Most designers of stamped parts don’t need to be convinced of the value of specifying tolerances as a regular practice. This example in Machine Design illustrates why tolerancing is always a good idea. “A machine shop that sees an untoleranced diameter, without knowing the design intent, may apply a standard tolerance for three-decimal-place untoleranced dimensions, ±0.005 in. [which] may result in interference, where the hole is smaller than the shaft diameter, which prevents the parts from sliding together … if too large of an interference exists, it will degrade performance.” As the article notes, the end result is usually extra time and money to rework the parts.
But including dimensions and tolerances isn’t a foolproof solution. When you spend time thoughtfully developing a part with features as elegant as they are functional, it’s all too easy to focus on design to the exclusion of manufacturing realities. This approach makes for tidy design on paper but often presents a challenge when it’s time to form the part.
Design for Manufacturability
Ask many manufacturers and they’ll lament there’s a gap in knowledge between designed perfection and actual fabrication in terms of dimensions and tolerances. In fact, “research suggests that manufacturers spend 30% to 50% of their time fixing errors and almost 24% of those errors are related to manufacturability.” The key point is this: just because you can design it doesn’t mean it is practical or achievable to build in terms of budget, time, or tooling.
In progressive stamping applications, consider these aspects of manufacturability:
- How critical is it to achieve each specified dimension, and what variances are acceptable?
- How do the variances impact each other in the finished part and its performance?
- Is the time and effort required to create the part scalable, that is, is it realistic to do a long, high-volume run of the part or would it become cumbersome, cost-prohibitive, or time consuming to make more than a handful?
- How can we extend the life of the tooling as long as possible for a cost-effective and efficient run?
The more familiar a designer is with manufacturing processes and what techniques are required to achieve various features, the better design decisions he or she can make. Knowing how features like leg length, width, hole diameter, and complex angles wear over time and affect tool life is essential to proper dimensioning in design.
Industry Standards and Tolerance Analysis
Designers apply industry standard tolerances (e.g. ANSI, ISO, DIN) and may also work with their company’s own in-house standards. Common standards in precision stamping include:
- ISO 2768 parts 1 and 2 (tolerances for linear and angular dimensions and other features without individual tolerance indications)
- ASME Y14.5 (the theory, rules and procedures for Geometric Dimensioning and Tolerancing, or GD&T, a common language developed to describe the engineering intent of parts and assemblies)
- ANSI B4.1 (limits and fits for nonthreaded cylindrical parts)
Perhaps more important than individual dimensions and tolerances is understanding what happens when all of a part’s features are taken as a whole. Variations in each feature accumulate or “stack up” and impact each other. They are caused by both the material and the manufacturing process, so designers need a way to anticipate what can happen when things like metal strip thickness, and location of holes or other features come into play. These include:
- worst case analysis (the sum of each individual tolerance for a grand total)
- root-sum square analysis (a distribution of how each dimension varies and the probability that it will stay within acceptable limits)
- second order tolerance analysis (looking at changes in the feature, part, and dimensions during the manufacturing process, as well as how all of these variations effect the final part)
So how does tolerance analysis apply to the industry standard tolerances many designers use? With GD&T principles (as defined in ASME Y14.5) the actual tolerance depends on the associated features. Consider a leg feature with a 90-degree angle +/-1 degree. If it is further defined by a perpendicularity callout with a tolerance of 0.004 in., depending on the length of the leg, that tolerance might translate into +/- 0.2 deg. or less, which is far harder to achieve. While the difference of a single degree may seem feasible, the actual tolerance according to GD&T requirements makes it unfeasible. This is because perpendicularity is a much tighter requirement, so it overrides the angular tolerance.
Another method of analysis, Statistical Process Control (SPC) factors in process capability, or Cpk (the ability of a given process to meet requirements) and further impacts actual tolerances. For example, given a Cpk requirement of 1.67, imposing an SPC requirement for a 0.250 in. hole with a tolerance of +/-0.004 in. can have the effect of restricting the working tolerance by roughly 50 percent to +/-0.002 in., which impacts manufacturability as well as cost.
Stamping in the Real World: An Example
Here’s how a stamping manufacturer can make adjustments to tolerances for a 0.250 in. diameter hole punched in steel while anticipating tooling wear and maintenance. The end result is a successful balance of cost, time, and part performance:
- Approximately the first few thousand pieces produced should be nearly identical, within tenths (0.0001 in.) of each other. But the punch wears a little with each punch, and over time the punch and the hole get smaller and smaller. By the 10,000th punch, the hole’s diameter measures 0.249 in., and by 20,000 it’s 0.248 in. and so on over time. If the tolerance of the hole is 0.250 in. +/-0.002 in., production stops for tool maintenance every 20,000 punches.
- One adjustment stampers make is to start at the top of the tolerance, to allow for the wear. So a punch sized to 0.252 in. is used, which extends the run to 40,000 before maintenance (because now the punch can wear down an additional 0.002 in.).
- A more ideal run would be 80,000 punches, and achieved by expanding the tolerance to a more manufacturable +/-0.004 in.
The takeaway here is that in this case, the wider tolerance provides more production time between maintenance cycles and allows for less complex inspection, less often, which saves money.
Best practices for designers and engineers include recognizing the interconnectedness of a part’s features and understanding the give and take between features and their corresponding manufacturing processes. We can help make sense of the many factors in play.