Tracking Offset Changes During Tool Maintenance

For companies that have long production runs on their turning centers, tool maintenance (changing inserts and replacing dull tools) must be performed several times during the production run. And during a given tool's life, it is likely that tool wear will cause workpiece attributes to vary, requiring offset changes to be made at several points during the tool's life.

Columns From: 5/1/1997 Modern Machine Shop,

For companies that have long production runs on their turning centers, tool maintenance (changing inserts and replacing dull tools) must be performed several times during the production run. And during a given tool's life, it is likely that tool wear will cause workpiece attributes to vary, requiring offset changes to be made at several points during the tool's life.

Many turning centers use geometry offsets to assign program zero and wear offsets to allow for sizing problems as the tool becomes progressively smaller over the course of its life. All experienced CNC operators will easily agree with this statement, but some unknowingly use wear offsets for the additional purpose of compensating for cutting tool placement imperfections during setup.

For example, say you have a finish turning tool in station number two. During setup, the geometry offset is measured and entered. If it is measured perfectly, the wear offset for this tool should be zero.

However, during cutting, other factors (like tool pressure) can cause the finish turning tool to be off slightly. In our example, say after cutting the first workpiece, the diameter machined is 0.0006 inch oversize. Most setup people will use the wear offsets to compensate for this imperfection, storing -0.0006 in offset number two. If this procedure is repeated for each tool, the production run will be started with an odd value in each tool offset.

Tool two continues cutting workpieces. After 50 parts have been finished the operator notices that the diameter machined by tool number two has grown by 0.0002 inch, a deviation that is clearly caused by tool wear. The operator adjusts offset number two, reducing it by 0.0002 inch (now this offset value is -0.0008). This reduction in offset value may have to be repeated several times before the insert edge is dull. When the insert must be indexed or changed, the operator must remember to reset the tool's offset to its original value (-0.0006 in our case) prior to running the next workpiece. If several finishing tools are being used, it can be difficult to remember the tool's original offset value.

What really caused the original imperfection? Surely it wasn't tool wear. In our case, it was caused by tool pressure. But you may also see larger imperfections if the tool is not placed perfectly in the turret during setup. Either way, since the wear offset is being used for the additional purpose of compensating for setup imperfection, the operator must deal with odd values in their wear offset table. Values that must be remembered during tool maintenance.

Truly, this problem is more related to program zero assignment (handled by geometry offsets) than it is to tool wear. If you make the original -0.0006 adjustment in the geometry offset, the wear offset value will be zero for each tool when an insert is new. Zero is a very easy number for the operator to remember during tool maintenance.

Admittedly, many setup people do not want to manipulate the very large geometry offsets with tiny changes because entry mistakes cannot be easily found. If your machine allows parametric programming, remember that you have total access to your tool offsets. You can write a parametric program that transfers the values of wear offsets into geometry offsets and then clears the wear offset values. This program can be run once, after the first workpiece is sized and when all cutting tools are still new.

Included here is an example written in custom macro B format. It assumes the machine has twelve tool stations.

 

O5001 (Program number)
#101 = 1 (Counter)
N1 IF [#101 GT 12] GOTO 99 (Test if finished)
#[2400 + #101] = #[2400 + #101] + #[2000 + #101] (Set X geometry offset to itself plus X wear offset)
#[2500 + #101] = #[2500 + #101] + #[2100 + #101] (Set Z geometry offset to itself plus Z wear offset)
#[2000 + #101] = 0 (Reset X wear offset to zero)
#[2100 + #101] = 0 (Reset Z wear offset to zero)
#101 = #101 + 1 (Step counter)
GOTO 1 (Go back to test)
N99 M99 (End of custom macro)

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