Although the components have near-net-shape properties of investment casting, they still need to be machined. Most of you may be wondering which machining process you should choose: CNC milling or CNC turning? Different components require different machining processes; choosing the wrong process will lead to doubled fixture costs, severely worn cutting tools, and delayed delivery dates. This article will explain the differences between CNC milling and CNC turning, help you choose the right one, and help you reduce costs.
What is CNC Milling?
Before examining the two in terms of machining characteristics, applications, and cost, it is helpful to understand their basic principles, as this will highlight their differences in other areas.
CNC milling is a machining process in which the workpiece and cutting tool move along programmed coordinates, and a high-speed rotating milling cutter uses its multiple peripheral cutting edges to remove material from the workpiece, enabling castings to achieve complex geometric shapes.
Due to its multi-point and interrupted cutting characteristics, milling excels in machining flat mounting surfaces, complex 3D contours, and intricate features such as bolt holes on flanges and internal flow passages within valve bodies. Building on the near-net-shape process of investment casting, CNC milling ensures that your castings meet positional tolerance and geometric accuracy standards at lower costs. Common equipment includes vertical, horizontal, and multi-axis machining centers.
What is CNC Turning?
CNC turning is a machining process in which the workpiece rotates by the spindle, while the cutting tool moves in a straight or curved path to machine the casting into a symmetrical part with high concentricity.
Through single-point and continuous cutting, CNC turning can achieve extremely tight diameter tolerances for pump shafts, valve spools, and other axially symmetrical parts, thereby improving the sealing performance and service life of your castings. Common equipment includes horizontal slanted-bed CNC lathes for small- to medium-sized castings and vertical CNC lathes for heavy-duty disc-shaped castings.
Differences in Engineering Features
When performing secondary finishing on investment castings, the choice between CNC milling and CNC turning is largely determined by the casting’s geometric features and machining requirements. Understanding which method is best suited for castings will help you select the appropriate CNC machining method based on your casting’s structure, thereby avoiding severe tool wear and doubled costs resulting from an incorrect choice.
Geometric Features
- CNC Milling
CNC milling is primarily used to machine complex castings with multiple reference planes and asymmetrical shapes that cannot be machined by rotating around a single axis.
- CNC Turning
CNC turning is primarily used to machine rotationally symmetric or axis-symmetric castings whose external contours or internal cavities can be perfectly superimposed around a single axis.
Machining Structures
The essence of CNC machining is the relative motion between the cutting tool and the workpiece. Because CNC milling and CNC turning involve different rotating components, the types of structure they can produce also differ. For example, when machining bolt holes in a flange on a casting, CNC turning is clearly not suitable, as it can only produce concentric circles. Therefore, understanding the types of structures each CNC machining method can produce will help you select the appropriate method for your component design.
| Structures | CNC Milling | CNC Turning |
| External Structures | Flat Mounting Faces Non-standard profiles Cam surfaces Asymmetric ribs | Step Diameters Outer conical surface External Grooves External Threads |
| Internal Structures | Non-standard Groove Complex Internal Channels Off-center blind holes | Center Through-Hole Blind Hole Internal Taper Internal Groove Internal Thread |
| Drilling Locations | Bolt holes Counterbores Locating keyways | Only the center hole on the shaft |
| Function Locations | Equipment Installation Reference Plane Valve Actuator Connection Port | High-Precision Bearing Seats Valve Annular Sealing Surfaces |
Difference in Manufacturing Applications
Based on the geometric characteristics and structural differences of the components that can be machined using CNC milling and CNC turning, the specific products they can produce also vary greatly. Knowing which products each of these two machining methods can produce will help you quickly determine which method is best suited for your product, thereby saving time and avoiding the additional financial costs associated with making the wrong choice.
When to Choose CNC Milling for Your Castings
- Components with Asymmetrical Shapes
If your castings have asymmetrical shapes and multiple mounting surfaces, these characteristics prevent them from rotating on a single axis, making them more suitable for CNC milling than for CNC turning. Common examples of such castings include pump housings, valve handwheel mounting brackets, sensor bodies, and so on.
