On a drawing it looks trivial: you put a dimension on, the part gets machined, done. But anyone who has stood at a machine knows that no two parts ever come out exactly the same, not even on the best CNC in the shop. That is where tolerances and fits come in: they are the way of telling the machinist how far a dimension may deviate before the part stops being usable.
What a tolerance is and why it exists
A tolerance is the allowable range of variation on a dimension. Making a dimension exact, to the micron, is impossible in practice, and even if it could be done it would cost a fortune. The tool wears, the machine has its play, the part expands with the heat of the cut and the material is not always homogeneous. All of that makes the real dimension swing within a band.
The tolerance defines that band. Instead of demanding a shaft of exactly 20 mm, the drawing allows, say, between 19.98 and 20.00 mm. If the part falls inside, it is good; if it falls outside, it is scrap or rework. If you are not yet clear on what the trade involves, it is worth going over what a machinist is before getting into this.
Nominal size and deviations
To speak the same language it helps to fix the vocabulary:
- Nominal size: the theoretical reference dimension, the one printed large on the drawing (20 mm, for example).
- Upper deviation: the difference between the maximum allowable size and the nominal.
- Lower deviation: the difference between the minimum allowable size and the nominal.
- Maximum and minimum size: the two ends of the band. The difference between them is the tolerance.
Example: a dimension written as 20 +0.02 0 has nominal 20, upper deviation +0.02 and lower deviation 0. The maximum size is 20.02 and the minimum 20.00. The total tolerance is 0.02 mm, that is, twenty microns of margin.
The ISO tolerance system
So that numbers do not have to be written against every dimension, there is a standardised (ISO) system built on two elements: a number giving the width of the tolerance and a letter giving its position relative to the nominal line.
IT grades (width)
The standard tolerance grade, or IT, sets how wide the band is. It runs from the finest grades to the coarsest: the lower the number, the tighter the tolerance. Roughly:
- IT01 to IT4: gauges, go/no-go gauges and very high precision parts.
- IT5 to IT7: the precision fits common in mechanical engineering (bearings, bushes, guideways).
- IT8 to IT11: general machining of parts that fit together without great demands.
- IT12 and above: coarse parts, forgings, cut stock, features where the size is not critical.
Important: for the same IT grade, the tolerance in millimetres grows with the size of the part. An IT7 on a 10 mm shaft is tighter than an IT7 on a 200 mm shaft, because the standard scales the margin with the diameter.
The position (letters)
The letter says where the tolerance band sits relative to the nominal size. Capitals are used for holes (H, G, F and so on) and lower case for shafts (h, g, f). The key reference is H/h, which puts one end of the band right on the nominal line.
So a designation like H7 is a hole with position H and grade IT7, and g6 is a shaft with position g and grade IT6. Putting letter and number together defines the tolerance completely without writing a single decimal.
Types of fit: clearance, transition and interference
A fit is the relationship between a shaft and a hole assembled one inside the other. Depending on how their tolerance bands overlap, there are three families:
| Type of fit | What happens on assembly | Typical use |
|---|---|---|
| Clearance (running) | The shaft always goes in loose, a gap is left | Parts that rotate or slide |
| Transition | It may end up slightly loose or slightly tight | Located joints needing accurate positioning |
| Interference (press) | The shaft is larger than the hole and is pressed in | Fixed joints without a key |
The normal shop approach is the hole-basis system: the hole is fixed at H and the shaft letter is varied to get clearance, interference or something in between. It is more practical because the hole, harder to rework, is kept at a fixed tolerance.
A shaft-and-hole example
Take an H7/g6 assembly at 20 mm, a classic for something that rotates smoothly with little play. Roughly:
- H7 hole: between about 20.000 and 20.021 mm.
- g6 shaft: between about 19.993 and 19.980 mm.
Since the hole is always larger than the shaft, a small clearance is left that allows rotation without slop. If instead you wanted the shaft to press in, you would change the shaft letter for one that makes it larger than the hole, and you would need a press or thermal expansion to assemble it. These concepts come up daily when working with the CNC lathe and milling machine, where controlling diameter is the bread and butter.
How they are read on a drawing
On the drawing the tolerance can appear in three forms:
- With numerical deviations: 20 +0.02 0, the most direct.
- With an ISO designation: 20 H7 or 20 g6; you have to look up tables to get the millimetres.
- With a general tolerance: a note in the title block applying to every dimension without an explicit tolerance.
A shop-floor tip: before you touch the machine, always find the tightest dimension on the drawing. That one sets the pace of the work, the tooling and the checks. If a term gives you trouble, pull up the glossary rather than assuming.
Tolerance and cost
Here is the lesson that saves the most money: the tighter the tolerance, the more expensive the part. A narrow band means lighter passes, more checking, more metrology, possibly a grinding operation afterwards and a higher reject rate. Asking for IT5 where IT9 would do can multiply the time and the cost without adding anything to the function.
So the sound approach is to tolerance as wide as the part allows: tighten only where it genuinely matters (bearing seats, guideways, fits) and leave everything else loose. Being able to read and interpret this is one of the skills that makes a difference if you are considering how to become a machinist and want to stand out in the trade.