Tolerance: Difference between MMC and LMC limits of a single dimension. Flatness. Straightness. Cylindricity. Circularity. Form Features Examples . are required; Shape of tolerance zone: 2 parallel lines, 2 parallel planes, and cylindrical. Flatness and parallelism which have relations and differences between them are two important subjects in shape-position tolerance theory. (4) Examples. For a object with given flatness and parallelism tolerance, the. The surface form is controlled similar to flatness with two parallel planes Two parallel planes or lines which are oriented parallel to the datum feature or surface . between two parallel planes = flatness) However the biggest difference is.
Parallelism does not directly control the angle of the referenced surface; it controls the envelope like flatness where the surface needs to be. The goal is to ensure all points are within a specified tolerance distance away from their corresponding datum points.
Parallelism | GD&T Basics
Parallelism is quite simple to measure. Like flatnessa gauge is run across the reference surface or feature. However unlike flatness, the part is constrained against a granite block or flat plane that acts as the datum surface where it is measured.
All the profiles of orientation and are used in the exact same way. All of the orientation symbols angularity, perpendicularityand parallelism all call out the particular feature envelope referenced to a datum. Whenever two surfaces or features need to work in sync with each other and constant distance must be maintained, parallelism is effective.
Even though surface parallelism seems to ensure that a flat surface is mating with another flat surface, it can also be applied to two sides of a hole or cylinder to avoid a taper. Any part with two critical flat surfaces such as gears, would call out parallelism.
A gear has to maintain constant axial load on both faces.
To ensure even contact one side of the gear is held parallel to the other side. To do this without parallelism, the gear width would have to be tightly controlled, which could be very difficult to do.Writing Skills: Parallel Structure
Combination of Orientation and Form: Parallelism is very common in its surface form. As seen in the first image below, the plate had a "bottom"face that was designated as Datum A. The "top"face was toleranced for parallelism to Datum A as well as flatness.
So far, nothing seemed out of the ordinary.
GDT: Parallelism vs. Flatness
As the customer and I reviewed the drawing in more detail, I noticed in the specific instructions toward the corner of the drawing what I believe to be a very common misperception: It's not rocket science. It's some of the most basic geometry you'll need to understand.
Just do everyone a favor and don't make it any more difficult than you need to. Flatness is a feature compared to itself, while parallelism requires that a feature be compared to a datum. To clear up the concept, consider the example of a kitchen table.
We purchase a new table from a furniture store and are fairly certain that the flatness of the tabletop to be less than a fraction of an inch.
If the table legs are the same lengths within a few fractions of an inch, we can be confident the tabletop is parallel to the kitchen floor. The table top is flat and because the length of the legs is equal the top is ALSO parallel to the kitchen floor.