Payback time is the primary component applied by accountants in investment justification formulas. A payback period of one year is considered by most people to indicate a very cost-effective investment, less than one year payback is a "no-brainer" decision.
Most companies become preoccupied massaging investment numbers that focus on large-ticket items such as new machinery, and understandably so, because these items represent the major portion of capital equipment budgets. Easily overlooked however, are relatively low-cost accessories that can significantly improve machine output, and in some cases, even double machine output.
W. A. Whitney Co. is one of the astute companies that long ago recognized the contribution accessories can make to increasing machine output. This was demonstrated when they purchased chucks, indexers and barfeeds to upgrade the efficiency of machinery they already own, which in some cases averted the need to purchase new machines just to meet production increases. Their experience with quick-changeover chucks is a prime example.
In the past few years W. A. Whitney has purchased six such chucks for their CNC lathes. Manufactured by SMW Systems, (Santa Fe Springs, California) and called Ultimate CNC Chuck, these state-of-the-art workholding devices have solved their major problem, and one that also faces other companies today: increased changeover downtime caused by J.I.T.-related short runs. States Whitney's Roger Fisher, "our batch sizes have diminished to a range of from 1-20 pieces per run. Accordingly, our changeovers have increased to an average of three per eight-hour shift."
The quick changeover chuck offers two advantages that are crucial to reducing changeover downtime: First, they allow all three jaws to be changed, adjusted or reversed in less than one minute. Equally important, they eliminate the need to rebore remounted soft jaws. Typically, the time saved in a hard-to-soft jaw changeover amounts to 30-45 minutes. Multiply these savings by three and there is a very substantial increase in machine output in an eight-hour shift.
The jaw quick-change features are attributed to a special jaw engagement and release mechanism operated by a simple hex key that is inserted into the chuck body. Turning the key in the clockwise direction disengages the jaw from the actuating mechanism which allows the operator to remove (slide) the jaw from its guideway, or reverse or reposition the jaw in accordance to the requirements of the next operation. When the new jaw is inserted or the existing jaw is repositioned, turning the hex key counter-clockwise re-engages the jaw with the actuating mechanism.
Aside from the quick changeover advantages, this design provides a measure of safety specific to this chuck that helps prevent "loose-jaw" accidents: Once the hex key is inserted and rotated for any reason, it cannot be removed until it is returned to a position that ensures that the jaw is engaged. Thus, when the key is removed and the machine spindle starts to rotate, there is no danger of a loose jaw being propelled out of the chuck body by centrifugal force causing potential harm to the operator or damage to the equipment.
The second chuck design feature that eliminates the need to rebore remounted soft jaws also centers around the jaws, jaw mounting and actuating mechanism. The accuracy tolerances to which these components are manufactured allows previously bored soft jaws to be remounted and the clamped workpiece to rotate concentric to the spindle axis within 0.001" TIR. With few exceptions this degree of accuracy is sufficient to eliminate the need to rebore. In the experience of W. A. Whitney, Roger Fisher states, "the runout on remounted soft jaws falls well within our tolerance requirements. Eliminating reboring downtime is the major contribution to reducing changeover downtime."
The SMW chuck offers other non-changeover related advantages. The rigid design of the chuck body minimizes deflection in the chuck body caused by centrifugal force. The result of this feature is a relatively flat grip-force curve. As rotating speed increases, many chucks lose a significant amount of grip force due to deflections within the chuck body. To avoid the workpiece slipping in the jaws, at high machine speeds and feeds, the work must be overclamped which can cause distortion in the workpiece and subsequent loss of accuracy. To avoid this problem, feeds and speeds are reduced which in turn reduces machine output and drives workpiece-cost up.
Another operating advantage involves the design of the chuck actuating system. It allows two different grip forces to be applied to the workpiece in the same machining cycle without having to release the workpiece. This allows high grip-force roughing work and low grip-force finishing work to be performed in the same cycle without stopping to rechuck the workpiece.
Collectively, these advantages have allowed W. A. Whitney to pay for the chuck in less than six months through elimination of changeover downtime and the permitting of higher machine feeds and speeds, the net effect of which is significantly higher machine output. Similar productivity increases have been realized at W. A. Whitney on lathes that employ SMW hydrodynamic barfeeds. These systems eliminate second-end changeover and rechucking the workpiece by allowing complete machining of the workpiece in one setup. Further, SMW high-performance indexers have improved the company's vertical machining center output by allowing the machining of multiple sides in one set up, thus reducing the number of setups required to produce multi-sided workpieces.
The examples in this story point out the need to take selection of machine accessories seriously. Considering that machine speeds are close or at their maximum potential, major productivity increases may only come from elimination of changeover downtime.