What are some problems that occur with holes and pins in conventional tolerancing?

Question

What will be an ideal response?

Answer 1

A potential problem occurs when holes are located using conventional tolerance dimensions. Consider a case where parts are designed so that there will be a clearance fit between pins and holes. The location dimensions for the holes will yield square tolerance zones for each part. Now consider some examples where the holes and pins fall at the extreme edges of their tolerance zones.


When a pin is machined to the far right of the square zone and the hole is machined to the far left of the square zone, the result is surface contact between the two parts. This is acceptable for the assembly since the parts will still function under these conditions. Surface contact is also the result when the two features are located at the extreme ends of the y-axis. Again, this is acceptable. The parts will still work.


A problem occurs when the center of the hole and the center of the pin are located at the extreme diagonals within the tolerance zone. In this situation, material interference will result, which will not work for the assembly. The parts will not fit or function properly. You may be thinking that all that needs to be done to make things work is to specify a larger hole size or a smaller pin size. Usually, this is not a good idea since tolerances between holes and pins are taken from standard tables. The only other way to correct this problem when using limit dimensions to locate the center of holes is to reduce the size of the square tolerance zone. Any time you need to reduce a tolerance value, it usually costs more to produce the parts. If you stay with the same values for the diameters of the holes and pins and continue to use conventional tolerancing to locate the centers, you must reduce the tolerances on the location dimensions from .006 to .004. The result is that you always pass parts that will function. Unfortunately, you also reject parts that will function properly.