Tooling Tips: ABC’s of Machine Movement

What are the ABC’s of CNC machine tool movement? A fundamental or foundational skill is understanding the Cartesian coordinate or rectangular coordinate system.

Inside of the Cartesian coordinate system exist the Axes Alphabet — A, B, C, P, Q, R, U, V, W, X, Y, and Z. With the increased demand for 5-axes machines coupled with high-speed machining capabilities, there is a need to help apprentices and trades people to understand some fundamentals in axes relationships.

My discussions with individual apprentices and trades people over the past few years indicate a fundamental misunderstanding of the relationship of the axes.

So what is the Axes Alphabet? Let’s begin with an overview of the Cartesian Coordinate or rectangular coordinate system and build on this.

All CNC and CAD systems define their work envelope in terms of the three basic axes X, Y, and Z which meet at right angle at a point referred to as the point of “origin” The origin is always zero. 3D space in this work envelope can be defined as units along each axis corresponding to a physical point of (X, Y, Z), for example (-4.800, 5.862, 3.461).

What is the Right Hand Rule?

When I start to instruct a student, apprentice, or trades person on CAD or CNC equipment, I always begin with an explanation of two important rules: “The Right Hand Rule,” and “The Right Hand Rotational Rule”.

Your right hand is one of the most valuable reference points to help illustrate the concept of both these rules.

To help you understand how these two rules can be applied to the movement of machine tools we will start with a verbal description. The “Right Hand Rule” states: turn your right palm up and extend the thumb and forefinger to form an “L” shape. Then, by pointing the middle finger up you can represent a three-dimensional coordinate system. The thumb represents the positive direction of the X-axis; the forefinger represents the positive direction of the Y Axis, and the middle finger is the positive direction of the Z-axis. Note: The Z Axis always corresponds to the machine tool spindle. This holds true on vertical or horizontal machining centers.

What is the Right Hand Rotational Rule?

Now that we have a base, lets build on it.

The Cartesian coordinate system helps define every point in 3 dimensional space; as well there are situations when we need to be able to rotate these axis in order to increase the movement that may be required to machine a cavity.

Each axis has a corresponding rotation axis, which are A, B and C. The X-axis has a rotational A-axis, Y-axis has a rotational B axis, and Z-axis has a rotational C axis. The Right Hand Rotational Rule states: turn your right hand upward and extend your thumb.

The thumb extended can represent the positive direction of the X, Y, or Z-axis. Then pretend you are gripping the axis in the palm of your hand. Curling your fingers around the axis indicates the positive rotational direction of the corresponding axis X+ — A+ direction Y+ — B+, and Z+ — C+.

An alternate method to remember this by is to place yourself at the origin point and look out along the axis. The positive rotation of the axis is in the clockwise direction. As the whole coordinate system rotates, as in the example of a horizontal machining center, the relationship of the axis A, B and C follow along. Many CNC machining centers have switches often not used labeled A and B.

We will stop here for now and continue this lesson on Cartesian coordinate fundamentals in the next Tooling Tips column in March.

Mark is the chair of manufacturing and transportation at St.Clair College in Windsor, Ont.