Machining centers traditionally have three linear axes—X, Y and Z. If the machine is equipped with additional axes, they are usually rotary axes (A, B and/or C). An additional linear axis that provides some very unique application possibilities is commonly called the U axis. The U axis allows a cutting tool (commonly a boring-bar-type tool) to move in a direction perpendicular to the spindle centerline. This allows the diameter being machined by the tool to change during operation.
While the drawing in Figure 1 is a rather crude depiction of a U axis tool, the inner workings of this tool are usually quite complex. At least some of the components making up the U axis are actually inside the toolholder. Due to space constraints and depending upon the size of the tool, the U axis travel distance is quite short—commonly under one inch.
The ability to change the diameter being machined during the machining operation has two primary applications. The first is related to holding size when boring holes. As you know, traditional boring bars must be manually adjusted. During setup, the setup person will commonly set the boring bar undersize, try machining a hole, measure the hole and re-adjust the boring bar based upon the measurement. This trial machining of the hole may have to be repeated several times before the hole is on size. During the production run, as the boring bar shows signs of wear, the hole will get smaller and manual adjustments to the boring bar will have to be made. Then when the boring bar eventually dulls and is replaced, the whole process must be repeated. Conventional boring bars almost eliminate the possibility of attaining fully unattended operation.
In this first application category for the U axis, the user is simply gaining the ability to achieve unattended operation for the critical boring operations (and/or making it easier to adjust hole size with attended operation). Since the diameter of the hole being machined is now programmable, hole size can be easily (and automatically) adjusted. The same kind of automatic sizing currently being done for other tools now can be done for boring bars.
The second application for the U axis is more complex and requires much greater U axis travel. A hole contour must be machined in the machining center, though this may look more like a turning center application. This kind of hole is conventionally machined with special form tooling.
In essence, the U axis for this application gives a machining center the ability to machine contours that would normally be machined in turning centers. Several special features are included, like threading.