Hub · Interactive
How Tolerances and Fits Work
Every machined part comes out a little bigger or smaller than the drawing says. A tolerance tells the shop how much is acceptable, and a fit tells it how two parts should go together. This page covers both, with a fit simulator and a thermal growth calculator you can try.
Start here
1What a tolerance is
No machine cuts to an exact size. Tools wear, and the part warms up as it is cut. So a drawing gives each size as a range: an upper limit and a lower limit. Any part that measures between the two is good. The difference between them is the tolerance.
There are three common ways to write it on a drawing:
Bilateral
Equal amounts either side of the nominal size. Good part: 19.95 to 20.05 mm.
Unilateral
All of the tolerance on one side. Used when a part can be oversize but never undersize. Good part: 20.00 to 20.05 mm.
Limit dimensions
Both limits written out. Nobody has to add or subtract, so the machinist and the inspector read it the same way.
Upper limit − lower limit = tolerance (20.05 − 19.95 = 0.10 mm)
Machinists usually talk in microns for fits. 1 µm is 0.001 mm. A human hair is about 70 µm thick.
The title block
2General tolerances
Most dimensions on a drawing have no tolerance written next to them. Those fall back on the general tolerance in the title block. A common one is ISO 2768-m, the “medium” class. It loosens as the part gets bigger:
| Nominal size (mm) | ISO 2768-m | ISO 2768-f (fine) |
|---|---|---|
| 0.5 to 3 | ±0.1 | ±0.05 |
| over 3 to 6 | ±0.1 | ±0.05 |
| over 6 to 30 | ±0.2 | ±0.1 |
| over 30 to 120 | ±0.3 | ±0.15 |
| over 120 to 400 | ±0.5 | ±0.2 |
| over 400 to 1000 | ±0.8 | ±0.3 |
Those numbers are easy for any decent mill to hold. Most general machine parts are fine at ±0.05 mm on the features that matter and the general tolerance everywhere else. Only the features that locate, slide, seal or press together need anything tighter, and those get a fit code.
Fit codes
3Reading a fit like H7/g6
A fit code describes a hole and a shaft that go together. The capital letter is the hole, the small letter is the shaft. The letter sets where the tolerance band sits relative to the nominal size. The number is the IT grade, which sets how wide the band is. A smaller number means a narrower band.
H7 = hole, band starts exactly at nominal and goes up / g6 = shaft, band sits a little below nominal
Almost every shop and designer works hole-basis: the hole is H (its lower limit is the nominal size) and the shaft letter picks the fit. The reason is tooling. A 10 mm H7 reamer and a 10 mm H7 plug gauge are stock items. A shaft can be turned or ground to any size just as easily, so it makes sense to vary the shaft and keep the hole standard.
Clearance fit
The shaft is always smaller than the hole. Parts slide or turn. Shaft letters a to h; H7/g6 and H7/h6 are the common ones.
Transition fit
The bands overlap. A given pair may slide or may need a tap. Used for accurate location. H7/k6 and H7/n6.
Interference fit
The shaft is always bigger than the hole. The parts are pressed or shrunk together and friction holds them. H7/p6 and H7/s6.
Interactive
4Fit simulator
Pick a size and a fit. The diagram shows the hole and shaft tolerance bands against the nominal size, using the ISO 286 limits. Then move the two sliders to try a real measured hole and shaft.
Bands drawn to scale in microns. The dashed line is the nominal size. The white lines are the measured sizes from the sliders below; they turn red when outside the band.
- Hole
- Shaft
- Loosest pair
- Tightest pair
- Fit type
Try a measured pair
The assembly notes are a rough guide. Real press force depends on the length of engagement, the materials, the wall thickness of the outer part and the surface finish.