- Components with Bolt Hole Patterns
If your castings require multiple positioning holes, countersunk holes, or bolt holes drilled at different angles on flange faces or flat surfaces, or if complex flow channels need to be cut inside your castings, CNC milling is required. Because the workpiece moves along the X, Y, and Z axes with the worktable, the milling cutter can easily machine holes and flow channel structures into the castings. Common examples of such castings include fluid castings, open impeller vanes, square valve bodies, and so on.
- Components with Complex 3D Surfaces
If your castings require machining discontinuous, irregular surfaces, CNC milling machines with 3-axis or even 5-axis simultaneous cutting are your best choice. 5-axis CNC milling machines enable all five axes to move continuously with extremely high precision, making it easy to machine twisted and uneven surfaces. Common examples of such castings include impeller blades, mixer blades, and medical implants.
When to Choose CNC Turning for Your Castings
- Components with High Coaxiality Requirements
If your castings have rotational characteristics and need to be symmetrical, the high-speed rotational capabilities of CNC turning ensure extremely high coaxiality between the outer diameter and the inner bore. Common examples of such castings include pump shafts, bushings, valve plugs, and symmetrical bearing housings.
- Components with Sealing Surfaces Requirements
If your castings require a good sealing surface, CNC turning is the best option. Its continuous cutting characteristic makes it easier to achieve an excellent surface finish, which helps your castings prevent fluid leaks during operation. Common examples of such castings include pipe flanges, camlock couplings, and valve seat rings.
- Components with Large-Diameter Requirements
If your castings have large-diameter disc-shaped designs, they are prone to sagging and deform under gravity when suspended from a horizontal lathe. However, using a vertical CNC lathe for CNC turning allows disc-shaped castings to be placed flat on the lathe’s worktable, allowing gravity to position the casting. This ensures even force distribution and extremely stable machining, guaranteeing that your castings achieve exceptional roundness after CNC turning. Common examples of such castings include large-diameter industrial valve bodies, large-diameter valve seats, and pump wear rings.
When to Choose CNC Milling and CNC Turning Together
Although CNC milling and CNC turning have distinct machining processes, some castings feature more than one structure. Therefore, it is necessary to combine the two processes in certain situations.
Such castings typically have one characteristic: the main body is rotationally symmetric, but certain sections feature asymmetric structures, eccentric holes, or fixed planes. Flange castings are one of the most typical examples; their outer circumference and sealing surfaces require CNC turning to ensure coaxiality and sealing integrity, while the multiple bolt holes and complex structures must be machined with CNC milling.
In addition to flanges, various other castings require a turn-mill multi-tasking machining center to complete the manufacturing process in a single operation, including valve bodies and bonnets with actuator interfaces, sensor housings, and fluid interfaces. Ensures your castings have high geometric tolerances, high coaxiality, and surface finish requirements.
Difference in Costs
Because the machining processes and characteristics of CNC milling and CNC turning differ significantly, the costs also differ. Taking 316 stainless steel casting as an example, which is one of the most commonly used materials in investment casting. Its high unit price, tendency to stick to the tool, and susceptibility to work hardening can further amplify the cost difference between the two methods. This allows you to design the casting structure based on your chosen finishing method by budget, thereby significantly reducing your overall costs.
Machining Efficiency
The machining time per part directly determines CNC machine time and work efficiency.
In terms of their machining characteristics, CNC milling involves intermittent cutting, requiring the milling tool to enter and exit the workpiece frequently; it also involves multiple direction changes or helical cuts. In contrast, CNC turning involves continuous cutting, where the cutting tool constantly contacts the rapidly rotating workpiece. Consequently, for symmetrical rotating components such as shafts, cutting speeds are higher, and the machine time per part is often 30%–50% shorter than that of milling. Therefore, using turning for high-volume production of axisymmetric castings can significantly reduce your costs.
