Quick Jaw Change Chucks – SMW Autoblok KNCS series

July 20, 2010

The Quick Jaw Change chuck gives the machinist the ability to change chuck jaws in one minute or less. Changing chuck jaws quickly results in a reduction is spindle idle time and more revenue generated by the machine tool. The KNCS-N or NB can easily pay for itself in less than a year.

The advantages of the QJC chuck are as follows:

  • Change jaws quickly, rapid setup
  • Great variety of jaw types available
  • Accuracy
  • Centrifugal force compensation
  • High gripping force
  • High – low chucking
  • Ideal for small batch production

The jaw change mechanism on the KNCS-N (NB) uses an external key inserted into the chuck body that disengages the jaw from the internal drive mechanism in the chuck, allowing the jaw to be removed from the chuck body.


Variety of jaw types available

The KNCS-N chuck has the greatest variety of different types of jaws available compared with any other chuck. The KNCS-NB has master jaws with differing serrations designed to use any inventory of existing top jaws already in stock. The KNCS-N offers a large variety of soft-jaw and hard-jaw types and sizes. In addition, the KNCS-N has available monoblock soft and hard jaws. The monoblock jaws offer greater accuracy (no bolts to attach the top jaw to the master jaw) and lower cost.


The wedge-bar drive of the KNCS-N (NB) gives these chucks much better accuracy than standard wedge hook chucks. The best accuracy possible on a wedge hook chuck is generally given as T.I.R of .001”. The KNCS-N can operate in the T.I.R. range of .0002” to .0004”. As the chuck size increases from 16” to 20” to 24” diameter, the accuracy gradually degrades from .0008” to .0012” on the largest size. It is not possible to achieve this level of accuracy on a wedge hook chuck; the wedge/master jaw connection is not accurate enough to provide this level of precision.

Centrifugal force compensation

One of the characteristics of the wedge-bar drive on the KNCS-N (NB) is centrifugal force compensation, accomplished without counterweights. A counter-centrifugal wedge-hook chuck uses counterweights to offset the loss of grip force caused by centrifugal force. These counterweights can deform thin-wall or delicate parts at spindle stop.

A standard 8” wedge hook chuck will turn at 5,000-rpm maximum and lose 50% of its static grip force at this speed. An 8” QJC chuck will turn at 6,000-rpm maximum and lose only 30% of its static grip force. This represents a 20% increase in operating speed with much less loss of grip force compared to a standard chuck. This makes the QJC chuck the ideal tool for machining thin wall or delicate work pieces since the operation can begin with much lower static gripping force, thereby reducing the distortion on the part.

High Gripping Force

An 8” QJC chuck produces 22,480 pounds of static grip force. An 8” standard wedge hook chuck produces 24,730 pounds of static grip force. At maximum rpm, the QJC chuck is exerting 15,736 pounds of grip force against the part, while the wedge hook chuck is applying 13,488 pounds of grip force through all three jaws. The combination of higher speed and greater grip force at maximum rpm allow the QJC chuck to operate in applications with much greater metal removal rates than a standard chuck is capable of holding.

High – low chucking

One method of producing thin-wall or delicate parts is referred to as high – low chucking. The part is held at high pressure for the roughing operation. The clamping pressure causes distortion in the part, the machining removes that distortion and makes the part round. If the work piece were released at this point, it would spring back to its shape with a tri-lobe distortion caused by the gripping force. In high – low machining, the turning center is brought to full spindle stop after the roughing operation. A series of electronic and hydraulic controls reduce the clamping pressure to a very low level, eliminating the distortion caused by the high pressure clamping. This change in clamping pressure is accomplished without opening the chuck jaws or removing the work piece from the chuck. This maintains a consistent datum on the work piece and results in a very accurate finished part. The finishing turning is done at the low pressure setting, completing the part without any distortion.

High – low chucking cannot be done using a wedge hook chuck. The wedge locks on the master jaw tongue and the only way to change the clamping pressure applied to the part is to open the jaws and re-clamp the part. This will cause a change of position in the part and a loss of the clamping datum along with a great increase in the scrap rate.

Small batch production

In addition to its advantages for machining at high rpm and high metal removal rates, the QJC chuck is ideal for small batch production and prototyping work. The flexibility of the jaw change mechanism and the variety of jaw types available make the QJC chuck the most versatile small batch chuck available.