How to choose a CNC spindle?

Lots of different spindle motors are available for CNC routers. Each brand has its own options, which differs from another brand. So, when you shop around for your ideal spindle, make sure that it is the right one.

Depending on its brand, you may have to spend several thousand dollars for a spindle. But you can also find spindle motors with lower horsepower that cost considerably less. The right spindle will optimize the performance of your CNC router. Make sure that the spindle is made for the horsepower and rpm speed of your machine. The weight is also another important feature. Since some of these spindle can weigh as much as 20 pounds, you must ensure that your router can support it. You don’t want its weight to reduce machine performance.

Confirm that your workshop can support the spindle motor. As most spindles require 220 volt power, you may have to settle for less powerful ones if you have only 120 volt outlets.

Three-dimensional model of a HSK spindle

What’s Inside and Why It Matters

To the casual eye, the spindle doesn’t do much – the tool cuts the metal, the
table moves, the control system controls the movement, and the software runs everything. The spindle motor is just an tool holder with a built-in engine and takes orders from a servo.

Although the spindle might not be complex or active or dazzling, it works hard and takes a lot of abuse. Because of the wear and tear it must endure, a huge amount of effort goes into designing the interior of the spindle to ensure high performance and a long lifespan. It is truly the core of the machining centre.

Not only do quality components determine the longevity of the spindle but they also determine how the spindle manages vibration, speed, and torque. As you start to find out more about high-precision motor spindles, you will find that the bearing system is frequently being discussed. The acronyms are overwhelming: ABEC standards; dN value benchmarks and formulas; Back to Back Bearing Mounting (DB or O); Face to Face Bearing Mounting (DF X). Any study of a spindle’s bearing system must include material, type, arrangement, and lubrication.

Bearing System of the Spindle

A bearing system uses balls to maintain the separation between the inner and outer steel raceways. The operating temperature, vibration levels, and the lifespan of the spindle will depend on the material of the components. Hybrid ceramic bearings is definitely better than standard steel ball bearings.

Less mass.

Ceramic ball bearings are 60 percent lighter than steel balls. This is important because as ball bearings rotate, especially at high speed, centrifugal force pushes the balls against the outer race. This deforms the shape of the ball, and the deformation causes it to wear out faster. Because ceramic balls are lighter, they are less affected (at the same speed). So ceramic balls allows the ball bearings to spin 30 percent faster without sacrificing bearing life.

No contact welding.

Ceramic balls do not fuse to steel raceways, removing a condition known as contact welding – a significant cause of bearing failure. Contact welding occurs when pressure causes a general flow of metal to take place, and consequently, mating the surfaces of the metals. As the bearings continue to rotate, these microscopic welds break, creating surface roughness, which in turn, leads to heat generation. And finally, the bearing fails.

Work at lower temperatures.

Because of its sphericity is almost perfect, a ceramic bearing rotate at much lower temperatures, resulting in longer life.

Lower vibration levels.

Tests show that spindles with hybrid ceramic bearings are more rigid and less resonant. So they are less sensitive to vibration, which again results in longer life.

Types of Bearings

Bearings are designed to carry different types of loads, such as radial or thrust. In high-speed spindles, angular contact ball bearings are most commonly used. These provide the accuracy, the load carrying capacity, and the rotational speed necessary to cut metal. The precision balls fits between steel races and are strong enough to carry axial and radial loads.

Taper roller or cylindrical roller bearings are another type of bearings. These can carry a greater radial load. The roller (or cylindrical) bearings are stiffer than ball bearings and are designed for high-speed applications. The spindle motor manufacturer will often use both ball and roller bearings in different parts of the spindle, depending on the type of load the bearing must carry.

Lubrication

It is essential to lubricate the bearings correctly. There are lots of systems that machine tool manufacturers use to maintain proper lubrication of the bearings, such as oil-mist, oil-jet, oil-air, and pulsed oil-air.

