Many of today’s digital fabrication technologies are not all that new, just a variation on a theme. For instance, electric motors and methods of controlling their speed have been around for more than a century, but new technologies in motor types and their control are critical to the operation of CNC equipment.
One essential electric motor on a CNC multi-axis machine is called the spindle and it does all the cutting via various router bits. Many of the simpler or inexpensive CNC machines use a brush-type spindle motor, more commonly known as a router. The Porter-Cable 7518 Speedmatic is a popular choice, though it was never designed for continuous use on a CNC machine. The accuracy of its cuts, as measured by runout (variation in the armature’s rotation), deteriorate rapidly when under heavy continuous use. Also, when loads are applied, the router’s speed will tend to vary, leading to uneven cutting. Accuracy can also be affected by the quality of the collets and the bits.
The modern alternative is a brushless spindle motor coupled with a variable frequency drive.
Under the hood
In a brushless motor, there are no brushes or connection between the rotor (or armature) and stator, which in combination causes changes in the magnetic field around the rotor so the motor shaft spins. Instead, permanent magnets on the rotor and electromagnets on the stator control the magnetic field changes. The result is a more efficient, reliable, powerful and quiet motor.
Combined with a variable frequency drive, a kind of computerized rheostat, the speed of a brushless motor can be controlled in a very precise manner to ensure accurate and clean cuts no matter the material.
A variable frequency drive converts alternating current power to direct current, removing AC ripple that can cause uneven motor operation which, in turn, can affect the quality of cuts, then converts the power back to alternating current though the use of a series of electronic components. The outputted alternating current is changed in one permanent fashion and one variable fashion, the latter which can be controlled. The permanent change is that the power curve is changed from analog power, or what some call a roller coaster curve (actually a sinusoid curve; a smooth repeating oscillating curve), to digital power, or a so-called “square curve” where the power is either on or off.
The variable change of the power through the use of the variable frequency drive is that the frequency, or hertz (the number of times the current changes direction in one second), can now be controlled electronically, which in turn can change the speed (RPM) at which a brushless motor turns. Since modern brushless motors can feed back its speed to the variable speed drive, the actual instantaneous speed of the brushless motor under a load can compensate for with more or less power so it can stay at a near constant speed irrespective of changing loads, such as when a cut is made going around a corner. This, in turn, results in high-quality and consistent cuts.
Four good reasons
There are other advantages to brushless motor spindle/variable frequency drive setup:
First, as the only contact between the rotating and fixed parts of the brushless spindle are bearings, there is less friction resulting in less heat and electricity used relative to a typical router like a Speedmatic.
Second, because a brushless motor spindle generates less heat, it is made to operate continuously and accurately for hours on end. Brushless motor spindles that are cooled with water or compressed air can be operated under very heavy loads for hours on end. In fact, brushless motor spindle power ratings are for continuous horsepower rather than maximum or peak horsepower.
Third, brushless motor spindles are machined from aluminum billet — rather than aluminum castings — resulting in runout of usually less than .001”.
Finally, brushless motor spindles are much quieter.
You’ll pay more for a brushless motor with a variable frequency drive, but these components provide accurate and consistent cuts, which means a better final product from your shop to your customer.
This article originally appeared in the September 2015 issue.