Main Components and Axes of a CNC Machining Centre

Knowing the program commands is not enough to use a CNC machining centre safely and correctly. The operator must understand what each section of the machine does and how the axis movements in the program are physically executed by the machine.

A CNC machining centre generally consists of the following systems: the control unit and operator panel, the machine body, the spindle system, the tool magazine and tool-change mechanism, the work table, the X, Y and Z axes, servo motors, ball screws, guideway and bearing systems, encoders and measuring systems, the coolant system, the lubrication system, the chip conveyor, hydraulic and pneumatic systems, and the safety equipment.

Why Does a Machining Centre Consist of So Many Systems?

A simple movement written in a CNC program is executed by several systems working together. For example:

G01 X100 F500

When this command is given:

1. The control unit reads the command.
2. It calculates the target position for the X axis.
3. It sends the motion command to the servo drive.
4. The servo motor starts turning.
5. The motor turns the ball screw.
6. The table or slide moves linearly.
7. The encoder measures the actual position.
8. The actual position is compared with the target position.
9. The servo system corrects the motion if needed.
10. The axis reaches the X100 position.

For this reason, a defect on the part does not always come from the program. The servo system, the encoder, mechanical play, guideway friction or the workholding can also affect the result.

1. Machine Body

The machine body is the main structure that carries all moving and fixed systems of the machining centre. Its main duties are to carry the spindle and axis systems, resist cutting forces, damp vibration, preserve geometric accuracy and limit the effect of temperature changes.

A good machine body requires high rigidity, high vibration damping, low deformation, resistance to cutting forces and thermal stability. High machining quality requires high rigidity, low friction, minimum play and adequate damping.

What happens if the body is weak or damaged?

• Vibration marks
• Surface waviness
• Dimensions changing with cutting direction
• Increased noise in deep cuts
• Reduced tool life
• Chatter at corners
• Geometric errors on circles and precision surfaces

These symptoms are not caused only by the body. The tool, the workholding, the cutting parameters and the spindle condition must be checked together.

2. Column

On vertical machining centres the column is the main vertical structure that carries the spindle head and the Z-axis motion system. Its duties are to carry the spindle head, support the Z-axis guideways, transfer cutting forces to the body and preserve the squareness of the spindle axis.

If column rigidity is insufficient or its geometry is distorted, depth dimensions can change, surfaces may not come out square, the tool can deflect under load and dimensional differences can appear along the Z axis.

3. Table

The table is where the workpiece, the vice or the fixture is clamped. Tables usually have T-slots, to which a vice, clamps, studs, special fixtures or a rotary table can be mounted.

Why is table cleanliness important?

Even a small chip left under the vice or fixture can cause the part to be clamped at an angle, spoil surface parallelism, shift the workpiece zero and let the part move during cutting.

Before clamping:

1. Clean the table.
2. Clean under the vice.
3. Wipe the clamping surfaces.
4. Check for chips and burrs.
5. If required, check vice parallelism with a dial indicator.

How should the T-slots be used?

Clamping elements must be placed in a balanced way, kept as close to the part as possible, stay clear of the tool path, be tightened with proper torque and must not overload the edge of the table. When clamping with step clamps, the clamp surface should be slightly inclined towards the part so the part is pressed downwards.

4. Spindle

The spindle is the main shaft that rotates the cutting tool; it is the heart of metal removal in a machining centre. The basic components of the spindle system are the spindle motor, the spindle shaft, the spindle bearings, the tool clamping mechanism, the drawbar, the tool taper seat, the encoder and the cooling or lubrication system.

Spindle speed

S3000 M03

This command usually means the spindle runs clockwise at 3000 rpm. M03 is clockwise rotation, M04 counter-clockwise rotation and M05 spindle stop. The actual rotation direction must be judged according to the operator’s viewpoint and the machine builder’s definition.

Operator checks for the spindle

• Any unusual noise?
• Any vibration?
• Is the tool holder seating properly?
• Is the spindle taper clean?
• Does the tool change complete fully?
• Is the spindle load normal?
• Does it reach the commanded speed?
• Is the spindle heating up?
• Any cooling or lubrication alarm?

