Engineering Tolerances Calculator — Fits & Limits

Enter deviations directly or use IT-grade helper (H/h/js). Get limits, min/max clearance or interference, and fit type. Everything runs locally in your browser.

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How Fits & Limits Work (Quick Primer)

Fits describe how a shaft sits in a hole after manufacturing tolerances are applied. A clearance fit always has positive clearance; an interference fit always has overlap; and a transition fit can result in either depending on actual sizes.

Limits are the permitted extreme sizes. For a basic size \(D\) in mm, a hole has deviations \(E_i\) (lower) and \(E_s\) (upper) in micrometers; a shaft has \(e_i\) and \(e_s\). Nominal-to-limit conversions:

  • Hole limits: \(D_\text{hole,min} = D + E_i/1000\),   \(D_\text{hole,max} = D + E_s/1000\).
  • Shaft limits: \(D_\text{shaft,min} = D + e_i/1000\),   \(D_\text{shaft,max} = D + e_s/1000\).

Clearance and interference:

  • Minimum clearance \(C_\text{min} = D_\text{hole,min} - D_\text{shaft,max}\).
  • Maximum clearance \(C_\text{max} = D_\text{hole,max} - D_\text{shaft,min}\).

IT grades. ISO 286 defines tolerance grades using a “tolerance unit” \(i\) in µm (for \(D\) in mm): \( i \approx 0.45\,\sqrt[3]{D} + 0.001\,D \). Typical grades: IT5=7i, IT6=10i, IT7=16i, IT8=25i, IT9=40i, IT10=64i. This tool uses that classic approximation to build H/h/js helpers: H has \(E_i=0\), \(E_s=+IT\); h has \(e_s=0\), \(e_i=-IT\); js is centered \( \pm IT/2 \).

This is an educational helper, not a substitute for the full ISO tables (which include additional letters and subtle deviations). For certification or production drawings, always verify with the official standard or your QA handbook.

Engineering Tolerances, Fits & Limits — A Friendly Primer

In precision manufacturing, two parts rarely measure exactly their nominal size. Instead, drawings specify a basic size plus permitted tolerances, producing a range of acceptable dimensions called limits. For mating parts (a hole and a shaft), these limits determine the fit: whether assembly results in guaranteed clearance, guaranteed interference, or a transition where either outcome is possible. This calculator lets you explore limits and fits either by entering deviations directly (µm) or by using a quick ISO-style helper with IT grades and simple bases (H/h/js).

Key Terms (Plain English)

  • Basic size (D): The nominal size from which limits are derived (e.g., 25 mm).
  • Deviation: Offset from the basic size. For holes we use \(E_i, E_s\); for shafts \(e_i, e_s\) (in µm).
  • Limits: The smallest and largest permissible sizes (e.g., 25.000–25.025 mm).
  • Clearance: Hole diameter minus shaft diameter. Positive = “slack”, negative = overlap.
  • Fit types: Clearance (always +), Interference (always −), Transition (can be + or −).

How Limits & Fits Are Computed

Deviations are given in micrometres (µm). Converting to millimetres is simply division by 1000. If the basic size is \(D\) (mm), then:

  • Hole limits: \(D_\text{hole,min} = D + E_i/1000\),   \(D_\text{hole,max} = D + E_s/1000\).
  • Shaft limits: \(D_\text{shaft,min} = D + e_i/1000\),   \(D_\text{shaft,max} = D + e_s/1000\).
  • Minimum clearance: \(C_\text{min} = D_\text{hole,min} - D_\text{shaft,max}\).
  • Maximum clearance: \(C_\text{max} = D_\text{hole,max} - D_\text{shaft,min}\).

Classification is then straightforward: both clearances > 0 → clearance fit; both < 0 → interference fit; mixed signs → transition fit.

What Are IT Grades?

ISO 286 defines tolerance “grades” (IT5, IT6, … IT10) that scale with size using a tolerance unit \(i\) (µm). A common approximation is: \( i \approx 0.45\,\sqrt[3]{D} + 0.001\,D \) (with \(D\) in mm). Typical grade widths are: IT5 = 7\(i\), IT6 = 10\(i\), IT7 = 16\(i\), IT8 = 25\(i\), IT9 = 40\(i\), IT10 = 64\(i\). Our helper uses these to build simple hole/shaft limits for the popular bases:

  • H hole: lower deviation \(E_i = 0\); hole lies entirely above the basic size.
  • h shaft: upper deviation \(e_s = 0\); shaft lies entirely below the basic size.
  • js shaft: deviations are symmetric about 0 (±IT/2).

Real ISO tables include many more letters (a–zC) with specific fundamental deviations. Use official tables for certification work; this tool is an educational shortcut for quick planning and sanity checks.

Selecting a Fit (Rule-of-Thumb)

  • Free sliding: H8/h7 or H9/h9 for general assemblies that must move without binding.
  • Location/transition: H7/js6 where some parts may be tight, others loose; alignment prioritized.
  • Light press: H7/p6 or H7/k6 for locating with light interference (bearing inner rings on shafts often use k/m).
  • Heavy press/shrink: H7/u6 or similar for permanent joints. Thermal methods are common for assembly.

Common Pitfalls (and Easy Fixes)

  • Forgetting units: Deviations are in µm, basic size and limits in mm. Convert consistently.
  • Mismatched grade severity: A very tight hole with a very loose shaft can yield unpredictable fits. Pair grades sensibly (e.g., H7/h6).
  • Ignoring process capability: Choose grades your machining process and metrology can actually hit.
  • Skipping function: Rotating parts may need clearance for thermal growth and lubrication; static pins may require interference.

Practical Tips

  • Prototype with slightly looser combinations first; tighten once function is proven.
  • For bearings, consult the manufacturer’s fit recommendations—load direction and rotation matter.
  • Consider environment: temperature swings, corrosion allowance, coatings/plating thickness, and surface finish all influence real fits.

Disclaimer: This primer and calculator provide engineering approximations for education and early planning. For production drawings, QA, or regulated applications, verify with ISO 286 tables and your organization’s standards.

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