Engineering · Design for Manufacturing · 6 of 8
Tolerances, Fits and GD&T
Which of these tolerances does the part really need, and can the shop measure the ones it keeps?
Tolerances and GD&T are how a drawing says what has to be right and what does not. Tight where the function needs it and loose everywhere else is the cheapest way to make a good part. This page has a limits calculator, an ISO 286 fit calculator that draws the hole and shaft zones, a tolerance stack-up with worst case and RSS, the 14 GD&T symbols, a builder that draws a feature control frame, and a guide to which gauge can check a given tolerance.
1Why tolerances and GD&T?
Interchangeable parts
Any part from any batch fits any mating part.
Control function, fit and alignment
Say how the part must work, not only how big it is.
Reduce manufacturing cost
Open every tolerance that does not matter.
Improve quality and reliability
Fewer surprises at assembly and in service.
Clear communication
Design, manufacturing and inspection read the drawing the same way.
Specify only what is necessary
Over-tolerancing is one of the cheapest ways to waste money.
2Limits, tolerance and deviation
Limits calculator
lower limit = nominal + lower deviation
tolerance = upper limit − lower limit
| Type | Example |
|---|---|
| Bilateral tolerance | 50 ± 0.1 |
| Unilateral tolerance | 50 +0.1 / 0 |
| Limit dimensioning | 49.90 to 50.10 |
| Basic dimension (GD&T) | boxed 50, with tolerance in the frame |
3ISO fits: clearance, transition and interference
In the hole basis system the hole is H (lower deviation zero) and the shaft letter sets the fit. A fit is clearance if the shaft is always smaller, interference if always larger, and transition if either can happen.
ISO 286 fit calculator
| Application | Typical fit |
|---|---|
| Free running | H8/f7, H9/d9 |
| Sliding (easy movement) | H7/g6 |
| Location, clearance | H7/h6 |
| Location, transition (accurate) | H7/k6, H7/n6 |
| Press fit (light interference) | H7/p6 |
| Press fit (permanent) | H7/r6, H7/s6 (not in the calculator) |
The calculator uses the ISO 286 tables for sizes 1 to 500 mm, hole grades H6 to H11 and shaft letters d to p. Shaft grades are limited where the standard changes the rule: k, m, n and p use grades 5 to 7. For bearing seats use the Bearing Fit Calculator. Sizes at a range boundary (for example 30 mm) fall in the lower range.
Show the IT grade values used (micrometres)
4Tolerance stack-up (chain analysis)
Stack-up calculator
| Part | Nominal (mm) | ± Tol (mm) | Direction |
|---|
RSS (statistical): Tol = √(T1² + T2² + T3² ...)
Direction + adds the dimension to the chain, − subtracts it (use for a gap or clearance). Worst case means every part is at its extreme at once, so it is always safe. RSS assumes independent, centred, normally distributed dimensions, so it is more realistic for many parts but is not a guarantee. Use it only where the process is under statistical control.
5Datums and the datum reference frame
| Datum | Role | Degrees of freedom it removes |
|---|---|---|
| A | Primary datum: a plane | 3 (one translation, two rotations) |
| B | Secondary datum: a line or plane | 2 |
| C | Tertiary datum: a point or plane | 1 |
Together A, B and C remove all six degrees of freedom, so every measurement starts from the same place the part is held in assembly. Choose datums from how the part mounts and locates, not from how it is easiest to measure.
6GD&T symbols
ASME Y14.5-2018 no longer uses concentricity and symmetry (position or profile are used instead). They remain in ISO 1101, so they appear here.
7Feature control frame builder
Build a frame
8Inspection: which gauge can check it?
Measurement capability
| Method | Typical accuracy | Verdict | Good for |
|---|
The verdict compares typical accuracy with the total tolerance band (2 × the value above). The ideal is a gauge ten times finer than the band; four times is the usual minimum. Accuracy figures are typical for ordinary shop equipment, so use your calibrated equipment’s own specification.
9Functional tolerancing: tolerance what matters
Do
- Define functional datums from how the part is assembled and used.
- Use fits for mating features and GD&T for form, orientation and location.
- Use standard tolerance grades (ISO).
- Perform tolerance stack-up for critical chains.
- Specify the inspection method for critical features.
Avoid
- Tolerance everything to the same tight value.
- Datums chosen only for easy measurement.
- Notes that conflict with the drawing frames.
10Try it: tolerance and GD&T checklist
Tolerance scorecard
Remember
- Tolerance the function, not the whole drawing.
- Datums are how the part is held in assembly.
- Hole basis: H7 with g6 slides, h6 locates, k6 or n6 transitions, p6 presses.
- Gauge must be about ten times finer than the tolerance band; four times at the very least.
- Boothroyd, Dewhurst and Knight, Product Design for Manufacture and Assembly (CRC Press), for the DFM and DFA method.
- Machinery’s Handbook (Industrial Press), for machining, sheet metal and welding practice.
- ISO 286-1 and ISO 286-2: limits and fits (IT grades and shaft deviations used in the calculator).
- ISO 1101 and ASME Y14.5 for geometrical tolerancing; ISO 2768 for general tolerances.
Fit values come from ISO 286 tables for 1 to 500 mm. Check any critical fit against the standard or your bearing and gauge makers. Gauge accuracy figures are typical, not guarantees.