shapes. However, CNC milling rotates the cutting tool against a stationary workpiece, while CNC turning rotates the workpiece with the cutting tool moving linearly.
By rotating the workpiece on a fixed axis, precision CNC turning offers more workpiece movement freedom than CNC milling. CNC milling is suitable for complex and intricate designs of different shapes, while CNC turning is best suited for cylindrical parts.
Nowadays, manufacturers have CNC turning and milling centers that can perform both milling and turning operations.
What Are the Pros and Cons of Precision CNC Turning?
Precision CNC turning offers multiple advantages, but it also has limitations. Understanding the pros and cons of precision CNC turning allows engineers to make balanced decisions.
Pros of Precision CNC Turning
Here are some of the benefits of precision CNC turning:
- Accuracy: Precision CNC turning can produce parts with extremely tight tolerances (up to 0.001 mm), making it suitable for complex geometries and intricate designs.
- Batch-to-batch Consistency: Precision turning offers consistency in quality across multiple components in the same batch and across different batches.
- Efficiency: The precision CNC turning process is entirely automated, and there are no delays between steps.
- Versatility: Precision CNC turning is suitable for a wide variety of shapes, sizes, designs, and materials.
- Safety: The precision turning process is enclosed, and the operator is not exposed to the flying debris.
Cons of Precision CNC Turning
Here are some of the drawbacks of precision CNC turning:
- Cost: The cost to set up and maintain precision CNC turning machinery is high, which is especially disadvantageous for small-scale and one-off production.
- Material wastage: Precision CNC turning is more detailed than standard cutting, which translates to more material wastage.
- Design restriction: Precision CNC turning cannot effectively machine non-cylindrical geometries, like cuboids and curved holes.
What Materials Are Suitable for Precision CNC Turning?
Precision CNC turning is compatible with an array of materials, but these materials often respond differently to tight tolerances and extremely fine surface finishing. Here are some of the materials suitable for precision CNC turning, and important factors to consider when choosing the right one for your project.
Steels
Carbon steel, alloy steel, and stainless steel (303, 304, and 316) are suitable for precision CNC turning. Both carbon and low-alloy steels have high mechanical strength, making them a great choice for parts that bear a lot of load and are exposed to fatigue. These steels are also suitable for long shaft parts and threads.
The hardness of carbon and alloy steels may present some problems during machining, as the workpiece is abrasive and can hasten cutting tool degradation. These steels, particularly alloy steel, are also poor at dissipating heat. This may cause the workpiece to expand during machining, affecting the dimensional accuracy of the machining process.
Stainless steels are also strong, plus being highly corrosion-resistant due to their chromium content (at least 10%), making them suitable for many precision turning projects. However, stainless steel workpieces often harden during machining, requiring higher cutting forces, which may make the production process longer and more expensive.
Brass and Copper Alloys
Brass alloys are easy to machine, requiring low cutting forces, which allow for tight tolerances and excellent surface finishes, even on very small diameters. Copper alloys are even easier to cut than brass alloys, along with being ductile and having excellent thermal and electrical conductivity. For this reason, copper and brass CNC turned parts are common in the electrical industry, especially for fittings, valves, and connectors.
However, the easy machinability and high conductivity of these alloys come with disadvantages while machining. With copper alloys, there is a high risk of built-up edges (BUE), which can affect the quality of the surface finish and also accelerate tool wear. Deformation of the workpiece, uneven expansion, and workpiece hardening are other common problems that contribute to poor machining results.
Rapid tool wear is also a problem with brass alloys due to workpiece hardening, fine dust generation, and evaporation of its zinc content during machining. Brass alloys may also crack in contact with moisture.
Aluminum Alloys
The most commonly used aluminum alloys in precision CNC turning are 6061, 6082, and 7075. These alloys are preferred for their high strength-to-weight ratio and predictable machining. They are also compatible with the high cutting speeds and intricate control associated with many precision turning centers, allowing for extremely tight tolerances and fine surface finishing.
A downside to aluminum alloys, however, is their lower elastic modulus than other common precision turning metals. This makes them more susceptible to vibration and part deflection, which is particularly problematic when machining long or thin workpieces.
Titanium
Titanium (usually Grade 5) has an excellent strength-to-weight ratio, even better than steel, making it the preferred option in many aerospace and automotive parts. Furthermore, titanium is highly corrosion-resistant and can withstand extreme temperatures.
However, titanium has low thermal conductivity and poor heat dissipation, which means heat concentrates at the tool edge rather than dissipating with the chip. This can hasten tool wear, increasing the already high production costs. Titanium can also stick to the tool at high temperatures due to a chemical reaction called galling.
Plastics
Precision CNC turning is compatible with a lot of engineering plastics, including ABS, PEEK, PVC, PTFE, POM, and nylon. These plastics offer high accuracy and are easily machinable, making them suitable for customized, geometrically complex plastic parts.
Despite these advantages, plastics have low melting points, making them very susceptible to deformation, breaking, warping, and cracking in the presence of heat. If they must be used, proper thermal stabilization and secondary inspection are necessary, which add to production costs and cycle times.
Applications of CNC Precision Turned Parts
CNC precision turning is critical in industries and parts where rotational dimensional accuracy, tight tolerances, complex geometries, and high surface finishing are needed. Below are some of the applications of CNC precision turning.
