Chuck in CNC Explained: Types, Working

A CNC chuck is an automated work-holding device used on CNC machines like lathes and machining centers. It has a circular structure with radially arranged jaws, making it ideal for holding symmetrical workpieces. It can be programmed for faster work holding, providing great operational flexibility.

What kind of CNC chuck should you use? Are they versatile? What should you look for when buying one?

This article discusses CNC chucks in detail by explaining their parts, types, and differences. It also provides tips on picking the right chuck for your job.

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What is a CNC Chuck?

A CNC chuck is a mechanical device that securely holds the workpiece in its position during the machining process.

Unlike a regular chuck, a CNC chuck offers automated control of jaws. Also, it allows for precise jaw movement because of the programmable actuators.

Since the movement is controlled automatically with a powering device, they are also called a power chuck.

CNC chucks generally have a rigid configuration and are made of aluminum, brass/bronze, cast iron, steel, stainless steel, etc.

Parts of a Standard CNC Chuck

The above-given diagram shows an exploded view of a 3-jaw CNC power chuck.

Its actuator is housed inside the body of the chuck. Some of its main parts include the body, master jaw, wedge plunger, drawnut, plunger nut, etc.

Body

The body of the chuck houses all the integral components. It is a hollow circular piece that is clamped onto the spindle.

Master Jaw

A master jaw is made of toughened steel. They are attached to the wedge plunger.

The jaws slide over the grooves present on the cross-sectional face of the chuck.

Wedge plunger

The wedge plunger converts its Z-axis movement into the X-axis movement of the master jaws. It transfers optimal holding force to the jaws.

Drawnut

The drawnut acts as the actuator and transmits the motion from the hydraulic or pneumatic fluid to the chuck mechanism.

Plunger Nut

The plunger nut forms a link that connects the draw nut to the wedge plunger.

Working of a Standard CNC Chuck

The above-given image shows the working of a standard CNC pneumatic power chuck. It works using pressurized air.

When pressurized air is passed through the bottom valve, the actuator (marked in yellow) moves upward due to the built-up pressure.

Then the wedge plunger (marked in green) slides upward along the vertical axis, pushing the jaws radially away from the center. It is connected to the actuator.

Similarly, jaws move towards the center when the pressurized air is applied on the other side of the actuator, moving the wedge plunger downwards and letting the jaws move toward each other.

A similar setup is used in other CNC chucks, but they may have different actuators that control jaw movement. For example, hydraulic powered.

Specification - What should you look for?

The number of jaws, holding range, and size specify a chuck. Consider a 10", 300mm, 3-jaw chuck to understand the concept better.

Number of jaws

When selecting a CNC chuck, you should consider the number of jaws, as it determines the workpiece shapes it can hold.

For example, if the chuck only has 3 jaws, then you'll only be able to hold cylindrical stocks, but a 4 jaw chuck can hold cubic, cuboid, or hexagonal stock in addition to cylindrical ones.

Consider the jaws on a chuck as the number of fingers holding a pencil. Using more fingers will give you a better grip on the pencil. The jaws on a chuck work in a similar fashion.

However, you should also note that with more jaws, they may become functionally redundant. So, as with most machines, you need to optimize for cost and function.

Capacity

The capacity of a chuck indicates the size (diameter) of the workpiece it can hold.

In the above example, 300mm indicates the maximum workpiece diameter it can hold.

There is also the question of weight. But that is dependent on the power of the machine itself.

Chuck size

Chuck size refers to the chuck's outside diameter specified in inches. So from the example mentioned above, 10" refers to the outer diameter of the chuck.

The outer diameter of the chuck will influence its rotating speed and weight.

Difference between CNC and Manual Chuck

Parameters	CNC Chuck	Manual Chuck
Gripping force	Uniform	Varies
Operational time	Fast	Slow
Error due to eccentricity	Low	High
Sensitive to hold thin-walled parts	Depends on the set holding pressure	Sensitive

Gripping Force

A power chuck offers a constant gripping force. The smooth transmission of forces to the jaws makes the power jaw chuck the most sort after chuck.

In a manual chuck, the jaws are manually controlled. Hence, the chances of nonuniform pressure distribution are high.

Operational Time

The holding and releasing of the workpiece by the jaws on a CNC chuck is smooth and fast compared to a manual chuck.

Unfortunately, if the jaws on a CNC chuck are not centered properly, fixing it'll be harder than in a manual chuck.

Error

The error arising while holding the workpiece is minimal in a power or CNC chuck as all the jaws move together at the same distance.

But on a manual chuck, The workpiece can go off its center easily. Then you'll have to use a dial indicator to align the stock concentric to the chuck's central axis.

