Consistency in tolerancing methods is every bit as important as consistency in dimensioning. Seeing tolerances specified the same way among all drawings will make it easier for the operator to interpret blueprints. But more importantly, the style of tolerance specification can have a big impact on the operator every time an offset must be changed on a CNC machine to hold size (which can be quite often, especially on turning centers).
Most manufacturing companies try to hold the mean dimension of a tolerance band on their CNC machine tools. The programmer will program this mean value and the machine operators will strive to hold their workpieces to the mean of each tolerance as production is run. If this is the case in your company, how hard is it for your people to determine the mean (target) value of a tolerance band? If your design engineers consistently dimension with simple plus/minus (+/-) tolerances, the mean value will be specified right on the print for every dimension. For example, with the dimension and tolerance
3.125 +/- 0.001
the mean value is, of course, 3.125. The CNC operator need not perform any calculations to determine the mean value. On the other hand, consider the dimension and tolerance
3.2248 +0.0013 / -0.0007
3.1261 / 3.1241
which are both very common tolerancing methods. With these two specifications, the operator must perform a calculation just to determine the mean value. While we agree that all CNC operators should easily be able to calculate these values, the calculations take time and can be error prone, especially for inexperienced people.
If your design engineers do not use simple plus/minus tolerance specifications, your CNC operators must perform calculations every time an offset adjustment is required or a workpiece is measured. And if you have drawings with many dimensions having difficult-to-interpret tolerances, a great deal of time can be wasted and avoidable mistakes are probable.
Some companies make it even more difficult for their operators by requiring that a dimension be started to the high side or low side of the tolerance band in order to allow the dimension to grow through its tolerance band during the life of the cutting tool. This means that the mean value of the tolerance band is not the target value for offset adjustments or measuring purposes. If you use this method, be sure your CNC operators have a clear-cut method for determining the target value for each dimension. It is best if you specify the target value for each dimension in the program running documentation so that it doesn't need to be calculated. In too many companies I visit is left to the operator to determine the target value and, believe it or not, operators commonly disagree on how to determine this target value. This not only wastes time and is error prone, it leads to inconsistencies in quality, especially among operators running the same job on different shifts.
Do not be too quick to discount the benefits consistent and easy-to-interpret tolerances can have on your CNC environment. Considering the number of offset adjustments and measurements a CNC operator must perform on a daily, weekly or monthly basis, your savings in time and reduced mistakes can be sizable.
A word about process drawings: Admittedly, the above comments assume that your company has control of the design engineers responsible for making the drawings. This, of course, may not be the case, particularly in contract shops.
However, you still can gain the benefits of consistent dimensioning and tolerancing even if you do not have direct control of the design process. One way is simply to mark-up your drawings with appropriate dimensions and tolerances prior to releasing them to the shop. Another way is to use process drawings as part of your program run documentation. With process drawings, the manufacturing engineer can translate the blueprint into a format that is considerably easier for the CNC operator to interpret. Of course, the same points we've made about consistent and logical dimensioning and tolerancing apply to process drawings just as they do to blueprints.