The Part That Didn’t Fit (Tight Tolerance, No Need)
We designed a machined bracket. The drawing called for a ±0.01 mm tolerance (very tight) on a hole. The machine shop machined it (it was expensive, and slow). On the floor, the hole didn’t need that tolerance (it was a clearance hole, a bolt goes through). The ±0.01 mm was overkill (it cost more, and delayed). We loosened the tolerance (to ±0.1 mm, for a clearance hole). The part was cheaper (and fast). The mistake was specifying a tight tolerance (without needing it). Tight tolerances cost money (and time).
Machining tolerances — use the right tolerance (not too tight). This article covers the selection.
The Tolerance Grades (IT)
ISO defines tolerance grades (IT01 to IT18). The lower the number, the tighter:
- IT5–IT7 (tight): For a fit (a bearing, a precision hole). ±0.01 mm. Expensive.
- IT8–IT10 (medium): For a general hole (a bolt clearance). ±0.05 mm. Normal.
- IT11–IT13 (loose): For a non-critical dimension (the outside). ±0.5 mm. Cheap.
Step 1: The Fit (Critical?)
Is the dimension critical (a fit, a bearing)? Use a tight tolerance (IT7). Is it a clearance (a bolt hole)? Use a medium tolerance (IT10). Is it non-critical (the outside)? Use a loose tolerance (IT13). Don’t use IT7 on a clearance hole (overkill).
The tolerance rule: Use the loosest tolerance that works. The bracket that was expensive had ±0.01 mm (IT7) on a clearance hole. Use ±0.1 mm (IT10) for a clearance. Tight tolerances cost money (and time). Only use tight where it’s needed (a fit).
Step 2: The Standard (Default)
Use a default tolerance (the drawing’s general tolerance, e.g., ±0.1 mm). The dimensions that need tighter (a fit) are specified (on the drawing). The rest (the default) is loose. Don’t specify every dimension (let the default cover it).
Step 3: The Machining Process
A milling machine (CNC) holds ±0.05 mm (normal). A grinding machine holds ±0.005 mm (tight). For a tight tolerance (±0.01 mm), you need grinding (expensive). For a normal tolerance (±0.1 mm), milling is fine. Match the tolerance to the process (don’t ask milling for ±0.005 mm).
Step 4: The Cost
A tight tolerance (IT7) costs 2–3× more (more setups, slower). A loose tolerance (IT13) is cheap. For a production part (1000 pieces), the tight tolerance adds up (a lot). Use the loosest (that works).
| Grade | Tolerance | Use |
|---|---|---|
| IT7 (tight) | ±0.01 mm | Fit (bearing) |
| IT10 (medium) | ±0.1 mm | Clearance (bolt) |
| IT13 (loose) | ±0.5 mm | Outside (non-critical) |
A Tolerance Checklist
- Is the dimension critical? (A fit?)
- Is it a clearance? (Bolt?)
- Is it non-critical? (Outside?)
- Is the default tolerance set? (±0.1?)
- Is the tight tolerance only where needed?
- Is the process right? (Milling? Grinding?)
- Is the cost right? (Not overkill?)
- Does the part fit? (Assembly?)
- Is the gauge R&R? (Article 112?)
- Is the drawing clear? (Tolerances?)
The Bottom Line
Machining tolerances use the loosest that works. The bracket that was expensive had a tight tolerance (on a clearance hole). Use the default (±0.1). Tight only for a fit. The part that was cheap (and fit) wasn’t the tightest tolerance — it was the right one.
