The SolidWorks Geek

Today will be a short article continuing on where we left off last week when we covering the Limits of Size. Section 2.7 of ASME Y14.5-1994 only relates to the Limits of Size and limits of size do not control the orientation or location of features. Section 2.7.3 goes into detail about this and we will quickly examine the contents of this section today.

Relationship Between Individual Features

As I mentioned above, the limits of size do not have any effect on the actual location or orientation of the features as they relate to each other.  If you have features shown that are perpendicular, symmetrical or even coaxial to each other then you must provide more controls.  The location or orientation of features must be controlled one way or another or you will have an under-defined drawing and that ain’t good.

Available Controls for Orientation and Location

Some of the controls available for the orientation and location of features as they relate to each other can be found in Section 5 & 6 of ASME Y14.5-1994. Section 5 of ASME Y14.5 relates to the Tolerances of Location. The section illustrates the usage of the GD&T controls: Position, Concentricity and symmetry to control the center distance between features, the location of features from a group or datum, coaxiality of features and the concentricity or symmetry of features. Section 6 of ASME Y14.5 relates to the Tolerances of Form, Profile, Orientation and Runout. The section illustrates the usage of the GD&T controls: Straightness, Flatness, Circularity, Cylindricity, Profile, Profile of a Surface, Profile of a Line, Coplanarity and more. We will be spending a significant amount time later on each of the GD&T controls but it doesn’t hurt to at least know where they are.

Establish a Boundary of Perfect Form at MMC for Relationships Between Features

Last week, you may recall, we discussed how a feature must have perfect form when the feature is at MMC. There will be times that it may be necessary to have the features indicated to have an exact location in relation to each other when at MMC. One of example of this could be that you have a flange plate with an array of holes. When the holes are at their smallest, MMC, they must be perfectly located off of each other to ensure that the mating component will fit. As the holes get bigger, deviate from MMC, the location of the holes can loosen up a little. There are a few ways to do this that are described in this section but they all require some knowledge of using the controls described above.

Option 1

The first way to control the relationship between features at MMC is to establish a tolerance of zero to the orientation at MMC including a reference to a datum. Do this to control the angularity, perpendicularity or parallelism of the feature. Section 6.6.12 of the standard applies to this method. We will spend some time on this when we get to Orientation Tolerances later.

Option 2

The second way to control the relationship between features at MMC is the establish a tolerance of zero to the position at MMC including a reference to a datum. Do this to control the coaxial or symmetrical features. Sections 5.11.1.3 and 5.13.2 of the standard apply to this method. We will spend some time on this as well when we get to Tolerances of Locations.

Option 3

The third way to control the relationship between features at MMC is by using a note . For the orientation use: PERFECT ORIENTATION AT MMC REQUIRED FOR RELATED FEATURES. For the coaxiality use: PERFECT COAXIALITY AT MMC REQUIRED FOR RELATED FEATURES. For the location of symmetrical features use: PERFECT LOCATION OF SYMMETRICAL FEATURES AT MMC REQUIRED FOR RELATED FEATURES.

Option 4

Lastly, the forth way to control the relationship between features at MMC is by the use of datum features. By indicating the datum preference with a datum reference frame or with a note the noted dimensions define the MMC envelope. Hopefully this will make more sense we actually discuss Datum Referencing.

Told You It Would Be Short…

Today’s article probably won’t be very helpful to you unless you already understand the GD&T controls mentioned but it still needed to be addressed since we were talking about limits of size. The next Standards Wednesday will introduce you to the applicability of RFS, MMC and LMC, so make sure your well rested. See you next time…