When To Use A Collet Chuck

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The three-jaw powered chuck is the standard workholding device for most CNC lathe users. This type of chuck is versatile enough to be used in a wide range of turning applications. However, it's not the best chuck for all jobs. The collet chuck is an alternate workholding device that, like the jaw chuck, also uses mechanical force to hold the part being turned. While a collet chuck lacks the capacity for the same wide range of workpiece sizes that a jaw chuck can accommodate, it offers advantages related to speed, accuracy and productivity that may be crucial for certain jobs.

Several factors figure into the determination of which type of chuck would work better. When evaluating a collet chuck versus a jaw chuck for a given lathe application, take all of the following factors into account.

Spindle Load Capacity

The lathe spindle has a maximum allowable weight based on bearing load capacity. If the combination of the chuck and the work accounts for too much weight, the bearings may be overloaded. In applications where there is a danger of exceeding this limit, this very danger may dictate the choice of workholding. Jaw chucks tend to be more massive than comparable collet chucks, making the collet chuck an appropriate choice where weight control is needed

Spindle Speed

A collet chuck tends to be the better choice for turning at particularly high levels of spindle rpm. There are two reasons for this.

One reason relates to the mass of the chuck. Given the same spindle horsepower driving a jaw chuck and a collet chuck, the more massive jaw chuck would take longer to accelerate up to speed. The acceleration time would extend cycle time and reduce productivity.

Another reason relates to centrifugal force, which becomes a significant concern at high speeds because it increases as the square of rpm. For example, doubling the spindle speed causes centrifugal force to quadruple. This force pulls chuck jaws out from the center and tends to reduce clamping force. But with a collet chuck, centrifugal force does not have a significant effect. Therefore, clamping force is more constant across the speed range.

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Operation To Be Performed

A collet chuck applies clamping force all around the circumference of the part instead of just at select contact areas. The result is tight concentricity. This can be particularly significant for second-operation work where accuracy relative to the first operation is a concern. Even when a jaw chuck is used for the first operation, a collet chuck may be used for the second operation because of its precision clamping. A jaw chuck with bored soft jaws repeats within 0. to 0. inch TIR. A collet chuck typically provides repeatability of 0. inch TIR or better. The collet chuck can also be adjusted for concentricity during installation to further improve secondary operation accuracy.

Workpiece Dimensions

Collet chucks are best suited to workpieces smaller than 3 inches in diameter.

A collet chuck may also impose a limitation on the workpiece length. Specifically, a collet chuck limits the machine's range of axial (Z-axis) travel, because its length is longer than that of a jaw chuck. When the machining length of a workpiece is so long that just about all of the available travel of the machine is needed to cut it, then this requirement will probably dictate the use of a jaw chuck.

Lot Size

Very large and very small lot sizes both help make the case for a collet chuck.

Where there are small lot sizes and lots of them, the collet chuck's advantage relates to changeover time. Swapping jaws takes around 15 to 20 minutes for a standard jaw chuck or 1 minute on a jaw chuck specially designed for quick change, but the collet in a quick-change collet chuck can be changed in 15 to 20 seconds. The time savings add up where changeovers are frequent.

Similar time savings related to clamping add up where lot sizes are large. A collet chuck takes less time to open and close than a jaw chuck, shaving cycle time by reducing the non-cutting time from one piece to the next.

Workpiece Size Range

Part of the reason a collet chuck opens and closes more quickly is that its actuation stroke is shorter. Compared to a jaw chuck, a collet chuck is more limited in the range of workpiece sizes it can accommodate.
Collet chucks essentially trade flexibility for speed. If part size is consistent, a collet chuck is faster. But where workpieces vary significantly in size, it may take a jaw chuck to accommodate the complete range of work.

Types Of Materials

For hot rolled steel, castings, forgings and extrusions, standard jaw chucks tend to work better because of the diameter variations inherent in all of these types of parts. On the other hand, cold rolled material tends to be more consistent in size and therefore better suited to collet chucks.

However, the absence of any diameter measurement is not necessarily an obstacle to using a collet chuck. Collets designed for non-round cross sections can be provided for extruded bars that are made to custom shapes.

