Milling and turning machines
Milling and turning differ in tool and workpiece motion, resulting in distinct capabilities for part geometry, production efficiency, and material removal rates when choosing between milling vs turning in turning parts manufacturing. These differences influence the selection of milling vs turning based on design requirements, such as cylindrical symmetry versus complex contours in modern CNC turning and milling services.
What Is CNC Turning?
CNC lathe machine
CNC turning process is a subtractive manufacturing process, where subtractive means the removal of material. In CNC turning, the workpiece is gripped in the jaws of a chuck that rotates at high speeds. A stationary tool with a sharp single point feeds into the rotating material, removing micron or mm thick chips depending on the tolerance and pass.
First, a bar or cylindrical stock is secured in the chuck or collet of the lathe spindle, which now receives good rotational motion. The cutting tool is fixed on the turret in such a way that it travels linearly along the axis of the workpiece, usually in the X or Z-direction, to reduce diameter, induce tapers, or thread it.
Important parameters in CNC turning are:
- Spindle speed
- Feed rate
- Depth of cut
Common operations are straight turning for uniform diameters, facing to produce flat ends perpendicular to the axis, boring for internal diameters, threading for screw-like features, and grooving for separations or recesses.
The materials include metals such as aluminum, steel, and titanium, as well as plastics and composites, with tolerances of up to ±0.01 mm.
The process excels in the production of axisymmetric parts, such as shafts, bushings, and fittings, as the tool remains in continuous contact with the workpiece, resulting in a smooth surface finish (Ra 0.8–3.2 µm).
For large-scale productions, the cycle time is short as setups require very little repositioning. Non-rotating features pose that limitation and require secondary milling operations on hybrid machines.
What Is CNC Milling?
CNC Milling cutter and coolant
CNC milling process is the movement of a multi-point cutting tool rotating on various axes for removing material from a stationary workpiece, creating complex three-dimensional geometries.
The workpiece is mounted on the table or bed, while the spindle-driven cutter makes cuts along the X, Y, and Z dimensions. Some advanced machines feature a rotary A-axis and a rotating B-axis, providing five-axis capability.
CNC milling services use CAM software to optimize toolpaths along with other parameters, including spindle RPM, feed rate, and stepover (the distance the tool will step sideways with each pass).
Milling operations include face milling for flat surfaces, peripheral milling for slots and contours, pocketing for cavities, and drilling for holes. Materials range from ferrous alloys to polymers. The tolerance is ±0.005–0.02 mm, with surface finishes ranging from Ra 0.4 to 6.3 µm, depending on the tool type and operational parameters used.
Milling is used on surfaces, pockets, and planes with irregular profiles such as brackets, moulds, and turbine-blade assemblies. The multi-axis movement has the advantages of creating undercuts and angled features without refixturing.
Production rates depend upon complexity. For instance, simpler 2D contours run quickly, while elaborate 3D surfaces require long cycle times.
Tool wear is high due to intermittent cuts, resulting in a drastic need for frequent tool changes. Half-sheet coolant systems play a significant role in heat dissipation.
10 Different Factors Between Milling VS Turning
CNC milling and turning services rely on some key factors to select the most efficient method for part machining when deciding between milling vs turning. Each factor includes a comparative analysis to guide selection for specific applications.
Primary Motion
In the CNC turning process, the workpiece rotates while the tool remains stationary, allowing for continuous cutting along its length. Milling, conversely, offers multidirectional tool access, where the tool rotates, and the workpiece remains stationary. However, such an arrangement also causes interrupted cuts that produce continuous chips.
Machine Configuration
Turning uses lathes equipped with chucks for rotational holding and turrets for tool indexing, typically on two axes, X and Z. Milling uses vertical or horizontal machining centers with tables for linear motion of the workpiece and spindles for rotation of the tool, supporting between three and five axes for greater flexibility.
Suitable Geometries
Turning creates rotational bodies capable of symmetry, such as cylinders, cones, and threads, suitable for certain shafts and valves. Milling, on the other hand, is used in producing non-symmetric features such as slots, pockets, and angled surfaces, which are suitable for complex housings and prototypes.
Cutting Tools
Turning uses single-point inserts, such as those made of carbide or ceramic, for longitudinal feeds. Milling employs multi-flute end mills or face mills. These mills distribute forces along the edges but require frequent indexing due to edgewear.
Material Removal Rate (MRR)
For long, slender parts, turning yields higher MRs (up to 200 cm³/min) because it entails deep cuts. Milling provides MRs of 50–150 cm³/min per pass. This is limited by planar constraints but compensated by the ability to operate on multiple
surfaces in parallel.
Surface Finish
Turning finishes cylindrical surfaces better (Ra 0.4–1.6 µm) because continuous shear is involved. Milling ends produce rough finishes (Ra 1.6 –6.3µm) with tool marks and require secondary honing for critical tolerances.
Setup Time and Fixturing
Turning setups are shorter (10–20 minutes) with collet clamping for bar stock. Milling requires very precise fixturing (whether vices or clamps) and alignment, and setups can last 30–60 minutes, especially if multi-sided access is involved.
Production Volume Suitability
Turning is optimal for high-volume cylindrical runs (thousands of parts) due to the use of bar feeders and automation. Milling handles low-to-medium volumes better for custom geometries, though high-volume flat parts benefit from gang tooling.
Lathe work is preferred for high-volume cylindrical runs (thousands of pieces) due to the use of bar feeders and automation systems. Milling handles lower to medium volumes for custom geometries, although high-volume planar parts might benefit from gang tooling.
Cost Implications
Turning is a less expensive alternative on a per-piece basis for axisymmetric parts, with one machine costing $50,000 and another costing $150,000. However, for complex parts, expensive milling equipment ($100,000-$500,000) and tooling are required, with economies of scale following complexity.
Precision and Tolerances
In both cases, the tolerance is ±0.01 mm, but turning guarantees tighter concentricity (±0.005 mm) for diameters.
Conversely, milling offers better positional accuracy (±0.002 mm) for features, especially when multi-axis control is used. Vibration would otherwise adversely affect flatness.
In engineering practice, hybrid mill-turn centers integrate both processes, reducing setups by 50% for parts like turbocharger housings. The choice depends on the design intent. The choice lies between turning for optimized manufacturing of rotationally symmetric parts or milling for more versatile manufacturing forms.
Additionally, parameter optimization is performed in an advanced manner using simulation software in CAM to further aid in maintaining dimensional integrity during loading.
Conclusion
In conclusion, milling and turning are both important processes, but the main differentiating principle is that milling has a moving tool and a rotating spindle that can move in three axes, while turning has a fixed cutting tool and a high-speed rotating workpiece. The part geometry can simplify the choice between them.