Sometimes you may be able to go ahead and machine the first part of a run on a machining center without having to take trial cuts. When tolerance bands are large, operators or setup people can afford to run the program with the confidence that the cuts will come out somewhere in their tolerance bands and the workpiece will not be scrapped. Consider, for example, having to counterbore a hole with a +0.010-inch depth tolerance. It's quite likely that a setup person or tool setter will be able to measure and enter the tool-length compensation value in an acceptable manner, thus eliminating the need to trial machine.
After machining the first workpiece, most setup people will measure what each tool has done and make a sizing offset adjustment to make the tool cut to the mean value of its tolerance band for the next workpiece. In this manner the operator can begin the production run with all tools cutting right to size. After machining the first workpiece with our counterbore, for example, say the hole comes out 0.004 inch deeper than the mean value of the hole's tolerance band. While the workpiece is acceptable, since it falls well within the +0.010-inch tolerance, most setup people will adjust the tool length compensation offset value by 0.004 inch in the plus direction to allow the tool to begin the production run cutting right to size.
As a surface's tolerance band shrinks, however, so does the confidence that a tool will cut in an acceptable manner on the very first workpiece. At some point, it will become necessary to trial machine if the first workpiece is to be acceptable. Trial machining involves making an adjustment, usually to the tool length or cutter radius offset, which ensures additional stock will be left on the surfaces machined by a tool. Once the tool has machined, the setup person can stop the machine and measure what the tool has done. In this manner, he will know precisely how much to adjust the offset in order to make the tool cut in an acceptable fashion, usually right to the mean value of the tolerance band.
Holding size is a somewhat subjective topic because the size of tolerance bands varies from company to company. What one company may consider a tight tolerance, another may hold every day. The skill level of the people involved as well as the quality of setup-related measuring devices also have a lot to do with when trial machining is required. If, for example, the tolerance band for our counterbored hole depth is plus or minus 0.005 inch, some setup people will say they can still measure the tool length compensation value in an accurate enough manner to machine the first workpiece to size while others will say they cannot.
Since trial machining takes time, and since it's part of setup time (we consider running the first good workpiece and getting it to pass inspection ), anything you can do to eliminate the need for trial machining will, of course, reduce setup time. In some cases this may be as simple as improving the skill level of the people involved or improving the quality of your tool-setting devices. So don't be too quick to give up on the goal of cutting the first workpiece without trial machining. A little ingenuity and up-front work could save a lot of setup time.
When trial machining is required, you should do everything you can to make it as quick and easy as possible for your setup people to perform. Left to their own devices, most setup people will adjust the related offsets, allow the tool to cut, stop the machine, measure what the tool has done, readjust the offset, and rerun the entire tool. While this method is almost fail-safe (though it doesn't allow for tool pressure differences), trial machining in this fashion is very time consuming.
Optional block skip (also called block delete) can often help. Use the slash code and specify just enough commands to allow setup people to take a measurement. They commonly do not have to run the entire tool. If the optional block skip switch is left off, trial machining and an appropriate movement to a convenient measuring position will be done. At this point the machine will stop for a measurement. If the switch is turned on (as it would be during normal production), no trial machining will be done.
As an example, consider having to counterbore 15 holes having a tight tolerance. If trial machining, it would be wasteful to wait until all fifteen holes are done before the measurement is taken. Consider the program segment shown in the box which trial machines but one hole.
N150 T05 M06 (Counterbore)
N155 G54 G90 S900 M03 T06
N160 G00 1.0 Y1.0
N165 G43 H05 Z0.1 M08
/N170 G82 R0.1 Z-0.25 P500 F4.0
/N175 G80 M09
/N180 G00 Z4.0 (Move up to clear obstructions)
/N185 X4.0 Y4.0 (Move to convenient measuring position)
/N190 M00 (Stop for measurement. Depth should be 0.25)
/N195 G00 X1.0 Y1.0 M03
/N200 G43 H05 Z0.1 M08 (Invoke new offset value)
N205 G82 R0.1 Z-0.25 P500 F4.0