Programming Efficient Tool Changes

Though machining center builders have come up with some pretty unique devices to automatically change tools, the programming words related to tool changing are relatively consistent from one machining center to the next.

The "T word" is commonly used to rotate the machine's magazine (or tool storage device) to bring a tool into the ready or waiting position. An M06 exchanges the tool in the spindle with the tool in the ready position (though on some machines, the T word also does this). And an M19 rotates the spindle to align the key in the toolchanger arm with the keyway in the toolholder. Though most automatic tool changing devices are easy to program, there are some techniques you can use that will minimize tool changing time.

• Where is the tool change position?—First of all, machine tool builders vary when it comes to where you must send the machine prior to commanding a tool change. A few can change tools anywhere as long as the tool change will clear the workholding setup. If so, you can dramatically minimize tool-changing time by keeping the tool change position close to the workpiece being machined. This minimizes the rapid movements required to get to the tool change position.
  However, most machining center builders require that you send the machine to its reference position in at least one axis in order to line up the toolchanger mechanism with the tool in the spindle. With this kind of machine, be sure to limit the number of moving axes only to those required. Most vertical machining centers, for example, require that you send only the Z axis to its reference position. Additionally sending X and Y to their reference position would be wasteful unless there is some obstruction that would cause interference during the tool change.
• Get the next tool ready—Many machining centers allow you to rotate the magazine to get the next tool ready while the tool in the spindle is machining the workpiece. This saves magazine rotation time, since the next tool will likely be ready when the spindle tool is finished. With most machines this simply involves including a T word in the program after each tool change to specify the tool number for the next tool.
• Orient the spindle on the tool's retract to the tool change position—As stated, M06 is the word that causes the tool change. It will perform all motions (including spindle orientation) related to tool changing. However, spindle orientation takes time, usually 1-3 seconds, and waiting for the machine to retract to its tool change position before the spindle begins orienting is wasteful. You can include an M19 with the machine's motion to the tool change position, which will save 1-3 seconds per tool change.
• Load tools sequentially for short cutting operations—All current model toolchangers are random access. You can select any tool at any time within the CNC program. While this is a great feature, there are times when selecting tools sequentially (tools next to each other in the magazine) will minimize cycle time. If, for example, you have a center drill that machines but one hole in aluminum, it is likely that it will finish its operation and the machine will be back at the tool change position before the magazine can rotate to the next tool.

By the way, this can also cause inconsistency in cycle time from one time a job is run to the next. One time tools are placed into the magazine in an efficient manner (right next to each other). The next time the job is run tools may be less efficiently placed in the magazine. This is one good reason to document cycle time right in the CNC program with a message. The setup person can easily time the current cycle to confirm that it is running as efficiently as it should.

• Look for machines that allow tool replacement during automatic operation—Note that many machine tool builders completely lock out operation of the toolchanger magazine during automatic operation. While this is done for obvious safety reasons, there are times when having access to the magazine for tool replacement can increase productivity. If, for example, a tool is dull or if the setup person would like to start loading tools for the next job, doing so during automatic operation can minimize machine downtime. More and more machine tool builders are providing an interlock for the toolchanger magazine that allows safe manual operation of the toolchanger magazine even during the machine's automatic operation.