Holding Sensitivity

Manual chuck is highly sensitive in holding thin-walled tubes due to varying pressure applied through its jaws.

Power jaw chucks are optimal for holding thin-walled tubes as they facilitate uniform pressure.

Types of CNC Chucks

The CNC chucks are classified into three basic categories based on their structure, mode of actuation, and diameter.

Based on The Structure

Based on the type of structure, CNC chucks are classified into three types: self-centering chuck, independent chuck, and combination chuck.

Self-centering Chuck

Self-centering chucks, commonly known as universal chucks, have their jaws engaged to a scroll wheel, ensuring equidistance movement along the radial direction.

Most 3-jaw chucks are self-centering in nature.

Jaw-independent Chuck

Generally, 4-jaw and 6-jaw chucks come under the category of jaw-independent chucks. They can move each jaw independently, allowing them to hold irregular shapes.

The concentric circles engraved on the chuck's cross-sectional face help identify the distance moved by the jaw.

Combination Chuck

A combination chuck includes the features of both an independent and a self-centering chuck.

The jaws of a combination chuck can be moved together or individually.

You can use this type of chuck when working on stocks having varying shapes.

Based on The Mode of Actuation

Based on the type of actuating forces, chucks are classified mainly into three: hydraulic chucks, pneumatic chucks, and magnetic chucks.

These chucks are largely used for CNC-specific jobs as they facilitate easy and quick clamping of the workpieces.

Hydraulic Chuck

The hydraulic power chuck works by the pressure applied by the working fluid.

Based on the direction of the flow of working fluid, the pressure forces the chuck's jaws to move outward or inwards radially.

These types of chuck help hold the workpieces fast and accurately without any eccentricity.

Pneumatic Chuck

The working of the pneumatic chucks is similar to that of the hydraulic chucks. The only difference is that it uses air instead of a working fluid.

When compared to a hydraulic chuck, pneumatic chucks are durable and inexpensive. In addition, the maintenance cost is far less than that of a hydraulic chuck.

Both hydraulic and pneumatic chucks are largely used in mass-production setups where frequent changes switching of workpieces is required.

Magnetic Chuck

Magnetic chucks are commonly used to hold ferromagnetic workpieces.

Thin structures, which are more prone to bending and twisting, can be machined without distortion with the support of a magnetic chuck.

Electromagnets housed inside the chuck create the magnetic field. When a workpiece comes in the vicinity of this field, the magnetic circuit is closed, forming a safe and secure hold on the workpiece.

Based on Diameter

Based on the size of the workpiece that the chuck can hold, chucks are classified into conventional chucks (either 3-jaw or 4-jaw chuck) and collet chucks.

Collet chucks can hold thin workpieces ranging from 1/16" to 4" in diameter.

These are usually used with a bar feeder where a circumferential hold of the workpiece is needed. They offer better gripping force at higher rpm than a conventional chuck.

Use a collet chuck when you need faster changing time and have less expertise to perform the task.

If you want to hold heavier and large-sized stocks, go for a jaw chuck. Some jaw chucks can hold up to 16" diameter stocks.

Maintenance of a CNC Chuck

Maintenance is an essential preventive measure to enhance the durability of any machine part.

So it is important to regularly clean and repair the chuck to keep it in good working condition.

On a CNC Chuck, the chances of leftover machined chips clogging the path of a wedge plunger are high.

For a smooth and equidistant movement of jaws, you'll have to clean the chuck regularly, depending on the machining cycle.

If the operation speed is high or the manufactured lot is large, cleaning the chuck every 4 hours is recommended.

Greasing the chuck with the right lubrication will prolong its lifespan while working with high frictional forces.

Molybdenum-disulfide-based lubricants are the most preferred to keep surfaces from rubbing against each other.

Final Thoughts

A CNC chuck has good stability, uniform grabbing force, and excellent dynamic balance, making them an ideal choice for working with CNC machines.

They are widely used in modern-day machines as they are reliable and fast compared to manual chucks.

CNC chucks can help speed up the process by significantly cutting down work holding time if you have a huge production lot.

Frequently Asked Questions

Can a chuck hold square-shaped bars?

Yes, a chuck can hold a square-shaped bar. 4-jaw chucks facilitate holding bars of regular symmetrical shapes like squares, octagons, etc.

What is a chuck key used for?

A chuck key is used to change the position of the jaws in a chuck. Turning the chuck key in the slots helps move the jaws radially inward and outwards.

How to choose the right type of chuck?

An important caution undertaken while choosing a chuck is - Always choose a chuck having the same speed rating as that of a lathe spindle. You can choose the remaining specifications, like holding range, jaw types, etc., based on the machining need.