What a 0.02 position tolerance does to the route

What changes between Ø0.1 and Ø0.02?

A subcontractor with twelve people receives two requests for the same 6082 block, 120 by 80 by 60 mm. It has two Ø12 H7 bores: one in the top face, one in the side. The STEP models are identical; the difference is on the drawing alone. On one drawing the side bore carries a position tolerance of Ø0.1 mm to datum A, the underside, and datum B, the bore in the top. On the other it says Ø0.02 mm. The order of the datums in the feature control frame counts: A then B is a different requirement from B then A, and can give a different measured result on the same part.

Keep three things apart. The bore is the feature. H7 and the position are requirements: H7 is a dimensional tolerance that says how large the bore may be, while the position tolerance says where its axis may lie. Drilling, reaming or boring are the operations that meet them. ISO 1101 places position among the tolerances of location, alongside form, orientation and run-out.

The Ø symbol in front of the value makes the tolerance zone a cylinder. At Ø0.02 mm the axis of the bore may therefore stray no more than 0.01 mm from its true position, in any direction. The machine, the workholding, the measurement and the heat all have to share that room.

On the CMM you convert the measured axis back to that diameter: position deviation = 2 × √(Δx² + Δy²), where Δx and Δy are the offsets from the true position in the plane square to the bore. The factor 2 turns a distance into a diameter, so the result reads straight against the Ø value on the drawing.

One setup or two?

On a 5-axis machine both bores are made in one setup: the rotary axes turn the block, the work origin stays put and both bore axes sit in the same coordinate system. On a 3-axis machine you bore the top, reclamp and bore the side. The reclamping adds a stack-up: the repeatability of the fixture plus the error of probing the work origin again.

That stack has to stay below the share of the tolerance your shop's rule set reserves for clamping. At Ø0.1 mm it fits with room to spare, and the second setup costs only setting up and measuring. What each extra setup adds is worked out in count the setups before you name a price. At Ø0.02 mm almost nothing is left for it.

In our piece on why the answer depends on the factory (opens in a new tab) we described the two routes that remain: one setup on a 5-axis machine, or two setups on a 3-axis machine with a jig-bored fixture, verified repeatable reclamping and firm discipline on the CMM. Both hold the tolerance. Which one you choose depends on the machines and the measuring room you have, not on the drawing. The order of setups is worked out in the setup plan, back to front.

How much room does Ø0.02 leave?

The worked example below lists, for each route, how much of the side bore's tolerance zone each contribution uses, expressed as zone diameter.

Against Ø0.1 mm both routes leave at least 0.08 mm. Against Ø0.02 mm the 5-axis route keeps 0.004 mm of margin and the 3-axis route keeps none: anything not in the table, such as a chip under the fixture, turns the margin negative. That is why, in this example, the second route needs every piece measured.

The contributions are added as a worst case; added in quadrature, the total comes out lower. Which of the two applies is fixed in your shop's rule set.

Check, too, whether a circled M follows the Ø value. That is the maximum material requirement from ISO 2692, meant for features that have to fit at assembly: a bore that comes out larger than its smallest permitted size gains the difference as position tolerance. The worked example assumes a value without the M; Ø0.02 mm then applies whatever size the bore is made to.

Which machines are still in the running?

The first question on any request is whether the part fits your machines: work envelope, axes, workholding. At Ø0.1 mm every machining centre the block fits on qualifies. At Ø0.02 mm machine selection turns into a question of accuracy: which machine holds this position, warmed up, with that fixture on the table?

The data sheet gives positioning accuracy and repeatability, measured to ISO 230-2 one axis at a time, directly on the machine. A part asks for more: two or three axes moving together, sometimes a rotary axis, with tooling, workholding and heat in the mix. What a machine holds under those conditions is in your own measurements.

Often the one machine that holds Ø0.02 mm is also the one everybody wants. The lead time of this part then hangs on the loading of that single machine, and that belongs in the quote before the customer places the order.

Can you measure it and prove it?

Making it is half the job; the other half is measuring it and showing the customer. A common rule of thumb is the 10:1 rule: the measurement uncertainty is at most a tenth of the tolerance. At Ø0.1 mm that is 0.01 mm; at Ø0.02 mm it is 0.002 mm.

The decision rule in ISO 14253-1 puts the burden of proof on whoever makes the claim. A supplier proving a feature conforms subtracts its measurement uncertainty from each limit; a customer rejecting it adds its own. With two limits and an uncertainty of a tenth of the tolerance, that leaves 80 per cent of the zone for making the part; at a fifth, 60 per cent. The 10:1 rule predates the standard.

A probe on the machine is often enough at Ø0.1 mm, but it measures with the errors of the same machine that made the bore. At Ø0.02 mm you measure on a CMM in a temperature-controlled room. What the CMM achieves there shows in its acceptance and reverification test to ISO 10360-2.

Each critical feature gets a measuring moment in the inspection plan, an instrument, the nominal value with its limits and an offset field. A deviation found at first article inspection then leads straight to an offset correction. On a batch of 25 you then choose between a report per piece and a sample.

Give the surcharge an address

Two common ways of pricing miss this difference. A price taken from the 3D model alone sees two identical blocks, because the STEP models are identical. A flat percentage for tight tolerances in one cost engineer's spreadsheet puts the surcharge on the whole part, with no address. In the worked example the 3-axis route sends every piece to the measuring room; on a batch of 25 that is 25 measurements neither price contains, and you pay for them out of your margin.

The margin report from the process plan sets the error budget against each tolerance, as in the worked example. A feature with thin margin is flagged, and that flag is the basis of the risk surcharge. The surcharge then has an address, that one side bore, not a feeling about the customer.

If the budget exceeds the tolerance, the margin is negative and the part cannot be made with these means. Manufacturability is therefore not a property of the geometry; in our research on manufacturability (opens in a new tab) we describe it as a relationship between part, requirements, process and factory. That goes into the quote as a reservation, or earlier, back to the customer with the question of whether Ø0.02 mm is functionally needed.

Asking that question is part of the craft. A tight value is sometimes carried over from an older drawing; if the customer accepts Ø0.05 mm, the route gets simpler and the lead time often shorter.

Count the toleranced dimensions

For a quick read of the measuring load, count the dimensions with their own tolerance. We use that count as our measure of inspection load: every toleranced dimension is a feature in the inspection plan, with a measuring moment and an instrument. The remaining dimensions fall under the general tolerance in the title block.

Two features at Ø0.1 mm are measured at the machine. Two features at Ø0.02 mm mean a trip to the measuring room. How long that trip takes is a figure of your own shop: the distance, the queue for the CMM and the report the customer asks for. You find that figure in your post-calculations.