About the author: Michael Minton and Michael Sullivan work for ATS Workholding of Anaheim, California. Mr. Minton is eastern regional sales manager. Mr. Sullivan is vice president.

Collet Chuck Advantages

• Light weight
• Fast acceleration
• Less affected by centrifugal force
• Tight concentricity
• Fast clamping
• Fast collet change for part changeover

Collet Chuck Drawbacks

• Limited range of workpiece sizes
• Large axial dimension
• Better suited to small parts
• Better suited to workpieces with consistent diameter

The Subspindle Scenario

Turning machines with subspindles are often used in the kinds of high-volume applications where collet chucks can realize significant savings. With their ability to machine all of a part's faces in one cycle, these machines are often coupled to bar feeders for unattended production of a continuous succession of workpieces. In these applications, the savings in chuck actuation time, which may be small for one piece, may add up to considerable time savings when multiplied across the production run as a whole.

 

The Chuck Arsenal

When choosing the most appropriate workholding between jaw chuck and collet chuck, a third option is also important to consider. It may be cost-effective to keep both chuck types on hand and change from one to the other as circumstances warrant. Changing from a jaw chuck to a collet chuck, or vice versa, should take no more than 20 minutes. The jaw chuck can remain on the machine to handle an unpredictable range of parts. But when the machine is to run a large batch, or several batches of parts that are consistent in size, then the productivity gain from a collet chuck may more than make up for the time spent on changing chucks.

 

Workholding Maintenance

Proper maintenance of workholding systems is important. In fact, it may be more important than machine tool maintenance. Most modern CNC machine tools have extensive logic programs designed to watch and protect the machine tool from machine-related problems. But similar automatic safeguards for machine accessories generally don't exist. What's more, a failure of the workholding system&#;whether due to operator error or damage from some past crash&#;can be catastrophic to the machine and work, and dangerous to the operator. Standard practice should include a brief check of the workholding, making sure everything is tight and nothing has moved, every time a new component is loaded into the workholding device.

 

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CNC collets are a key member of the modern workholding family. They&#;re used across a wide range of industries and crafts, providing the ideal solution to various clamping requirements.

In this blog article, we&#;ll dive into the subject of collets and examine their uses, pros, and cons. Furthermore, we&#;ll break down the different types of collets available for the metalworking industry in particular.

Discover how collets can enhance your workholding processes with exceptional precision.

Which type of collets can Hyfore offer? 

At Hyfore Workholding, we supply a leading range of collet options. While the options available are primarily for the metalworking industry, the applications of collets, as listed here, are far-reaching.

We supply the following collet workholding: 

1. C-style collets

C-style collets are cylindrical workholding devices used to clamp machine spindles for accurate, high-speed rotary machining. The spindle is inserted into the collet, and the collet is tightened using a nut, which squeezes the slits in the collet shut so it firmly grips the spindle. C-style collets are available in various sizes for a variety of applications.

C-Style Collets

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2. Collet blocks

Collet blocks are used to secure the collet before machining. The collet is placed inside the block, and then the block is clamped into the desired position on the machine using a vice. Collet blocks come in different sizes and shapes to allow for various indexing options. Standard collet blocks are usually either square or hexagonal, but custom collet blocks are also available for machining workpieces that require unusual indexing.

Collet blocks

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3. Expanding collets

Expanding collets are specialised workholding devices for securing workpieces during machining. They use a cylindrical construction with slits that expand to take the tool when force is applied, allowing them to grip the workpiece or tool securely.

Expanding collets

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HQC (Hardinge&#;s unique system)

This quick-change collet system from US precision part manufacturer Hardinge is designed for use on automatic screw machines, Hardinge collet-ready CNC lathes, and all other chuck-style CNC lathes with A2-5, A2-6 and A2-8 spindles.

S-style master collets and pads

Designed for multi-spindle automatic screw machines, S-style master collets can also be used on CNC lathes and vertical machining centres. The collet pads allow for quicker changeovers than when using a standard collet and are also easier to store.

Swiss-style collets

Specifically for use with Swiss-style machines, Swiss-style collets are accurate workholding devices ideally suited for precision work.

FlexC Quick Change Collet System 

The precisely engineered Hardinge® FlexC Collet Systems mount to A2-5, A2-6, A2-8, and certain flat-back spindle noses.

