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In the 2018 standard, concentricity was removed. Now there are 4 ways, instead of 5 to control coaxiality. We will discuss position in this tip and the other three in Tip 10A.
With so many options it is no wonder that many people find the differences hard to understand. Each control has its place, although some are more difficult and time consuming than others. By comparing sample inspection methods, the differences may become clearer. This month’s tip will illustrate how position can be used. Next month’s Tip will continue with the runout tolerances and profile.
Position may use the MMC and LMC modifiers on the tolerance and Datum references. The illustration above shows a possible gage (ignoring gage tolerance and wear allowance) that could be used when MMC is the modifier. If the sizes are within the size tolerances but the part doesn’t fit the gage, the features position (coaxiality) is out of spec. This type of control works well when the main concern is the assembly of this part to another. The gage may usually be thought of as representing the worst mating part.
When no modifiers are present, the implied condition is regardless of feature size as shown below. In this case, the axis of the datum feature and the feature being controlled must be determined to find the error in coaxiality. Although this control may be applied to bearings and dynamic balance applications, the job can usually be accomplished at a lower overall cost by using one of the runout controls.
This tip is in accordance with ASME Y14.5-2018. This tip was originally released in February 1998.