August 2012 / Volume 64 / Issue 8|
Importance of tool presetting
By Dr. Scott Smith, University of North Carolina at Charlotte
To most machine tool users, the meaning of endmill presetting is actually closer to endmill length measuring. The endmill is mounted in the toolholder, with the tool length approximately correct. The gage length of the tool is then measured in a presetting station or by a touch-off procedure in the machine tool. The difference between the actual tool length and the tool length used when the NC program was prepared is entered in the CNC as a tool length offset. The control uses the offset to adjust the NC program to correctly position the tool tip.
The U.S. standard for overall length tolerance for endmills is specified in ANSI B94.19-1985. For all endmills except those for heavy-duty use, such as roughing, the overall length tolerance is ±0.062 ", about 1⁄16 ". Many users control the installed tool length with a substantially worse tolerance than that.
Courtesy of BlueSwarf
From the point of view of NC programming, the tool is a cylinder and absolutely rigid. If the cylinder length differs from the length that was used during programming, it is a simple matter to adjust the Z-axis position of the machine and place the tool tip in the geometrically correct position. If the geometry of the intersection of the cylinder and workpiece were all that mattered, this strategy would work. However, the length of the tool affects much more than the geometry.
The cutting performance of the machine-and-tool combination strongly depends on tool length (see my June 2009 and March 2010 columns). Tools set at the wrong length can easily cause chatter. Random procedures in tool length setting lead to random machining performance.
Courtesy of BlueSwarf
Controlling tool length is important, and, fortunately, there are some simple procedures to set the tool length repeatedly with a tolerance at least an order of magnitude better than the ANSI standard.
One strategy is to use a shaft collar (Figure 1). Prior to the mating of the tool and holder, the collar is clamped on the endmill. When the endmill is inserted into the holder, the collar provides a stop for the axial position of the tool. With the collar in place, the tool length is held constant for all types of holder styles, including shrink fit, collet and hydraulic.
In shrink-fit holders, the collar should be left in place until the holder is cool. In collets or hydraulic holders, the collar should be left in place until the collet is tightened or until the hydraulic clamping is achieved. The collar can also be used to accurately position the flutes and improve balance repeatability.
Courtesy of Stop-Loc
A cutting edge can be aligned with the slit in the collar (like a gun sight), and then the slit can be aligned with a witness mark on the toolholder (see Figure 2). This sets the teeth in the same angular position with respect to the holder every time the tool is changed.
The position of the shaft collar on an endmill can be set, for example, using a Stop-Loc device (Figure 3). The sliding stop on the Stop-Loc is set to the desired length of the tool minus the thickness of the collar. The tool tip is placed against the sliding stop, and the collar is attached to the endmill at the end of the device. The collar is held in position with a screw clamp. The collar then provides a stop for the tool when it is inserted in the holder. After the tool is inserted, but before cutting, the collar is removed. Experiments have shown that most operators can use this procedure to quickly and reliably set the tool length with a tolerance of ±0.004 ". CTEAbout the Author: Dr. Scott Smith is a professor and chair ofthe Department of Mechanical Engineering at the William States Lee College of Engineering, University of North Carolina at Charlotte, specializing in machine tool structural dynamics. Contact him via e-mail at firstname.lastname@example.org.
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