Workholding can affect many aspects of the machining process, but it often is not given enough attention. Many shops are content to use whichever workholding system comes with the machine. If it gets the job done, they don't give it another thought'unless they have to. But once a shop does take a closer look at its workholding, it can lead to improvements in several different areas of the part production process.
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One such workholding option is a collet chuck. To get more information, we spoke with Matt Block, a regional business manager with Royal Products. Royal provides a range of workholding options, including several lines of collet chucks.
Collet chucks are traditionally thought of as an effective workholding solution for applications involving bar feeders, small diameters and thin-walled, delicate or irregularly shaped parts. But collet chucks can also provide a range of benefits that apply to shops that produce other types of parts.
Fast Change-over
Almost every shop running more than one job on a machine could benefit from reduced change-over time. According to Mr. Block, reducing setup time for workholding is often overlooked, but possible to accomplish. 'One of the reasons many shops have not pressed into this area is related to part accuracy,' he says. 'Increasingly, part tolerances have become tighter and more difficult to achieve, and the common school of thought dictates that one must freshly cut the jaws on a three-jaw chuck to have any hope of holding 0.001 inch total indicated runout. This is a process that can take 25 to 45 minutes or more, depending on the job and skill level of the person involved. Reducing that time to 10 seconds with a TIR of 0. inch puts the machine in the position of making more parts.'
Royal has a line of collet chucks that is specially designed to provide fast change-over. The Quick-Grip (QG) system includes a special tool that enables users to quickly switch collets. The tool has a series of steel pins that pick up and compress the collet. The user can then position the collet in the machine and release the collet to install it.
Compared with a conventional collet chuck, which requires users to thread in a collet and then find a groove to lock into with a set screw, the Quick-Grip's collet changing tool is not only faster, it helps improve repeatability between operators. 'You don't have the inconsistency of one operator's feel and how well it's set up versus another, so any operator can achieve the same results when it comes to setup time,' Mr. Block says.
Increased Gripping Range
According to Mr. Block, the Quick-Grip collets also have a wider gripping range than traditional collets. This is because the Quick-Grip collets use vulcanized rubber between the metal segments to open and close instead of the spring-type action used by conventional collets. 'That allows for more freedom of movement,' he says.
The wider gripping range of the Royal Quick-Grip collets enables customers to run materials with higher variances and still achieve the benefits of using collet chucks. Mr. Block gives hot-rolled steel barstock as an example. While it is often a less expensive material, the diameter varies more than conventional collet chucks can handle (typically ± 0.010 inch) and the inherent scale on the outside can cause conventional collet chucks to jam up. The ± 0.031-inch gripping range of Quick-Grip can help customers run this material profitably.
Another way shops can take advantage of the wide gripping range is by performing two machining operations using the same collet. If a part's roughed and finished diameters are close enough to be held in the same collet, users can flip the part and clamp the second diameter without having to change the collet. 'This is how some shops move into single-piece flow versus batch and work-in-progress operations,' Mr. Block says.
Accuracy/Rigidity
Collet chucks can also improve the quality of parts being produced. These systems have full contact with the workpiece, as opposed to a three-jaw chuck, which only has three points of contact with the workpiece. The vulcanized rubber in the Royal QG collets provides parallel clamping along the entire portion of the workpiece inside the collet, compared with conventional spring-action collets, which only pinch the part at the end of the collet. 'That's where we get our accuracy and additional rigidity,' Mr. Block explains.
Accurate and rigid workholding is essential to most machining processes. Increasing the accuracy of the workholding system not only enables the production of highly accurate parts, it can also increase tool life. 'If the part is more accurately and consistently placed in the exact same position, the cutting tools have a predictable chip load and can be optimized for longer cutting life,' Mr. Block explains. The increase in cutting tool life can reduce tooling cost per job. An increase in rigidity without sacrificing accuracy also enables users to increase speeds, feeds and depth of cut. Increasing the material removal rates reduces cycle times, which in turn increases throughput.
Reduced Maintenance
Finally, the Quick-Grip collet system requires less maintenance than a three-jaw chuck, according to Mr. Block. 'Three-jaw chucks typically require lubrication at least once per day, and, depending on use, as often as every six to eight hours of operation,' he says. 'Failure to grease a three-jaw chuck can result in a significant reduction in applied clamping force. Most collet chucks do not require the same level of attention, and the Quick-Grip chucks are greased and sealed from the factory.'
The other cause of chuck maintenance is chip contamination. 'Chips are probably the most clever items in any given shop'they always seem to get into exactly where you don't want them,' Mr. Block jokes. Cleaning the chips out of most three-jaw chucks can cause significant downtime on the machine. And this problem is not unique to three-jaw chucks'conventional collet chucks are also prone to having chips packed between the collet segments. When chips build up in the workholding system, the chuck's clamping ability is reduced until the chips are removed. The vulcanized rubber between the segments of the Quick-Grip collets helps prevent chip contamination. 'While nothing is 100-percent impervious to chips in a machine, this system is a significant step above the alternatives when it comes to reducing maintenance,' Mr. Block adds.
Putting It All Together
The benefits of collet chucks are illustrated by one of Royal Products' customers, an Illinois-based equipment manufacturer.
The company has set up cells in different parts of its machine shop to produce different components. Within these cells, one employee is responsible for running several machines, which include CNC and manual lathes, robotic and plasma welding, fabrication, horizontal machining centers, vertical machining centers, and five-axis machines, along with an assembly department. About 40 employees work in the 70,000-square-foot machine shop across two shifts.
Some of the parts produced by the company have bearing journals with tight tolerances. These parts need to fit together with other parts, and in some applications the clearances can be as little as 0.003 to 0.004 inch. Runout is another issue, as it can ruin the functionality of the final product.
All of the shop's lathes had the same three-jaw, soft-jaw, quick-change chucks. Eventually, the chucks on the two lathes used for finishing operations began to wear out and rebuilding them was not an option. Once the company realized that it was going to have to spend money on a new workholding system, it reached out to its distributor, who recommended the Quick-Grip collet chuck system from Royal Products.
Initially, some of the company's employees were skeptical about switching to a collet system. There was some concern about how collet chucks would affect change-over time. But once the employees saw a video of the Quick-Change collet chucks, they became convinced that the system could work for their needs.
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The shop started off by installing the new workholding system on one of its machines for a three-month study. The company was able to see that the collet system was a success, so it implemented it on the second lathe. In the year or so since the company has been using the system, it has seen improvements in several areas.
The biggest benefit the company has seen since switching to the collet system is reduced change-over time. Changing a part on the old system required the operator to adjust the jaws, move the tailstock and occasionally turn the soft jaws so they would stay true. With the collet chucks, the operator only has to unplug the old collet, plug in the new one and adjust the tailstock.
Unlike the jaw chucks, the new system also requires less maintenance, which has contributed to increased efficiency. The shop's employees no longer need to turn the lathes' soft jaws to ensure they remained true, which they described as a 'constant battle.' The soft jaws also required greasing every day, which the collets do not, saving additional time.
In addition to time savings, the company has reported an improvement in part quality. It has also seen less rework and scrapped parts since implementing the new system.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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