Be Careful With Unwritten Rules

Every company I’ve visited has at least some unwritten rules. These rules are related to things that experienced CNC people sometimes take for granted. Certain company methods may be considered so basic and obvious that there is no need to document them—or even to talk about them.


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One example of an unwritten rule is related to tool length compensation offsets. With machining centers, it is quite common—almost universal—to make the tool length compensation offset number for a cutting tool the same as its tool station number. This makes it easy for the setup person or operator to remember which tool length compensation offset is related to each tool (as long as that person knows the rule).

This method is logical to experienced CNC people, so tool length compensation offsets are often not documented on the setup sheet. However, there is nothing keeping a CNC programmer from breaking the rule. In a program, tool station number three could use tool length compensation offset number five and no alarm would sound. Admittedly, it makes no sense to use offsets in this manner. My point is that a given unwritten rule may not be so obvious to everyone, especially company newcomers.

Similar offset-related unwritten rules are related to other forms of compensation. With cutter radius compensation, and when the offsets for a machine contain only one register per offset, the offset numbers corresponding to tool station numbers are already taken for tool length compensation. So the programmer will add a constant number (such as 30) to the tool station number to determine the cutter radius compensation offset number. Tool number three will use offset number three in which to store the tool length compensation value and offset number 33 in which to store the cutter radius compensation value.

This method is not nearly so obvious. Everyone must know the constant number (30 in our case) in order to correctly specify cutter radius compensation offset values. Again, many companies won’t even mention cutter radius compensation offset numbers in setup documentation. Instead, all setup people and operators are simply expected to know the unwritten rule.

Here are some other examples of common unwritten rules. As you read them, think about making a list of those used in your own company.

• For the people assembling machining center cutting tools: Unless a minimum length is specified on the setup sheet, keep the overall length of all cutting tools as short as possible.

• For setup people and operators: When making sizing adjustments, always target the mean value of the tolerance band.

• For turning center setup people: When soft jaws are used in the setup, always machine jaws 0.005 inch smaller than the clamping diameter for external chucking and 0.005 inch bigger for internal chucking.

• For CNC operators: When machining critical surfaces that need sizing adjustments, allow the surface to grow or shrink to within 10 percent of its tolerance limit before making a sizing adjustment.

• For CNC programmers: Specify each coordinate in the program with the mean value of its tolerance band.

• For setup people and operators: Deburr all sharp edges from CNC machined workpieces.

These are but a few of the countless unwritten rules I’ve seen at the companies I’ve visited. It shouldn’t be too difficult to write a pretty sizeable list of your own.

If you’re having problems creating a list of unwritten rules, think about problems your company has faced. It’s likely that some scrapped parts, wasted time and even damaged machines may be attributed to people not knowing an unwritten rule.

Also, think about the things setup people and operators must do to be proficient. What must they know that has not been documented?

There is nothing wrong with having unwritten rules as long as they are explained to the appropriate people and as long as people understand them. However, before you’re too quick to assume that everyone understands your company’s unwritten rules, take a tour of your company. You may be surprised to find that some people —possibly even experienced people—are unfamiliar with some of the unwritten rules.

When hiring new people, provide them with this list and be sure they understand the rules that apply to them. Also, be sure that they are not bringing undesirable or inappropriate unwritten rules with them from their previous employer.