In the previous article, we had discussed the basics of setting up a true position evaluation with Zeiss Calypso. In that we touched base on selecting the proper datum reference frame (DRF) and explained the difference between a base alignment and a proper datum reference frame. In this issue, we will delve deeper into a true position evaluation with the application of the positional modifier tools.
All of us have run across the material modifiers on a drawing. These modifiers will allow additional “bonus” tolerance or mobility depending on the application of these modifiers. Each modifier has it’s own unique function. If one is not aware of the function of these modifiers, we could be either cheating ourselves out of allowed tolerance or applying additional tolerance where it shouldn’t be. Let’s start with a basic explanation of each modifier.
Maximum Material Condition (MMC). When a feature has the most material applied to it, no bonus tolerance is in effect. For example, an ID hole has .500” + .010” requirement. At the smallest size, .500”, no bonus tolerance can be applied. Once you deviate from the maximum material size, you can apply a 1 for 1 bonus to your positional tolerance. If this hole measures .501”, you get an additional .001” tolerance in addition to the established tolerance in your Feature Control Frame. A .510” measured diameter would allow an additional .010” of positional tolerance.
Least Material Condition (LMC). Opposite of the MMC, when a feature has the least amount of material it can, no bonus tolerance is in effect. For the same .500” ID hole, a .510” hole would not have any additional bonus tolerance applied. A .500” measurement would allow an additional .010” of positional tolerance. This application is seen most often when needing to protect a thin- wall condition.
Regardless of Feature Size (RFS). When this symbol is applied, no bonus tolerance is allowed regardless of any size conditions. The current ASME Y14.5 standard states that if no material modifier is indicated within the feature control frame, RFS applies. MMC and LMC must be specified. For all those engineers out there, I recommend placing the RFS symbol when necessary to be absolutely clear on the design intents.
The required placement of the material modifier conditions has changed throughout the revisions of the ASME Y14.5. In the 1966 and 1973 standards, the RFS had to be specified, while the MMC was implied or specified. This only applied to a True Position Feature Control Frame. For all other geometric controls, the RFS is implied and MMC must be specified. Keep this in mind if you run across older drawings that still refer to the older ASME Y14.5 standards, but use caution when applying. We don’t want to apply bonus tolerance if it can affect the functionality of the parts. In the current iteration of the Y14.5 standard, MMC must be specified while the RFS is implied.
Let us begin with putting these into application. Here we have a part with a hole requiring a true position evaluation.
Our print Feature Control Frame (FCF) looks like this.
Open up the true position and plug in all the applicable information.
As you can see, Calypso defaults to the RFS and the part is out of tolerance. With a deviation of .004”, however, our FCF indicated we can apply the MMC material modifier.
Let’s report the diameter and turn on the MMC condition.
Now let’s look at the result.
We are still out of tolerance, but now we are only out by a deviation of .0009”. The report lists nominal tolerance and the total tolerance with the bonus applied. We could have a result of .0081” and be in tolerance with the MMC condition applied. So the part is bad, correct? In review of the FCF, we can also apply MMC to datum feature D.
Report the datum feature D diameter and turn on the MMC
So did the allowed coordinate system shift put the feature within tolerance?
It did! We have good parts. However, you may be looking at your results and bonus tolerance and wonder why the results changed but my allowed bonus tolerance didn’t? Look back at the original results.
When MMC is applied to a feature, the resulting deviation is added to your nominal tolerance as bonus. The tolerance zone increases in size. When MMC is applied to a datum feature, the tolerance zone doesn’t increase, but the coordinate system is allowed to shift and attempt to fit all the features within the tolerance zone. Remember this, for a feature, more tolerance, for a datum feature, more mobility.
Be careful when using the material modifiers on datum features. If you are attempting to provide feedback to manufacturing on where to adjust their machine, they will be chasing numbers back and forth. The results and additional position results do not reflect the actual feature location. If allowed, Calypso will translate and / or rotate the coordinate system and fit the features into tolerance. The results will give you a “good enough” number. Think of using a functional position gage. You will wiggle and rotate the gage until the gage fits, than you will stop. If the program is used for production, I recommend not using the MMC conditions to help ensure manufacturing can dial in their process as close to nominal as possible. When attempting to sell product, take full advantage of the tolerances allowed.
Chairman of the Zeiss North American User’s Group
Assistant Quality Manager for Seiler Instruments and Mfg.