A drawing says Ø25 H7 on a bore and Ø25 g6 on the mating pin. The operator wants a number, not a letter. The quality engineer wants to know which gauge to issue. And the person quoting wants to know whether this needs reaming, grinding or just a good boring pass. All three questions are answered by one standard: ISO 286, the system of limits and fits.

What the letter and the number mean

Every ISO fit callout has the same parts. Take Ø40 H7/g6, the example used by Roymech:

  • 40 is the basic size in mm.
  • H is the position of the hole's tolerance zone. Capital letters are for holes.
  • 7 is the IT grade of the hole, which sets the width of the zone.
  • g is the position of the shaft's zone. Small letters are for shafts.
  • 6 is the IT grade of the shaft.

So the letter tells you where the band sits relative to the basic size, and the number tells you how wide it is. For an H hole the lower deviation is zero, so the smallest hole equals the basic size. For an h shaft the upper deviation is zero.

From callout to real limits: a worked example

Roymech works through Ø40 H7/g6 step by step:

  • H7 hole: 40.000 to 40.025 mm (IT7 at 40 mm is 25 µm).
  • g6 shaft: 39.975 to 39.991 mm (upper deviation -9 µm, lower -25 µm).
  • Minimum clearance: 40.000 - 39.991 = 0.009 mm.
  • Maximum clearance: 40.025 - 39.975 = 0.050 mm.

The same pair at Ø25 falls in the 18-30 mm band. From the Machining Doctor H7/g6 table, the hole is +0 to +21 µm and the shaft is -7 to -20 µm. That gives a bore of 25.000 to 25.021 mm and a pin of 24.980 to 24.993 mm, with clearance between 0.007 and 0.041 mm.

The tolerance band in microns grows with size, so always look up the correct size band. A value exactly on a boundary belongs to the lower band, so a Ø30 shaft uses the "over 18 up to 30" row.

A fit callout is not a tolerance until you have looked up the size band. Write the actual limits on the router and the inspection plan, not just the letters.

Clearance, transition and interference fits

ISO 286 fits fall into three families:

  • Clearance fit: the shaft is always smaller than the hole. H7/g6 and H7/h6 are common examples.
  • Transition fit: depending on the actual sizes, you may get a small clearance or a small interference. H7/k6 and H7/n6 are typical.
  • Interference fit: the shaft is always larger than the hole. H7/p6, H7/s6 and H7/u6 are in this group.

The preferred-fits list published by AmesWeb gives a one-line purpose for each. For example, H7/g6 is a sliding fit meant to move and turn freely and locate accurately; H7/h6 is a locational clearance fit for stationary parts that must still be assembled by hand; H7/k6 is a locational transition fit; and H7/p6 is a locational interference fit for rigidity and alignment.

Hole basis vs shaft basis

Most Indian machine shops work on the hole-basis system: the hole is always H and the shaft letter changes to get the fit. Holes are made with fixed-size tools such as reamers and plug gauges, so keeping the hole constant means fewer tools. Shaft-basis fits (h shaft, varying hole letter) appear where one ground shaft carries several parts, such as long drive shafts or bright bar used as-is.

What to check before you quote a fit

A fit callout quietly decides your process, your gauges and your scrap risk. Before quoting, check:

  • Process capability: an IT6 shaft band is narrow. Ask whether your turning holds it, or whether you need grinding. An IT7 bore often means reaming or fine boring.
  • Gauging: H7 bores usually need a plug gauge or bore gauge; g6 and k6 shafts need a snap gauge or a micrometer with suitable resolution. Include gauge cost if needed.
  • Plating and coating: if the part is plated after machining, the coating thickness eats into the fit. The drawing should say whether the limits apply before or after plating.
  • Temperature and material: interference fits on aluminium or thin sections can distort. Clarify assembly method (press, shrink) with the customer.
  • Missing callouts: a bore marked "Ø25 (for bearing)" with no fit is a spec gap. Ask before you quote, not after the first rejection.

Illustrative example: if you plan a drilled and bored Ø25 hole at your normal ±0.02 mm, an H7 requirement (0 to +0.021 mm) leaves you only half the band you are used to. That one line can add a reaming operation and a gauge to the job.

How SpecReview helps

When you upload a 2D drawing PDF, a STEP model or both, SpecReview lists spec gaps and unclear notes, such as a bore with no fit class or a fit with no plating note, and flags DFM and rejection risks with a short reason why each matters. Its layout inspection plan lists each fitted diameter as a characteristic to check, so the limits are on paper before the first piece is cut.