ASME Y14.5M-1995, is a whole set of standards that limit the technical drawings in the techniques of dimensioning and tolerance. In other words, it describes how to apply the Dimension and the proper way of using tolerance in the technical drawings. Because of the drawings might be using in other countries, so for the ASME, they have to developed a set of standards that will “translate” the drawing which originally drawn on local language to foreign languages. That is the purpose of GD&T (ASME Y14.5M-1995).
GD&T stands for Geometric Dimensioning and Tolerance. In this whole department of mechanical engineering, it deeply describes how to use some sort of symbol which is called GDT symbol and how to apply tolerance to your designs. In use of GDT, we have xx component and all of these are based on symbols. These symbols are best described the surface, parallel and flatness of the object inside the drawings. So even the engineers from other countries that we assume not very well at English, can easily understand the concept and the design intent. So its original desire of this development fits to the international standards.
TOLERANCE. When we are talking about tolerance, we are talking about the dimension of the geometric object that is allowed to vary. That is, the variable allowance of dimensioning. What it really means and how it implied? Luckily, that we have a specific department of ME that release a set of dimension variable table, which can be use in day to day engineering design. For example, slight force fit is one of the design concepts. When Marker pens first designed, the cap of body is design to be fitted and no need to twist. The outside diameter of shaft (body) is a little bit bigger then the inside diameter of hole (CAP). That is, when we are squeeze the body into the cap, we have to really joint a little bit of presure to push the body into the cap, so it won’t move and loose. That is the desire of design intent of this slight force fit. There is a little problem in this design. How much of little bigger of shat compare to the hole is enough? How much of little smaller of cap is enough of cap to hold the shaft? In this case, we are going to imply the TOLERANCE. Tolerance will determine the max and the min dimension of both inside diameter and outside diameter. There we also have one situation, material select consideration. This consideration also include of the implementation of tolerance. Which we have MMC and LMC. MMC stands for maximum material condition; LMC stands for least material condition.
For shaft, if we applied MMC and LMC. The bigger of diameter, the bigger number of MMC cause it will cost more material to fit the design. In another side, smaller of number of dimension, will be the LMC of the object. There is yet to be different for the Hole (CAP). Bigger number of dimension will be the LMC of the hole. Cause there is a gap between the outside diameter and inside diameter. Bigger number of inside diameter, means smaller gap and less material to be used in the design.
To determine the better solution of your design intent, you should use tolerance properly and carefully handle the symbols of GDT.