Aerospace
Aerospace components typically require high length-to-diameter ratios, stringent coaxiality standards, in addition to tight tolerances and part diameter accuracies. These parts also often need to operate under extensive vibrations, and must be able to withstand them.
Therefore, titanium and steel alloys are often in high demand in these industries. However, because of the problem associated with these alloys, proper thermal control and tool inspection are critical during machining.
Some of the common parts machined under the aerospace industry include:
- Gear
- Couplers
- Nozzles
- Spacers
- Bushings
- Fasteners
Medical
CNC precision turning is useful in producing medical and dental instruments, ensuring that materials meet biocompatibility and accuracy requirements. The most typical problem here is ensuring surface finish requirements are met with materials like titanium and stainless steel, which may harden or become abrasive during machining.
Common precision turned CNC components include:
- Bone screws
- Prosthetics
- Pins
- Cannulas
- Implants
Automotive
In the automotive industry, precision CNC turning is used to manufacture components where tight tolerances, dimensional precision and accuracy, and wear resistance are critical. In these parts, even small errors in diameter or concentricity can affect NVH, sealing, and sensor alignment.
Some of the common automotive parts machined with precision CNC turning include:
- Transmission shafts
- Valve bodies
- Bushings
- Pistons
- Sensor housings
- Drivetrain components
Electronics
Brass, copper, and aluminum alloys are the most typically used materials for precision CNC turning in the electronics industry. Parts in this industry require small diameters and high ductility, in addition to tight tolerances. The biggest problems with precision turning in this industry are suboptimal surface finish, burrs, and inconsistent insertion forces.
Some of the common components for precision CNC turning in this industry are:
- Rotors
- Pins
- Terminals
- Connectors
- Housings
Hydraulics
Precision turning is useful in producing hydraulic components where sealing is a function of how precise the dimensions and surface finish are. For these parts, the main issues are maintaining roundness and straightness in long parts, ensuring that roughness is minimal to prevent the seal from wearing out, and maintaining the part diameter under high pressures.
Typical hydraulic parts machined with precision turning are:
- Pistons
- Cylinders
- Spools
- Sleeves
- Couplings
- Nozzles
Design Considerations of Precision CNC Turning
There are key design considerations associated with quality and cost-effective precision CNC turning, and they include the following:
- Cutting speed: This affects the rate at which the tool wears, the stability of the surface finish, and heat generation and control. Higher speeds increase thermal degradation of the tool and workpiece, while low speeds can increase BUE.
- Feed rate: The higher the feed rate, the more efficient the removal of materials is, but there’s a corresponding decrease in roundness of the part. In contrast, a low feed rate can guarantee better surface finishing, but cycle time is longer, and the material is exposed to heat longer, increasing the risk of degradation.
- Workpiece material: Softer materials are easier to machine, but their susceptibility to deflection is higher. On the other hand, harder materials are more difficult to machine, but tool wear is faster.
How to Choose the Right CNC Turned Parts Manufacturer?
The success and failure of precision CNC turning is highly dependent on the parts supplier. As such, engineers and manufacturers need to be as careful and particular about who they assign their CNC turning services.
Here are some of the key factors to consider.
Machine Capability
Engineers should confirm that their potential CNC turning services supplier can meet tolerance limits across multiple cycles. There should be verifiable data of tolerance and thermal stability, as well as repeatability of around 5 µm. Also, the supplier should be willing and able to provide CMM reports and surface roughness tests.
Material and Application Knowledge
Engineers need to confirm that the supplier can handle the specific material and final products needed. For instance, a project of brass CNC turned parts should be awarded to suppliers that have verifiable knowledge and results in previous brass turning processes and products.
Experience with Tight-Tolerance Turned Parts
Experience is valuable in CNC machining, and this is not just measured in years of service, but also in the number of projects the supplier has successfully managed. Suppliers should be experienced with managing different risks and problems associated with cylindrical parts, such as vibration, tapering, coaxiality errors, and uneven surfaces.
Quality Systems
The supplier should comply with industry regulatory standards. For example, there should be proof of ISO 9001 or ISO 2768 certifications, as well as GD&T certification. Specific industries may also have certain certifications, like AS9100D for aerospace parts.
Lead Time and Scalability
Generally, the tighter the tolerance, the longer the production process takes. A common red flag with suppliers is failure to adjust lead times with the complexity of the projects. While the goal is typically to supply products as fast as possible, suppliers that are unrealistic with their turnaround times are likely to cut corners on quality.
Also, the ability of the suppliers to maintain product quality over an increased workload is a critical factor when choosing the right supplier. Tolerance and thermal stability, geometric precision, and adherence to lead times are essential factors to consider when assessing the scalability potential of the supplier.
Conclusion
Precision CNC turning is the preferred manufacturing method for cylindrical parts that demand tight tolerances and superior surface finishes, particularly in the aerospace, medical, automotive, electronics, and hydraulics industries. To fully realize these performance advantages, selecting the right machining partner is essential.
FastPreci specializes in high-precision CNC turning, delivering tolerances as tight as ±0.005 mm, lead times as short as 3 days, and cost-effective solutions for both prototypes and production parts. With extensive experience across a wide range of materials and part geometries, FastPreci supports engineers throughout the entire machining process.
