CNC milling vs CNC turning

The advent of CNC technologies (Computer Numerical Control) has brought speed, precision, and repeatability to industrial production. Milling and turning are widely used in the automotive, fabrication, and electrical industries.
In CNC milling vs CNC turning, the difference lies in the moving part: the machine or the raw material. CNC milling produces components by removing shavings from the stationary workpiece through cutting tools. It uses three degrees of freedom (X, Y, and Z).

But CNC turning, or subtraction machining, keeps the tools stationery while the materials (in a chuck) rotate. A tool gets fed to the pieces iteratively to create the required component. Some facilities use one turret, but an additional sub-spindle bolsters volumes.

  • Surface Type: Milling is preferred for flat and irregular surfaces. Contrastingly, turning works on cylindrical, round, or conical surfaces.
  • Continuity: Turning uses a continuous cutting style. But the tools intermittently disengage from the material in milling.
  • Material Wastage: Turning leads to more wastage, while milling is superior at conservation.
  • Versatility: Turning is more amenable to components with axial symmetry, like disks, hollow tubes, shafts, and cylinders. It performs grooving, boring, knurling, etc. Milling delivers a broader range for chemical, electrical, mechanical, and thermal processes.
  • Material Removal Rate (MRR): It denotes the rate at which raw material gets removed in cubic inches per minute. Milling tends to be slower than turning.
  • Cost & Efficiency: Turning is cheaper and more efficient. But its cost-effectiveness reduces with volumes.
  • Volume & Prototyping: While milling is better for larger scales, turning is perfect for prototyping and specialized parts like screws or bolts.
  • Application: Milling is ideal for fittings, engine parts, gears, and hydraulic equipment. Turning suits ball joints, nozzles, flanges, and firearms.
  • Tooling: Milling uses multi-point cutting tools like reamers. But turning uses single-point tools like fly cutters.
  • Setup Time: Turning takes precedence as it does not need too many tools.
  • Tolerances: Turning can adhere to stricter tolerances, making it better for high dimensional accuracy.
  • Surface Finish (Roughness average or Ra): Turning produces a finer finish of 64 to 125 Ra due to the continuous workpiece rotation. But milling may leave tool patterns.

What are the common challenges and solutions in CNC milling?

  • Improper cutting
    Tools with mismatched strength for the material, poor operator training
    Use appropriate cutting allowances (side, bottom), tolerance, surface feet parameters, tools and speeds.
  • Incorrect center position
    Inaccurate manual operations, issues in the optical edge finder, rough edges and non-vertical sides in the mold
    Conduct de-burring using a whetstone and demagnetize the edge finder.
  • Poor surface precision
    Parameters too strict for the surface hardness and the tool sharpness, poor tool clamping, inadequate chip removal, burring
    Inspect and replace worn-out tools.
  • Collisions
    Inaccurate program with wrong Z-axis (depth) values, poorly judged safety height, mis-turning the handwheel
    Program and measure the coordinates carefully.
  • Wrong tool coordinates
    Faulty manual operations, inherent tool errors
    Use a tool program to prevent errors between the R knife, flat knife, and fly-cutter.

Advantages of CNC milling and CNC turning

The difference between CNC milling and CNC turning is significant, but both offer distinct advantages.

CNC milling

  • Can produce complicated shapes with multi-point cutting tools
  • Good accuracy, speed, consistency, and efficiency
  • Versatile
  • Is affordable if the volumes are lower

CNC turning

  • Can create round profiles – challenging for milling or routing.
  • Can perform polygonal turning using rotating tools.
  • Excellent for rapid prototyping services
  • Delivers smooth surface finish: It is possible to reach 32Ra by grinding. You must also monitor factors like heat treatment and passivation.

CNC milling & turning machine types

These machines are available in various types to address versatile industrial requirements.

Based on Machine Types Use
Orientation
  • Horizontal
Useful for heavier, deeper grooves
  • Vertical
Suited to greater details
Axes
  • 3-, 4-, and 5-axis
Can machine complicated parts by working at different angles
Priority
  • High speed
Can handle small diameters with excellent surface precision
  • High capacity
Better for sturdy materials like titanium and Inconel
Spindles
  • Double or multi-spindle (may have automatic pallet changers)
Offers high yield and productivity for complex parts
Guide Ways
  • Box
Provides stability and dampens vibrations
  • Linear
Better for speed and dynamic movements
Column
  • Double Column
Accurately produces large components
  • Travelling Column
Used for long Y-axis parts for defence, energy, etc.

Is there any difference in tolerances and precision?

Between CNC turning and milling, the former shows higher accuracy and has a precision of +/- 0.001 inches (0.025 mm). Turning is preferred for boring holes of accurate dimensions in environments needing tight tolerances.
Turning machines let you adjust the speeds and feed rates to adhere to specified tolerances. These metrics for CNC turning determine the rotation of the workpiece. For instance, a high turning speed and a low feed rate are ideal for stringent tolerance demands.

CNC turning & milling cost estimation. What are the 4 cost reduction tips?

CNC machining costs depend on the complexity of the component the time taken.

  • Utilize fillets and undercuts
    Adding a fillet to vertical edges lets you use a larger tool and speed up machining. You will need an undercut for a 90-degree internal corner.
  • Reduce machine orientations
    Multiple setups involve manual labour, time, and cost. Multi-axis machines speed things up at a cost. One option is to split the components by geometries and conduct assembly.
  • Accurately measure turning speed and feed rate accurately
    An expert must estimate these figures as errors will affect production and lead to the wastage of material and money.
  • Select budget-friendly materials
    Picking affordable materials by comparing prices across vendors is crucial for cost management.
    Manufacturing centers frequently integrate the two technologies for seamless production. For instance, round components with some asymmetrical features can use CNC milling followed by turning. However, if you run into size limitations, prefer other manufacturing processes, like die casting and injection molding.