A defining component of the Swiss-type turning center is its guide bushing and sliding headstock. Originally designed to enable the machine to turn long, slender shafts by supporting the workpiece at the guide bushing as the headstock slides the work past a close-coupled cutting tool, today’s multitasking CNC machine’s guide bushing is both a strength and a weakness of the Swiss-type.
All Swiss-type machines have a guide bushing to support the barstock as the sliding headstock moves the material forward and the guide bushing bearing performs the function of a spindle. Most of the time, the user has to manually adjust the pressure of the bushing on the bar prior to starting the machining cycle—it’s a single adjustment.
Any variance in the bar quality—diameter or concentricity—during a machining run means that the guide bushing will be tighter or looser as the bar is fed into the machine, affecting part quality. That’s why some applications require centerless ground stock to ensure consistent tolerance of incoming stock. That’s an added step in the process and thus, an added cost.
An answer to this “set it and forget it” guide bushing adjustment is a programmable guide bushing available on Traub’s TNL series by Index Corp. It offers the ability to program the CNC to provide a series of guide bushing pressures on the bar based on dynamic conditions.
Instead of simply setting the guide bushing and moving on, the TNL machines use a series of air or hydraulic pressures that allow the guide bushing to consistently support the bar despite variances along the length of the bar. The bushing actually moves depending on the pressure range set within the program of the machine. As the bar diameter varies, the guide bushing follows the variance.
Experience shows that a guide bushing pressure from 2 to 4 bar is optimum for the TNL machine—good enough to support the bar, but not to hold it too tight as the headstock slides. However, when the machine needs to mill or drill or perform heavier turning work without the sliding headstock movement, the program calls for a higher pressure, and the guide bushing clamps the bar as a colleted fixed headstock machine would do. After those spindle stop operations, the program calls for a lower pressure, and the guide bushing is ready for Swiss-type turning again.
Actuation for the programmable guide bushing works like a collet, which is closed by a pusher tube. The guide bushing, however, is not a fixed bushing. Instead, it is designed like a double-cone collet. The double-cone design of the guide bushing/collet assures parallel clamping over the length of the bushing and provides more accurate material support and less wear.
When the pusher tube moves forward, actuated by a higher pressure in the cylinder, the cone on the end of the pusher tube closes the double-cone collet-style bushing. The collet-style guide bushing is a hardened steel component with the collet slots filled with vulcanized rubber to assist in opening the collet when the pressure is released. Although there is always a small gap between the bar and the bushing, with a programmable guide bushing, the gap can be minimized, benefiting the final part quality.
The programmable guide bushing is available on two Traub models with slight differences. The TNL26 uses pneumatic pressure and can be programmed for a range of pressures for machining operations. The pressure can be varied as much as needed. However, the TNL12 is a hydraulic system, and on it there are two programmable settings (“support” and “clamp”) after it is adjusted manually prior to machining. The idea remains the same: The guide bushing is always supporting the bar, and when you need to do heavier work, the bushing clamps the bar like a fixed headstock.
Programming the TNL26 is simple, according to the company. On the CNC, the user calls up a code or function and inserts it into the part program to tell the bushing to exert a chosen pressure at that point. On the TNL12, there are two settings, and the user manually sets the pressure. On this machine, programming the guide bushing is one of the last mechanical operations to be automated on the modern Swiss-type machine tool. With lights-out and untended operations increasing, it is best to eliminate the guide bushing as a variable, according to the company.
Traub has offered this guide bushing option for more than 5 years, and it has been a popular function. In its training classes, the company helps customers to put it to their advantage using experience in deciding what pressure to program for virtually any part process and stock diameter.