Terms and Definitions
Common Terminology Mistakes:
Flat & Parallel
Many use the terms "flat" and "parallel" interchangeably, when in fact, they are two totally different specifications. Using these terms incorrectly can cost you money!
Parallelism is notated on drawings with this symbol.
When two surfaces are parallel to one another, they are continuously equidistant. In precision grinding, parallelism is typically confirmed using a micrometer. After grinding, the critical dimension of a part is "checked" in several locations to confirm the surfaces are equidistant within the specified tolerance. Note that parallelism does not take any warpage or twisting into account. Use this analogy: railroad tracks are always parallel - but the tracks do go up and down inclines and go around curves. Even though the tracks are parallel, they are not necessarily flat or straight!
Flatness in notated on drawings with this symbol.
Flatness is the condition of a surface falling in one continuous plane. In most cases, a flatness specification on a part requires that a critical surface lies within a zone (a tolerance) defined by two parallel planes. In precision grinding, flatness of a critical surface is typically confirmed by mounting the part on three "zeroed" points and passing a dial indicator over the entire surface. This will indicate the degree of warpage or twist in the critical surface of the part. Note that this has absolutely nothing to do with parallelism. Use this analogy: a wedge-shaped part can have two surfaces that are "dead flat," but the surfaces are definitely not parallel to one another.
Total Indicator Reading (T.I.R.) is a common notation that requires a critical surface to be both flat, and parallel to another surface. In precision grinding, T.I.R. is confirmed by laying the part on a master granite surface plate and passing a dial indicator across the entire surface of the part. This will indicate the combined degree of taper (parallelism) and warpage or twist (flatness) of the critical dimension. It is important to note that the phrase "parallel within .001 T.I.R" is technically not a correct notation - and it causes confusion. Does the part need to be parallel within .001 or does it also need to be flat? To avoid confusion, the proper terms used should be "flat & parallel .001," "flat and parallel within .001 T.I.R.," or simply ".001 T.I.R."
The distinction between flatness and parallelism is very important. In the case of larger parts, achieving flatness is much more time consuming, and therefore more expensive, than achieving parallelism. Keep this in mind when designing parts or when asking for the specification on a quotation request when it really may not be necessary.