Everything You Need To Know To Find The Best Profile of a Surface

Profile of a surface

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Introduction

A profile of a surface specification controls how much the features' surfaces can deviate from being the nominal shapes, which are located and oriented to zero or more datums.

For this geometric tolerance, these three aspects work together:

• Each considered feature's surface data

• Each considered feature's nominal shape and each resulting tolerance zone

• The datum features (if any are referenced)

To evaluate this tolerance, PC-DMIS optimizes each feature's surface data into its respective tolerance zone. The optimization process respects whatever constraints each datum imposes. With multiple considered features, the optimization process simultaneously considers those features. In this way it fits all the toleranced features into their tolerance zones at once.

Allowed Feature Types

You can use these feature types if they have surface data:

cylinders, spheres, 3D and 1D widths, scans, planes, cones, sets, and tori.

When no datums are referenced, there is no workplane option. Sometimes, the measured data were only measured in a single cross section. This is typically a case where profile of a surface to no datums was specified but the surface is too shallow to measure more than one cross section. In this case, the geometric tolerance command automatically detects the workplane of the cross section. It also uses that workplane as an invisible primary datum to restrict the degrees of freedom to that workplane.

Tolerance Zones and Allowed Modifiers

The tolerance zone is based on the nominal surface of the feature. By default (without any modifiers), the tolerance zone is equal bilateral. This means that half of the tolerance value is on each side of the nominal surface:

Suppose you have this profile of a surface specification:

With the above specification, the actual value looks like this:

Since there are no modifiers, the tolerance zone is centered on the nominal surface, which is nominally oriented and located to each actual datum. The solid line indicates the actual surface, the dashed lines represent the nominal surfaces (including the actual datums) and the gray shaded area represents the minimum-size tolerance zone centered on the nominal surface that contains the actual surface.

The measured value (with DEFAULT datum math) look like this:

The measured tolerance zone center remains the nominal surface, which is nominally oriented and located to each measured datum. In this case, the measured points were not measured densely enough, and so the measured value is smaller than the actual value.

Modifiers may change the nature of the tolerance zone. Under ASME Y14.5, PC-DMIS supports the modifier (unequally disposed profile) and the modifier (dynamic profile). Under ISO 1101, PC-DMIS supports the UZ modifier (specified tolerance zone offset) and the OZ modifier (unspecified linear tolerance zone offset). While they are not equivalent, the and UZ modifiers have similar functionality. They offset the center of the tolerance zone from the nominal surface. Likewise, the and OZ modifiers have similar functionality. They allow the center of the tolerance zone to progress in the plus-material or minus-material direction.

Here is an example of a modifier specification. The equivalent ISO specification would be 0.08 UZ+0.04.

With the above specification, the specified tolerance zone looks like this:

Because this is the specified tolerance zone, the tolerance zone is not minimized, and so it does not represent the actual value. The center of the tolerance zone is offset from the nominal surface and is shown in the dashed-dotted line.

The actual value looks like this:

The center of the tolerance zone stays the same, but the zone is minimized until it just contains the actual surface.

Actual Value and Measured Value

Profile tolerance zones have a defined center. They also have a mechanism to grow and shrink the zone around that center until it just envelops the actual surface.

Actual Value:
Each considered feature has its own actual value. This is the size of the smallest tolerance zone that contains the actual surface. The zone is nominally oriented and located to each actual datum, with some exceptions detailed in "How PC-DMIS Solves Datums".

If you have more than one considered feature, and the datum reference frame is not fully constrained, the optimization procedure must simultaneously fit all the features' surfaces in their respective tolerance zones, if possible.

Measured Value:
Each considered feature has its own measured value. This is the size of the smallest tolerance zone that contains the measured surface points. The zone is nominally oriented and located to each measured datum, with some exceptions detailed in "How PC-DMIS Solves Datums".

If you have more than one considered feature, and the datum reference frame is not fully constrained, the PC-DMIS optimization procedure simultaneously fits all features' surface points into their respective tolerance zones. It does this in a proportional way. This guarantees that all toleranced features will fit in their respective tolerance zones if at all possible.

Validity Rules

All input features (considered and datum) must have the correct specified nominal values and shapes. This ensures that PC-DMIS calculates the measured values correctly, and it ensures that the tolerance command correctly identifies the optimizable degrees of freedom.

