One of the most common mistakes in mechanical design is treating tolerance as decoration on a drawing. A designer adds a small number because the part “needs to be accurate,” but there is no clear reason for the value. That approach can make a drawing expensive to manufacture without making the product better.
Engineering tolerance selection should start with function. The right tolerance is the amount of variation the part can accept while still performing its job.
Start with the function
Before choosing a tolerance, ask one question: what does this dimension control? A dimension may control clearance, alignment, fit, appearance, motion, sealing, structural position, or simply the overall envelope.
If a 100 mm cover dimension can vary from 99.5 to 100.5 mm without affecting anything, there is little value in specifying ±0.05 mm. If a bearing seat controls the position of a rotating shaft, the tolerance may need to be much tighter.
Do not make every dimension tight
A drawing full of tight tolerances often looks precise, but it can be a sign that the designer has not separated critical dimensions from non-critical dimensions. Tight tolerances increase machining time, inspection effort, rejection risk, and sometimes tooling cost.
Use general tolerances for ordinary dimensions and reserve tighter limits for functional features. This makes the drawing easier to read and gives manufacturing teams a clear signal about what actually matters.
Consider the manufacturing process
The process should influence the tolerance. A laser-cut sheet-metal profile, a CNC-milled hole, a turned shaft, and a plastic injection-moulded feature naturally have different capability ranges.
For example, asking a low-cost fabrication process to hold a tolerance that normally requires precision machining creates unnecessary production problems. The designer should know the supplier’s normal capability before placing unusually tight limits on the drawing.
Fit is different from ordinary tolerance
For mating holes and shafts, a simple plus-or-minus tolerance may not communicate the functional requirement well. Standardized fit systems such as ISO hole-basis fits provide a better way to describe clearance, transition, and interference relationships.
The important concept is not memorizing every fit code. It is understanding what the assembly needs. A rotating bearing seat, a removable dowel, a sliding guide, and a permanent press fit should not be treated as the same type of connection.
Think about tolerance stack-up
Individual dimensions can each be acceptable while the final assembly still becomes unacceptable. Imagine a bracket with five dimensions contributing to the position of a mounting hole. If every dimension is near its maximum or minimum limit, the final hole position can move significantly.
This is why tolerance stack-up analysis matters. For a simple worst-case stack, the individual tolerances can be added according to their effect on the final dimension. Statistical approaches can be used when the production process is stable and the assumptions are appropriate.
Temperature matters too
Metal parts change size with temperature. Aluminium is particularly noticeable because its coefficient of thermal expansion is higher than that of steel. If two parts operate at different temperatures, a fit that works on the inspection bench may behave differently in service.
For precision assemblies, consider operating temperature, material pairing, and the location where dimensions are measured.
Inspection capability is part of tolerance selection
A tolerance should be measurable. If a drawing specifies a very small limit but the inspection method cannot reliably resolve it, the specification becomes difficult to control.
Think about the actual gauge, CMM, micrometer, bore gauge, height gauge, or functional fixture that will be used. Measurement uncertainty is not an afterthought; it is part of the quality system.
A practical tolerance-selection workflow
- Identify the function of the feature.
- Decide whether the dimension affects fit, motion, strength, appearance, or interchangeability.
- Choose the manufacturing process.
- Check the normal capability of that process.
- Perform tolerance stack-up for important assemblies.
- Consider temperature and material expansion where relevant.
- Confirm that the tolerance can be inspected economically.
- Use standards and supplier feedback instead of guessing.
A useful drawing-review question
During a design review, point to every tight tolerance and ask: “What fails if this becomes the other side of the tolerance?” If nobody can answer, the tolerance deserves another look.
This simple habit can remove unnecessary precision from a drawing and make the product easier to manufacture.
Final takeaway
Engineering tolerance selection is a balance between function, manufacturing capability, inspection, cost, and assembly. The goal is not the smallest possible tolerance. The goal is a tolerance that gives the product enough freedom to manufacture reliably while protecting the features that actually matter.
Example: choosing a tolerance for a mounting hole
Suppose a bracket has a 12 mm clearance hole for an M10 bolt. The hole is not locating the assembly; a separate dowel controls position. In that case, making the hole position extremely precise may add cost without improving the assembly. The functional requirement is simply enough clearance for the bolt and enough material around the hole for strength.
Now change the situation. The same hole is being used to locate a bearing housing relative to another component. Suddenly position becomes important. A tighter location requirement, a datum scheme, or a dedicated locating feature may be justified. The correct tolerance changed because the function changed.
Use supplier capability early
If a component will be outsourced, ask the supplier what tolerances they normally hold for the selected process. A capable supplier can often suggest a small design change that removes a difficult tolerance while keeping the function unchanged.
General tolerances make drawings cleaner
A sensible general tolerance standard can keep ordinary dimensions from being covered with individual plus/minus values. Then the drawing can highlight only the dimensions that are functionally important.
Do a worst-case thought experiment
Take every critical dimension and imagine it at the least favorable end of its tolerance. Does the assembly still work? If not, tighten the appropriate dimensions, change the design, or introduce adjustment.
The cost of over-tolerancing
Over-tolerancing can mean slower machining, additional grinding, more inspection, tighter process control, more scrap, and longer supplier lead times. Those costs eventually appear in the product price and schedule.
Good drawings do not say “make everything accurate.” They tell the manufacturer exactly where accuracy matters and where it does not.