4 Advice to Choose a Customized Collet Chuck Size

(This article is an updated version of the original that was published in .)

If you are looking for more details, kindly visit DRAKE.

When considering the purchase of a CNC lathe or turning center, it is important to ensure that the workholding system is matched to both the machine&#;s capabilities and the type of work that it will be doing. For example, the workholding system for a large-bore oil-patch lathe that will be machining pipe threads is probably not going to be the right solution for a high-speed multitasking machine making hip joints.     

Many CNC lathes and turning centers come equipped from the factory with a standard workholding system consisting of a three-jaw power chuck, a hydraulic actuating cylinder and a drawtube. The hydraulic cylinder is fixed to the rear end of the machine&#;s spindle, while the chuck is mounted to the front side, or working side, of the spindle. These two components are typically connected together via the hollow steel drawtube that passes through the spindle bore.  

The opening and closing of the chuck jaws are accomplished through the axial motion of the hydraulic actuator&#;s piston. As the piston slides back and forth, the drawtube moves in unison, driving a wedge-shaped plunger within the chuck body. This plunger converts axial motion to radial motion, causing the jaws to open and close. The amount of gripping force that the jaws apply to a workpiece is directly related to the actuator output force and is easily regulated with a pressure valve. 

As a universal workholding device, a three-jaw chuck functions well for many common turning applications. It can hold a range of part sizes, is capable of operating at reasonable rotational speeds and achieves good accuracies&#;particularly when the jaws are bored in place. 

A CNC collet chuck mounts to the working side of a CNC lathe spindle in the same manner as a three-jaw chuck and uses the machine&#;s existing hydraulic cylinder and drawtube for actuation.

However, there are many applications where a jaw chuck is not the best workholding option, which has led to the development of a range of alternative solutions. The most common among these is the CNC collet chuck.

A CNC collet chuck mounts to the working side of a CNC lathe spindle in the same manner as a three-jaw chuck and uses the machine&#;s existing hydraulic cylinder and drawtube for actuation. The installation of a collet chuck usually requires the fitment of an intermediate adapter between the chuck and drawtube to compensate for thread and position disparities. This adapter is custom machined for the specific lathe/chuck combination and is normally included in the price of the collet chuck.           

Small Diameter Parts

CNC collet chucks were originally developed to facilitate the machining of smaller parts. Although collet chucks are available with capacities of as large as about 6 inches, the majority of applications are for workpiece diameters measuring 3 inches or less. In fact, the benefits achieved by using a collet chuck on parts in this size range are so significant, many lathe manufacturers and machine tool distributors now allow customers to purchase their machines with a collet chuck already installed as the standard workholding device.         

The most obvious advantage when machining parts in the 0 to 3-inch range is the additional tool clearance provided by a collet chuck&#;s streamlined shape and reduced nose diameter. This arrangement enables machining to take place much closer to the chuck, providing maximum rigidity and better surface finishes. In contrast, the large diameter of a three-jaw chuck and its jaws typically requires the part being machined to extend further into the work zone, thereby increasing the likelihood of deflection.    

Higher RPM

Collet chucks also lend themselves to small diameter work because their lower mass and symmetrical geometries enable them to run at higher speeds than conventional three-jaw chucks. Being comparatively lighter in weight, collet chucks are less prone to the negative effects of centrifugal force and, therefore, tend to produce more consistent gripping force over the entire rpm range.

Tubes and Thin-Walled Parts

One of the major differences between a collet chuck and a three-jaw chuck is evident in the area of workpiece engagement. A jaw chuck typically provides three points of contact&#;one for each jaw. Conversely, a collet chuck and collet offers 360-degree support. This additional contact helps reduce the chance of part slippage, and also plays an important role in the machining of tubes and thin-walled parts. By distributing the gripping force evenly over the entire workpiece circumference, the risks of pinching and crushing are reduced. 

Some additional benefits of using a collet chuck when bar feeding include reduced vibration and faster clamp/unclamp times.

Bar Feeding

Collet chucks are the preferred workholding method for most bar feeding applications. Aside from the previously outlined advantages related to small diameter turning, some additional benefits of using a collet chuck when bar feeding include reduced vibration and faster clamp/unclamp times. The 360-degree contact between the collet and workpiece again comes into play in bar feeding applications by ensuring that the barstock remains on centerline for concentric re-gripping after being advanced by the feeder.

Additional Advantages

Some additional advantages to using collet chucks include faster setup times and/or higher accuracies because there is no need to bore soft jaws in place; availability of standard collets for gripping round, square and hex-shaped stock; internal expanding collets for gripping on workpiece internal diameters; and bores as small as ¼ inch can be accommodated. Holding odd-shaped parts and performing off-center turning are among some of the more specialized applications that can be easily handled with custom collets.

Pullback Versus Dead-Length Operation

As its name implies, a pullback collet chuck operates by drawing a collet back into the chuck body. As the external taper on the collet makes contact with the internal taper of the chuck, the collet compresses to grip the workpiece. A downside of this simple design is that as the collet draws back, it often starts pulling the barstock or workpiece with it, resulting in variable Z-axis positioning. On first operation jobs, this part movement can usually be remedied with a simple facing cut, but may present more of a problem with secondary operation work or subspindle backworking operations where length features must be accurately controlled. 

