CNC Machining — Do you need 3/4/5 axis machining?

Computer Numerical Control (CNC) machining is a manufacturing process in which pre-programmed computer software dictates the movement of factory tools and machinery. CNC machines come in many different forms with the most common being the 3-, 4- and 5-axis CNC machines. With each additional axis of freedom, the complexity of parts that can be produced increases. We shall explore these 3 types of CNC machines further in this article.

Before that, do check out our previous articles on CNC manufacturing listed below for a better understanding of the guidelines when designing parts for CNC machining.

Factorem’s ISO 2768 Machining Guidelines

Drafting technical drawings for CNC Fabrication

Dowel Pin Tolerance and Fit Standards

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Introduction

In CNC manufacturing, machines are operated via numerical control (NC), wherein a software program is designated to control an object. The language behind CNC machining is alternately referred to as G-code, and it’s written to control the various behaviors of a corresponding machine, such as the speed, feed rate and coordination.

Today, most CNC machines are fully automated. Computer-Aided Design (CAD) files dictate the dimensions of the finished part accurately. These files are passed through Computer-Aided Manufacturing (CAM) software to produce the tool path required for the machine to carry out the fabrication.

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Conventional Drilling Machine in Operation

3-Axis CNC Machine

In a 3-axis CNC machine, the workpiece is held stationary as the cutting tool moves in the X-, Y- and Z- directions to achieve the desired cuts.

Parts that can be produced using a 3-axis CNC machine are simple parts with planar milled profiles, drillings & threaded holes in-line with an axis. Intricate details such as angled features and compound angled features cannot be produced. While all 6 sides of a part can be cut using a 3-axis machine, if a part has features on multiple planes, each surface would have to be manually set-up. This would increase the costs associated with the fabrication process as labour time is required to change orientation of the workpiece and it would make more economical sense to use a higher axis CNC machine instead.

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4-Axis CNC Machine

The operation of the cutting tool remains the same as in the 3-axis CNC machine, but in a 4-axis CNC machine, the workpiece is now able to rotate about the X-axis, called the A-axis.

As mentioned earlier, using a 3-axis machine would require the workpiece to be manually rotated to access different surfaces of the workpiece. This process is eliminated with the use of a 4-axis CNC machine. 4-axis CNC machines are able to access 4 out of the 6 sides of any part due to the extra freedom of rotation.

4-axis machining gives us the ability to machine angled features, otherwise not possible with a 3-axis machine. The limitation being that all angled features must be angled about the same axes, or additional fixtures would have to be put in place.

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5-Axis CNC Machine

In addition to automatic movements along the X, Y, and Z axes, 5 axis CNC machines can select two of the three rotational axes (A,B,C) to use. The A, B, and C axes perform a 180° rotation around the X, Y, and Z axes, correspondingly.

The advantages of a 5-axis CNC machine include increased feature accuracy, intricate details manufacturable as well as giving the designer freedom to make a wider variety of prototype parts without being limited to machine capabilities.

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Which one should you choose?

Whilst a 5-axis machine is able to manufacture most of parts with high levels of complexity, it is important to note that these machines come at a much higher price. This translates to higher operation and maintenance costs and thus the product price will undoubtedly be more expensive than if the part can be manufactured using a 3-axis machine.

5-axis machines are also not common in small shops due to the high skill level required to operate them as the programming of the machines has to account for 5 degrees of freedom to trace out the tool path. Larger machine shops which can afford and operate a 5-axis machine will be less likely to have free machine time for small quantity parts.

Hence, it is vital to simplify the part for production as much as possible to ensure the fastest machining lead times and most cost-efficient prices. Best is to design the part for manufacturing using a simple 3-axis CNC by considering the turning, milling and drilling process required during fabrication. Some tips to design more easily machinable parts are as follows:

1. Avoid angled cuts. This refers to holes which are not perpendicular to the surface of the object e.g. diagonal cavities.
2. Avoid features on multiple surfaces. A unique set-up is required for each of the 6 faces of the part to be fabricated and hence, having features on more than 1 face will increase set-up cost and machining time.
3. Avoid undercuts. As mentioned earlier, holes can only be cut in one direction. Designing a hollow cavity at a certain depth into the part should be avoided as much as possible. If necessary, T-slots and Dovetails are available but the implications of longer waiting time and higher cost apply.

Thank you for reading!