There are two broad categories of spindles used on CNC machines: brush types and brushless types. The brush type spindles, as the name implies, use commutating brushes to transfer current to the coils that cause the shaft to spin. The brushes tend to wear over time under normal use, and consequently wear out faster under heavier applications. Another drawback to brush type spindles is the noise associated with their commutating brushes. The main advantage, however, is that they are generally much lower in cost. In fact, in some CNC type applications, they are considered disposable.

This low cost also tends to make these spindles less precise. TIR, or Total Indicator Runout, is an industry measure of spindle accuracy. TIR measures how true the rotation of the shaft is. It is typically measured at a fixed distance from the tip of the spindle shaft and, as the measurement name indicates, determines how out-of- round the rotation is. Brush type spindle motors generally have TIR significantly greater than the brushless type spindles. In many applications, such as woodworking, this is not of concern.

The other broad category of CNC spindles are brushless, also known as AC spindles. Brushless AC spindles do not have the maintenance issue of brushes wearing out or the noise associated with brush type spindles. Because AC spindles require less maintenance, have a better TIR rating, they are also a more expensive alternative to brush type spindles. The cost is usually justified when looking at the overall comparison between the two spindle types.

In the category of AC spindle motors, there are 3 types: fixed collet spindles, manual quick change spindles, and automatic tool change spindles.

The fixed collet spindle requires the collet to be changed with a manual operation that usually requires a couple of wrenches. This is a fairly simple process but takes a few minutes each time it is required. The manual quick change and automatic tool change spindles have an electro/ pneumatic system that releases the current tool and replaces it with another in a matter of seconds. The automatic tool change system, as the name implies, is done without any operator intervention while the manual quick change system usually requires the operator to activate a signal to release the tool and to activate another button to reload the next tool into the spindle.

The question of which spindle should be selected depends primarily on the applications and quantities of parts to be made. The benefit of the automatic tool change system is that a great deal of time can be saved for parts that are to be made in large quantities that also require a number of different tools. If the quantities are not so large, the manual quick change might represent an economical compromise. In the case where the quantities of the parts are small or only a small number of tools are required, there is no clear benefit to using either a quick change or automatic tool change system.

Spindle Power

The traditional way that the spindle power is “measured” is by the HP rating. This rating has to be carefully considered since the HP rating is generally proportional to the spindle rpm. For example, if a 3HP spindle is rated at 3HP at 18,000 rpm, it would only have 2HP at 12,000 rpm. The speed at which the rating is specified is therefore extremely important.

The other aspect of spindle power that must be considered is the nature of the power rating. The power rating on the brush type spindles is generally specified as a momentary peak rather than a continuous rating. The rating for AC spindle motors is usually considered a continuous rating but even this might be specified as a function of duty cycle. Some spindles are rated for 100% duty cycle – able to maintain the rated power continuously, while others are rated for a 60% or 80% duty cycle. In the latter case, the expectation is that the spindle will be used to its rated power for a few minutes and then allowed to “rest” for a brief period before the next part. This duty cycle rating is associated with the required cooling of the spindle.

Spindle Cooling

Three common methods of spindle cooling include: fan, compressed air or liquid. There are two types of fan-cooled spindles. First is an electric fan which will blow air through the spindle body. The second is a fan blade attached to the spindle arbor. This method of air flow is dependent on spindle rpm. Both methods of cooling have drawbacks. With fan-cooled systems, the duty cycle of the spindle is approximately 60% to 70%.

Another is the noise generated by the arbor fans which tends to be in the upper 70’s to lower 80’s decibel range; however, electric fan-cooled spindles do not have noise related issues. These spindle offerings usually do not exceed 24,000 rpm and are usually the most economical.

Compressed-air-cooled spindles allow for a 90% duty cycle. These require a constant stream of clean nonfluctuating source of compressed air. These spindles are typically used for greater then 24,000 rpm. These spindles usually incorporate ceramic bearings.This method is the most efficient and allows for duty cycles of 100%. These spindles are ideal for very demanding applications such as production of very hard materials in a 24/7/365 day operation. The constant loads on the spindle generate a lot of heat, and the only method to remove this excess heat is through a separate liquid chiller unit.

Choosing a Spindle

There are many spindle options available for Techno CNC Routers. Correct selection of a spindle is imperative for optimum machine performance. The electric spindle is the heart of the machine. Many variables must be considered when selecting the correct spindle such as material to be cut, production volume, tooling, machine feed rate, and spindle rpm. Generally, each material and cut has an ideal tool profile and cutting speed. Larger diameter tools require slower speeds. Smaller diameter tools require higher speeds. Spindle speed and feed rate for a given cut must be balanced for best quality, tool life and spindle life. Incorrect spindle speed is a common error in CNC machining. Machine feed rate and spindle rpm are directly related to one another. The higher the rpm, the faster the machine must be cutting. The typical question is how fast should I be cutting? This can be determined by the chip load. Simple formulas can be used to predetermine feed rate.

Formula:  (chipload) x (# of cutting edges) x (rpm) = feed rate

Many cutter manufacturers supply this information with the specific cutter to be used for different types of material to be cut. This will get you close and the optimal feed rate can be fine- tuned at the machine. Typically, feed rates that are too slow will decrease tool life due to the increased friction. This increased friction will not only wear out the cutter, but will also heat up or burn the material being routed. Other considerations that must be taken is how to enter the part. Ramping into the part is the preferred method. The ideal ramp should be between 0 and 20 degrees from the table surface. This angle will allow you to enter into the material at 100% of the feed rate. At any angle greater then 20 degrees, the feed rate should be reduced accordingly. Entering into the part on a ramp will greatly increase spindle bearing and tool life.