Creating Better GD&T Datum Coordinate Systems | Design for Inspection, Part 3

Never underestimate the power of GD&T datums. As we reviewed in Part 1 and Part 2 of this series, by understanding how parts are manufactured and inspected, we use GD&T to improve both the drawing of our designed parts and the communication of our design intent.

By working with the quality inspector early, we also tune our drawings to the equipment, tools, and methods that they will use to confirm part dimensions and quality.

We called this approach “Design for Inspection” (DFI), because we are using GD&T to drive successful inspection outcomes.

Aaron Nelson AC Mechanical Engineer & Technical Lead

“To me, the advantage is that AC has a huge depth and breadth of engineering knowledge. It’s kind of like having our own inside consultancy within the company where our most experienced and mature engineers are available to pass down their knowledge. I’m a resource for other engineers who reach out to me for feedback and ways to improve their drawings using GD&T. That’s the whole purpose behind our company Lunch & Learns. We’re helping each other get better at what we do.”

As we move into this next part of our series, Aaron Nelson demonstrates how we can use GD&T to create a better datum coordinate system for this DFI approach.

Identifying the Optimal Measurement State: Free or Restrained

measurement and coordination

One of the foremost considerations that impacts how we define our datums is whether a part or individual feature should be measured in the free or restrained state.

Because the default measurement state is the free state, if we don’t specify how the part is to be restrained, the inspector will simply measure it in the free state, which may not be our intent.

Determine the measurement state

Certain factors like a part’s material, rigidity, or assembly can help us determine the optimal measurement state. Without careful consideration and planning, some parts will be distorted, deformed, or misaligned because the part is not tuned to the inspection technique, which results in failed parts.

Part attributes

Measurement state recommendations

Parts in an assembly with interfacing parts that could distort the part of interest

> Measure in the restrained state

> Use an inspection fixture to restrain the part the same way it is restrained in the assembly, this may deform the part, but in the actual assembly it would be deformed as well.

Parts that are small, rigid plastics (for example, gears)

> Measure in the free state by securing the part with clay to prevent unintentional movement by a CMM probe Do not use clamps that might distort the part

Parts that are larger and flexible


> Measure in the free state using a non-contact method like an OMM or laser point-cloud

> Avoid CMM probe hits that can deform the part

Specify restrained or free notation.

When parts or a feature are to be restrained in a fixture, we add a Restrained Condition Note to the drawing.

Then, when distinguishing features that should be measured in the free state from those that are to be restrained, we can add the Free state symbol in the feature control frame:

For a quick illustration showing how to use GD&T to add a Restrained Condition Note and Free State Modifier, link to the video below published by GeoTol.

Image Credit: GeoTol

Specify the part or individual feature that is to be measured either in the restrained or free state.

Selecting the All-Important Base Datum A

We need three datums (A, B, and C) to create a datum coordinate system where all the features related to that coordinate system will be measured. Datum A is the most important datum and serves as the base for the rest of our datums.

We have a number of design features to select from for Datum A – it can be any of these:

> Parallel (or not)
> Coplanar (or not)
> On the same surface (or not)
> Lines
> Points
> Surfaces

Datum A features do not need to be coplanar.

So, let’s examine some general guidelines to help us define our base Datum A.

Choose the largest surface area that is significant to the assembly.

Probably one of the most important features of Datum A is that it should cover a large area of our part that is significant to the assembly. Because imperfections on small surface areas result in multiplicative errors, we avoid using them for our base datum.

Add a flatness specification for control.

It’s common to add a flatness specification to control the Datum A feature for parts. This is especially true for flexible, plastic parts that may be prone to distortion.

flatness specification

Tip: If the part is restrained with a fixture to Datum A, then a flatness tolerance on Datum A is unwarranted.

For example, if Datum A is supposed to be a planar surface, but the part comes out warped, the inspector may pick three or four points for a best fit. But a best fit doesn’t tell us how far out of flatness Datum A is, which might be important to control later.

Consider features that interface with other parts.

Consider features that interface with other parts for Datum A. In general, we should try to select surfaces for datums that interface with other parts, such as screw bosses. Using screw bosses as a datum is discussed in more detail below: Fixturing for Datum A.

Add tooling flats to create coplanar or parallel surfaces for organic shapes.

With organic shapes, selecting Datum A can be challenging because the curved surface is dependent on the XY position, so we don’t have the benefit of planar surfaces. Planar surfaces are preferred because they’re less intolerant of the position where you measure them. But with a curved surface, as the XY position changes, the curvature changes.

For organic shapes that lack planar surfaces, we can solve the challenge of curvature by adding tooling flats to intentionally create coplanar or parallel surfaces for Datum A. These small planar surfaces can also be used for fixturing.