Exposed Options

Several types of features expose an ITERATEANDREPIERCE option. These are points, scans, sets, and torii (except for 2D profile vision auto-features and edge point features) when a CAD model is available. When available, the option is set to YES by default, because that ensures the center of the tolerance zone is the CAD model surface. When the option is unavailable, or when NO is selected, those feature types create a separate planar tolerance zone for each measured point, defined by theoretical point and vector associated with that measured point. This is called the "piecewise planar" approximation, which is excellent in many circumstances. It is poor in these cases:

• If the alignment used to find nominals is significantly different than the optimized datum reference frame

• If the measured data include sharp corners or radii

Due to the sometimes-poor behavior of the piecewise-planar approximation, in most cases we recommend that you use a CAD model and keep the ITERATEANDREPIERCE option set to YES. In some circumstances, it makes sense to set it to NO if the computation time is too long. When you set it to NO in this way, it usually improves computation speed, but you are responsible to ensure that the piecewise planar approximation is a good approximation.

Cylinders, spheres, widths, planes, and cones do not expose the ITERATEANDREPIERCE option because the geometric tolerance command internally represents the tolerance zones exactly. It is not possible to use the piecewise-planar approximation for those feature types. By contrast, 2D profile vision auto-features, edge point features, scans made out of edge point features, and "adjust filter" constructed set features do not expose the ITERATEANDREPIERCE options because they always use the piecewise-planar approximation.

When at least one datum feature has surface data, the datum math type controls how to compute the measured datums from the datum features' surface data.

For information, see "How PC-DMIS Solves and Uses Datums".

When there are no datum features, the tolerance zone math type controls how the measured surface points are optimized into their respective tolerance zones:

DEFAULT - This does a minimum-zone best fit (also called min-max). This best-fit finds the smallest tolerance zone that contains the surface points. Thus, the DEFAULT option produces the smallest measured value for evaluating profile of a surface. It is also mathematically very similar to the specification, because if you measure points densely and with high accuracy, the measured value closely approximates the actual value.

LSQ - This does a least-squares best fit. It minimizes the sum of the squares of the deviations to the center of the zone. This option produces a larger measured value (it is more conservative than the DEFAULT option). But in general, this option computes more quickly.

Lower Segments of Composite Profile of a Surface

A profile of a surface tolerance with multiple segments is called a “composite profile of a surface”. The first (or upper) segment of a composite profile of a surface tolerance is the same as a single segment profile of a surface as described above at the beginning of this topic. All lower segments of a composite profile of a surface are subtly different. This is because the tolerance zones have unlocked translation compared to the datum reference frame. However, the tolerance zones remain nominally located and oriented to each other.

The datum reference frames for the lower segments of a composite profile of a surface follow these rules:

• Each datum reference frame must only use the same datums as the reference frame above it.

• The datums must be in the same order.

• The datums must have the same modifiers.

• A lower segment can have fewer datums than the segment above.

Suppose the upper segment has datums ABC. The lower segment then could reference no datums, datum A, datums AB, or datums ABC. But it could not reference datums BA nor AC nor ABD.

Here are some examples of allowed composite position tolerances:

        

Here are some examples of not-allowed composite position tolerances:

     

Report

Here is an example report for a profile of a surface tolerance of a plane.

GD&T consists of 14 geometrical tolerances that can be applied to any part feature to control them. These tolerances are described in detail in ASME Y14.5-2009.

Many of these 14 tolerances have features similar to each other in the way they control a part’s shape and dimensions, therefore it is important to understand what is the intended use of the part beforehand.

For ease of use, these 14 tolerances are divided into five main groups.

1. Form

2. Orientation

3. Location

4. Profile

5. Runout

In this article, we shall study the profile of a surface (or surface profile) callout that is part of the profile group mentioned above. These two callouts can help us manufacture geometries that have complex outer shapes with impressive details.

What Is the Profile of a Surface in GD&T?

The profile of a surface is an extremely powerful and versatile GD&T callout. It can be used on nearly all kinds of complex outer shapes where other tolerances are not easily applicable.

Some examples of surface profile use:

1. Aerospace: airplane wing, air intake for engine, turbine blades

2. Automotive sector: BiW, A-pillar, complex outer shape

3. Product design: design of complex outer shapes for consumer appliances such as coffee makers, smartphones and displays.

GD&T Profile of a Surface

Profile of a surface controls a part’s surface profile in accordance with the CAD model or the drawing. Many engineering parts such as turbine blades, car BiWs and airplane wings have highly complex surfaces. These surfaces need to be manufactured to great detail for functional reasons. The profile of a surface callout can help us manufacture these parts by controlling the surface profile during manufacturing and measuring results after production.

