Reading Engineering Tolerances
A drawing that simply says a shaft is 25 mm in diameter is an instruction no machinist can actually follow, because no real part is ever exactly 25.000 mm. Every manufacturing process leaves some variation, so engineers specify not a single perfect size but an allowable range. That range is the tolerance, and learning to read it is fundamental to making parts that fit together and function as intended.
Nominal, limits, and deviations
A dimension has a few named pieces. The nominal size is the round reference figure, say 25 mm. The deviations are how far above and below that the part may stray, given as an upper and a lower value. The actual boundaries they define are the limits of size: the largest and smallest the part is allowed to be. The gap between those two limits is the tolerance itself.
For example, 25 with deviations of +0.02 and −0.01 gives an upper limit of 25.02 mm and a lower limit of 24.99 mm, for a tolerance of 0.03 mm. Any part measuring between those two figures is acceptable; anything outside is scrap.
Unilateral versus bilateral
- A bilateral tolerance allows variation in both directions, such as 25 ±0.05, spanning 24.95 to 25.05.
- A unilateral tolerance allows it in one direction only, such as 25 with +0.00 and −0.03. Unilateral tolerances are common where a part must never exceed a critical size, like a shaft that has to enter a bore.
Fits: how two parts meet
Tolerances truly earn their keep when two parts assemble. The relationship between a hole and the shaft that goes into it is called a fit, and there are three families:
- Clearance fit: the hole is always larger than the shaft, so the parts slide freely. Think of a bolt in a clearance hole, or a rotating axle in a plain bearing.
- Interference fit: the shaft is always larger than the hole, so the parts must be pressed or heat-shrunk together and then grip permanently. Think of a bearing race pressed onto a shaft.
- Transition fit: the tolerance zones overlap, so the assembly may come out slightly loose or slightly tight — used for accurate location of parts that still need to come apart.
The basic hole and basic shaft systems
Rather than tolerancing both mating parts arbitrarily, engineers usually fix one and vary the other. In the basic hole system, the hole's lower limit equals the nominal size, and the different fits are produced by changing the shaft. This is the more common choice, because holes are made with fixed-size tools like drills and reamers, so it is cheaper to standardise the hole and adjust the shaft on a lathe. The basic shaft system does the reverse and suits designs built around standard-diameter bar stock.
ISO IT grades
The ISO system standardises the width of a tolerance through IT grades, numbered IT01 through IT18. A lower number means a tighter tolerance. Importantly, the physical size of any grade widens as the part gets larger, because it is harder to hold a given accuracy on a big part. The table shows how a few grades map onto practical work.
| IT grade | Typical application |
|---|---|
| IT01–IT4 | Gauge blocks, precision instruments |
| IT5–IT7 | Bearings, machine tools, precision fits |
| IT8–IT11 | General machining, everyday fits |
| IT12–IT18 | Rough, non-critical dimensions |
A fit is written by combining a letter for the position of the tolerance zone with an IT grade, as in H7 for a hole or g6 for a shaft; the pair H7/g6 is a classic close-running clearance fit.
Why tolerances cost money
Every extra bit of precision has a price. Halving a tolerance can mean slower machining, better tooling, more skilled operators, tighter temperature control, and more parts rejected at inspection. The guiding principle is to specify the loosest tolerance that still lets the part do its job. Over-tolerancing a non-critical dimension quietly inflates cost for no functional gain, and it is one of the most common mistakes on a beginner's drawing.
Beyond size: GD&T
Size alone does not guarantee a part works, because it says nothing about form or orientation — a shaft can be the correct diameter yet bent or oval. Geometric Dimensioning and Tolerancing (GD&T) extends the idea to control features like flatness, roundness, perpendicularity, and true position, giving a far richer and less ambiguous description of what "correct" means. It is a language of its own, but it rests on the same foundation of limits and deviations covered here.
Getting comfortable with limits and fits makes drawings far easier to read. Check nominal sizes and their limits against the Thread & Fastener Size Reference on MechKit.