How does a spindle fault appear on the workpiece?

• Vibration lines on the surface
• Circular tool marks
• Constantly changing dimensions
• Oversized holes
• Very short tool life
• Noise at high speed
• Degraded surface quality
• Runout at the tool holder

These symptoms can be related to the spindle bearings, the tool holder, tool runout or the cutting parameters.

5. Tool Holder and Spindle Taper

The cutting tool is not mounted directly in the spindle; a tool holder is normally used. Common tool holder systems are BT, CAT, SK, HSK, ISO taper, ER collet chucks, Weldon holders, hydraulic holders and shrink-fit holders. The correct holder type is selected according to the spindle standard of the machine.

Why must the tool holder be clean?

If there are chips, rust, oil residue, scratches or burrs on the spindle taper or the tool holder, the tool may not seat fully. As a result, tool runout increases, the surface degrades, hole dimensions grow, the tool can break and the spindle taper can be damaged. Before every tool change the taper surfaces must be clean and undamaged.

6. Tool Magazine

The tool magazine stores the tools used by the CNC program. Its capacity varies by machine. Common magazine types are umbrella type, disc type, chain type, rack type and large-capacity tool storage systems.

Tool change command

T05 M06

This command usually means tool number 5 is selected and the tool change is performed. On some controls the tool is pre-selected on a separate line; on others T and M06 are used on the same line.

What the operator must check in the magazine

• Is the tool in the magazine the same as the one in the program?
• Is the pot number correct?
• Is the tool length within the magazine limit?
• Does the tool diameter interfere with neighbouring pots?
• Is a suitable pot used for heavy tools?
• Is the holder seated properly?
• Is the pull stud the correct type?
• Is the tool broken or worn?
• Is the tool offset stored under the correct number?

A wrong tool number can make even a correct program machine the part wrongly or cause a collision.

7. Automatic Tool Changer (ATC)

The automatic tool changer exchanges tools between the spindle and the magazine. The system may consist of a tool-change arm, a magazine motor, pots, sensors, a spindle orient system and the tool clamp/unclamp mechanism. During a tool change the spindle stops at a defined angle; this positioning is called spindle orientation on most systems. On many controls this positioning is performed with the M19 command; the tool changer expects the spindle drive key to be at the correct angle. Operators should not use M19 experimentally during normal production; its behaviour differs between machine builders.

Symptoms of tool-change problems

• The machine waits at the M06 command.
• The magazine cannot find the correct pot.
• The arm stops mid-cycle.
• The tool does not release from the spindle.
• The tool drops.
• A spindle orient alarm occurs.
• A sensor alarm occurs.
• The tool pot does not move up or down.

The operator must not reach into the tool-change mechanism or force the sensors manually. Recovery must follow the machine builder’s procedure.

8. X, Y and Z Axes

Axis directions are covered in detail in our training series, so here is a short reminder: X is the left–right movement, Y is the front–back movement and Z is the movement along the spindle. On a vertical machining centre Z+ moves the tool away from the part and Z- moves it towards the part; Z is the most critical axis for collision risk. Programming direction is always considered as the movement of the tool relative to the part – even if the table appears to move the opposite way.

What matters most on this page is how axis errors appear on the workpiece:

How does an X-axis error appear on the part?

• The part width comes out wrong.
• Pockets are over- or undersized in the X direction.
• The X coordinates of holes shift.
• Circles become oval in the X direction.
• Dimensional differences appear at direction reversals.
• Opposite wall dimensions do not match.

How does a Y-axis error appear on the part?

• Pocket length is wrong in the Y direction.
• Hole rows shift.
• Circles become oval in the Y direction.
• Dimensional differences appear between parallel walls.
• Diagonal dimensions do not match.
• The part zero appears wrong in the Y direction.

