The horsepower and accuracy potential of a lathe can be significantly impacted by the design of the jaw chuck used during machining operations. The assumption that all 3-jaw chucks for CNC lathes have similar performance characteristics can result in severe under-utilization of the lathe’s performance capabilities.
There are four major design categories that are the basis for jaw chuck evaluation:
• Method of jaw actuation or closure
• Jaw force loss at high RPM
• Hysteresis (jaw force increase from inertia shift)
• Accuracy or repeatability
Method of Actuation – There are two basic jaw actuation systems — the “wedge or sleeve” system and the “lever” system.
The wedge system is most commonly found on lower cost, non-counterbalanced chucks, often those that are supplied as standard equipment with some CNC lathes. The jaws on such chucks are actuated by the inclined plane principle, where the drawtube is connected to an angled sleeve that slides inside the chuck body against a matching wedge. The matching wedge is connected permanently to the chuck jaws, forcing the jaws open or closed.
There are two basic drawbacks to this style of actuation. In many cases, mechanical advantage is sacrificed in order to gain acceptable jaw travel. Secondly, the wedge design results in a large amount of bearing surface required to actuate the chuck, which can lead to rapid wear of the actuating surfaces by the components. Wear accelerates if the chuck is not kept constantly lubricated, which can be the case in some shop operations. Gripping power also degrades rapidly if the chuck is not kept lubricated.
A preferred method of jaw actuation is the lever type design. In this system, the drawtube is connected to the jaws through a rocking lever that is mounted on pins inside the chuck body. Lubrication is important for lever-operated chucks also, but less sensitive due to the design. The lever system typically has reduced friction and increased mechanical advantage compared to a wedge operated chuck for a given draw bar pressure. In other words, a lever will always have greater actual gripping power at the jaws than a wedge operated chuck, for any given draw bar pressure. The lever system also has reduced internal bearing surface than the wedge type closure, making it less sensitive to lack of lubrication. The greater efficiency of the design results in reduced wear on the operating cylinder, drastically increasing component life. For this reason, a lever actuated chuck system (chuck and operating cylinder) will usually have a longer life than that of a wedge operated system.
Jaw Force Loss
All jaw chucks lose gripping force as spindle RPM increases due to the action of centrifugal force on the top jaws. Inexpensive, low cost chucks can lose a substantial amount of their static (zero RPM) gripping power when run at higher RPM. Because of this tremendous loss of gripping power, full horsepower cuts on a workpiece may not be possible. This can compromise the ability of the lathe to remove material, resulting in increased cycle times and higher part production costs. Damage to the machine could result if the workpiece is forced from the chuck.
It is often suggested by suppliers of non-counterbalanced chucks that this loss of gripping power can be offset by simply increasing the draw bar force to compensate. However, increasing the draw bar force puts additional strain on the actuating cylinder and the chuck mechanism, resulting in a drastically shortened life of the system.
A design that significantly improves on this condition is the counterbalanced chuck. In this design, weights are incorporated into the actuating levers of the chucks at the opposite end of the fulcrum or pivot-point of the jaws. Centrifugal force acts upon this weight just as it does on the top jaws. However, since the weight is at the opposite side of the lever from the top jaws, the upward thrust generated counteracts some of the jaw force loss. Thus, the counterbalanced lever design has substantially more gripping force at high RPM than that of a non-counterbalanced style.
Another critical factor in chuck design is hysteresis. This condition occurs when a chuck is being decelerated to a stop, or the spindle direction is rapidly reversed, a common practice in CNC turning. In this situation, jaw forces actually increase due to the inertia shift that occurs when direction is changed (similar to a person flying over the handle bars of a bicycle when it is brought to a sudden stop).
There are many factors, such as rotational mass of the chuck, weight of the top jaws, etc., that influence the amount of hysteresis generated. On the low-cost “giveaway” chucks that are supplied with many CNC lathes, hysteresis can run as high as 50% over the static gripping power. This can distort parts out of tolerance, or actually crush them in the case of thin-wall parts. When selecting a chuck, the amount of hysteresis is a key factor to be considered since it will influence the capability of the machine’s performance and the outcome of the parts.
The final evaluation of chuck performance is repeatability. The repeatability of a chuck is a measure of its ability to repeat the performance, either from job-to-job or from part-to-part. The industry acceptable standard for jaw chuck accuracy is approximately .001 inch. The majority of the chucks on the market today are built to these standards. There are some specialty chuck builders who can supply chucks with .00005 inch repeatability. However, these chucks are primarily light-duty, second operation chucks and cannot be considered as general purpose, main spindle CNC chucks.
Most models of Hardinge® SURE-GRIP® power chucks have an accuracy (T.I.R.) of .0005 inch and repeatability of .0005 inch—better than most other builders. This makes them ideal for close tolerance turning requirements. A major advantage of the SURE-GRIP power chuck is the configuration of the drawtube that actuates the chuck. The drawtube is configured exactly the same as the corresponding collet for each particular machine. Since Hardinge machines all have collet spindles that do not require a collet chuck or adaptor, it is a simple matter to remove the collet and quickly mount the 3-jaw SURE-GRIP power chuck when needed. The chuck’s drawtube threads directly into the machine’s collet closer, just as a collet. This changeover can be accomplished in ten minutes or less. Other designs can take hours.
The quick-change ability between the collet and jaw chuck, as well as step chuck, Dead-Length® collets and internal expanding collet systems, makes the Hardinge spindle tooling system unique in the marketplace. Such versatility and accuracy are essential to meeting just-in-time (JIT) requirements and more precise accuracy requirements. The SURE-GRIP power chuck is also available for non-Hardinge lathes which do not have collet style spindles.
In terms of component life and efficient use of the lathe’s horsepower, it is apparent that the less desirable style of chuck design is a non-counterbalanced, wedge operated chuck. Unfortunately, this type of chuck is the cheapest to produce and is most often supplied as “standard equipment.” Considering the factors of component life and ability to utilize all of the machine’s power, the most desirable style of chuck would be a lever-operated, counterbalanced design. The SURE-GRIP power chuck is this type and one of the premiere chucks on the market today.
When comparing chucks, hysteresis must also be considered. SURE-GRIP power chucks incorporate design features that limit hysteresis to one of the lowest in the industry. In the past the primary advantage in purchasing a non-counterbalanced chuck was purchase price. The SURE-GRIP power chucks are priced below or close to the price of wedge style chucks, and have many advantages to the user.
For more information on lever-operated, counterbalanced power chucks phone Hardinge at 800-843-8801.