There are some pros and cons to using tooling flat features for Datum A:

> Pro: They are planar and less intolerant to position, and if spaced out widely enough, they also encompass the entire part for Datum A.
> Con: They are not non-functional features, so not considered ideal for Datum A.

Add a fourth point to create a best fit plane and control relative position.

Although we only need three points to define a plane, by using four points we can create a best fit plane. A fourth point also enables us to measure the location of all four points relative to each other to determine when one point is out significantly.

For example, we can use the tops of the four screw bosses of this gaming controller to define Datum A and establish our datum coordinate system.

the Fourth GD&T Datum

Tip: Add “All” to datums to specify a cluster of points.

Defining Datum A this way does over-constrain it, but using four points enables a best fit plane. And later we we’re able to control the position of the screw bosses with these basic dimensions.

Intentionally create coplanar or parallel non-functional surfaces to aid in datum creation and fixturing.

Add an offset to use non-coplanar features

When our Datum A features are not coplanar or continuous, we can simply add a basic offset to call out the distance between the datum surfaces.

In this example, we added an offset dimension to the lower two screw bosses because they are not coplanar to the top two screw bosses.

GD&T Datum A Offset

Fixturing for GD&T Datum A

Remember that one of our guidelines for selecting Datum A is to pick a surface that is significant to the assembly. For example, for this battery cover held down by four screws, there are three surface options for Datum A: two coplanar surfaces and one parallel surface.

Battery Cover GD&T Datums

The challenge is, how do we enable measurement of the screw bosses?

Use fixture options that enable measurement of screw bosses.

Looking at this drawing of the battery cover, we can see it’s constrained in a fixture using four gray clamps over our screw holes. Since the best measurement state is retrained for an assembly where parts are bonded using screws, this fixture was designed to simulate the state of the assembly.

Battery Cover Screw Bosses

But if we use these four surfaces for Datum A, the inspector won’t be able to measure the diameter of the screw holes because the fixture clamps are likely to be in the way. So, let’s consider a few different options to understand the optimal solution for designing a Datum A fixture.

Fixture Option 1 (least preferred)

Represent the perfect position of the screw holes with a pin

Fixture Option 2 (better, but not optimal)

Constrain the part against another features

Fixture Option 3 (best solution)

Restrain and enable measurement through a single datum

Our first option is to use pins in the fixture to represent the screw boss holes.

Pro: Perfectly positioned pins simulate the use of screws to control the position. The pins are forced into the correct position in the fixture through the screw boss holes in the assembly.

Pro: There’s need to measure the position of the pins.

Con: Because of the threads, the screws cannot be used to position the part.

Con: It does not allow a measurement of the screw holes.

Our second option is to constrain the part against another feature, such as a non-functional one.

Pro: Screw bosses are free to be measured.

Con: The part would not follow the guideline to constrain a part in the fixture similarly to its constraint in the assembly. But, in certain cases, this may be our best option.

The best option overall is to create a fixture that enables restraint and measurement of multiple points of access through a single datum.

Pro: Two spring-loaded clamps restrain the two bosses.

Pro: A cutaway in the clamp allows the CMM to probe the holes.

This solution gives us the best of both worlds: fixturing that restrains, while enabling unobstructed measurement.

GD&T Datum A Fixture Clamp Cutaway Bosses

Tip: Enable multiple points of measurement from a single setup.

Tip: Vertical fixtures enable multiple sides of access from a single setup.

At first glance, because we’re looking at a horizontal view of this fixture, we may miss a key design feature. The fixture has two feet on its base that enable it to be measured vertically, not horizontally. This design provides the CMM with access to both sides of the part without requiring a change to the datum coordinate system. This solution creates a more efficient inspection by enabling multiple measurements of all features from a single setup.

Simplify fixtures to enable faster inspection

There’s actually a disincentive for inspectors to inspect parts well. If the vendor begins to reject parts, there’s less yield which means their costs go up. So, whenever we can simply fixtures to enable faster inspection, we make it more likely that the inspection will be efficient and more complete.

Our goal should be to enable easier and quicker inspection by designing a less cumbersome fixture.

Here’s an example of how to enable a faster, more efficient inspection:
> By selecting a cosmetic, non-functional surface to restrain in the fixture, we provide the inspector will full access to all the internal features.

> In our drawing, we show the part flipped upside down and call out all the cosmetic surfaces as Datum F with a note to restrain the part for inspection.
> In the fixture, we mirror the non-functional, cosmetic surfaces and use rubber clamps to hold the part to a steel structure.

Cosmetic Surface GD&T
Cosmetic Surface Fixture

TIP: In some cases, using non-functional, non-interfacing surfaces can provide the best fixturing solution.

It’s worth pointing out that non-functional surfaces aren’t generally recommended for datums because they don’t interface with other parts. But sometimes a solution like this is worth the tradeoff because it provides full access to all the features of interest.