Common Z-axis mistakes

• Wrong tool length offset
• Wrong workpiece Z zero
• Wrong H number
• Not using G43
• Insufficient safe approach height
• Not accounting for the vice and clamps
• Long tools hitting the magazine or the part
• Incorrect use of G53, G28 or G30 movements

How does a Z-axis error appear on the part?

• The part is machined too deep.
• The pocket floor remains high.
• Hole depths are wrong.
• The part thickness is out of tolerance.
• Too much stock is removed in face milling.
• The tool hits the part or the clamping elements.

9. A, B and C Rotary Axes

Rotary axes can be added to the three linear axes: A rotates around X, B around Y and C around Z. Rotary axes are used to machine several faces of a part without re-clamping, drill angled holes, produce multi-face parts and perform simultaneous 4- or 5-axis machining.

3+2-axis versus simultaneous 5-axis machining

In 3+2-axis machining the rotary axes bring the part to a fixed angle and stop; machining is then done with the X, Y and Z axes.

In simultaneous 5-axis machining X, Y, Z and the two rotary axes move at the same time. This method is used for complex surfaces such as turbine blades, impellers, mold surfaces and aerospace parts.

10. Servo Motors

Servo motors move the CNC axes precisely. The servo system’s job is to bring the axis to the target position, move it at the commanded speed, monitor the position continuously, compensate load changes and correct the difference between the target and the actual position. In a closed-loop control system the actual axis position is measured, compared with the target and the difference is corrected by the servo system.

Why does the servo load rise?

• Guideway friction
• Insufficient lubrication
• Ball screw problems
• Very high acceleration
• Heavy workpiece
• Chip jamming
• Mechanical impact
• Brake not releasing fully
• Bearing or coupling faults

If the servo load rises, do not change parameters alone – check the mechanical movement as well.

11. Ball Screw

The ball screw converts the rotary motion of the servo motor into linear axis motion. Its advantages are low friction, high efficiency, precise movement, high repeatability, high speed and the possibility of preloading. Some axial play can develop in ball nuts at direction reversal; rigidity can be increased by preloading with a double nut.

What happens when the ball screw wears?

• Play appears at direction reversals.
• Circles come out oval.
• Pocket dimensions change with approach direction.
• Axis repeatability degrades.
• Noise and vibration increase.
• The servo load changes.
• Reversal marks appear on the surface.

In this case simply increasing backlash compensation may not be enough. The mechanical play must be measured and the system checked.

12. Guideways and Bearings

Guideways allow the moving axes to travel in a straight line. Common guideway types are box ways, linear guideways, roller-type linear guideways and hydrostatic guideways. Friction in bearings and guideways causes wear, energy loss and temperature rise, which is why lubrication is important.

What happens if guideway lubrication is insufficient?

• Axis movement becomes heavy.
• The servo load rises.
• The axis may move in a jerky way.
• Surface quality degrades.
• The guideway and carriages wear.
• Positioning accuracy drops.
• Axis alarms can occur.

The operator must monitor the central lubrication level and lubrication alarms.

13. Encoder and Measuring System

The encoder measures the movement of the axis or motor and reports it to the control unit. Basic encoder types are incremental encoders, absolute encoders, motor encoders, linear scales and rotary scales.

Direct and indirect measuring systems

In a direct measuring system the measuring element measures the slide or table movement directly – for example a linear scale. Its advantage is that most ball screw and transmission errors are included in the actual position measurement.

In an indirect measuring system the rotation of the motor or the ball screw is measured and the axis position is calculated from it. Direct measurement works from the slide; indirect measurement works through the screw mechanism.

Symptoms of encoder faults

• Reference loss
• Position alarms
• Changing dimensions
• The axis not returning to the same point
• Sudden axis movement
• Unstable actual-position readings
• Wrong spindle speed reading
• Measuring system difference alarms

Encoder adjustment or replacement is a job for the service engineer.

14. Coolant System

Coolant is pumped to the cutting zone. Its duties are to cool the tool and the part, reduce friction, flush chips away, improve surface quality and support tool life.

What should the operator check?

