The forging process produces near-net-shape parts where strength is maximized through metal grain flow. After forging, these parts typically move into machining operations that create the geometry and surface finishes needed.
Machining is a necessary step because forging alone can’t hold the tight tolerances and achieve the fine surfaces that many finished parts need. Part-to-part variation is an inevitable part of forging and is addressed through machining allowances and forging tolerances.
Machining allowances put additional material on the forged part. This ensures the finished part can be machined from the forged shape. Forging tolerances are added to these allowances to ensure features and surfaces are of the required size and have the correct relationships.
To help forging manufacturers apply realistic tolerances, the Forging Industry Association (FIA) has established the “Guideline Tolerances For Hot Forged Impression Die Forgings”. These are derived from industry experience of what is actually achievable.
For anyone who’s asked why tolerances are important in forging, what tolerances are in forging, or how they are derived, here’s an explanation.
Dimensional Tolerance
A dimensional tolerance is the range over which a length, width, radius, or height can vary and still be acceptable. Dimensional tolerances are used to control the size of specific features. They are usually expressed as a nominal value, plus or minus the amount by which it can vary.
Adding the upper and lower variation permitted yields the tolerance range.
As an example, for forging, the FIA recommends a tolerance on length measurements of +/-0.003” per linear inch.
Shape Tolerance
Shape tolerances, sometimes known as form tolerances, control the form of a part. The main characteristics covered by shape tolerances are flatness, straightness, roundness, and cylindricity.
To illustrate how shape tolerances are used, consider a long forged beam. Length, height, and flange width might all be within limits (the dimensional tolerances), but the part could be bowed or twisted enough to make it unusable. Shape tolerances provide a way to put limits on the amount of deformation that is acceptable.
Position Tolerance
The location of features like bosses is controlled through position tolerances. These set limits on the amount by which location can vary and still produce a good part.
For example, the center-to-center relationship of two bosses might be important for placing tapped holes for mounting the part to a bracket. A position tolerance will be defined by how much the relative position of the bosses can vary and still enable machining of the holes that are needed for correct mounting.
In forging, die mismatch can cause variation in the relative positions of parts formed by upper and lower dies. Die mismatch – a lateral shift in relative position – can lead to bosses being in the wrong position, or shafts not being cylindrical. Position tolerances provide a way of managing problems like these.
What Influences Tolerances
Forging Temperature
Forging temperature influences the ease with which a metal will deform under pressure. Die temperature affects how quickly the metal cools and which areas cool fastest.
If both die and billet temperatures are held within tight limits, they will contribute very little to part-to-part variation. However, fluctuations in temperature will cause changes in how metal flows and the speed and direction in which the forged part cools. Both of these will lead to shape and dimension variations that could put key features outside the tolerance limits. Alternatively, limits may need to be increased to avoid scrapping large numbers of forged parts.
Material Used/Steel Category
Some metals are easier to forge than others. Flow resistance, tendency to work-harden, coefficient of thermal expansion, and grain flow behavior all affect forgability.
Carbon steels tend to forge well. They have good flowability and produce consistent results, which allows the application of relatively tight tolerances. Aluminum, despite good ductility, is more challenging to forge because of high thermal conductivity and a relatively narrow forging temperature window. Cooling can therefore make it hard to maintain tight shape tolerances.
Titanium is an example of an alloy that’s harder to forge. It resists deformation, making it difficult to get it to flow into more distant areas of the die. Wider tolerances will result in a higher yield.
Part Geometry
The size of the part being forged affects the tolerances applied. With larger parts, there’s more distance or space for variation, so the effects are larger. This is why tolerances are often defined as a tolerance range per unit of measurement (such as +/-0.003” per linear inch).
Another factor is that with larger parts there’s more contraction during cooling. This can create distortion that affects shape.
Draft angles also pose challenges for how tolerances are applied to forged parts. Draft is needed to ensure the forged part releases easily from the die, but it changes the dimensions of features it’s applied to through their height or depth.
Draft angles and varying feature dimensions can be handled through machining allowances. If no machining is needed, though, dimensions that change due to draft should be toleranced like other dimensional features.
Part Mass
Mass affects temperature gradients, contraction, and forgability. In larger, heavier parts, the impact of these factors is greater, so tolerances are increased accordingly.
Parting Line
The position of the parting line affects the relative position of features on either side of the die. Accordingly, tolerances are often increased to allow for the greater variation that’s likely.
This issue is more significant when forging asymmetric parts. In these, the forces acting on the die are uneven, which can increase loads in certain directions. This can exacerbate problems like die shift. Accordingly, designs for asymmetric parts may need wider tolerances across the parting line.
Learn More From Trenton Forging
FIA guidelines for tolerances on forged parts provide a starting point, but part design strongly influences what can be held in production. Accordingly, forging businesses tackle each project on a case-by-case basis, determining what tolerances to apply to dimensions, shape, and feature position.
As a proven leader in the industry, Trenton Forging has the skills and expertise necessary to make these decisions in ways that optimize part performance and manufacturability. In addition, our advanced equipment, materials knowledge, and in-house tool design and manufacture capabilities give us the ability to forge high-quality, cost-effective parts.
If you would like to learn more about forging and the impression die process in particular, more information is available on our blog. If you have a specific project you would like to discuss, please contact us.