These systems come in pull-back thru-hole, pull-back dead-length®, and push-to-close dead-length® styles, suitable for single and dual-spindle CNC lathes. With just one spindle mount, you can handle many different part diameters by interchanging vulcanised collet heads between jobs.

When using FlexC, collet changeover takes less than 10 seconds.

Through our trusted partners, we can supply an unrivalled selection of collet workholding options, as well as a variety of chucks, clamps and vices. Contact us today to learn more about our advanced solutions or head over to our online store to browse our workholding range.

What is a collet?

Collets are a type of chuck used to form a collar around an object and to clamp onto the object when tightened firmly. They&#;re mainly used within the industry to secure cutting tools and workpieces at high speeds and pressures.

Primarily, collets can be divided into two categories; external and internal collets.

• External collets &#; an external collet is, in essence, a sleeve, usually with a cylindrical internal surface and a conical external surface. External collets are the most commonly used type of collet. They hold the external surface of the tool or workpiece that&#;s being clamped such as a drill bit shank.
• Internal collets &#; an internal collet is presented in the form of a truncated cone, which has been drilled and threaded along the centreline. They expand inside the desired objects, securing them in place or holding two separate items, such as telescoping tubes, together. They are most often used to hold workpieces securely from the inside while the surface of the workpiece is machined on the outside.

Advantages and disadvantages of collets

Collets find a wide variety of uses for a host of reasons, but like all workholding options, they have their advantages and disadvantages when compared with the chuck alternatives.

Advantages of collets:

Lightweight &#; Collets are a modern, lightweight clamping option when compared to chucks. Their light weight allows them to accelerate up to higher speeds than rotary-type chucks. Collets are also less affected by centrifugal force, making them a more accurate and reliable choice for high-speed machining.

Faster set-up time &#; Additionally, their springy construction and functional design makes collets much quicker and easier to set up than jaw chucks. Their fast-clamping action and quick-release mechanism will save you even more time when changing similarly sized tooling.

Secure workholding &#; Collets offer a more secure workholding solution than jaw chucks, as there is an almost continuous and equal amount of pressure exerted on the circumference of the shaft by way of the collet nut. This tight concentricity allows for greater accuracy and improved rigidity when in use, reducing runout and helping to prolong tool life. Not to mention this also produces a better finish for your workpiece.

Clamp small diameters &#; Collets give you the ability to clamp smaller diameters than chucks. They excel at clamping parts with extremely small diameters, allowing for versatile workholding options across a wide range of applications. Custom collets can be used for off-centre turning or holding irregular-shaped parts.

Some more benefits include:

• Less affected by centrifugal force
• Tight concentricity

Disadvantages of collets

Collets are limited in the range of workpiece sizes they can accommodate and are ideally suited for repetitive jobs where the clamping requirements don&#;t vary greatly. They are also better suited for clamping tooling with longer shanks and workpieces with consistent diameters.

Where are collets used?

While metalworking utilises the broadest range of collet varieties, collets are used in a surprising number of applications:

Woodworking

In woodworking, collets are used in various handheld and machine-operated rotating tools, from drill presses to routers. Furthermore, collets are also used to secure larger woodworking pieces during machining. Collets are favoured in woodworking applications as they are hardwearing and ideal for performing repetitive tasks.

Metalworking

Collets have a variety of applications in the metalworking industry. Their ability to secure small-diameter precision parts operating at very high speeds makes them especially suited to this type of work. Equally, their fast setup and release times make them ideal for use with multiple tools of a similar size.     

Craft hobbies

Collets aren&#;t just used in industrial workholding. Hobbyist, artists, and craftspeople use tiny collets to secure interchangeable parts within handheld tools such as craft knives, drills, and files.  

Watchmaking

Another use for tiny collets is in watchmaking, where watchmakers use collets that are sized in 0.1mm increments to secure the delicate workpieces to their lathes.

Combustion engines

Collets are used within combustion engines to regulate engine valves. Also known as valve locks, keepers, or clips, the collet attaches to the valve through the spring retainer and sits between the valve spring and the valve.

Semiconductors

Collets are used in the semiconductor industry to pick up a die from a wafer after a die-cutting process.

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