Remember: Parallelism does not directly control the angle of the referenced surface it controls the envelope (like flatness) where the surface needs to be. All points that are on the referenced feature must in the tolerance zone. Two parallel planes or lines which are oriented parallel to the datum feature or surface. It is important to determine what the reference feature is (surface or axis) and then what is acting as the datum (surface or axis) to determine how the parallelism is to be controlled. Note: Parallelism does not control the angle of the referenced feature, but only creates an envelope in which the feature must lie.
See the tolerance zone below for more details. The tolerance indirectly controls the 0° angle between the parts by controlling where the surface can lie based on the datum. It can reference a 2D line referenced to another element, but more commonly it relates the orientation of one surface plane parallel to another datum plane in a 3-Dimensional tolerance zone. Parallelism is a fairly common symbol that describes a parallel orientation of one referenced feature to a datum surface or line. GD&T Symbol: Relative to Datum: Yes MMC or LMC applicable: Yes GD&T Drawing Callout: However, be sure to pay attention if it is referencing a central axis because it is different! We will only discuss surface parallelism on this page but be sure to check out our page on Perpendicularity to see how an axis is controlled with GD&T. Parallelism is most commonly called out as surface parallelism. The axis form is controlled by a cylinder around a theoretical perfectly parallel axis. Axis Parallelism is a tolerance that controls how parallel a specific parts central axis needs to be to a datum plane or axis. The surface form is controlled similarly to flatness with two parallel planes acting as its tolerance zone. The normal form or Surface Parallelism is a tolerance that controls parallelism between two surfaces or features. It could also apply to a line and a plane and two planes.Parallelism actually has two different functions in GD&T depending on which reference feature is called out. And because there's only 3 pairs I decided to write them all up, so don't just think that parallelism applies only to two coplanar lines. And we can say that two planes can be parallel if they never intersect. We said that we could have a line parallel to a plane again there's many and I just chose one. So two coplanar lines if we look at our cube, there's lots of them but I only named one pair and that was a, b, and c, d. So we have a, b, f, e is parallel to this bottom face which is c, d, h, g. a, e, h, d is parallel to this other side face b, f, g, c and last we could say our two bottom faces or the top and the bottom face. So I'm going to say a, b, c, d is parallel to the face that is opposite to it e, f, g, h, so I'm going to say e, f, g, h but we could also consider the other 2 pairs, so we could say this side face a, e, h, d. So we can say that line segment a, b is parallel to plane c, d, h, g so that line will never intersect that plane they're considered parallel.Īnd last what about two planes? Well since we have a cube we have 3 pairs of parallel planes, so we could start off with front plane a, b, c, d. Which means it could be parallel to this bottom face, so the bottom face is c, d, h and g. Now what about a line and a plane? How can those be parallel? Well taking that same plane a, b, c, d if I took one edge let's say a, b so I'm going to say line segment a, b line segment a, b intersects this plane a, b, c, d it also intersects this plane a, b, e, f. So those will be 2 that are in the same plane that will never intersect. I could say that this segment a, b so I'm going to write segment a, b is parallel to segment c, d. So if we start off by saying well two coplanar lines, if I look at this front face, so that's going to be one plane. So this cube we'll assume that we have 6 congruent faces and that opposite faces are parallel. So let's start off by identifying two coplanar lines in this cube right here. But a line and plane can be parallel to each other and two planes can be parallel to each other. We could be talking about well, the obvious the two coplanar lines that's what we're going to see the most. In Geometry when we talk about this concept of two things being parallel, we aren't just talking about two parallel lines.
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