Eliminating Tool-Change Scrap in CNC Machining Using FANUC Macro Offset

Problem

100% of production processes in IATF 16949 or ISO 9001 certified companies fear the first part after a tool change being transferred down the line. Companies usually implement solutions such as creating clearly labeled holding areas to place these parts so operators do not accidentally pass them along.

However, non-conforming products are still occasionally detected during the process or even at the customer's site. Subsequent investigations using methods like 5 Why consistently conclude that the product was indeed the first part after a tool change. Yet, finding a permanent countermeasure seems almost impossible - because human error (forgetting) is involved, even Poka-yoke-style measures cannot thoroughly eliminate the root cause.

Breakdown

When using 5 Why to investigate this type of defective product, have you ever stopped at a "Why" whose countermeasure is to simply not produce non-conforming products in the first place? Most people assume that as long as machining happens, NG parts will inevitably be generated - but that is not actually impossible. It is precisely because we think it's impossible that we stop looking for solutions.

A skilled technician, maintenance engineer, or operator will offset the tool (moving it away from the product by a certain amount so it doesn't touch the finish machining area) to run a trial part after a tool change. After machining, this part has larger dimensions (for outer diameters) or smaller dimensions (for inner diameters), allowing it to be re-machined. This works, but doing it manually takes too much time and effort - which is why it's rarely adopted as standard practice.

Real Case

It took 3 years of thinking to find a way to make this practical: figuring out how to perform the tool offset faster, using automation instead of manual adjustment.

Solution

The idea came from a macro program project on the FANUC operating system. Using macro programming allows the tool-offset process to be automated.

Example: to machine a finish outer diameter, if a new tool immediately runs the first part, the tool cuts right to the finish line and that part becomes NG. Simply offsetting by a fixed amount and adjusting from measurement carries risk and cannot be officially adopted under ISO standards.

The solution instead lets the tool cut only an oversized section - with larger dimensions in both X and Z axes than the finished part. This is done precisely via a macro program: selecting the tool nose radius offset location (e.g. #2201 for Tool 1) and writing a simple value (1) into it to call the oversized cutting path.

After machining the oversized section, the part is measured and the machine adjusted to reach the oversized target. Once that target is reached, the next part is run and measured again - reaching the final finish dimension. Parts run in the oversized section only cut the outer layer of raw material, so they don't need to be isolated separately and can simply be returned to the stock bin for re-machining. This reduces both the risk of generating non-conforming products and scrap cost.

Results (worked example, one product line)

Investment Cost

$0 - utilizes existing macro programming capability and tool offset variables already present on the FANUC control system.

Implementation Time

1-3 days: drafting macro code, testing the oversized-section cut, and instructing operators on variable entry.

Implementation Difficulty

Medium - requires understanding of FANUC Custom Macro B programming and writing data to system offset variables such as #2201.