Tooling & Fixture Investments
The costs of cutting tools and fixtures are essential in CNC machining. This is because when machining castings, particularly stainless steel castings, which exhibit work-hardening properties, cutting tools must be replaced frequently. Furthermore, since castings come in various shapes, the CNC machines must be equipped with a variety of fixtures to secure them.
CNC milling requires a variety of cutting tools, including solid carbide end mills, indexable face mills, ball-end mills, drill bits, and taps. CNC turning uses standardized single-edge carbide inserts, which are less expensive to replace. The large number and the high cost of replacement make milling more expensive than turning in terms of tooling costs.
For fixture costs, because only CNC milling can machine stainless steel castings with irregular shapes, the manufacturer usually needs to custom-workholding fixtures for investment castings, which can incur hundreds to thousands of dollars. In contrast to CNC milling, CNC turning requires only simple adjustments to standard soft-jaw chucks before operation, resulting in extremely low or even zero costs for you.
Material Utilization
If stainless steel investment casting is not used and the part is machined directly from a solid metal block, up to 60%–80% of the expensive stainless steel will be wasted due to CNC milling, which can machine complex structures. In contrast, turning is typically used to cut outer circles, inner bores, and end faces, which generally produces fewer chips and results in higher material utilization.
However, by optimizing the stainless steel investment casting machining allowance when using the near-net-shape investment casting process, whether turning or milling, only a machining allowance of 0.5 mm to 2 mm needs to be removed, increasing the material utilization rate of stainless steel castings to 85%–95%. Investment casting can help you save a significant amount on material costs.
Final Thoughts
We believe you now understand the differences between CNC milling and CNC turning. Each machining method can offer significant benefits for your castings, and choosing the right one can help you save costs. If you’re looking for a stainless steel investment casting foundry with comprehensive CNC machining capabilities, please contact us. Pingheng Machinery, with its multiple CNC finishing workshops, strict quality control, and one-stop customized services, will provide you with castings that meet your expectations.
FAQ
Q1. What’s the main difference between CNC Milling and CNC Turning?
The fundamental difference between the two lies in their distinct relative motion modes: in CNC milling, the cutting tool rotates at high speed while cutting into the workpiece on the worktable, and is good at machining asymmetric structures; in CNC turning, the workpiece rotates at high speed while the cutting tool remains stationary, and is suited to machining axially symmetric structures and ensuring extremely high roundness and concentricity.
Q2. Can I skip the investment casting process and rely entirely on CNC machining from solid bar stock?
Yes, but only for quantities of 1 to 20 pieces. Direct finishing using CNC machining for small pieces can significantly reduce lead times and tooling costs associated with investment casting. However, for more than 50 stainless steel castings with complex geometries, using CNC milling directly would result in significant material waste and long machining cycles, thereby substantially increasing per-part production costs.
Q3. Which CNC process is more cost-effective for my investment castings?
This depends entirely on the geometric characteristics, the batch size, and the tolerance requirements of the castings.
If your castings are axisymmetric parts, such as pipe flanges, CNC turning is a more cost-effective option. It can reduce your machining time by 30%–50%, requires virtually no additional custom fixtures, and saves you costs while ensuring extremely high concentricity.
If your castings have complex structures, such as pump housings or valve bodies, CNC milling is more cost-effective. Although milling requires a longer machining time per part and may need custom fixtures, it is the most efficient method for achieving geometric and positional tolerances.
Q4. Can CNC milling and CNC turning be used together?
Absolutely, they can. Many complex stainless steel castings feature both rotational and asymmetrical characteristics. You can either machine them in separate steps or use a combined turning-and-milling process, that is, completing all turning and milling operations in a single setup. This approach can eliminate cumulative positioning errors caused by multiple setups, guarantee 100% compliance with geometric and positional tolerances, significantly shorten production lead times, and reduce your total procurement costs.
Q5. Since CNC turning is cheaper than milling, can turning replace milling?
No. Although CNC turning is cheaper than milling in terms of machine time per part, tooling costs, and other factors, the two processes are suited for different workpiece structures. For complex irregularly shaped castings, CNC milling is the only method that can ensure high geometric and positional tolerances.
