The Benefits of Collect Chuck

Three-jaw chucks are the standard work holding device for most CNC lathe users. It is among the most widely used types of chuck in a variety of turning applications. However, that does not mean it is the best chuck for all jobs. A collet chuck is an alternate work holding device that also uses mechanical forces to hold a workpiece.

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What is a Collet Chuck?

A collet chuck is an alternate work holding device that also uses mechanical forces to hold a workpiece. While a collet chuck does not work with the same wide range of workpiece sizes that a jaw chuck can accommodate, it offers advantages in speed, accuracy, and productivity that may be crucial for certain jobs.

 

When is a Collet Chuck a Better Choice than a Three-Jaw Chuck?

Bar Feeding: A collet chuck also helps to minimize vibration, and opens and closes faster than a three-jaw chuck. Collet chucks are ideal for machines equipped with bar feeders because they provide full 360° contact, ensuring that the bar stock remains on centerline and allowing for accurate re-gripping after being advanced.

Odd Shaped Parts: With a bit of customization, collets can easily be made to hold off-shaped or off-center parts. There is a wide selection of standard collets designed for round, square, and hex stock.

Short / Thin Parts: A collet chuck&#;s low-profile design provides better tool clearance and allows for machining very close to the chuck face, which helps prevent thin parts from sagging or bending.

High Speed Operation: Compared to three-jaw chucks, collet chucks hold up far better to the negative effects of centrifugal force.

Reduced Setup Times: A collet can be changed much faster than chuck jaws, and without boring. Collet chucks keep your machines making chips &#; and profits!

Small Diameter Work: Collet chucks offer much better tool clearance for small parts ranging in size from 1/16&#; to 4&#; in diameter. Collet chucks can also be run at higher speeds, and provide superior gripping force, than three-jaw chucks.

Pipe and Tube Work: Collets provide 360° contact with the part, distributing the gripping force evenly around the part&#;s entire circumference and reducing the risk of crushing or distorting thin-walled parts.

Better Tool Life: The paddlewheel-like shape of a three-jaw chuck can deflect coolant before it reaches the cutting tool tip, shortening tool life, whereas the streamlined shape of a collet chuck enables precise coolant delivery. Collet chucks also produce far less mist than three-jaw chucks.

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.

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.

Contact us to discuss your requirements of Power Chuck Supplier. Our experienced sales team can help you identify the options that best suit your needs.

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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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