Surfaces and sets of parallel surfaces associated with a size dimension are called features-of-size FOS. Typical examples of features of size include:. Maximum Material Condition MMC refers to a feature-of-size that contains the greatest amount of material, yet remains within its tolerance zone. Some examples of MMC include:. It is the smallest hole diameter because a larger hole removes material, hence the smallest diameter provides for the greatest amount of material. Likewise, it is the largest pin diameter because a smaller diameter would remove material.
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Tolerance Of Position. Back to Table of Contents. Tolerance of Position RFS. Projected Tolerance Zone. Composite and multiple single segment Tolerance. Bonus Tolerance Introduction. Tolerance of Position MMC. Calculating whether or not a hole is in spec with bonus tolerance. Datum Feature Shift Introduction. Zero tolerance at MMC. Six different datums derived from one datum feature. A quick summary of how Tolerance Of Position controls Perpendicularity.
Datum Feature Shift relative to a pattern of holes. Boundaries used with Tolerance of Position. Locating datum features relative to a dash datum derived from those datum features. Advanced topic - calculating whether holes are in spec with bonus and datum feature shift. The feature control frame for Tolerance Of Position is shown below:. This chapter is about Tolerance Of Position. It is not about True Position Tolerance. Tolerance Of Position is a geometric control that specifies how far away from True Position a feature of size is allowed to be.
Tolerance Of Position has some fundamental rules: 1. Tolerance Of Position must always be applied to a Feature of Size 2. Tolerance Of Position must always be located by Basic Dimensions. The Basic Dimensions may be explicitly called out or implied.
The figure below shows the two cases in which a Tolerance Of Position does not require a datum reference. On the left, we have three coaxial cylinders. They are located relative to each other, not relative to anything else.
On the right, the pattern of four holes is a primary datum. This is an unusual situation, but it is possible. The tolerance of position locates the holes within the pattern relative to each other. The large hole in the middle of the part is then located relative to the pattern of four holes.
Therefore the large hole in the center does require a datum reference. For our discussion of Tolerance Of Position, we will first look at our holes that we are digging for fence posts to keep our dog in our yard. In previous chapters we talked about the orientation of the holes, but we never talked about the location of the holes. In the figure below, the hole on the right is located relative to the hole on the left.
Note the addition of a basic dimension and a tolerance of position control. In order to understand our tolerance of position control, we must thoroughly understand our datum.
For now, focus on the datum hole. We must establish datum axis [B]. The hole on the right will be located relative to datum axis [B]. Our datum callouts in the feature control frame for the Tolerance Of Position tells us that datum [B] must be perpendicular to datum [A].
The datum hole is not necessarily perpendicular to datum [A]. The datum hole has a perpendicularity control that controls the orientation of the datum hole. The perpendicularity control says that there is a tolerance cylinder that is 0. The axis of the unrelated actual mating envelope of the datum hole must fall within this tolerance cylinder. The axis of the unrelated actual mating envelope cannot be datum [B] because this axis is not necessarily perpendicular to datum [A].
The Related Actual Mating Envelope is the largest perfect cylinder that is exactly perpendicular to datum [A] and still fits within the datum hole. Having established our datums, we can now look at how the tolerance of position locates and orients the hole relative to the datums. The tolerance of position callout tells us that there is a tolerance zone cylinder that is 0.
This cylinder is exactly perpendicular to datum [A], and the center of this cylinder is exactly 50 from datum [B]. The axis of the unrelated actual mating envelope of the hole must fall within this tolerance cylinder. We can see from the figure below that the requirement for the axis of the unrelated actual mating envelope falling within this cylinder allows the hole to move 0.
The requirement also allows the hole to tilt relative to datum [A] with the tilting being restricted by the axis of the unrelated actual mating envelope staying within the tolerance cylinder. The fact that tolerance of position allows some tilting should not be ignored.
Recall that the purpose of these holes is to plant fence posts in them for our fence that will keep the dog in the yard. See the figure below that includes the fence posts. The fence will be okay with the fence post on the right being out of position to the extent allowed by the tolerance of position. However, the tilting has a geometric effect.
This geometric effect will allow the top of the fence post to move much more than is acceptable. In order to get our fence post to do what we want it to do, we will use the Projected Tolerance Zone modifier. The Projected Tolerance Zone modifier is the letter P in the circle in the feature control frame. The Projected Tolerance Zone modifier specifies that the tolerance zone is projected above the part. It is no longer inside the part.