These systems are required if spindle speeds exceed 18,000 rpm, but they cost more to upkeep and are more expensive to replace. Furthermore, these lubrication systems must be monitored regularly to ensure the right amount and mixture (of oil-mist, oil-jet, or oil-air) is applied.

Sintered bearings are the best option to lower maintenance and replacement costs. Lubricated once during the assembly of the spindle, and sealed immediately, you do not have to bother with lubricating these bearings.

Types of Spindles

Spindles are driven in various ways, by belts, gears, inline drives, and built-in motors. With a belt-driven spindle, maintenance costs is reduced if the belt is easily accessible. Futhermore, some types of belt design produce a lower noise level. The diagonal teeth of herringbone belts dissipates trapped air to reduce noise.

Gear-driven spindles

increase the total cost of the machine. It is noisier and need more upkeep than belt-driven spindles. Although gear-driven spindles were favored over a belt-driven spindle in the past, improvements in material technology and design of belts have made it a favorite for low-maintenance alternatives.

Direct drive spindles

are designed to couple directly with motors. These spindles are smoother and quieter to operate, and they provide excellent surface finishes.

Built-in motorized spindles

have motors built into them. Although they are more expensive than the belted ones, they are used when higher speeds (more than 16,000 rpm) are required.

But no matter what kind of spindle you’d like to buy, the motor is an important consideration. Dual wound spindle motors have an extra set of windings, called booster windings to provide more torque on cutting edge. Single wound motors are used when high torque is not required.

T

hings that Damage the Spindle

Heat and contaminants are two major causes of spindle failure. Find out what has been designed to protect the bearing system from contaminants (often chips and coolant.) Spindles usually fail because its bearings failed. And bearing failure is usually caused by condensation, contamination, coolant ingress, or chip damage. You will want to keep the spindle temperature low and the contaminants out.

Contaminants

mostly enter the spindle because the seal leaked. Find out which method is used to keep the seal tight. In an air purge system, a labyrinth seal is used to prevent leakage, and an air purge used to remove contaminants. A dual air purge system has two ports (upper and lower) to keep contaminants out.

Heat

is another cause of spindle problems. Since steel expands when heated, manufacturers should explain how spindle growth and head growth is warded.

Heat exchangers (or more commonly, chillers) keep the spindle cool and control the expansion caused by heat dissipation. This type of system lengthen spindle life, and it is typically used for continuous running, or for long durations. The type of chiller used depends on the application. You may want to use a Thermo Active Stabilizer for extended high-speed applications. This system uses temperature sensors to provide real-time feedback for the CNC control to compensate for thermal deformation.

Unbalanced tools can also damage the spindle. DIN 69888 specifies that the bearing load caused by unbalance must be less than 1% of the bearing’s dynamic load capacity. Moreover, worn tools or tools that are too long or both can affect the lifespan of your spindle.

Tooling Considerations

Like the spindle, heat can also have a a bad effect on tooling. Find out if a coolant ring (of flexible coolant nozzles) is provided with the spindle. If that is the case, check the number of nozzles and whether they are adjustable. It is obviously better to have more nozzles. And it would be more convenient if the direction of the nozzles can be adjusted for a larger tool radius.

The CTS (Coolant Through Spindle) option is commonly recommended when routing at more than 12,000 rpm. This will ensure that your custom or expensive tools are protected. CTS is also recommended at lower speed for better hole quality and greater throughput. CTS prices vary depending on its pressure and its design.

Replacement Costs

Just like tires on a car, you will need to replace the spindle on your router someday. When buying a brand new router, curb your enthusiasm for making a hasty purchase. Think about what will happen in the future when you need to replace the spindle: its cost and your machine downtime if the spindle is not easily available.

Summary

The spindle is a machining workhorse. Ensure that the manufacturer has given the spindle design careful attention, and that they have put in quality components that will increase the lifespan of the spindle.

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