Overview: How CNC Milling works?

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CNC milling is a machining process that uses pre-programmed computer software and a specialized machine, called a CNC milling machine, to automatically remove material from a block of metal, plastic, or other materials to create a part. The CNC refers to Computer Numerical Control, which is how these machines are programmed and operated.

CNC milling is an extremely precise process thanks to the procedural nature of the programming that controls all the machine’s movements. By using coded instructions, the computer controls the cutting tools that removes metal from stock material. This allows for the automatic and highly accurate creation of complex parts that would be difficult to produce manually.

Below we will explore how CNC milling machines work and the key components that make CNC milling such an efficient and precise manufacturing process.

How CNC Milling Machines Work

CNC milling machines have the following main components:

  • Machine frame and bed – Provides a rigid, stable platform for the machine components. Made of cast iron or steel for damping vibration.
  • Spindle – The rotating shaft that holds the cutting tool. Spindle speed is variable and can reach several thousand RPM. Powered by an electric motor.
  • Automatic tool changer – Swaps cutting tools in and out of the spindle automatically under program control.Allows for multiple tool types/sizes to be used.
  • Cutting tools – Typically end mills that cut with their end or side. Made of carbide, cobalt, or high speed steel. Multiple tools used for rough and finish cuts.
  • X, Y and Z axis – Control movement and position of cutting tool. Linear motion guideways provide precise positioning.
  • Controller – The computer that stores the machining operation program. Converts code into electrical signals to control axis motion, spindle speed, coolant, etc.
  • Coolant system – Directs compressed air/water mixture to cutting area to cool and lubricate tool and workpiece.
  • Workpiece holding – Various workholding devices (vises, clamps, fixtures) can be used to secure and precisely position the raw stock material that will be machined into a part.

CNC machines follow these basic milling steps to create a part:

  1. The stock material, usually a block of metal, plastic, or other material is loaded and clamped onto the machine bed.
  2. The dimensions of the stock material are input into the controller software to establish a reference point.
  3. The machinist uploads the program with instructions for the cutting path into the machine controller.
  4. The cutting tools needed are loaded into the automatic tool changer rack depending on the sequence of cuts required.
  5. The controller moves the tool, workpiece, or spindle as needed to position the tool at the starting point defined in the program.
  6. The spindle spins the cutting tool at the specified RPM and feed rate as the axes move to cut the material in the programmed path.
  7. The controller positions the tool at incremental depths to remove more material until finishing the operation.
  8. For multiple operations, the machine will swap tools automatically based on the program sequence.
  9. After completing all cutting operations, the finished machined part is unloaded. Any scrap material is cleared from the machine.

Now let’s take a closer look at the key components and functioning of a CNC milling machine.

Key Components of a CNC Milling Machine

Machine Frame and Bed

The frame and bed form the foundation of the CNC milling machine providing the rigidity and vibration dampening needed for precision metal removal.

  • Made of cast iron or heavy steel construction to minimize distortion under cutting forces.
  • Weight provides mass to reduce chatter by absorbing vibrations.
  • Guide rails on the bed allow precise positioning of the table in the X and Y axes.
  • Other components like spindle, tool changer, and enclosures are mounted on the frame.


The spindle holds and rotates the cutting tool at high RPM for material removal operations.

  • Consists of a precisely machined steel shaft running on high precision bearings.
  • Driven by an integral AC or DC electric motor at speeds from 2000 to 24,000 RPM.
  • Enclosed in a spindle housing that has the drive motor and sometimes the automatic tool changing mechanism.
  • The nose holds the tool holders into which the cutting tools are clamped.
  • Program controls spindle speed since optimum speeds vary for different materials and tools.

Automatic Tool Changer

The automatic tool changer swaps cutting tools in and out of the spindle under CNC program control.

  • A rack or rotating carousel that stores multiple tools needed for the sequential operations.
  • Tools are deposited and retrieved from this storage area automatically.
  • Permits long periods of unattended operation as all required tool changes are pre-programmed.
  • Help maximize productivity by reducing setup time between operations.

Cutting Tools

The cutting tools do the actual machining of the material by removing metal from the stock material.

  • Made of very hard materials like carbide, cobalt steel, ceramics, diamond, etc. to handle high-force metal cutting.
  • Come in many styles such as square end mill, ball end mill, drill bits, taps, reamers, face mill, etc.
  • Multiple tools needed for roughing cuts, semi-finishing, and finishing passes. Also specific tools for special features.
  • Insert style cutters allow indexable cutting inserts to be swapped once worn instead of replacing the whole tool.

X, Y, and Z Axes

The motion of the axes positively locate the cutting tool and/or workpiece for accurate machining.

  • Each axis slide mechanism has a servo motor, drive mechanism (ballscrew, rack/pinion, linear motor), encoder feedback, and sturdy guideways.
  • X and Y axes control table motion. Z axis vertically positions cutting head.
  • Precise coordination of the three axes allows complex 3D movement of the cutter through space.
  • Ballscrews with precision ground ball bearings are the most common drive system for high accuracy positioning.

