Clamping

Achieving low deformation in workpiece clamping

| Author / Editor: Markus Michelberger / Rosemarie Stahl

Whether they are round, square or cube-shaped, made of steel or cast iron – the doubled square pole plates in this application facilitate a highly flexible and at the same time deformation-free workpiece clamping.
Whether they are round, square or cube-shaped, made of steel or cast iron – the doubled square pole plates in this application facilitate a highly flexible and at the same time deformation-free workpiece clamping. (Source: Schunk)

Conventional chucks or face plates reach their limits in cutting applications where thin walled parts are concerned. the parts are deformed in the clamping process, shaping and geometric tolerances are difficult to adhere to and the interference contours of the clamping devices restrict the accessibility. However, there are alternatives.

As is so often the case for low-deformation workpiece clamping: there is no one-size-fits-all solution. The requirements of the workpieces, type and frequency of the machining as well as the integration into the process change vary so severely that an individual observation at the user level is always worthwhile. The following questions are primarily important here: what parts are to be machined in what batch sizes? How flexible must the machine be able to be used? What shape and position tolerances must be adhered to? How will the parts spectrum develop in the future? Only when these and other aspects have been clarified can the optimal clamping solution be defined.

The following clamping technologies are explained in this article:

  • Increasing the number of contact points: Pendulum jaws
  • Compensation jaw with centrifugal force and vibration compensation
  • The most cost-efficient alternative: Plastic jaws
  • Oscillating mounted 6-jaw chuck for high repeat accuracy
  • Radial pole technology for interfering contour-free machining
  • Square pole technology for increased flexibility
  • Hybrid chucks: A combination of radial pole technology and a centering chuck

Pendulum jaws increase the number of contact points

For pendulum jaws, users take advantage of the number of clamping points in order to minimize deformations on the workpiece. This way, with identical clamping force and a doubling of the clamping points, the achievable roundness accuracy increases in practice by a factor of 10. Usually, pendulum jaws are made of a movably mounted pendulum body with two clamping inserts, which are mounted on a supporting jaw with the aid of a bearing bolt. For demanding applications, there may be four or six clamping inserts, either soft or case-hardened.

Compared to a conventional set-up in the 3-jaw chuck, a 6-point pendulum improves the calculated run-out accuracy by factor 17.
Compared to a conventional set-up in the 3-jaw chuck, a 6-point pendulum improves the calculated run-out accuracy by factor 17. (Source: Schunk)

As pendulum jaws are mounted like one-piece top jaws, a conventional 3-jaw chuck can be converted into a 6-point clamping in no time at all. As the jaws are adjusted to the workpiece, they can compensate for form tolerances within a certain range, for instance with cast bodies. It is beneficial if such pendulum jaws can be fixed for finish machining or for clamping pre-turned surfaces. Furthermore, the jaws should be as light as possible so as to achieve faster speeds. Calculation programs can be used for calculating what compensation is required for achieving specified concentricity tolerances. Primarily for small tolerances, experience pays off, because with matured special solutions, enormous effects can be attained. But there are also restrictions: pendulum jaws are comparably expensive, they wear out easily, are sometimes quite heavy and restricted when it comes to flexibility.

Compensation jaw with centrifugal force and vibration compensation

Hydraulic compensation jaws set a new benchmark in hydraulic compensation jaws for low-deformation workpiece clamping. They combine a balancing workpiece clamping with centrifugal force compensation, vibration-damping characteristics and micron-precise concentricity. The integrated oil chamber system is a central feature, above which two or more oscillating clamping elements for clamping rough or finished parts are mounted. As they individually adapt to the workpiece, form tolerances of cast bodies, for example, are reliably compensated. Compared to conventional 3-point clamping, the number of clamping points is increased, which also lowers the deformation of the workpiece and the obtainable roundness is significantly increased. At high speeds, the hydraulic clamping system automatically ensures centrifugal force compensation so that the clamping force is always reliably maintained.

Pendulum jaws like this 12-point pendulum are available as a special solution for I.D. or O.D. clamping.
Pendulum jaws like this 12-point pendulum are available as a special solution for I.D. or O.D. clamping. (Source: Schunk)

In order to minimize workpiece deformation, the clamping force can be considerably reduced in comparison with previous solutions without restrictions on process reliability. For maximum precision on the component, the concentricity can also be adjusted micron-precisely on the chuck jaws. Additionally, the vibration-damping characteristics of the oil chamber system result in a better surface quality of the workpiece and the tool life. Hydraulic compensation jaws are suitable for low-deformation O.D. clamping of rough and finished parts and are available for all lathe chuck sizes and serration types.

Plastic jaws are the most cost-effective alternative

Hydraulic compensation jaws combine a high-precision, low-deformation workpiece clamping with vibration-damping characteristics. Even at high speeds, the clamping force is completely retained.
Hydraulic compensation jaws combine a high-precision, low-deformation workpiece clamping with vibration-damping characteristics. Even at high speeds, the clamping force is completely retained. (Source: Schunk)

Special jaws made of glass fiber reinforced plastic are an often underestimated problem solver. Their high coefficient of friction of 0.3 to 0.4 as well as a large angle of wrap ensure that high machining forces can be transmitted even with low clamping forces. Here, the support structure of the aluminum supporting jaw ensures stability. As a result of the low weight, only low jaw centrifugal forces are created during turning operations. As a result, plastic jaws are also suitable for high machining speeds up to 6,000 rpm. In addition, they do not leave any clamping marks even on ground parts or parts with a surface finish. Exchangeable clamping inserts also make the solution attractive from cost aspects as well.

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