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When to Choose Spring Clamp in CNC Milling
Three-jaw electric chuck is the standard workpiece clamping device for most lathe users. This chuck has enough versatility and can be used in various turning processes. However, it is not a good fixture for all processing tasks. Spring chuck is a kind of spare workpiece clamping device, similar to claw chuck, which also uses mechanical force to fix parts that need turning. Although the size range of the workpiece provided by the spring chuck is not as wide as that of the claw chuck, the advantages it provides in relation to speed, accuracy and productivity may be extremely important for some processing tasks.
Several factors need to be considered when deciding which fixture works better. For a given lathe processing task, it is necessary to consider all the following factors when evaluating whether to choose spring chuck or claw chuck.
Spindle Load Capacity
The larger allowable weight of lathe spindle is based on bearing load capacity. If the weight of chuck and workpiece combination is too large, the bearing may be overloaded. For those processing tasks that are dangerous beyond the limit, this danger may determine the choice of workpiece fixture. Claw chuck is often heavier than the same spring chuck. Therefore, spring chuck is the appropriate choice when weight control is needed.
Spindle speed
Spring chuck is often a good choice for turning at very high spindle speed, mainly for two reasons:
One reason is related to the quality of chucks. Assuming that the chucks and spring chucks are driven by the same spindle horsepower, thicker chucks need longer time to speed up to the required speed. Long acceleration time will prolong the working cycle and reduce productivity.
Another reason is related to centrifugal force, because it increases with the increase of RPM square value, so this value is very important in the case of high speed cutting. For example, if the spindle speed is doubled, the centrifugal force will be quadrupled. This force pulls the chuck claw away from the center and often reduces the clamping force. But with spring chuck, centrifugal force will not cause obvious influence. Therefore, the clamping force will be more stable in the whole processing speed range.
Machining operation
The spring chuck exerts clamping force on the whole circumference of the part, not only in the selected contact area, so that a good concentricity can be obtained. This is particularly important for the project of secondary processing. Secondary processing needs to consider the accuracy related to primary processing, because the spring chuck has a strong ability to clamp accurately, even if it is clamped. When the claw chuck is used for primary processing, the spring chuck can also be used for secondary processing. The chuck with hollow soft claw can achieve TIR (total reading) repetition accuracy in the range of 0.0006 to 0.0012 inches, while the typical repetition accuracy of spring chuck is 0.0005 inches TIR or better. In order to further improve the secondary processing accuracy, the concentricity of spring chuck can be adjusted during installation.
Workpiece size
Spring chucks are ideal for workpieces with diameters less than 3 inches. Spring chucks restrict the length of the workpiece. In particular, spring chucks restrict the axial (Z-axis) travel range of the machine tool because they are longer than chucks. When the processing length of the workpiece almost needs the whole available travel of the machine tool, it is probably necessary to use the claw chuck.
Processing batch size
Spring chucks are suitable for large batch and small batch processing tasks.
In small batch and multi-task processing occasions, the advantages of spring chucks are related to product conversion time. It takes about 15 to 20 minutes for the claw replacement of standard claw chucks, 1 minute for quick replacement of claw chucks, and 15 to 20 seconds for quick replacement of spring chucks. It saves when products change frequently. The cumulative amount of time is considerable.
When the processing batch is large, the time saved related to clamping can also be accumulated. The opening and closing time of spring chuck is less than that of claw chuck. By reducing the non-cutting time from one workpiece to the next, the processing cycle time can be reduced.
Size range of workpiece
Part of the reason for the faster opening and closing of spring chucks is that the driving stroke of spring chucks is shorter, and the scope of workpiece size applicable to spring chucks is more limited than that of claw chucks.
Subspindle condition
Turning machines equipped with secondary spindles are often used for various mass processing. In these applications, spring chucks can significantly save processing time. They can process all facets of parts in a working cycle. These machines are often combined with bar feeders to realize unattended production and continuous processing of parts. In these applications, the time saved for a workpiece may be very small, but in the whole production process, the time saved for each workpiece is multiplied by the number of workpieces processed, and the accumulated time saved is considerable.
Chuck tool library
It is also important to consider the third option when choosing an appropriate workpiece clamping device between the claw chuck and the spring chuck. With permission, it may be cost-effective to retain two fixtures from one to the other. Change from claw chuck to spring chuck or vice versa, usually no more than 20 minutes. Claw chuck can be retained on the machine to deal with uncertain parts. But when machine tools process large quantities of workpieces, or several batches of parts of the same size, the productivity gained by using spring chucks is greatly increased, which greatly exceeds the production caused by the time spent in replacing fixtures.
Machining operation
The spring chuck exerts clamping force on the whole circumference of the part, not only in the selected contact area, so that a good concentricity can be obtained. This is particularly important for the project of secondary processing. Secondary processing needs to consider the accuracy related to primary processing, because the spring chuck has a strong ability to clamp accurately, even if it is clamped. When the claw chuck is used for primary processing, the spring chuck can also be used for secondary processing. The chuck with hollow soft claw can achieve TIR (total reading) repetition accuracy in the range of 0.0006 to 0.0012 inches, while the typical repetition accuracy of spring chuck is 0.0005 inches TIR or better. In order to further improve the secondary processing accuracy, the concentricity of spring chuck can be adjusted during installation.
Workpiece size
Spring chucks are ideal for workpieces with diameters less than 3 inches. Spring chucks restrict the length of the workpiece. In particular, spring chucks restrict the axial (Z-axis) travel range of the machine tool because they are longer than chucks. When the processing length of the workpiece almost needs the whole available travel of the machine tool, it is probably necessary to use the claw chuck.
Processing batch size
Spring chucks are suitable for large batch and small batch processing tasks.
In small batch and multi-task processing occasions, the advantages of spring chucks are related to product conversion time. It takes about 15 to 20 minutes for the claw replacement of standard claw chucks, 1 minute for quick replacement of claw chucks, and 15 to 20 seconds for quick replacement of spring chucks. It saves when products change frequently. The cumulative amount of time is considerable.
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