Specifying tolerances on mechanical components is a direct trade-off between assembly fitment and manufacturing cost. Over-tolerancing non-critical features forces machine shops to run slower feeds, conduct frequent tool offset adjustments, and perform 100% CMM inspection, driving part prices up by 30% to 70% unnecessarily.
The Standard: ISO 2768-m
Unless your component requires a precision bearing press-fit (such as an H7/p6 tolerance class) or optical alignment, standardizing your general drawing tolerance block on ISO 2768-m (medium) is global best practice:
- Linear dimensions up to 30 mm: ±0.1 mm tolerance, perfectly achievable on modern 3-axis CNC milling centers without secondary grinding.
- Dimensions from 30 mm to 120 mm: ±0.2 mm tolerance.
- Dimensions from 120 mm to 400 mm: ±0.3 mm tolerance.
Critical Dimensioning Rules
When generating 2D engineering PDFs from SolidWorks, Fusion 360, or Inventor, follow these essential guidelines:
- Establish Clear Datums (A, B, C): Select functional mating faces as primary, secondary, and tertiary datums rather than dimensioning from rough stock edges.
- Do Not Over-Constrain Closed Loops: Always leave one dimension in a chain open or mark it as reference (REF) to prevent tolerance stack-up conflicts.
- Specify Thread Classes Explicitly: Use standard ISO metric thread callouts (e.g., M6x1.0 - 6H) and indicate minimum usable full thread depth.
How FamilyMake Inspects Machined Parts
Through our hybrid manufacturing model, FamilyMake verifies all custom CNC milled parts against your supplied 2D drawing block before dispatch, verifying bore diameters with calibrated three-point internal micrometers and pitch gauges.