The profile of a surface generates a virtual surface that acts as the baseline to measure the actual surface. Thus, it is possible to create highly intricate surface profiles as long as they can be created as a CAD model. The required surface’s specification must also be within the capabilities of the fabrication process to be used.

Surface Profile Tolerance Zone

The surface profile tolerance zone consists of two parallel planes, bilaterally disposed on each side of the ideal curved surface (aka the true profile). Both planes follow the shape of the ideal surface and the distance between them is the tolerance limit for the callout. A smaller tolerance limit gives tighter control but may be difficult to manufacture. The points on the part’s entire surface must lie within the specified tolerance zone for it to be accepted.

Surface Profile Relation to Other GD&T Symbols

As mentioned above, the profile of a surface can effectively replace almost all of the GD&T Callouts whether they are curved or flat. Let’s see a few examples:

Profile of a surface vs form controls

When used without a datum, the profile of a surface can replace all the form controls.

The four form controls are

These form controls constrain the form of a part by creating a tolerance zone between two parallel surfaces. These planes can be either flat or cylindrical.

Profile of a surface vs orientation controls

When used with a datum callout, the profile of a surface can replace all the orientation controls.

The orientation controls are:

In order to orient a part feature in GD&T, we need a reference point such as a datum plane, line, or axis. By specifying a datum in the profile of a surface’s feature control frame, we can replicate the function of all orientation controls to maintain a surface in a particular position.

Profile of a surface vs line profile tolerance

The profile of a surface tolerance is the 3D equivalent of the line profile tolerance. While the profile of a line controls a specific cross-section on the part, the profile of a surface controls every cross-section across the entire length of the surface.

There are times when both profile controls may be applied together. In such a case the line profile’s tolerance is tighter than the profile of a surface. This is done to achieve greater control at critical cross-sections while the profile of a surface maintains a looser overall control over the full surface.

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Profile of a Surface Feature Control Frame

A leader arrow connects the feature control frame (further referred to as FCF) of the profile of a surface tolerance to the part feature. Like any other FCFs in GD&T, the profile of a surface FCF can be divided into three distinct blocks. These are:

• Geometrical tolerance block

• Feature tolerance block

• Datum block

Geometrical tolerance block

This block defines the geometrical tolerance applied to a feature by housing its symbol. The profile of a surface symbol is an inverted semicircle with a horizontal diameter connecting its two ends.

Feature tolerance block

This block contains specific information about how a tolerance applies to a feature. Since for the profile of a surface, the tolerance zone is a total wide tolerance zone which is the default zone, there is no special symbol for it. This is followed by the tolerance limit. This limit, also known as the tolerance value, is the distance between the two planes of the tolerance zone and is represented by its numerical value in the FCF.

This number is typically followed by material condition modifiers (MMC, LMC, etc.), but these do not apply to the profile of surface control.

Datum block

This is the third block in GD&T that houses the reference planes for the tolerances. In the case of a profile of surface control, the use of a datum is optional. When a datum is not present, the callout only controls the feature’s form. But if we want to control additional aspects such as the orientation, location, and size, we must define one or more datums as needed.

When to Use Profile of a Surface Tolerance?

As mentioned in the above examples, the profile of a surface can effectively replace almost all of the form and location tolerances. So the question then becomes:

Why isn’t it used everywhere?

The answer boils down to pricing. In order to measure the surface profile, the part has to be measured via a CMM machine. CMM machines can measure either with a probe or with a laser. Regardless both of these options are expensive because:

1. It takes time to program the machine and then measure. This is not feasible for parts in running production.

2. They require skilled operators: The operator should not just be trained in measurement, but he/she should also know how to interpret the drawings. Such operators don’t come cheap.

Hence it is strongly advised to engineers to make sure that the surface profile tolerance is necessary and no other GD&T can fulfil the requirement.

Important Points to Remember

Here are some best practices and important points to consider when using the profile of surface tolerance.

1. As with all GD&T, make sure that basic dimensions are used, when adding numerical dimensions for features controlled with surface profile control.

2. The numerical tolerance range for the surface profile depends on the following factor:

   1. Is the intended surface exposed or assembled?

   2. What kind of environmental factors will the part be subject to? Putting tight profile control on a bumper panel for example does not make sense if the part will expand and contract depending on the temperature

Therefore it is extremely important that tolerance chain analysis is conducted beforehand to find the numerical values that are put in the drawing.

Conclusion

The surface profile is an extremely powerful GD&T symbol that can be used for complex surface profiles for all kinds of parts. However, it should be used keeping in mind the associated costs for measuring and verification of parts.