Most tolerance issues look harmless on paper.
But in production and assembly, they will increase your part costs, rework, and scrap.
Here are 7 mistakes to avoid when tolerancing your sheet metal projects.
Why tolerance mistakes are so common
Most tolerance issues start with design. They’re generally caused by a lack of understanding of fabrication processes and how sheet metal behaves.
Drawings only describe intent. While fabrication introduces many unknown variables such as material behavior, tooling limitations, and other features and variables. And when that gap isn’t accounted for, tolerance issues will appear later, and often more expensively.
Mistake #1: Applying the same tolerances
Applying a tight tolerance globally across an entire part is one of the most common mistakes.
Why it happens
- It feels consistent and safe
- It avoids identifying functional features
- It “saves” design time only to reveal problems later in manufacturing
- It’s often inherited from previous drawings
Why it’s a problem
Not every feature affects fit or function. Applying the same tolerance everywhere triggers more inspections, more time, tighter controls, and higher scrap risk. Often, not worth it.
Mistake #2: Reusing legacy tolerances
Many OEMs carry forward default tolerances from past projects to save time.
Why it happens
- Drawings are copied and modified
- Defaults are rarely questioned
- “It worked before” becomes the justification
Why it’s a problem
- Processes and volumes change
- Design features change
Legacy tolerances often reflect old constraints; not current ones leading to unnecessary costs.
Mistake #3: Tight tolerances everywhere
Using tight tolerances is often a substitute for uncertainty.
Why it happens
- Limited production feedback
- Limited manufacturing experience
Why it’s a problem
- Tighter tolerances increase setup time, tooling costs, inspection effort, and scrap risk.
Most times, it does not improve performance, just adds costs.
Mistake #4: Ignoring feature design
Tolerance decisions are often made without considering the process that creates the dimension.
Every process behaves differently:
- Laser cutting vs forming
- Flat features vs bended features
- Single parts vs assemblies
And when tolerances don’t align with the process, instability will show up in production.
Mistake #5: Not factoring assembly stack ups
Most individual tolerances look reasonable in isolation.
But their combined stack up effect on an assembly will cause issues on other areas. All of this must be accounted for. Every small tolerance variation accumulates across features and parts.
So, what passes inspection individually will often fail in final assembly.
Mistake #6: Assuming part or feature tolerance capabilities transfer over
A common mistake is assuming that because a specific tolerance has held before for a specific part or function, it can be achieved everywhere.
Why it happens
- “We’ve held this tolerance before”
- Capability assumed to be universal
Why it’s a problem
Capability largely depends on:
- Machines
- Current setup
- Current tooling
- Process selection
Assuming tolerance transferability leads to cost escalation and further product instability.
Mistake #7: Not identifying critical tolerances
Everything has a tolerance, but not everything is critical.
Why it happens
- Critical features aren’t explicitly called out
- All tolerances are equally important
Why it’s a problem
- Non-critical features get over-controlled
- Truly critical dimensions don’t get the focus they need
This is often a communication design error, not math error.
The most important tolerance rule
If a sheet metal part tolerance does not directly impact:
- Fit
- Function
- Safety
- Compliance
It likely doesn’t need to be overly tight. This is the most important rule of them all.
Tighten only the features that are critical to your application and nothing more. If unsure, confirm with your suppliers who often can often spot the issues before production.
Summary
- Most tolerance mistakes come from hidden assumptions
- Global and inherited legacy tolerances quietly increase cost
- Over-tolerancing is often used as a safety blanket
- Processes and stack-up tolerances matter
- Capability does not automatically transfer
- Critical features must be explicitly identified
Good tolerances don’t make parts harder to build. They make your parts faster, cheaper, and easier to produce – while meeting your requirements for form, fit & function.