In Previous articles, we explained the meaning and types of Geometric Dimensioning and Tolerancing (GD&T), and their differences from size tolerances.
This time, we will explain Form tolerance (aka shape tolerance), which is a type of geometric tolerance. It is an important and fundamental characteristic that determines the function of the part, so please learn about it.
1. What Is Form Tolerance (Shape Tolerance)?
“Form tolerance” regulates how far a shape (*) such as an indicated plane or straight line can be biased from the ideal shape. This tolerance is characterized by not regulating the shape based on any part of the target part, but only the shape of the target part. Therefore, it does not require a datum as a reference to regulate tolerance zones.
A shape is a point, line, plane, etc. that is subject to geometric tolerances. In particular, a shape for which shape tolerances are indicated is called a “single form”.
2. Types Of Form Tolerances in GD&T
It is good to know that bias from the ideal shape can be regulated by the above four types. In terms of frequency of use, the order is (1) straightness, (2) flatness, (3) roundness, and (4) cylindricity.
“Contour tolerance” is also included in the shape tolerance when regulating the shape of a single shape, but it is treated separately in this series. Now, we will explain each tolerance.
(1) Straightness
What is straightness? “Straightness” is a geometric tolerance that tells a shape to “be straight”. The only shape that can be regulated by straightness is a “linear body”, and this includes ridges, busbars, and axes. Since straightness regulates a single straight line, the tolerance zone is the space between two parallel planes or a cylindrical space that covers the straight linear body.
How to use straightness
Straightness is often used for cylindrical parts. It instructs axes that are functionally not allowed to warp or wavigate. Suppose that a cylindrical shaft is inserted into the hole of the mating part, and the bending of the shaft is allowed up to 0.1 mm. When inserting an axis into a hole, it is designed based on the axis line, so it is a good idea to regulate the straightness of the axis line. In this case, the straightness is indicated on the extension line of the diameter dimension. In addition, the straightness of the axis of a cylinder is often regulated by a tolerance zone regardless of direction. Therefore, precede the tolerance value with a diameter symbol φ to indicate that the tolerance zone is within a cylinder with a diameter of 0.1 mm.
(2) Flatness
What is flatness? “Flatness” is a geometric tolerance that tells a shape to be “straight flat”. Since flatness is regulated by a single plane, the flatness tolerance zone is between the two plates that sandwich the planform.
How to use flatness
Flatness indicates mounting surfaces that require surface accuracy, for example. If the degree of warpage with the mating part can be tolerated up to 0.1 mm, it is a good idea to indicate. You can also indicate flatness for distant faces. One thing to keep in mind is when you want both faces to be the same plane. In that case, attach the symbol “CZ (Common Tolerance Area)” along with the tolerance value. This symbol ensures that distant shapes are included in a common tolerance zone. An example of determining the necessity of CZ is whether there is a single part attached to the mounting surface or a separate part.
(3) Roundness
What is roundness? “Roundness” is a geometric tolerance that tells a shape to “be round”.
As shown in Fig. 7, roundness is regulated by one circle, and the tolerance zone is between two concentric large and small circles between the circular body.
How to use roundness
Roundness is applicable not only to cylindrical axes, but also to tapered axes. Assume that the cross section of the cylindrical shaft or tapered axis should be a perfect circle, and deviation from the perfect circle can be allowed up to 0.1 mm. In this case, it would be nice to indicate. It is regulated that any cross section of each axis falls into the tolerance zone of concentric 2 circles. At this time, do not put a φ before the tolerance value. If the name is roundness, it is tempting to put a φ before the tolerance value, but since roundness is a radius difference of 2 yen large and small, it does not φ. In addition, since roundness is not regulated by the axis line, it is indicated by removing it from the extension line of the dimension line representing the axis line.
(4) Cylindricity
What is cylindricity? “Cylindricity” is the geometric tolerance that indicates when “I want the shape to be straight and round”. The regulation of the cylindricity is one cylinder, and the tolerance zone is between the two cylinders sandwiching the cylindrical body.
How to use cylindricity
Suppose you want an axis with a perfect cross section and no warping, and you want to allow deviation from the perfect circle and warpage along the axis up to 0.1 mm. In this case, you can instruct. It regulates that the surface of the entire cylinder is between two cylinders with a radius difference of 0.1 mm. Like straightness, it’s tempting to give cylindricity a diameter symbol φ, but the correct answer is not to. Also, since it does not instruct the axis line, it is instructed by removing it from the extension of the dimension line.
Points to note when using cylindricity
Finally, I would like to tell you about the points to keep in mind when using cylindricity.
Cylindricity cannot be indicated for tapered shapes, etc.
Both cylindricity and roundness are tolerances that regulate the roundness of a shape, but cylindricity evaluates the entire cylinder, while roundness evaluates arbitrary cross-sections. Note that roundness is also reducible for shapes with varying diameters, such as tapered shapes, but not for cylindricity.
Cervirulence inspection is generally difficult
In general, cylindricity tolerances are difficult tolerances to inspect, and a coordinate measuring machine is required. If you cannot prepare a coordinate measuring machine, you will have to perform a simple inspection. Therefore, if the design allows, it is recommended to combine straightness and roundness to regulate straightness and roundness.
So far, we have explained the characteristics of form tolerance (shape deviation) and how to use it.