Machine Controller

The controller is the computer that runs the CNC machine. It converts the programmed instructions into electrical signals to drive the machine tool.

  • Reads the part program storage media and interprets the coded instructions.
  • Sends signals to motors, amplifiers, and actuators to move the machine axes and operate other components.
  • Manages all the machine functions – axes positions, spindle control, coolant, tool changer, etc.
  • Has full interface for machine operation, programming, and status feedback.

Coolant System

The coolant system delivers compressed air or a water-soluble coolant/lubricant mixture to the cutting area via hoses and nozzles.

  • Keeps cutting tool and workpiece from overheating.
  • Washes away chips and allows smoother cutting action.
  • Prevents built up edge (BUE) on the cutting tool from high heat.
  • Prolongs tool life, improves surface finish, and supports higher feed rates.

Workpiece Holding

Workholding devices securely clamp the stock material in place on the machine table to allow accurate machining.

  • Various vises, clamps, jigs, fixtures, etc can be used depending on the part requirements.
  • Ensures the raw material does not move during cutting operations.
  • Positions stock accurately relative to the machine axes.
  • Common workholders are vises, pneumatic/hydraulic/magnetic clamping, modular fixture plates, tombstones, etc.

Now that we have covered the key components of a CNC milling machine, next we will learn about the machining capabilities that make CNC so versatile for part production.

Machining Capabilities of CNC Milling

CNC milling utilizes multi-point rotary cutters to selectively remove material and accurately produce parts based on programmed cutting paths. Here are some of the machining operations possible with CNC milling:

1. Facing – Machining a flat surface perpendicular to the spindle axis using a face milling cutter. This squares up the sides of a workpiece.

2. Plain milling – Moving the cutter parallel to the axis of the cutter. Used for surfacing, slotting, side milling.

3. Angular milling – Machining angled surfaces by tilting the workpiece or cutter. The cutter moves on an angle relative to the part surface.

4. Form milling – Cutting irregular contours by feeding the workpiece against a rotating multi-edge cutter. The cutter width determines the width of the finished cut.

5. End milling – Machining slots, keyways, pockets with the sides of an end mill. End mills can produce flat, squared surfaces or complex 3D shapes.

6. Chamfer milling – Beveling sharp edges by using a cutting tool with a 45 degree lead angle.

**7. Face grooving – Cutting grooves and recess pockets on a vertical face. Accomplished with specially shaped cutters.

8. Thread milling – Milling external and internal threads using dedicated thread milling cutters. Produces accurate, high quality threads.

9. Drilling – Drills holes by advancing a rotating drill bit along its axis into the material. Can drill, counterbore, countersink, tap, and ream holes.

10. Boring – Enlarging an existing hole to specific dimensions by using single point boring bars or boring head tools. Used to obtain accurate hole size and finish.

11. Tapping – Cutting internal screw threads by feeding a rotating tap into a pre-drilled hole. Done with tap chucks or by programming spindle speed/feed.

These and even more milling capability makes CNC extremely versatile for producing precision parts out of various materials. Complex 3D contours, pockets, slot features can be machined with shorter cycle times versus manual milling.

Next we will look at the key steps in the CNC milling programming process that makes all this automated machining possible.

CNC Milling Programming Process

CNC milling machines are programmed and operated using a set of computer instructions that follow specific coding formats and syntax. Here are the main steps in the programming process:

1. Design Part Geometry

  • Create a CAD solid or surface model that defines the part design features, dimensions, and tolerances.
  • CAD files can be imported into CAM software for programming. STL files from 3D printing also work.

2. Define Machining Operations

  • Using CAM software, select tools, speeds, feed rates, machining sequences needed to mill the part.
  • Different operations like face milling, contouring, pocketing, drilling are virtually simulated.
  • CAM optimizes the toolpaths for efficiency and collision avoidance.

3. Post-processing

  • The toolpaths are converted (“posted”) into a syntax that the machine control can interpret.
  • Post-processors output standard G & M codes but formatted for a particular machine brand.
  • Tool names, retract positions, canned cycles, subprograms can be incorporated.

4. Set up Machine

  • The proper workholding fixtures are mounted and the stock material is secured.
  • Required cutting tools are loaded in the carousel and tool length offsets are set.
  • Program is loaded into the CNC controller and all offsets and parameters entered.

5. Perform Machining

  • The operator starts running the program and the machine begins making the part automatically.
  • Coolant applied to cutting area for chip flushing and heat reduction.
  • Operator supervises for issues but no manual intervention needed if program is right.

6. Part Completion

  • After all toolpaths complete, the finished part is unloaded from the machine.
  • Some post-processing like deburring, cleanup, surface treatments may be needed.
  • The part is then inspected to verify it matches the design specifications.

This summarizes the main steps in going from CAD design to machined part using the automated programming and machining made possible with CNC milling technology.

Next we will cover the advantages and benefits of CNC milling compared to manual machining processes.

