This video is in response to a question that we received on our question line from Lokesh. Lokesh asked us if a material modifier can be used with cylindricity.
GD&T modifiers are maximum material condition, or MMC, and least material condition, or LMC. The MMC and LMC modifiers cannot be used with cylindricity, but according to the ASME Y14.5 standard, regardless of feature size or RFS, they always apply to cylindricity. We sometimes think of RFS as a modifier, but technically it is not. So, the answer to Lokesh’s question is no; modifiers are not used with cylindricity. The same explanation also applies to circularity.
Cylindricity is a form constraint that measures how well the shape of a part matches a perfectly cylindrical form. This constraint’s tolerance zone is a three-dimensional shape described by two cylindrical shells with perfect form. The entire surface of the part to be evaluated must fit between the two perfect cylinders. Cylindricity is not used very often because it only applies to cylinders and is very difficult to measure, usually requiring a coordinate measuring machine or CMM.
GD&T Rule #1 requires a part to have perfect form at MMC, but it is essential to remember that this is dealing with the part’s size, not another constraint. Every feature of size will have an LMC and MMC condition. For example, consider a cylinder with a nominal diameter of 20mm and a diameter tolerance of +/- 0.5mm. The cylinder also has a cylindricity callout of 0.03mm. The LMC occurs at a diameter of 19.5mm, while the MMC occurs at a diameter of 20.5mm. Rule #1 requires our part to have perfect form at MMC, so the size tolerance alone would effectively allow us a maximum cylindricity error of 1mm. However, the addition of a cylindricity tolerance means that the designer wanted to further refine the form requirement for this part.
Because Rule #1 states that a part must have perfect form at MMC, the example part with a diameter of 20.5mm would be required to have perfect form, allowing no cylindricity error. As the size decreases from 20.5mm, more cylindricity error is permitted. At sizes below 20.47mm, the full cylindricity tolerance of 0.03mm would apply. This is true all the way down to the lower end of the size tolerance at 19.5mm (LMC).
Finally, our free wall chart is a great resource that can answer questions like this. We recommend that you download this chart and print it in 11×17 size for a quick reference document to keep near your workstation. The chart provides a wealth of information, including which callouts allow the use of modifiers.