Royal Products&#;s dead-length collet chucks (branded as Accu-Length) have been designed to overcome the issue of part movement during the closing operation. With a dead-length chuck, the collet is fixed to the chuck body, and a tapered sleeve pushes forward over the collet to compress it. Because there is no axial movement of the collet with this design, workpiece positioning remains constant. As machine tools become more sophisticated with features such as live tooling, multiple spindles, multiple turrets and in-process gaging, there has been a shift in the market away from pullback-style chucks in favor of the more advanced dead-length model of collet chuck for both main and subspindle operations.    

Low-Profile CNC Collet Chucks

Royal&#;s low-profile CNC collet chucks are characterized by both a distinctive streamlined shape and their use of traditional spring-type collets. Collet choices include 5C, 16C, and 3J models, while the larger models use master collets that work in conjunction with S-type collet pads. 

With this type of workholding system, the collets have a limited range of a few thousandths of an inch, so it is important to closely match the collet bore to the workpiece size for optimal accuracy and grip force. Installing a spring-type collet into a low-profile chuck is accomplished by threading it into the chuck and engaging a key with the collet&#;s keyway. Low-profile chucks are available in both pullback and dead-length configurations. 

Quick-Change Collet Chucks

Most shops are dealing with short lot sizes across the production spectrum. The days of dedicating a machine to a single long run part are, for the most part, over. The capability for quickly changing over a machine from one job to the next is in high demand.

Workholding manufacturers, such as Royal, have developed quick-change collets (Royal brands its Quick-Grip) in response to this demand. These collet chucks are a relatively new innovation in the workholding market and offer a number of advantages over traditional collet chuck designs. (To read an article about the Quick-Grip collet, visit &#;Discovering the Benefits of Collet Chucks.&#;)

Another advantage offered by Royal&#;s quick-change system is the 1/16-inch gripping range of the collets.

One of the most functional features of this design has to do with the way the collet is installed. A special installation tool compresses the rear end of the collet so that it can be inserted into the chuck. Upon expansion, the collet locks into place via a hook and groove arrangement.   

Another advantage offered by Royal&#;s quick-change system is the 1/16-inch gripping range of the collets. This range is possible because of the fact that each segment of the quick-change collet is completely separated from the other segments by a vulcanized rubber spacer. With this design, all collet segments are installed at the same time.

This design allows for parallel gripping of the workpiece over the entire closing range, providing a significant advantage over spring-type collets where the segments begin to form an angle after being compressed by a few thousandths of an inch. The range and parallel grip of the system enables it to easily handle both oversized and undersized barstock, as well as over-the-shoulder gripping of pins, fasteners, and so on. 

One final advantage of the Quick-Grip collet chuck is its extremely short overall length. This provides maximum work zone usage, a benefit that is especially valuable on smaller machines with limited Z-axis capacity. The short length also improves rigidity by keeping the workpiece closer to the machine&#;s spindle bearings.   

Choosing the Right Collet Chuck

Choosing the right collet chuck begins by identifying the spindle nose of the machine on which the chuck will be mounted. All CNC lathe spindle noses follow international standards that clearly define the mounting interface dimensions. Common spindle nose configurations for the North American market include A2-6, A2-8, and 140 mm. 

If you are looking for more details, kindly visit Customized Collet Chuck Size.

Further reading:
Woven Wire Mesh for Vibratory Screens (Definition, Types, ...
4 Tips to Select the Best Rotary Disc Brush Supplier
How to Select a Reliable Rotary Disc Brush Exporter?
Top Scrubbing End Brush Exporters: Your Ultimate Buying Guide
10 Questions You Should Know About Electric Vehicle Battery Technology
How Does Industrial Cleaning Equipment Custom Work?
How to Choose Industrial Disc Brush Custom Options?

The next step is to determine the chuck&#;s required capacity, based upon the size of the parts that need to be held. For bar feeding applications, it is customary to match, or slightly exceed, the machine&#;s drawtube bore size.

Once the spindle mount and required capacity have been identified, the selection can be further narrowed based on specific features and benefits of the various models, including pullback versus dead-length operation; external clearance dimensions; RPM capability; and setup time requirements.

When properly matched to a machine, bar feeder and application, CNC collet chucks provide an economical and effective way of holding material less than 4 inches in diameter. Because collet chucks are generally matched with a bar feeder, some of which have magazine storage and are designed to maintain consistent gripping force at high speeds, they are the workholding choice for lights-out operations. 

Collet chucks&#; compact size improves tool clearance, maximizes Z-axis capacity and aids in rigidity. Readily available round, square, hex, internal expanding and emergency collets offer broad flexibility.

For more information about choosing the right workholding option for a company, read &#;The Many Sides of Workholding.&#;

Photo Credit: Getty Images

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.

Photo Credit: Getty Images

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.

 

The company is the world’s best Customized Collet Chuck supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.