Introduction to CNC Machining

Coordinates, Axes, and Motion

Overview

We’ll first learn about axis systems.

Then we’ll investigate how the machines understand where to move,

and the kinds of moves they can make getting there.

Machining and CNC Technology by McGraw-Hill Higher Education

Machining and CNC Technology by McGraw-Hill Higher Education

World Axis Standards

There are 14 standard axes defined by the Electronics Industries Association (EIA) used for motion and position.

In this course we’ll study 9 of them. 3 Primary Linear Axes X, Y and Z 3 Primary Rotary Axes A, B and C 3 Secondary Linear Axes U,V and W

Unless it’s a multiplexed machine with several auxiliary rotary and linear axes, these nine are adequate to define most of the equipment in industry today.

However, for tomorrow’s manufacturing world, that’s another question? Machines continue to evolve as central processors are able to handle more and more calculations per nanosecond, thus more functions simultaneously.

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Identifying Machine Axes

Whenever you are assigned to a new CNC machine, the axis set must be identified as the first order of business.

Here are the sets for three common machines.

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Quick Tips on Axis ID

It’s easy to identify the spindle, which is the Z axis or it faces Z.

Then apply the Right Hand Rule by pointing your right middle finger in the positive Z direction.

Your fingers and thumb then form the orthogonal axis frame (mutually at 90º)

Machining and CNC Technology by McGraw-Hill Higher Education

The Right Hand Rule

First identify the Z axis. It’s parallel to the spindle axis, and brings the work toward and away from the spindle.

Pointing your middle finger in the positive Z direction, your index finger and thumb form the other positive axes.

Machining and CNC Technology by McGraw-Hill Higher Education

Orthogonal but not always level

All CNC machines use the X-Z or X-Y-Z frame, with each axis perpendicular to the others.

That relationship stays the same no matter how the axis set is rotated to suit the machine.

Toward stronger or more efficient machines manufacturers arrange the set any way convenient, but not the inter-relationship between axes.

The set (my fingers) remain in the same orientation to each other no matter their world orientation

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Now a serious example: A slant-bed lathe

The X axis on many turning centers, is not parallel to the floor, it slants forward.

That provides easy access to the turret for setup work, since the machine isn’t as wide as level X axis machines.

Plus chips and coolants slide right off to the catch basin below.

Z

X Slanted

This lathe’s world axis orientation is not level but it’s still an orthogonal set.

90º

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Primary Rotary Axes A,B & C

Whenever a machine features a rotary axis, we identify it this way:

If it rotates around a line parallel toX it’s an A axisY it’s BZ it’s C

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Rotate Cutter or Part

Rotary axes can move a cutter head in an arc

Or they can move the workpiece in an arc.

In this film we see A and B auxiliary axes moving simultaneously with X, Y and Z, to cut this complex turbine blade.

Machining and CNC Technology by McGraw-Hill Higher Education

Which Direction?

To determine the direction of rotary motion, either plus or minus A,B or C, we use the Rule of Thumb.

It’s based on the line about which the rotary axis pivots, X, Y or Z

Point the thumb of your right hand in the direction of the rotary axis’ line of rotation, X, Y or Z positive direction.

Positive C direction

Z+ X+

Positive A direction

Y+

What motion?Positive B

Machining and CNC Technology by McGraw-Hill Higher Education

Absolute Value Coordinates

In CNC work, the origin, X0, Y0, Z0, is known as the Program Reference Zero (PRZ)

It’s the starting point for coordinates Most coordinates in the program

refer their distance from the PRZ. For example.

The tip of the drill is at

X2.500, Y1.00, Z-1.00

Relative to the PRZ which is the lower left corner on this part

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Incremental Value Coordinates

Occasionally we encounter the need for a different kind of coordinate.

They do not refer to the PRZ but rather, to their last position.

Incremental coordinates are jumps from where you are to where you wish to go next.

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CNC Motions

CNC machines move their axes in five ways:

Rapid TravelLinear single or multi axis straight line

motionCircular motion within a single plane.Circular/Linear, also called 2 ½ dim. motion.

Two axes move in an arc while the third moves in a straight line.

3-D motion few controls have the ability to move in an arc using 3 axes simultaneously. Most approximate these arcs through the power of the cam software.

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Rapid Motion

Rapid – as fast as the machine can move but with the ability to reduce speed through operator over-ride control.

Trade Tip Caution! Depending on the power of the CPU, your machine will rapid in one of two ways. Older controllers take an unexpected nonlinear path! Newer controllers with 16-bit or higher microprocessors follow the true linear motion.

Machining and CNC Technology by McGraw-Hill Higher Education

Feed Rate Motions

The next four motions all move one or more axes at the rate specified in the program.

The differences lies in how many axes are involved, in a straight line, or arc.

As motions become more complex, the CPU must handle far more calculations per second by interpolating each axes drive commands.

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Interpolating

To move axes simultaneously, to produce a constant velocity along the line A-B, say at 400 inches per minute.

neither the X or Y axis will be moving at 400 IPM.

They will run at lower speeds that combine to create the tool motion of 400 IPM.

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Linear Interpolation Point A to B

Programmed Rate 400 Inches Per Minute

A

B

375.87 IPM, X Axis Motion

137.81 IPM Y Axis

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Feed Rate Over-ride

The linear interpolation required the control to set each axis moving at constant values but different rates.

The operator can over-ride the resultant tool motion from 0% (no movement) up to 100% or 150% on some machines.

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Circular and Above

For arc motion at feed rate, the controller is also interpolating as with linear.

The difference is it is constantly changing the ratio between the axes involved, as the curvature changes slants.

Machining and CNC Technology by McGraw-Hill Higher Education

Polar Coordinates

Sometimes engineering information comes not in the form of rectangular dimensions, but rather as the radius and angle from a starting point.

Those points are more easily defined using polar coordinates – a bolt circle for example.

Polar Coordinates aren’t used inside CAM generated programs, but they are very useful for drawing the part geometry or when doing hand program writing of polar entities.

Trade Tip

Using polar coordinates, often saves a trigonometry step during drawing or hand program writing!

If the needed significant point is defined in radius-angle, rather than X-Y, why do an unnecessary calculation? Define it with an R-A coordinate on your geometry drawing.

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