CNC Machines and Automated Motion

Machines still move through physical space

A CNC machine does not understand an object in the same way a person does.

It works with positions, axes, coordinates and commands.

A simple machine may have movement along:

• X — left and right;
• Y — forward and backward;
• Z — up and down.

More complex machines may add rotational axes or additional controlled mechanisms.

The controller moves the machine through these coordinates according to a program.

CAD, CAM and CNC

Several stages may sit between an idea and a finished CNC part.

CAD — Computer-Aided Design

CAD describes the intended geometry.

CAM — Computer-Aided Manufacturing

CAM interprets that geometry and develops manufacturing operations or toolpaths.

Machine instructions

The manufacturing information is converted into commands the controller understands.

CNC execution

The machine physically performs the programmed movement.

These stages are related, but they are not interchangeable.

A geometrically correct CAD drawing can still produce a poor result if the CAM operation, tooling, workholding, machine setup or material assumptions are wrong.

Coordinates and origins

The machine needs a known reference system.

Its internal machine coordinates describe positions within the machine's available travel.

The operator or manufacturing process also establishes a useful reference for the workpiece.

This may be called a work origin, work zero or datum.

Once that relationship is established, a programmed point such as:

X = 50 mm, Y = 20 mm

has physical meaning relative to the workpiece.

If the assumed origin and the actual workpiece position disagree, the entire operation can be displaced.

Toolpaths

A toolpath describes where the active tool moves.

But the desired finished edge and the toolpath are not always the same line.

A router cutter has a diameter.

A laser produces a finite-width cut.

A plasma arc has a cut width.

A waterjet produces a stream with measurable width.

CAM systems can compensate for these physical characteristics so the finished object more closely matches the intended geometry.

This is one reason clean CAD geometry matters.

The manufacturing system ultimately has to convert that geometry into real machine motion.

CNC is a category, not a single machine

Computer control can be applied to many manufacturing processes.

Examples include:

• CNC routers;
• CNC mills;
• CNC lathes;
• laser cutters and engravers;
• plasma cutters;
• waterjets;
• CNC saws;
• automated drilling and machining centres.

The cutting mechanism may be completely different, but the machine still depends on programmed motion and a known relationship between the machine, tool and workpiece.

Automation changes the hazard

Automation can remove people from some dangerous manual operations, but it does not make machinery harmless.

A CNC machine may start moving rapidly and precisely once a cycle begins.

It does not know that somebody has placed a hand, tool or loose object in the wrong location unless the machine has a system specifically capable of detecting that condition.

This makes guarding, interlocks, emergency stops, correct setup and safe loading practices especially important.

Operators need to understand the machine's travel, tooling, workholding, programmed operation and safe operating area before a cycle begins.

An incorrect command can still be executed very accurately.

Workholding becomes part of the coordinate system

Fixtures are especially important in CNC manufacturing.

A fixture can do more than stop a part moving. It can establish exactly where the workpiece sits relative to the machine.

Production fixtures allow multiple parts to be placed repeatedly into the same known position.

This means good fixturing contributes to:

• safety;
• accuracy;
• repeatability;
• production speed;
• reliable automation.

The machine assumes the physical world matches the setup it was given.

Good CNC manufacturing is largely about making that assumption true.