The actual act of replacing dull cutting tools on turning centers is usually pretty simple. Most cutting tools use carbide inserts, which, aside from requiring an awkward position of the CNC operator, can be removed, indexed and replaced with relative ease. However, the physical act of indexing or replacing dull inserts may be just half the task. Depending on the tolerances you expect to hold, your CNC operators may have to consider offset adjustments made during the previous tool’s life (caused by tool wear) and set the offset back to its original value. They may also need to perform some trial machining (also called gauge cutting) for the new/indexed insert if tolerances are extremely close.
One often overlooked reason operators need to trial machine when an insert is replaced is the consistency of the inserts. Frankly speaking, inserts used by most companies are not very consistent. Note that trial machining should not be necessary when inserts are simply indexed and reused, as long as operators correctly reset the wear offset. Because the insert is being reused, there will be no inconsistency. You can easily determine the consistency of carbide inserts because insert tolerance is part of insert specification. It is specified by the third letter in the insert’s identification number. The following list shows standard tolerances (in inches). You can find this list in most cutting tool manufacturer’s technical information.
A—I.C. = ±0.0002, thickness = ±0.001
B—I.C. = ±0.0002, thickness = ±0.005
C—I.C. = ±0.0005, thickness = ±0.001
D—I.C. = ±0.0005, thickness = ±0.005
E—I.C. = ±0.001, thickness = ±0.001
G—I.C. = ±0.001, thickness = ±0.005
M—I.C. = ±0.002, thickness = ±0.005
U—I.C. = ±0.005, thickness = ±0.005
Consider, for example, a popular 80-degree diamond-shaped insert, the CNMG-432. Many companies use this insert for finishing operations, even when holding very tight tolerances. However, this insert will not be very consistent from one insert to the next. Notice that tolerance for the included circle of this insert (specified with the third letter, M) is ±0.002 inch. The included circle is a circle tangent to all sides of the insert. Because this insert has an 80-degree nose angle, the variation on the included circle of this insert (0.004 overall) will almost translate directly to the position of the tool tip position, and in turn, will directly affect workpiece size.
If close tolerances must be held, something must be done when replacing inserts to allow for this inconsistency. Again, many companies will have their CNC operators trial machine with the new insert, which takes time and can be error-prone. If you use a tool touch-off probe, it can be used to determine the new insert’s position (and reset the wear offset), eliminating the need to trial machine. But either way, additional time will be spent during dull tool replacement.
You should at least evaluate the possibility that using a more consistent insert may eliminate the need to trial machine or use the tool touch-off probe when inserts are replaced. (Operator skill is involved, so this may not be right for everyone.) Consider using a CNAG-432 insert. It has a tolerance for its included circle of ±0.0002 inch (0.0004 overall). For some applications, using this insert will eliminate the need for trial machining when inserts are replaced. (Operators must reset the wear offset if adjustments were made during the previous tool’s life.) The added cost of the insert should be easy to justify considering the time, effort and potential for error you’ll save.