Tip: Make a duplicate fixture to correlate inspection data.

Whenever possible, creating a duplicate inspection fixture enables us to keep one fixture for our own use to correlate the measurement data between our fixture and the vendor’s.

Controlling Orientation with GD&T Datum B

With Datum A defined, it’s time to create Datum B relative to Datum A.

In this example, for Datum B, we selected one of the screw bosses.

GD&T Datum B
GD&T Datum B 2
Detail view of Datum B: Controlling perpendicularity to Datum A.

Because this is an injection molded part, let’s remember that nothing is going to be straight – everything is drafted. So, the screw boss is a conical surface.

Since Datum B is an axis, we can control the perpendicularity to Datum A. From our callout above, the inspector understands there is a cylinder of 0.10mm diameter which is perfectly perpendicular to A, and the B axis must lie within that cylinder.

We’re not controlling position; what we’re controlling is the orientation of Datum B relative to Datum A. This is an important method of control as we build up our datum coordinate system.

Tip: Remember the limitations of CMMs in measuring diameter.

Let’s also consider how many CMM hits it will take to control perpendicularity.

How many points does it take to create an axis?
> The rule of thumb is we need two circles – the center point of each circle creates the axis.
> So, if we need three points to create a circle, then we need a minimum of six points to create an axis from two circles.

But a CMM probably won’t represent the entire conical surface, because it can’t get to the bottom, and it won’t try to get too close to the top. So, this means the CMM will only be able to measure the axis at the middle of the conical section.

Controlling Relative Position with GD&T Datum C

For Datum C, we can use the other screw boss to control the position relative to Datum A and B.

GD&T Datum C
Datum C
Detail view of Datum B: Controlling perpendicularity to Datum A.

In our example below, the callout indicates control to Datum A is a perpendicularity tolerance (similar to Datum B) and control to Datum B is the distance, which is 112mm.

So, based on our specification, the inspector needs to ask: “At this distance, does the Datum C axis fall within a cylinder of the specified diameter?”

Orientation of Datum C is controlled to Datum A and B.

Tip: Request the CMM hit map to confirm accurate measurement.

There are different ways of defining the position of Datum C in this orientation. One is to confirm that the center point is actually about the axis of this feature. This is a good time to request the CMM hit map to ensure a minimum of six hits is planned in order to confirm our axis and measure the position.

Tip: Specify the diameter measurement for a drafted hole.

With Datums A, B, and C defined, let’s go back and measure our conical screw boss hole.

Here we specify a 2.83mm diameter, but we know that because of its conical shape, the diameter will change with the depth through the hole.

GD&T Datum C Diameter

So, how can we clarify what this diameter is actually referring to?

Let’s us an exaggerated side view of our conical screw hole to illustrate that it’s not clear if the diameter callout is referring to the top of the screw hole or the bottom. There’s a lot of concentric circles going on here.

Some challenges to resolving this are:

> We can’t measure the hole with a pin gauge because that would require a tapered pin which further complicates it.
> It’s doubtful that an OMM can measure it because these are blind holes; there is no light to enable a passthrough to create a high contrast edge.
> A CMM is the right tool, but it can’t touch the edge at the top, or the edge at the bottom, and it’s difficult for CMM probes to find corners. It needs to measure someplace in the middle.
> Some engineers create a plus draft or minus draft notation as an easy way to get around this situation, but that still leaves too much room for misinterpretation.

To specify our intent clearly, it’s much better to define the position where we want the diameter measured on our drawing.

For example, with Datums F and G, we can specify how to measure the position and diameter of the screw boss holes:

By defining the dimension from Datum A, we give the inspector the diameter for that depth based on our datum coordinate system, which:

> Removes ambiguity
> Makes it easy for the CMM to measure and hit the diameter
> Satisfies our diameter constraint
> Enables the inspector to use this circle as one of the two circles needed to create the axis to measure perpendicularity

Adding the Defined Datum Coordinate System to our CAD Drawings

We now have three datums necessary to constrain the part:

> Datum A is a plane
> Datum B is an axis
> Datum C is an axis

The coordinate system is finished when all six degrees of freedom are constrained – three translational and three rotational.

The coordinate system is finished when all six degrees of freedom are constrained – three translational and three rotational:

> Datum A constrains one translational degree of freedom and two rotational degrees of freedom.
> Datum B constrains two translational degrees of freedom.
> Datum C controls one rotational degree of freedom.

Although it’s not required by ASME, in order to clearly communicate the intention of our datums, let’s add the coordinate system to our CAD and drawing using a coordinate system symbol.

Tip: A good method of distinguishing multiple coordinate systems is to place the datum letters that were used to create the coordinate system on the axes.

In the next part in this series, we’ll take a look at several essential GD&T techniques that are worth the effort to learn and use regularly.

Next in Part 4: Using Advanced GD&T Techniques



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