Developing a Default Tolerancing Standard for CAD Drawings

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Several recent episodes of this column have discussed methods to speed up and standardize the CAD jockey’s data entry chores. Picklists, radio buttons and checkboxes have all been covered (see Figure 1). The overall goal behind the data entry form is less typing and more productive modeling.

As a CAD project that will have a lifespan of several years, the FMA cart is something you are now very familiar with (see Figure 2). The finished project will likely have dozens of drawings for fabrication and assembly that will be reviewed by many hands.

Please note this disclaimer: the shopping cart is a product in the sense that a CAD model of it exists. This product is imaginary in the sense that it is a DIY project, not for sale. It is used here for the discussion of CAD techniques.

An investment in designing templates and forms quickly pays for itself in projects like this. A good combination of forms and templates standardizes and speeds up work.

The topic of tolerances in the February 2022 episode caught the attention of Mike Matusky in Everett, Washington. With his help, we incorporate some of his thoughts into the design of our templates and forms.

The drawings (PDF) created for this project will be used to control the manufacturing process, so that interested parties (including inspectors) know what is required. The plans exist within the framework of a quality plan.

In this scenario, we stipulate that our corporate standards exist as part of a quality plan. This includes the policy and procedure for specifying the inspection equipment to be used, how it should be installed and what should be recorded and calibrated. The quality plan controls drafting standards and guides the design effort that goes into forms and templates. When it comes to drawing, geometric dimensioning and tolerancing (GD&T) is a key player in communicating pass/fail decision requirements.

A detailed quality plan and comprehensive GD&T training is beyond the scope of this article. Both are recommended and important.

Usability via tolerance

Matusky observed that CAD techniques for filling information fields in title blocks are well covered so far. How the default tolerance appearing in the title block ensures the usability of the finished product is an overlooked and important topic.

To paraphrase Matusky’s imperative, following ASME Y14.5 drafting standards and using GD&T appropriately can pave the way to fewer scraps, rejects, and failures. Beware of the perilous path of persisting in using tolerated (plus/minus) dimensions to locate, orient and define the shape of part features.

A data entry form appears.

FIGURE 1. A data entry form is shown at right. A drop-down selection list for the finish is active. Picklists reduce the amount of typing and creative writing.

picture 3 is an example of a marginal corporate norm. This is a drawing for a sheet metal battery tray. The title block displays a default tolerance. Dimensions have their precision (number of digits) adjusted to indicate the acceptable tolerance.

Additional note: The idea behind the default tolerance is to avoid having to add a tolerance to each drawing dimension. Another method is to declare in a standard note that intolerant the dimensions are basic. (Note that “base dimension” has a very specific meaning in Y14.5.)

It’s almost good in the sense that plus/minus dimensions aren’t always bad drawing technique. Pass/fail decisions can be made reliably if the plus/minus feature can be brought into contact with the inside or outside jaws of a caliper in a way that reflects the function of the feature. This is the case of figure 3. The measures of acceptance/rejection of these functionalities are easy to reach.

On the other hand, if a coordinate measuring machine (CMM) or similar test gauge is required in the quality plan, GD&T is a good method to communicate how to inspect the part.

Even though the part can be inspected with calipers, the company standard we have shown in Figure 3 is problematic.

Rounding error

Figure 3 echoes this company standard: a dimension displayed with more decimal places means a tighter tolerance. In this case, more precision is needed. An example is “X.XXX ±0.020, X.XX ±0.05, XX ±-0.3.”

This round tolerance pattern is labor saving for the CAD jockey. Simply increase or decrease the number of digits in the rating for more or less accuracy. While saving labor from a point of view, this is not a win for manufacturing.

As an example of a marginal result, rounding a value to three digits (0.425 inch for example) introduces a discrepancy. The drawing shows 0.43 inch with ±0.05 inch as the default tolerance, while the STEP file shows the nominal value of 0.425 inch. This permanently puts inspection and CNC programming at odds of 0.005 inches.

As another issue, the tolerable size allowed (0.380″ to 0.480″) may just represent laziness and not work in terms of room usage.

And to complicate matters further, nominal dimensions do not always have symmetrical tolerance bands on the drawing. Should the 3D model then show a limit or should it be changed to an average value? The answer lies in the quality plan.

The FMA cart is displayed.

FIGURE 2. This product will require many drawings and probably several people will work on it over time. At the time of the launch of this new project, the forms and templates are designed to reduce labor and improve consistency.

To avoid such confusion, GD&T makes a distinction between size, position, and profile tolerances, with position and profile being measured against a reference frame of reference (DRF).

Perhaps a better model design for the company standard title block involves showing default values ​​for each type of tolerance. Matusky suggests that incorporating a bit of GD&T into the company standard isn’t that painful and is likely to reduce manufacturing cost due to higher yields and less inspection time. .

In the proposed GD&T compatible enterprise standard, the various default tolerances in the title block would apply to all features. These default tolerances would be established to facilitate manufacturing. The default tolerances are trip wires to detect a process that is blatantly malfunctioning.

Features requiring tighter control would be called out on the drawing to override the defaults in the title block. As Matusky noted, such overriding notations automatically emerge. Routine inspection of only critical features not only saves time and money, but is also an achievable goal.

Goof Proof via Benchmarks

To quote Matusky, “The features that locate the part in the next assembly must be the same features that establish the default DRF.”

In this paradigm, the tolerances that are inspected against a DRF are actually inspected as the parts are assembled. DRF is a way to design quality instead of trying to inspect it later.

A corporate standard for GD&T implementation should specify how to select the reference functions used to establish the DRF for default tolerances. Features that locate the part should be designated as datum features.

Does a default tolerance belong to the title block? Would it be better to reduce the size of the title block (no default tolerance) or to enlarge the title block to display several types of tolerance? What notes hide in the shadow of the models?

Plus/minus dimensions are highlighted in a printout.

FIGURE 3. When the feature can be measured with the jaws of a vernier caliper, including plus/minus dimensions in the print does the job. Using a number of digits to indicate the required precision can be confusing.