How to Specify Tolerance Zones in Forging RFs

Why Tolerance Zones Matter in Forging

Forged components rarely rely on a single dimension. A turbine shaft, for example, depends on the fit between the shaft and the bearing, the alignment of the flange, and the wall thickness of the pressure boundary. If your request for quotation lists these dimensions without context, the supplier has to guess. They will either quote a loose tolerance that risks assembly failure or a tight one that increases material cost. The difference between those two outcomes is usually the difference between a smooth production run and a cycle of rework.

Defining tolerance zones means grouping dimensions by their function and assigning acceptable variation based on that function. It is a way of telling the supplier exactly where precision is needed and where it is not. The following steps outline how to build this specification into your RFQ.

1. Identify the Functional Dimensions

Start by listing every dimension on the part that affects its function. For a drive shaft, this includes the shaft diameter, the bearing journal length, the keyway width, and the total length. For a flange, it includes the bolt circle diameter, the bore, and the face flatness. Do not include cosmetic dimensions unless they are contractually required. Every dimension on the drawing should have a reason. If a dimension does not affect fit, function, or safety, it may not need a tolerance.

This step requires you to understand the part. If you are buying a complex assembly, work with your design engineer to identify the critical interfaces. If you are buying a simple part, the function is usually obvious, but it must be stated.

2. Assign Tolerance Zones by Function

Not all dimensions deserve the same level of precision. Group dimensions into zones based on their role. A typical breakdown looks like this:

Tolerance Zone	Function	Example Dimension	Typical Tolerance Range
Critical	Directly affects fit or safety	Shaft-to-bearing journal diameter	±0.05 mm
Important	Affects assembly or performance	Flange bolt circle diameter	±0.5 mm
Standard	General form and fit	Part length, non-critical width	±1.0 mm
Permissive	Cosmetic or non-functional	Surface finish, minor contour	±2.0 mm

The ranges in the table are examples. Your specific material, size, and process will dictate the actual values. The point is to categorize dimensions so the supplier can apply the correct level of effort. A critical zone demands a tighter control strategy than a permissive zone.

3. Select the Appropriate Tolerance Class

Once dimensions are zoned, assign a tolerance class or a specific value. For standard parts, you can reference a general standard for mechanical tolerances. However, for critical forged parts, it is often better to specify exact values rather than relying on a default general tolerance. Default tolerances on drawings are often too loose for precision forging. They are designed for machined parts where tolerances are easily controlled. Forgings have more variation due to the forming process.

If you specify a value, use a clear format. A diameter of 50.00 mm with a tolerance of ±0.10 mm is clear. A diameter of 50 mm with a general tolerance of ±0.5 mm is less precise. For critical dimensions, use the plus-minus format or a limit format (e.g., 49.90 to 50.10 mm). Avoid using words like “close” or “tight” without a numerical value.

4. Define the Inspection Method

A tolerance is only as good as the method used to measure it. If you specify a tolerance of ±0.05 mm, the supplier needs to know how they will measure it. For small diameters, a micrometer or a CMM is appropriate. For large flanges, a CMM or a profile projector may be needed. For flatness, a surface plate with a dial indicator might be sufficient.

Include the measurement method in the drawing or the RFQ notes. For example, “Measure diameter using a CMM. Report max deviation from nominal.” This prevents disputes. If the supplier uses a caliper for a dimension that requires CMM-level precision, you may reject a part that is actually within the tolerance but measured incorrectly. Clear measurement instructions reduce the need for second opinions.

5. Specify the Datum and Coordinate System

Tolerances are relative to a reference point. If you specify a position tolerance for a bolt hole, you need to define the datums. Datum A might be the top face. Datum B might be the centerline of the shaft. Without datums, the tolerance is ambiguous. The supplier may measure from a different reference, leading to a part that does not fit.

For complex parts, use a coordinate system. Define the origin, the X, Y, and Z axes, and the datums. This is standard practice in CAD drawings. If you are using a 2D drawing, label the datums clearly. A common mistake is to specify a position tolerance without referencing a datum. This forces the supplier to guess the reference, which often leads to rework.

6. Review the Tolerance Stack-Up

Tolerances add up. If a part has three dimensions that must fit together, the total variation is the sum of the individual tolerances. For example, if a shaft diameter has a tolerance of ±0.05 mm, and the bearing bore has a tolerance of ±0.05 mm, the total fit variation is up to 0.10 mm. If you need a tighter fit, you may need to reduce the individual tolerances or specify a tighter assembly requirement.

Perform a tolerance stack-up analysis before finalizing the RFQ. Use worst-case stacking for safety-critical parts. For less critical parts, statistical stacking may be acceptable if the supplier can prove process capability. This step prevents the situation where individual parts are within tolerance, but the assembly does not fit.

7. Define the Acceptance Criteria and Rejection Process

What happens if a part is outside the tolerance? You need to define this. Does a single deviation of 0.06 mm on a critical dimension reject the part? Or is there a concession process? Define the Acceptance Quality Plan. Specify the sampling method. For example, “Inspect 100% of critical dimensions. Sample 10% of standard dimensions.”

Also, define the rejection process. If a part fails, who is responsible for the cost of rework? Can the supplier machine the part to bring it within tolerance? If machining is allowed, specify the material removal limit. For example, “Machining allowed up to 0.5 mm of material removal.” This prevents the supplier from quoting a low price and then charging for extensive machining later.

Common Mistakes in Specifying Tolerances

Several errors appear repeatedly in forging RFQs. The first is using default general tolerances for critical dimensions. A general tolerance of ±0.5 mm is often too loose for a precision shaft. The second is mixing metric and imperial units on the same drawing. This causes confusion and errors. The third is omitting datums for position tolerances. The fourth is not defining the inspection method. The fifth is failing to account for tolerance stack-up in assemblies.

Each of these mistakes increases the risk of rejection. A rejected part costs the supplier time and material. It costs you time and delays. The cost of a clear RFQ is a few hours of engineering time. The cost of a vague RFQ is a week of back-and-forth, rework, and potential production delays.

Final Verification Step

Before issuing the RFQ, run a final check. Print the drawing. Read every dimension. Ask yourself: Does this dimension have a function? Is the tolerance appropriate for the function? Is the measurement method defined? Are the datums clear? Can the supplier understand what is required without asking questions?

If the answer to any of these questions is no, fix the drawing. A clear RFQ is not just a document. It is a tool for reducing risk. By specifying tolerance zones carefully, you align your expectations with the supplier’s capabilities. You reduce the number of questions. You reduce the number of rejected parts. You build a relationship based on clarity, not guesswork.

Conclusion

Specifying tolerance zones in a forging RFQ is a discipline. It requires understanding the part, the process, and the assembly. It requires attention to detail. But it pays off. A well-specified RFQ reduces rework. It clarifies costs. It builds trust. Use the steps above to create a drawing that works for you and your supplier. The result is a more efficient process and a better part.