• Coolant level
• Concentration
• Odour and bacteria growth
• Foaming
• Nozzle direction
• Pump operation
• Filter condition
• Chip accumulation
• Tramp oil

Wrong concentration can cause rusting, skin irritation, loss of tool life and poor surfaces.

15. Central Lubrication System

The central lubrication system automatically feeds oil to moving elements such as the guideways and ball screws. It can include an oil reservoir, a pump, distributors, a pressure sensor, a level sensor and lubrication lines.

Why is the lubrication alarm important?

If the machine keeps running without lubrication, the guideways can wear, the ball screw can be damaged, the axis load rises, accuracy degrades and expensive mechanical failures can develop. The alarm must not simply be RESET so production can continue.

16. Hydraulic System

On some machining centres a hydraulic system is used for tool clamping, pallet clamping, rotary-table locking, axis counterbalance, fixture clamping and auxiliary mechanisms. The operator should check the hydraulic level, pressure, leaks, oil temperature and filter warnings.

17. Pneumatic System

Compressed air can be used for cleaning the spindle taper, tool changing, sensor cleaning, door or auxiliary mechanism movement and tool-probe cleaning. Low air pressure can cause tool-change alarms, a dirty spindle taper, tools not seating fully and pneumatic mechanisms not working.

18. Chip Conveyor

The chip conveyor carries chips out of the machining area. It can be operated from the program or from the panel.

What happens if chips accumulate?

• Table movement can be obstructed.
• Coolant nozzles can clog.
• The tool can re-cut chips.
• Surfaces can degrade.
• Chips can scratch the part.
• The conveyor can jam.
• The coolant tank can fill with chips.

Long, stringy chips must not be pulled by hand; use suitable equipment.

19. Workpiece and Tool Probes

Modern machining centres can be equipped with probe systems. A workpiece probe can be used for finding the part zero, measuring edges, finding hole centres, checking part position and post-machining measurement. A tool-setting probe is used for measuring tool length, checking tool diameter, detecting tool breakage and monitoring tool wear.

Probing reduces manual touch-off errors, but correct calibration is required.

20. Guard Cabin and Safety Systems

A CNC machining centre operates inside an enclosed cabin. The cabin keeps flying chips, coolant, broken tool fragments and rotating parts away from the operator. Safety systems can include door locks, emergency stop, limit switches, overload monitoring, spindle protection, servo alarms and hydraulic and air pressure monitoring.

The Easiest Way to Learn the X, Y and Z Axes

A Safe Exercise for Checking Axis Directions

1. Send the machine to its reference point.
2. Move the tool to a safe distance from the part.
3. Switch to JOG mode.
4. Select a low axis speed.
5. Press X+ briefly.
6. Watch the change on the position screen.
7. Repeat for X-, Y+, Y-, Z+ and Z-.
8. Evaluate the physical motion together with the screen coordinates.

Be extremely careful when moving towards the part in the Z axis.

Quick Fault-Symptom Table for Operators

This table is for first diagnosis only. It does not mean the operator should change machine data or servo parameters.

Why Is It Important to Know the Sections of the Machine?

An operator who knows the machine’s sections interprets alarms more accurately, notices collision risks in advance, links a defect on the part to the related system, gives maintenance staff correct information, makes fewer tool and clamping mistakes, uses the machine more safely and performs daily checks more consistently. CNC operation is not just pressing keys – it is understanding how the machine works.

Summary

In this guide we covered the machine body, the column, the table, the spindle system, tool holders, the tool magazine and ATC, the X, Y, Z and rotary axes, servo motors, the ball screw, guideways, the encoder system, the coolant and lubrication systems, hydraulic and pneumatic systems, the chip conveyor and probe systems. Recognizing how each system’s faults appear on the workpiece is the first step of correct diagnosis.

Disclaimer

This content is prepared for general CNC machining centre training. The physical movement of the axes, the tool-change system and the hydraulic and pneumatic equipment differ between machine builders. The machine’s user and safety manuals must be followed during practice.