Could you please give me an example of programming a lathe using mean vs. nominal one that would include a couple of different diameters with different tolerances. Also I'm confused about target values. Thanks in advance. Tom Conway
Response:
Tom,
Tom, The mean value of a tolerance band is always the value that is right in the middle of the tolerance band. For example, for the dimension and tolerance: 3.000 +/- 0.002, the mean value is 3.000. For the dimension and tolerance 3.002, plus nothing minus 0.004, the mean value is still 3.000. As it is for the dimension and tolerance 2.998 plus 0.004, minus nothing. A CNC programmer should ALWAYS program the mean value for every coordinate they specify in a CNC program.
Other important tolerance naming includes high limit (3.002 in the examples above), and low limit (2.998 in the examples above).
To calculate the mean value, determine the total tolerance (0.004 in our examples), divide it by two, and either add the result to the low limit or subtract it from the high limit.
There may be a little confusion regarding the "nominal" value. I've seen manufacturing people refer to it differently from company to company. Most manufacturing people will refer to the nominal dimension as the closest common value to the dimension specified. For example, if the company commonly works with fractional dimensions, the nominal value for the dimension 3.124 will be 3.125 (3-1/8). The nominal value for 4.503 will be 4-1/2. In my opinion, this nominal value will be of little meaning to a CNC person trying to hold size on any piece of CNC equipment.
I have heard some manufacturing people refer to the nominal value of a dimension as the actual specified dimension itself. For example, for the dimension 3.374 +/- 0.003, they would say that 3.374 is the nominal value. I disagree with this definition of nominal value.
Probably the most important value in a dimension and tolerance for CNC operators is the target value. As the name implies, it's the value you're "shooting for". Many companies have an unwritten rule that the target value for every dimension and tolerance is always the mean value. They want CNC operators targeting a value that is right in the middle of the tolerance band for every surface being machined.
While this is a common practice, when you target the mean value, you're really only working with half the tolerance band. As cutting tools begin to show signs of wear (cutting edge begins wearing away), the surfaces they machine will begin to grow. As a dimension begins getting close to its high or low limit (based upon whether it's an internal or external surface), the CNC operator will have to make an offset adjustment. Again, if they target the mean value for the adjustment, they're only working with half the tolerance band.
Many tooling engineers and quality engineers would agree that if you target a value that is close to the opposite limit (high limit if machining an internal surface or low limit if machining an external surface), the cutting tool could machine for almost twice as long before an adjustment must be made.
Admittedly, this can get a little complicated. But I hope I've answered your questions. Feel free to contact me again for further clarification.