It is completely above the part. The number after Projected Tolerance Zone modifier in the circle is the distance above the part that we want the tolerance zone projected. In our case, our posts will stick above the ground a distance of Therefore we want to project our tolerance zone 40 above the part.
Now the axis of the unrelated actual mating envelope of the hole, and therefore the axis of the post, must stay within this tolerance zone that is above the part. This limits the tilting of the post that is allowed. It restricts the movement of the top of the post to an amount that is acceptable. The figure below shows the classic example of projected tolerance zone. On the left, the tolerance cylinder is inside the part. The bolt centers itself on the threads in the threaded hole.
As the bolt extends through the cover while tilted as much as the tolerance of position will allow, there is a conflict between the bolt and the side of the clearance hole in the cover. On the right, the tolerance zone is projected from the top of the base to the top of the cover.
The axis of the hole, and therefore the axis of the bolt, stays within the tolerance cylinder as it goes through the clearance hole in the cover and there is no conflict between the bolt and the side of the clearance hole in the cover. Now let's look at locating some windows in our dog house. To simplify things, we will only look at locating them vertically. The windows are features of size, so we can locate them with a tolerance of position.
We will dimension them as a pattern. The tolerance of position tells us that exactly 10 above datum [A], there is a perfect plane.
Centered on that plane are two parallel planes that are 0. The center planes of the unrelated actual mating envelopes of the window heights must fall between these two parallel planes. Note that since the tolerance zone is two parallel planes and not a cylinder, the diameter symbol is not used in the feature control frame. Since each window could be at the opposite end of the tolerance zone, the windows could be offset from each other.
The windows can also rotate relative to datum [A] and relative to each other. We would like to tighten up the alignment between the windows and the orientation relative to datum [A]. We do not need to tighten the location of the pattern relative to [A]. We will use a composite tolerance. The composite tolerance has two or more lines that share a single tolerance of position symbol. The composite tolerance is always applied to a pattern of features of size.
The top line controls the location of the pattern. Note that since the bottom line controls orientation and not location, we are allowed to have the same datums in the same order in the bottom line as in the top line. The top line controls the position of the pattern within the same 0. The bottom line holds the center planes of both windows within the red tolerance zone.
The red tolerance zone is two parallel planes that are 0. This tolerance zone is exactly parallel to datum [A], but this tolerance zone is allowed to float within the 0. It aligns the two windows with each other and controls the orientation to [A] while allowing the location of the pattern to vary within the 0. Now we want to mount a sign on our dog house.
How to Factor Bonus Tolerance into a Tolerance Stack
The maximum material condition is used when designing two mating parts. Taking a shaft designed to fit into a bore as an example, this specification ensures that the shaft actually fits into the bore under the maximum material condition MMC , while also preventing excessively strict size tolerance from being applied in order to avoid cases where the shaft does not fit into the bore. To apply the maximum material requirement to a dimension, you write after the size tolerance in the feature control frame. This symbol indicates the application of maximum material requirement. Size tolerance is always followed when the maximum material condition is used. However, if a size tolerance is deviated from the maximum material size , the difference can be added to a geometric tolerance to make a virtual size.
Bonus tolerance is the additional tolerance available for a geometric control attached to a feature of size with MMC maximum material condition and LMC least material condition modifier. When a component consists of maximum amount of materials it could possibly carry then the component is said to be in MMC condition. For a shaft and hole base system, a shaft will be in MMC when it has the maximum diameter and a hole will be in MMC when it has minimum diameter. In the above figure the size of the hole can vary from And at You already know that MMC for the hole will have the diameter of
Bonus Tolerance Calculation - GD&T
A bonus tolerance is arguably one of the most difficult concepts in Geometric Dimensioning and Tolerancing to explain to the powers that be. For almost all other characteristics called out on a drawing, here is our specification… this is our minimum, this is our maximum, and anything outside of these parameters is non-conforming product scrap or rework. If LMC or MMC is listed, we can calculate a wider tolerance, or bonus, depending on the actual size of the measured feature. We should start with understanding RFS because it allows no bonus to our location tolerance zone. If a drawing calls for positional location and does not specify LMC or MMC, the center of the measured feature must lie within the tolerance zone specified, regardless of the feature size. The concept of a bonus tolerance is simple.
Maximum Material Condition (MMC)
Tolerance Of Position. Back to Table of Contents. Tolerance of Position RFS. Projected Tolerance Zone. Composite and multiple single segment Tolerance.