Impact of tolerances in design – example

Cost reduction

Why are tolerances expensive?

Why does tight tolerances impact cost? This example will show you how a tight tolerance will impact the cost compared to the same design with a more easy tolerance setting (wider tolerance).

One of the reasons for tolerances to be expensive is that tolerances typically require more specialized tools when producing a product. This tool will be sensitive to wear. The specialized tool typically also requires an operator to execute the work. 

When eliminating the tolerances or making a design with an easier tolerance setting it will be possible to select a more general production method and possibly also remove the operator cost.

Overall, it can be concluded that the additional cost that generally is associated with tolerances is what makes them expensive.  

Ease of tolerances = cost reduction

Illustration of cost reduction levers and assets/efforts to drive cost reductions – set in relation to product levels.

Example - why tolerances are expensive (CNC)
Example - two similar designs - with a CNC drilled hole - one with fine (tight) tolerances and one with eased (wide) tolerances.

Imagine a hole in a CNC machined part. What is the difference in cost (illustrative) between a +/- 0.01 mm tolerance vs. a +/- 0.3 mm for an Ø8 mm measure?

The indicative tooling cost for the tight tolerance is app. 4-5 times greater than for the eased tolerance.

The indicative machining time (speed) for the tight tolerance is app. 10 times longer than the eased tolerance. To achieve the tight tolerance, first, a standard tool is needed to do the rough machining, then a tool change, for the fine carbide boring bar, which needs again more time to drill to achieve the desired tolerance. 

Slow machining time equals an increase in part cost (typical labor and machine depreciation are linked to cost/s or cost/min for machining).

For parts with tight tolerances “babysitting” is needed. Quality control/inspection and machine adjustment is needed to keep tolerances within limits. This adds to the part cost.

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