Eliminating Program Zero Assignment Tasks With Qualified Setups

A qualified setup is one that can be accurately repeated over and over again. When it comes to machining centers, a qualified setup can ensure that the workpiece location (and, specifically, the program zero point location) will be in exactly the same place every time the setup is made.

Columns From: 12/1/1997 Modern Machine Shop,

A qualified setup is one that can be accurately repeated over and over again. When it comes to machining centers, a qualified setup can ensure that the workpiece location (and, specifically, the program zero point location) will be in exactly the same place every time the setup is made. If the program zero point location is exactly the same time after time, all tasks related to program zero assignment can be eliminated which, of course, will reduce setup time.

Many companies go to great lengths to ensure that their setups are qualified. Generally speaking, this involves making fixtures that are keyed to the machining center table in some way (to key slots, in dowel holes, and so on). But few reap the total benefit of qualifying their setups because they still have to perform certain tasks during setup relative to the assignment of program zero.

If a setup is truly qualified, there is no reason to measure the program zero locations. The same program zero assignment values used on the last setup can again be used the next time the setup is made. At worst, if the programmer cannot predict the program zero assignment values as the program is written (possibly due to a fixture not being made perfectly to print), the program zero setting values need only be measured the very first time the setup is made.

However, some setup people have trouble believing that setups are truly qualified. They just cannot believe that a fixture can be replaced accurately enough. If you have this problem, at the very least, program the machine to go to each program zero position (a special series of commands can be included in your machining program). An M00 program stop can then be given so the setup person can simply confirm that the program zero point location is correct. Your setup people will eventually start trusting that your setups are truly qualified.

One reason people have trouble trusting qualified setups has to do with crashes. After a crash, some machines cannot be perfectly re-aligned (though most can with proper procedures). If an axis is not perfectly re-aligned, all program zero assignments previously made will no longer be correct. But remember that most controls have a feature that allows you to shift the machine's coordinate system with one simple fixture offset entry. One popular control calls this the common fixture offset. To use this feature, you must measure (one time) to find out how much misalignment exists and enter the misalignment values into the common fixture offset.

If setups are qualified, not only can you eliminate the program zero assignment value measurements, you can also eliminate the actual entry of the corresponding fixture offset values. Almost all current model CNC controls let you program fixture offset value entries. One popular command for this purpose is G10. An example of the G10 command for one popular control model is shown in the box.

The "L2" in this command specifies that fixture offsets are being set (as opposed to other offset types). The "P1" specifies the fixture offset number. And the X, Y and Z values are the values that are being entered into the fixture offset. Note that this command can be included in the CNC program and will set the values of fixture offset number one.

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