Tool-change specifications are another aspect that must be examined. They can represent a significant amount of cycle time, and it’s non-productive time because you’re not in the cut. If you have 50 tools in a part, you’re adding more and more non-productive time every time that tool is changed out. How you analyze this is by examining the interplay between various machine elements.

The rapid, acceleration, and deceleration rates will influence tool changes because you need to get from where you’re machining, to the tool changer, and back to the workpiece. Spindle spool-up and spool-down can’t be overlooked because every tool change requires the spindle to stop and restart. There are also many mechanical elements that must happen for tools to change, like the spindle drawbar and tool-changer components, which can slow down the process.

The bottom line is, tool-change speeds dramatically impact cycle time, and can increase cycle times and raise the cost per part, especially in complicated parts that require many tools.

Tool changes in the 1960s were often 30 seconds, tool-to-tool, and today many machines are changing tools in approximately two seconds.

Again, there are two tool-change specifications that must be examined. One is tool-to-tool, which is actually more a measurement of the physical elements of the tool-changer mechanism because there is no consideration for the axes’ movements.

The second measurement is chip-to-chip or metal-to-metal, which also takes into account the axes’ movements to and from the tool-exchange point. It also considers spindle impact depending on the specification. If you’re running a 20,000- or 30,000-rpm spindle, you’ll have to stop and restart the spindle before you engage. This measurement includes the tool-to-tool spec; however, if you think about it, chip-to-chip is actually more reflective on the machining non-cut time. It incorporates all the real-world elements of a tool change.

Once again, many standards exist to measure tool changes. There are three basic ways. The ISO method starts and stops the test at half the maximum rpm, utilizes half the machine axis stroke in X, Y, and Z, and is repeated 10 times to get an average value. The second way is the MAS method, which starts and ends at 1,000 rpm, utilizes the bottom 25 persent of Y and Z and X is centered, which creates more run-time, and finally it takes one measurement, not an average. There’s also a builder method that blends the two. It runs at half the maximum rpm, uses the center of X, Y, and Z stroke, and takes one measurement.

So, again, you’ll have to know if you’re looking at an ISO, MAS, or custom method of measurement, and whether it’s tool-to-tool or chip-to-chip.

If you plot out the results of the comparable machines, you can see that MAS, ISO, and custom methods often don’t compare to one another. Therefore, you can’t match them against each other to figure out which tool change is faster.

Another factor hidden that is rarely taken into account include tool “search” time. Particularly in a production environment with short tool times like spot drilling or single feature tools, your cycle times can get much longer if the machine is spending a lot of time searching for the new tool. You could possibly spend more time changing out the tool than actually doing the cutting, so it must be kept in mind. Many don’t even take that into consideration, and it can greatly impact productivity.