Process · Interactive
5IT grades, process and cost
The IT grade says how wide the band is for a given size. The same grade is wider on a big part than on a small one. The table shows the bands at 25 mm and the processes that normally reach them.
| Grade | Band at 18 to 30 mm | Typical process |
|---|---|---|
| IT5 | 9 µm | Precision grinding, honing, lapping |
| IT6 | 13 µm | Grinding, fine boring |
| IT7 | 21 µm | Reaming, fine boring, grinding, careful finish turning |
| IT8 | 33 µm | Finish turning, finish milling, boring |
| IT9 | 52 µm | Good milling and turning |
| IT10 | 84 µm | General milling, accurate drilling |
| IT11 | 130 µm | Drilling, rough turning and milling |
| IT12 and up | 210 µm and more | Roughing, sawing, punching, cut blanks |
Cost does not rise in a straight line. Going from ±0.2 to ±0.1 changes little. Going from ±0.02 to ±0.005 can mean adding a grinding step and measuring every part. Click through the steps:
Relative effort. This shows the shape of the curve, not a price.
- How it’s made
- How it’s checked
- Risk
Inspection
6Measuring a fit
A tolerance only means something if you can measure it. The usual rule is that the instrument should be several times finer than the tolerance it checks, ideally ten times. An H7 bore at 20 mm has a 21 µm band.
Vernier or digital calipers
Read to 0.01 or 0.02 mm, but the reading moves by about that much depending on how you hold them. Fine for ±0.1 work and general sizes. Not suitable for accepting an H7 bore.
Micrometers
Read to 0.01 mm, or 0.001 mm on digital and vernier types, with a ratchet so every reading uses the same force. The normal tool for a g6 or h6 shaft.
Bore gauges
A dial or digital bore gauge set to a ring gauge or a micrometer reads the bore at several depths and angles, so it also finds taper and out-of-round.
Plug and pin gauges
Go/no-go. The GO end is the lower limit and must enter. The NO-GO end is the upper limit and must not. No reading to interpret, which makes them quick on a batch.
Temperature · Interactive
7Heat changes the size
Metal grows when it warms. Drawing sizes are defined at 20 °C (ISO 1). A shop floor in Cavite is often 30 °C or more, and a part straight off the mill can be warmer than that. A hole grows with heat too, the same as a solid piece of the same metal.
Thermal growth calculator
Growth = expansion rate × length × temperature change. Rates are typical values; alloys vary a little.
A 100 mm aluminum part that warms by 10 °C grows about 23 µm. The whole H7 band at that size is 35 µm, so two-thirds of it is gone before anyone touches a tool. That is why a part should cool down before the final cut and again before it is measured.
Related
8Surface finish goes with the fit
A tight size on a rough surface doesn’t hold. On a press fit, the peaks of the surface get flattened as the parts go together, so some of the interference is lost. On a sliding fit, the peaks wear off in the first hours and the play grows. Bores in the H7 range are usually specified around Ra 0.8 to 1.6 µm, which reaming, fine boring and grinding can give. A normal milled face is more often Ra 1.6 to 3.2 µm.
For flat faces the same logic applies to flatness. Where a locating face or a die plate has to be flat and smooth, we finish it on the surface grinder after milling.
Working with us
9How we approach tolerances
We make precision machined components, jigs and fixtures, and die components on 3-axis CNC mills, with conventional milling and surface grinding for secondary work. When we design a machine ourselves, we choose the fits. When we machine to your drawing, these are the things that help most:
Tight tolerances only where they work
A locating bore, a bearing seat or a dowel hole needs a fit code. The outside of a bracket usually doesn’t. Every tight size costs time to cut and time to check.
Tell us what the feature does
“This pin has to come out by hand” or “this bushing must never turn” tells us more than a number alone, and lets us flag a fit that looks wrong.
Mention the mating part
If a shaft goes into a bought bearing or a standard dowel pin goes into the hole, say which one. Standard hardened dowel pins are made to m6, so the hole size decides whether the pin presses in or slips.
Say if it gets hardened
Heat treatment moves sizes. Fits on hardened parts are normally left with stock and ground after hardening, which changes the process plan.
Check yourself
10Quick quiz
1. In the fit code H7/g6, which part does H7 describe?
2. A 20 mm bushing is specified H7/s6. How does it go in?
3. Why shouldn’t you accept a 20 mm H7 bore with calipers?
4. A 100 mm aluminum part warms up by 10 °C. About how much does it grow?
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