Guides / Process

Feature Tolerance and Positional Tolerance Are Two Different Problems

September 15, 2026

The most expensive line on a large part drawing is usually not the tightest number. It is a positional tolerance stretched across a distance that nothing in the assembly actually needs.

Understanding why means separating two questions that look the same on paper.

Question one: can the machine hold the feature?

Can it bore 420 mm to H7 with a good finish? Almost always yes. A boring mill holding a few hundredths on a single bore is routine work, and it is not where your money goes.

Feature tolerance is a function of the spindle, the tool, the cut and the operator. Heavy machines are built for this and shops quote it without blinking.

Question two: will that feature still be in the right place tomorrow?

Can that bore sit within 0.08 mm of a second bore four metres away, after the part has been craned onto the table, clamped, machined, turned over, re-clamped, machined again, released and allowed to reach room temperature?

That is a completely different problem, and it brings in:

• Machine geometry over the full travel. Straightness and squareness degrade over distance. A machine that is excellent within 500 mm is merely good across 4 000 mm.
• Thermal growth. Steel moves roughly 11 or 12 micrometres per metre per degree. A four metre part that warms three degrees during a shift has moved more than a tenth of a millimetre on its own.
• Table deflection. A heavy part loads the guideways. The machine sags slightly and so does the geometry.
• Setups. Every re-clamp restates the datum, and the new statement is never identical to the old one.
• Residual stress. Covered in stress relief before finish machining.
• Clamping. Clamps distort the part. Release them and the geometry changes slightly.

None of these matter much at 300 mm. All of them matter at four metres.

Why this shows up as price

A shop reading a drawing with tight positional tolerance across the full length has to plan for it. That means some combination of:

• One setup instead of three, which means a bigger machine or a right angle head
• Stress relief in the sequence
• Temperature stabilisation before finishing, sometimes overnight
• In-process probing, and re-cutting if the probe says the part moved
• A laser tracker or portable arm to prove the result

Each of those is hours or days. That is why a positional callout that costs nothing to type can add a third to the quote.

How to tolerance a large part properly

Separate the critical from the inherited. Go through the drawing and mark the dimensions the assembly genuinely depends on. Usually there are two or three. Everything else can open up.

Say what the part mates to. A note like “bores carry a shaft on two bearings, alignment is what matters” tells the shop more than a table of numbers, and it lets the shop propose a better route.

Tolerance relationships, not coordinates. Two bores that must be coaxial should carry a coaxiality callout, not two position dimensions from a corner four metres away. The second version is harder to hold and harder to measure, and it is not what you meant.

Keep the datum scheme reachable in one setup. If your datums are on three faces that cannot be reached without turning the part over, you have specified a multi-setup part whether you meant to or not.

State the measurement condition. At what temperature, in what orientation, clamped or free. On a large part all three change the answer. A part measured in the fixture and a part measured lying on its side on the floor are different parts.

A worked example

A welded machine base, 3 800 mm long, with two bearing housings at each end.

The wrong drawing: every hole positioned to 0.05 mm from a corner datum, flatness 0.05 mm over the whole top face, surface finish called out globally.

The right drawing: the two bearing bores coaxial within 0.05 mm to each other, top face flat within 0.1 mm in the area where the linear rails mount, everything else ±0.5 mm and finish only where it is specified.

Same functioning machine. The second drawing can be run in fewer setups, on more machines, by more shops, and it will quote noticeably cheaper.

What to send with the RFQ

Tell the shop which single relationship you would protect if you could only protect one. That one sentence gets you a sequence designed around your requirement rather than around a defensive reading of the drawing.

Next: what to put in a large part RFQ and how large parts get inspected.

Frequently asked questions

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