Benefits and Advantages of CNC Milling

There are many important benefits that CNC milling provides over conventional manual milling machines:

Greater Accuracy

  • Programmed toolpaths and automated axes movement avoids human error in controlling the cutter positions.
  • Computer tracks and monitors axes location to micron level precision throughout program run.
  • Parts more consistent and accurate to print dimensions and tolerances.

Quicker Cycle Times

  • No time wasted on manual measurement, calculation, tool positioning for each step.
  • Optimal toolpaths maximize material removal rates.
  • Multiple tools used combines several setups into one continuous run.

Improved Surface Finishes

  • Precision programmed toolpaths, variable speed control, balanced end mills all help obtain excellent surface finishes.
  • Finishes of 10 Ra microinches or less are possible depending on capabilities of machine.
  • Less hand polishing and post-processing needed compared to manual methods.

Increased Repeatability

  • Once program is proven out, the same part can be machined over and over with zero variation.
  • Great for production volumes, allows scaling output easily.
  • Manual process varies each time due to human factors.

Complex Geometries

  • Intricate 3D shapes like sculpted surfaces, complex curved pockets are impossible to produce manually.
  • But are simple for CNC since toolpaths are programmed, not manipulated directly.
  • Open up new design possibilities outside manual methods limitations.

Multitasking Capabilities

  • Can switch between many operations – face milling, contouring, drilling, tapping without resetting or repositioning.
  • Automatic tool changer allows quick swaps between different cutting tools.
  • Unattended operation facilitates running large batch jobs.

Safer Operation

  • Software prevents tool crashing and protects from programming mistakes.
  • Operator does not need to directly manipulate cutter near spinning tool or chip hazards.
  • Enclosed work area and through spindle coolant systems provide a cleaner workspace.

This range of benefits makes CNC milling ideal for low volume production or prototyping needing tight tolerances and fast turnarounds. It expands the design possibilities over manual machining approaches.

Next let’s take a look at the various types of CNC milling machines on the market and their distinguishing features.

Types of CNC Milling Machines

There are several configurations of CNC milling machines designed for different types of machining operations and optimal usage scenarios:

1. Vertical Machining Centers (VMCs)

  • Most common CNC milling machines. Spindle is vertically oriented.
  • Table moves side-to-side and front-to-back to position workpiece. Z-axis moves cutter head up and down.
  • 3-axis, 4-axis, and 5-axis (with two rotary axes) VMC models. 5-axis allows angled milling.
  • Well suited for complex 3D parts and multi-sided machining in one setup.

2. Horizontal Machining Centers (HMCs)

  • Spindle is horizontally oriented. Cutting tool articulates from vertical to horizontal.
  • No table, the workpiece or pallet is positioned under the cutter. X-axis and Y-axis are on the cutter head.
  • Very rigidsince workpiece does not move. Good for larger parts.
  • Pallet changers allow automatic loading/unloading for unattended machining.

3. Gantry-Type Milling Machines

  • Table is stationary while an overhead gantry moves the vertical milling head in the X, Y and Z axes.
  • Designed for very large and heavy parts. Excellent rigidity.
  • Downside is the large machine footprint. Slow traverse speeds.
  • Used to machine extremely long bedways or machine inside deep cavities.

4. CNC Bed Mills

  • Similar to manual bed mills but CNC controlled. Has a fixed spindle and moving table.
  • Cost effective CNC for smaller work envelopes. Limited by weight on table.
  • Used for smaller parts requiring high precision. Convert manual mills to CNC.
  • Typically 3 axis. Lower rigidity than VMC design.

There are also CNC router machines used primarily for wood, plastics, and softer metals. Mini CNC milling machines with small work areas for prototyping and modelmaking. Knowing the application helps determine the best CNC configuration.

This covers the main styles of CNC milling machines. Next we will go over some key terms and definitions related to CNC machining centers.

CNC Milling Terminology and Definitions

Like any technology, CNC milling has its own specialized terms to describe components, parameters, and functions. Here are some of the key CNC milling terminology:

Axis – The directional movement of the milling machine table, workpiece, or cutting tool. The X, Y, and Z axis on a CNC machine correspond to the Cartesian coordinate system.

Pallet – The fixture that holds the workpiece or workpieces to be machined. Pallet systems allow quick loading/unloading to maximize spindle utilization on HMCs.

Tombstone – A type of pallet that holds multiple parts vertically for efficient use of the work area. The tombstone is indexed to put each part in position.

Fixture – A workholding device that securely and accurately locates a workpiece to ensure precise machining. Fixtures prevent workpiece movement and facilitate repeatability.

Zero point – The coordinate that all programmed movements are positioned from. The zero point is typically set at one corner/edge of the raw stock or fixture.

Datum – A fixed reference point on the stock material that dimensional coordinates are measured from. The machine zero point is aligned to the workpiece datum.

Tool offset – The distance the tip of the cutting tool is from the spindle centerline. Needs to be defined so the controller adjusts the toolpath by the offset value.

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