Machining centers, of course, have automatic tool changing devices to automate the tool changing process. Current models boast very fast tool changing times and you may be quite satisfied with tool changing time. But for older models, and if you're trying to minimize cycle time, there may be some things you can do to reduce tool change time. We offer a few suggestions.
From a cycle-time-reduction standpoint, tool changing is a non-productive on-line task. That is, it does not further the completion of the workpieces (only machining operations do this), and it adds to the time it takes to complete a production run. Anything that reduces tool changing time will, of course, make the machine more productive. When you consider how many tool changes a machining center will make during its lifetime, minimizing tool change time should be an important priority for any CNC-using company
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Spindle orientation
Most automatic tool changers require that the keyways in the cutting tool shank be aligned with the keys in the tool changing arm before a tool change can be made. This angular spindle position is called the spindle's orient position. Spindle orientation is done as part of the tool change command (M06).
Spindle orientation takes time (one to five seconds, depending upon the machine). Don't wait until the machine is at the tool change position before performing the spindle orient. A special M code (M19) can be used to cause the spindle to rotate to its orient position during the motion to the tool change position. Include an M19 in every command that sends the machine to its tool change position. If the machine's Z axis zero return position is the tool change position, as it is for most vertical machining centers, this command will cause the spindle to begin orienting as soon as the machine starts to move:
N105 G91 G28 Z0 M19
And again, this command will save time because the spindle will begin rotating to its orient position during the motion to the tool change position. Depending upon the machine's rapid rate, the distance to the tool change position, and how fast the spindle orientation occurs, it is possible that you can make spindle orientation time internal to the motion time. This, in effect, eliminates spindle orientation time from the program (possibly saving one to five seconds per tool change).
Note that machine tool builders vary when it comes to how they handle M codes. Given the command above, the vast majority of machines I've seen will begin to rotate the spindle to its orient position as soon as the Z axis starts to move. But I have seen some machine too builders that do not interface the M19 in this fashion. Instead, the spindle may orient before (or after) the motion occurs. This, of course, will not save any time.
If you happen to have one of these poorly interfaced machines, contact your machine tool builder and tell them that you want spindle orientation to occur during motion. Given the amount of time that can be saved over the machine's lifetime, don't take no for an answer. With today's programmable logic controllers, this should be pretty easy to do. You may even find that you have three special M codes that control when M code functions occur, before, during, or after motion commands.
How far does the machine have to move?
With most machining centers, the automatic tool changer arm is in a fixed location - and the machine must be sent to a special position in X, Y , and/or Z in order to make a tool change. With vertical machining centers, for example, the tool change position is commonly the Z axis zero return position. For horizontal machining centers, it is commonly the Y and Z axis zero return position.
Unless special interference problems exist with the job, be sure to minimize the distance the machine must move in order to make a tool change. With a vertical machining center, send the machine to (only) the Z axis zero return position. Sending the machine to the X and/or Y axis zero return position will be a waste of time (again, unless interference problems exist).
There are vertical machining centers available with which the tool change mechanisms move along with the Z axis. This means they can change tools in any position when a tool change is commanded. With these machines, you must be concerned with the lengths of cutting tools when commanding tool changes, but you can minimize the distance a machine must move in order to change tools.
Should you place tools in the tool changer magazine in sequential order?
Today's machining centers have random-access tool changers. This, of course, means that tools can be placed in the tool changer magazine in any fashion, yet they can be accessed in any order. This is a wonderful feature, allowing great flexibility in tool placement. But watch out for a time-wasting trade-off.
This is especially true for machines that have a single arm tool changer. With these machines, the cutting tool just used must be placed back in the tool changer magazine before the magazine can rotate to the next tool. Magazine rotation, of course, is not instantaneous. Magazine rotation time is a non-productive on-line task that can be minimized if tools are placed in the magazine in a sequential manner.
Machines with double-arm tool changers can have the magazine begin to rotate to the next cutting tool station while the current tool is working on the workpiece. But the program must be written in a fashion that allows this. Be sure to include a magazine rotation word (a T word with most controls) in the program close to the beginning of each tool:
O0001
N005 T01 M06 (Place tool one in spindle)
N010 G54 G90 S500 M03 T02 (Among other things, bring tool number two to the ready position)
Even with double-arm tool changers, of course, the tool changer magazine cannot rotate instantaneously. The larger the magazine, the slower it will move and the longer it will take to rotate. For machines having large magazines, magazine rotation time will be substantial. Consider a magazine that can hold one hundred tools or more. It may take over thirty seconds for the magazine to rotate to a tool that is on the other side of the magazine.
If a machining operation for a given tool takes a long time to complete (over thirty seconds for the example above), this will not create a time-wasting problem. The magazine will have rotated to its next position before the machine will be ready for its next tool change. But consider a short machining operation. Say a center drill must machine but one hole in a soft material. The tool may compete its operation in less than five seconds - and the machine may be back at the tool change position long before the magazine has completed its rotation to the next station.
This is one time when placing cutting tools in the magazine in close proximity will minimize tool changing time. Note that it will also ensure that cycle time will be consistent from one time a job is run to the next.
How heavy are your tools?
The actual act of tool changing requires a synchronized series of actions. The tool changer double-arm rotates to grab the tools in the ready station and spindle. The tools are a released, and the double arm pulls the tools out. It then rotates 180 degrees, bringing the tool in the ready station to the spindle and vise versa. The arm then pulls the tools into the spindle and ready station. Finally, the tools are clamped and the double arm rotates away.
We think of these actions as fixed and unchangeable - and with some machines, they may be. But remember that machine tool builders always strive to have their machines deal with the worst case scenario. If a machine tool builder claims that the automatic tool changer on a given machine can change a fifty-pound tool, they have designed the automatic tool changer mechanism accordingly - including the speed related to each action of the tool change.
Maybe you have a machine that has been designed to change a fifty-pound tool (or more), but your heaviest tool is less than twenty pounds. It is possible that the machine tool builder can speed up your machine's automatic tool changer to match your heaviest tool. Machine tool builders vary when it comes to how willing they will be to do so, but it sure doesn't hurt to ask.
