Tool Pressure Deviations During Touch-Offs

How to determine program-zero assignment during touch-offs.

Part of the process of setting up a CNC turning center is determining program-zero assignment values. The setup person must marry the programmed coordinates for each cutting tool to the tool’s actual position in the turret. This can be done in several ways, including with the help of a tool touch-off probe or by skim cutting an actual workpiece. This setup step is commonly referred to as touching off the cutting tools.

One problem with tool touch-offs is that the touch-off measurement is a static measurement. That is, it does not take into consideration the full dynamics of the machining operation that will be performed by the cutting tool when it machines an actual workpiece. While the skim-cutting method does require actual machining, the cutting conditions during skim cutting will not match the cutting conditions used to machine the actual workpiece. Also, if a tool touch-off probe is used for determining program-zero assignment values, again, no cutting is even taking place when program-zero assignment values are determined.

This difference in cutting conditions from tool touch-off to machining a workpiece, in turn, results in a difference in tool pressure between the two processes. A difference in tool pressure will result in a difference in sizing. While skim cutting during setup (or while using the tool touch-off probe), tool pressure will be negligible—if even measurable. While machining the actual workpiece, tool pressure can be substantial.

It is not uncommon, for example, to experience a 0.0004-inch-diameter deviation when finish turning an actual workpiece with aggressive cutting conditions. When tolerances are small—less than say, 0.0005 inch—this deviation can result in scrap workpieces. To avoid potential scrap, the setup person can trial-machine with each critical cutting tool to ensure that the first machined workpiece is a good one. 

The amount of size deviation between tool touch-off and machining a workpiece can vary from job to job based on several factors, including the rigidity of the workholding setup, the material being machined, the rigidity of the cutting tool and the cutting conditions. While this deviation can vary from job to job, it should remain pretty consistent for a given job. 

If you run jobs over and over (lots of repeated jobs), or if you have long-running jobs (high volumes), you should be able to determine, through experience, just how much size deviation your finishing tools are experiencing between touch-off and machining an actual workpiece. With this information, you can account for tool pressure deviation during tool touch-offs. If you are successful, you can eliminate the need for time-consuming trial machining whenever a workpiece is machined by a new tool for the first time. Eliminating trial machining can save a lot of time and effort.

The technique we’ll be showing is most helpful if you’re using a tool touch-off probe—and touching off every time an insert is indexed or replaced—during setup and the production run (the geometry offset value is reset each time this is done). If you’re using the skim-cutting approach, it’s likely that your operators do not touch off cutting tools every time an insert is indexed or replaced. So, monitoring tool pressure deviation will be helpful only during initial setup. Our technique may still be worthwhile with repeated jobs if lot sizes are small and cutting tools last for the entire production run.

Just knowing the amount of deviation for each critical finishing tool may be enough to eliminate the need for trial machining. With a long-running job, for example, say the operator knows that a given finish turning tool always cuts 0.0003 inch larger (due to tool pressure) when it machines its first workpiece after touch-off. The operator can simply reduce the geometry offset for this tool by 0.0003 inch every time they touch off the tool. The tool will cut properly when it machines its first workpiece, eliminating the need for trial machining. 

However, with a variety of shorter-running (though often repeating) jobs, it may be too much to expect operators to remember the deviation values. These values could be included in the setup documentation, but in the time it takes the operator to determine them, he or she could have completed a trial run. This would defeat the whole purpose of using this technique.

We offer a technique using custom macro B that makes accounting for size deviation transparent to the operator. It will work regardless of whether skim cutting or a tool touch-off probe is used for touch-off.

It will, however, require a change in the way programs are written. Before each turret index command (for critical finishing tools), the operator must include a command that will monitor whether a tool has just been touched off. (Note that we monitor the current value in the geometry offset for the tool. If it has changed, the tool has just been touched off.) This command will also include the amount of size deviation. The initial geometry offset setting will be automatically modified by this value.
N090 G65 P1000 T5.0 S-0.0003
N095 T0505 (Finish turning tool)
N090 is an example of the necessary command. The T word in this command specifies the tool station number of the finishing tool. The S word specifies the size deviation between touching off and machining an actual workpiece. It will be a negative value for external (turning) tools and a positive value for internal (boring) tools.

Here is the custom macro that does the monitoring and adjusts the geometry offset, if needed.

O1000 (Tool pressure deviation custom macro)
IF [#[2800 + #20] EQ #[530+#20]] GOTO 99 (If no change in geometry offset, exit)
#[2800 + #20] =#[2800 + #20] + #19 (Modify geometry offset))
#[530+#20] = #[2800 + #20] (Remember new geometry offset value)
N99 M99

We’re using permanent common variables (#531-#542) to monitor the current value of the geometry offset. The first time the program is run, and whenever the geometry offset in question has changed, the first IF statement in the program will be false. In this case, the appropriate geometry offset is modified by the amount of tool pressure deviation. Near the end of the custom macro, the monitoring permanent common variable is set to the geometry offset value. So, the next time the program is run (activating this custom macro just before the turret index), the first IF statement will be true, and nothing will happen to the geometry offset until it is changed again (by another tool touch-off).