Spindle, tool holder, and collet: how to choose the right one for your CNC router

2024/12/24

Reading Time: 8 minutes

Spindle

Spindles are available in both regular and automated tool change configurations. Each of these kinds may be cooled with either air or water.

Water cooling is silent, although it does require a chiller to function. Air conditioning, on the other hand, requires no additional equipment, although it is quite loud. Additionally, while a chiller will notify you of errors or low water levels, you’ll need to check the filter status of an air-cooled spindle manually. So you may make your choice based on your preferences (or noise tolerance).

Now, a few remarks regarding normal spindles with no automated tool change. There are other possibilities, but the primary difference is usually in the bearings and the materials they are built to operate with.

GDZ spindles: These are considered universal because they are suitable for milling wood, plastic, and soft metals. We use these universal spindles on some of our machines due to their ability to handle a wide range of materials.
GDK format: These spindles are mainly designed for working with non-ferrous metals and offer increased rigidity for such tasks. However, GDK spindles can also be used for milling other materials. Their RPM usually reaches up to 24,000 RPM.
DX spindles: This type has a maximum RPM range of 8,000 to 12,000 and is intended for processing non-ferrous metals like aluminum, as well as ferrous metals such as steel, cast iron, etc.

Let’s expand on this a bit. RPM (revolutions per minute) affects cutting efficiency, so pay attention to this parameter when choosing a spindle. Most spindles offer 18,000 or 24,000 RPM, but sometimes you can find options with 15,000 RPM (such as spindles with automatic tool change).

However, don’t chase the highest RPM value. Higher numbers don’t necessarily mean faster or better machine performance. For most tasks, 18,000 RPM is sufficient, while 24,000 RPM is only used in about 10% of cases. The higher RPM is needed for working with small-diameter cutters, which means it is intended for miniature operations.

Spindles can also vary by country of origin. There are Italian and Chinese spindles. In terms of quality, the former may be superior to the latter, but their price is five to ten times higher. As a result, repairing an Italian spindle can cost as much as buying a new Chinese one. Therefore, from the perspective of cost and performance, Chinese spindles are an optimal solution that ultimately allows for producing high-quality products.

Tool holders

At first glance, these tapers may look more or less the same. However, while they might appear similar, they are not interchangeable.

BT and SK look identical, but the first one is the standard for Asia (used in Japanese, Korean, Taiwanese, and occasionally American machines), while the second is common in Europe. These tapers are considered universal and are widely used.

ISO: Another visually similar tool holder, but with a different application. It is designed for working with large tools at low to medium RPM.

All three types have a steep taper. Steep taper tool holders can lose accuracy when operating at extremely high RPM.

BT and BBT. BT tool holders have a single contact point between the tool holder and the spindle. BBT tool holders feature a dual-contact design with additional contact surfaces. This dual-contact design increases rigidity and stability, providing better tool performance and accuracy (but they may not be compatible with all CNC machines unlike BT that are widely used).

HSK. A rigid and lightweight one. Its primary application is high-speed and high-precision machining. HSK holders have a hollow shank design that provides a more direct and efficient transmission of power to the cutting tool, resulting in better accuracy, stability, and cutting performance. They also have a dual contact system.

Note: Pay attention to this if you already have another machine with automatic tool change. Otherwise, you can temporarily disregard tapers, as this information is relevant only when you need to transfer them between machines.

Also, keep in mind that holders deteriorate over time. They become less accurate as they wear, which can lead to chatter and shorten the life of the cutting tool. Regularly inspect and replace them as they wear out. Don’t wait until they are completely worn out, as this could damage the spindle (which would be expensive to repair).

Collet

If BT, HSK, and others are the upper part of the tool holder, then the collet chuck, or simply ER, is its lower part. The name of the collet usually consists of three elements, for example: ER-11 A or ER-20 M. What does this mean?

The number after ER indicates the diameter of the collet and corresponds to the shank size of the tool that can be placed in it. For example, an ER-8 can be used with tools up to 5 mm in diameter. As you can see, the tool should be slightly smaller than the collet itself. Let’s list the main types of collets and the corresponding tools for each.

ER-8. Tool shank diameter from 0.5 to 5 mm.
ER-11 Diameter from 1 to 7 mm.
ER-16. To 10 mm.
ER-20. To 13.5 mm.
ER-25. To 16 mm.
ER-32. To 20 mm.
ER-40. To 26 mm.

Now, about the letter that follows. It mainly indicates the type and size of the nut used to secure the tool. This is important to know in order to choose the right wrench. For some collets, a special tool might be required. There are A, B, M, T1, and T2 types. So, take this into account when choosing, so you know how to tighten everything properly later.

Power

Collets are often categorized based on the power they are used with and the tasks they are typically applied to. This doesn’t mean they should only be used in this way, but we will provide this classification as it may help novice machinists.

ER-8: Used for miniature work, mainly for high-power engraving tools.
ER-11: Often used with power up to 1.5 kW. Applied for fine engraving on non-ferrous metals, plastics, and thin materials.
ER-16: Power up to 1.5 kW. Primarily used for solid wood, plywood, and plastic.
ER-20: Up to 3.5 kW. A widely used spindle type as it is suitable for working with wood, acrylic, metals, stone, etc. Almost any task can be handled with this size.
ER-25, ER-32: Power over 4.5 kW. Spindles for large-diameter tools, necessary for working with metals, stone, and thick wood.

Here’s another small tip: if your work is more hobby-oriented, you probably won’t need a machine with more than 3 kW of power. If you need a CNC machine for woodworking, you should choose a power range of 2.2–4.5 kW. For metal, spindles from 4.5 to 7.5 kW are generally suitable, but 5.5 kW is typically preferred. For stone, we recommend a power range of 3 to 4.5 kW.

You should choose collets and spindle power according to your specific tasks!

Important pieces

Spindle bearings

Let’s talk about this component as well. There are steel and ceramic bearings.

The main advantage of ceramic bearings is their higher hardness, better surface quality of the balls, and rings themselves. As a result, they offer higher precision and wear resistance. However, if these bearings break, they are harder to replace and are more expensive. Additionally, they are more sensitive to excessive forces (e.g., broken end mills).

The advantages of steel bearings are that they are widely available, easier to replace, and less expensive. They are also less sensitive to excessive forces during operation.

Bearings also have different working principles. Modern CNC machines most commonly use ball or roller bearings.

Roller bearings offer higher rigidity but are not practical for machines with low power or small loads. They are suitable for high-power spindles.

You will encounter this only when replacement is necessary, so there’s no need to worry about it for now. But keep in mind that there are different types!

Pull stud

Pull studs are the link between the machine and the tool holder. They guarantee precise positioning and safe locking of the holder in the spindle. Most machines require a standard pull stud design (ISO, BT, others) and some require a specific design.

You will encounter this when you need to buy additional tooling. You will have to choose the same ones as those you already have for the holder.