How to: Specify Sheet Metal GD&T Requirements

Prerequisites: Understanding the Foundation

Before adding geometric dimensioning and tolerancing symbols to a sheet metal RFQ, the engineering team must understand the part’s function. A flat panel, a folded bracket, or a formed housing each demands different precision levels. Procurement managers often default to general tolerances from industry standards because they are easy to apply. However, this approach can lead to parts that pass dimensional checks but fail in assembly.

The primary prerequisite is a clear understanding of the fit. Does the part mate with a precision machined surface? Does it rely on a formed flange to locate against a hole? The answer dictates which features require explicit GD&T requirements. If the part simply holds a component in place, a flatness or parallelism check may suffice. If it seals against a gasket, the criticality shifts to surface condition and edge squareness.

Second, the engineering team must have access to the actual sheet metal fabrication process. Roll forming, press bending, laser cutting, and hydroforming each introduce different variation patterns. A press formed bracket may have a predictable angular deviation along the bend line. A laser cut part may show consistent edge variation across the sheet. Knowing this helps in choosing the right tolerance type. You are not just specifying a number; you are specifying a method of control that the fab shop can achieve.

Finally, the team should review the cost implications. Every additional GD&T symbol increases the inspection effort. A part with five geometric tolerances requires more setup time and more measurement points than a part with two. The goal is to specify only what affects function, fit, or form.

Step 1: Identify Functional Features

Start by listing every feature that impacts assembly, function, or aesthetics. This is not a list of every dimension on the drawing. It is a list of points where variation causes a problem.

For a sheet metal enclosure, the functional features might include the flatness of the mounting surface, the perpendicularity of the side walls to the base, and the hole position to a mating part. For a heat sink, the feature list might focus on the parallelism between fins and the mounting face.

Write these features on a separate worksheet. Do not yet assign symbols. Just identify what matters. This step forces the team to separate noise from signal. Many sheet metal parts fail because the engineering team specified tolerances on non-critical features while leaving critical features to general tolerances.

Step 2: Establish a Datum System

A datum is the reference point against which all other measurements are made. In sheet metal fabrication, the datum choice is critical because it dictates how the part is held during inspection.

A common mistake is to use a feature as a datum that is difficult to measure or that varies significantly with the forming process. For example, using a thin, formed flange as a primary datum is problematic because that flange itself may have variation. Instead, use the largest, flattest, most stable surface.

For a typical folded bracket, the primary datum is often the largest flat face. The secondary datum is a perpendicular edge or hole. The tertiary datum is a third axis or feature that prevents rotation. The symbol order matters. A drawing might show a feature controlled by the primary and secondary datums, or by all three. The more datums involved, the more complex the inspection becomes.

When defining datums, consider how the fab shop will measure the part. If the part is held on a flat plate, the flat plate is the primary datum. If the part is clamped to a fixture, the fixture features are the datums. The drawing specs must match the inspection method.

Step 3: Select the Correct GD&T Symbol

Once the functional features and datums are defined, assign the appropriate geometric dimensioning symbol. The symbol must match the type of variation you are controlling.

A common error is to use a flatness symbol for a feature that is really a perpendicularity issue. Flatness controls the deviation of a surface from a plane. Perpendicularity controls the deviation of a surface from being exactly at 90 degrees to a datum. These are different problems. A surface can be flat but not perpendicular. A surface can be perpendicular but not flat.

For sheet metal parts, the most common symbols include flatness, perpendicularity, position, and true position. Position is often the most misunderstood. Position controls the location of a feature relative to a datum. It is not the same as a hole position tolerance. A hole position tolerance controls the hole center. Position controls the entire feature, including the hole, the slot, or the edge.

Use a table to map features to symbols. This keeps the process systematic and reduces the chance of selecting the wrong symbol.

Feature Type	Recommended GD&T Symbol	Reason
Mounting surface	Flatness	Controls surface deviation for gasket or mating fit
Side wall to base	Perpendicularity	Ensures 90 degree relationship for assembly
Hole location	Position	Controls hole center relative to datums
Formed flange	Parallelism	Ensures flange is parallel to a reference face
Edge squareness	Perpendicularity	Ensures edge is at 90 degrees to a reference plane

Step 4: Define Tolerance Values

Assigning tolerance values is where procurement managers often struggle. The value must be tight enough to ensure function but loose enough to be manufacturable.

Start with the general tolerance for the material thickness. Sheet metal fabrication has inherent variation in thickness, bend allowance, and forming. If the material is 1.5 millimeters, the bend line location will have a variation range. The tolerance on the bend angle must account for this.

For geometric tolerances, use a standard approach. Start with a value that covers the expected process variation. Then adjust based on the function. For a non-critical feature, a tolerance of two times the expected variation may be sufficient. For a critical feature, the tolerance may need to be one time the expected variation or less.

Avoid rounding. A tolerance of 0.5 millimeters is not the same as a tolerance of 0.4 millimeters. The difference may be the difference between a part that fits and a part that scrapes. Be specific.

Step 5: Align Inspection Levels with Criticality

Not every GD&T requirement needs the same inspection level. A critical feature that affects assembly may require 100 percent inspection. A non-critical feature may be covered by a sample inspection or a first article inspection.

Define the inspection method in the drawing specs. A flatness check on a 300 millimeter by 200 millimeter surface may require a flat plate and feeler gauges. A position check on a hole array may require a CMM. The inspection method determines the cost and the time.

For RFQs, specify the inspection level per feature. A part with ten geometric tolerances but only two critical features should not be inspected as if all ten are critical. This is a common source of cost inflation. The fab shop will quote based on the inspection effort. If the inspection level is vague, the shop will assume the worst case.

Common Mistakes in Specifying GD&T Requirements

The most common mistake is applying GD&T to every dimension. This creates a drawing that is difficult to read and expensive to inspect. The engineering team must resist the urge to control everything. Control only what matters.

Another mistake is using a datum that is not stable. If the primary datum is a formed flange, the entire tolerance stack-up becomes unstable. The flange may twist slightly during forming. This variation gets added to every other tolerance on the drawing. Use a stable, flat surface as the primary datum.

A third mistake is ignoring the material thickness. Sheet metal parts are thin. The tolerance on a bend angle is not independent of the thickness. A 1.5 millimeter sheet and a 3 millimeter sheet will have different bend variations. The drawing specs must reflect the actual material.

A fourth mistake is not specifying the inspection method. If the drawing says “flatness 0.25,” the fab shop does not know how to measure it. Does the part need to be held on a flat plate? Does it need to be measured at specific points? Specify the method.

Final Verification: Reviewing the Drawing Before Release

Before releasing the drawing for an RFQ, conduct a final review. Check each GD&T requirement against the functional feature list. Ensure the datum system is stable. Confirm the tolerance values are realistic for the fabrication process.

Ask the fab shop to review the drawing. A good shop will identify features that are difficult to measure or that are likely to cause problems. This is not a sign of weakness. It is a sign of experience. If the shop cannot measure a feature, the drawing is not complete.

The final verification step is to simulate the inspection. Pick a part. Measure the features. See if the inspection method makes sense. If it does not, revise the drawing before release.

A well-specified GD&T requirement is clear, measurable, and aligned with the part function. It reduces ambiguity in drawing specs and ensures that the sheet metal parts arrive on time, within budget, and ready for assembly.