What a Tolerance Actually Specifies
A tolerance defines the permissible variation in a manufactured dimension. A diameter called out as 1.000 ± 0.001 inches may be anywhere from 0.999 to 1.001 inches and still conform to the drawing. The tolerance is not a target — the machinist is not trying to hit exactly 1.000. They are producing a part that falls within the acceptable range as efficiently as possible.
This distinction matters because tolerance tightness directly affects manufacturing time and therefore cost. Achieving a dimension within ±0.001 inches requires slower cutting parameters, more careful setup, and more rigorous in-process measurement than achieving the same nominal dimension within ±0.010 inches.
Types of Tolerances on Engineering Drawings
General (Title Block) Tolerances
Most engineering drawings include a general tolerance note — often in the title block — that applies to all dimensions not individually toleranced. A typical general tolerance might be ±0.005 on two-place decimals and ±0.001 on three-place decimals. These defaults set the baseline expectation for the part and apply to every unspecified dimension automatically.
Feature-Specific Tolerances
Dimensions requiring tighter or looser control than the general tolerance are individually callout on the drawing. These explicit callouts override the general tolerance for that specific dimension and signal to the machinist and inspector that the feature deserves particular attention.
Geometric Dimensioning and Tolerancing (GD&T)
GD&T is a symbolic language for specifying not just size tolerances but geometric conditions — flatness, cylindricity, true position, perpendicularity, and more. GD&T is more precise than coordinate tolerancing for many applications because it directly defines the functional requirement rather than constraining individual coordinates. It does, however, require that both designer and manufacturer understand the system correctly — a GD&T callout applied incorrectly is worse than no callout at all.
How Tolerances Affect Machining Cost
Cutting Parameters
Tighter tolerances require lower material removal rates to control thermal expansion, tool deflection, and vibration. A feature toleranced to ±0.0002 inches may require a dedicated finishing pass at reduced feed and depth, adding significant cycle time compared to the same feature toleranced to ±0.002 inches.
Fixturing and Setup
High-precision features often require dedicated fixturing to provide the rigidity and repeatability needed to consistently achieve the tolerance. Custom fixtures add one-time cost to a job and may affect lead time on first article.
Inspection
Tight-tolerance features require more sophisticated inspection equipment — typically a coordinate measuring machine (CMM) rather than hand gauging — and more inspection time. CMM inspection adds both equipment cost and cycle time to a job.
Practical Guidelines for Tolerance Specification
The most cost-effective drawings specify tight tolerances only where functional requirements demand them. Before calling out a tight tolerance, ask: What happens if this dimension is at the maximum permissible variation? If the answer is "the part still functions correctly," a tighter tolerance is not justified. Reserve your tight callouts for genuinely critical features — bearing bores, sealing surfaces, precision locating features — and let everything else float to general tolerances.
Similarly, ensure that tolerance stacks across mating parts are analyzed before finalizing callouts. A tolerance that seems generous in isolation may produce interference or excessive clearance when combined with tolerances on three mating components.
Working With Your Manufacturer on Tolerances
If you are uncertain whether a particular tolerance is achievable or appropriate for a given feature, ask your manufacturer before releasing the drawing. Most experienced shops will give you straightforward feedback: "This is within our standard capability," or "This will require specialized processing — here is what that adds to the cost." That conversation is far less expensive than discovering a tolerance problem at first article inspection. Contact Mikron to discuss your specific tolerance requirements before finalizing your drawing package.
Ready to Put This Into Practice?
Upload your drawings or send Mikron a request for quote — the team will review your project and respond promptly.