Documentation

The tolerance walk-back

Last updated 6 August 2026

A print gives you the finished part. A machinist at the second of six operations does not need the finished part: they need to know what to leave so that after everything downstream has been done to it, the part lands where the print says. Working that out by hand, for every characteristic, on every revision, is the job this replaces.

You type the limits once, at the final inspection. ctrlPlanner walks backward through the routing and gives every station its own limits.

The rule

• An operation that adds material, such as plating or coating, means the part upstream of it has to be smaller.
• An operation that removes material, such as grinding or turning, means the part upstream of it has to be larger.
• An effect on both sides counts twice: plating a diameter adds the coat on the way in and on the way out.

A range narrows the band. A fixed amount does not.

This is the part that is genuinely hard to do by hand, and the reason the tool earns its place. A plating spec of 0.0002 to 0.0004 does not just shift the upstream limits: it eats 0.0002 per side out of the tolerance band, because the part has to be in spec whichever thickness the plater lands on. Grinding exactly 0.0100 shifts the limits and leaves the band alone.

Worked all the way through

Print spec      0.5000 / 0.5020, at final inspection
Routing         10 Turn    20 Grind    30 Plate    40 Final Inspection
                this characteristic is inspected at all four

  30 Plate      grows, both sides, 0.0002 to 0.0004 per side
  20 Grind      shrinks, both sides, 0.0100 exactly per side

Walking back from the final spec:

  40 Final      0.5000 / 0.5020    as typed

  30 Plate      0.5000 / 0.5020    unchanged: nothing dimensional
                                   happens after plating

  20 Grind      0.4996 / 0.5012    plating comes after this station
                                   0.5000 - (0.0002 x 2) = 0.4996
                                   0.5020 - (0.0004 x 2) = 0.5012
                                   band 0.0020 becomes 0.0016

  10 Turn       0.5196 / 0.5212    grinding comes after this station
                                   0.4996 + (0.0100 x 2) = 0.5196
                                   0.5012 + (0.0100 x 2) = 0.5212
                                   band 0.0016, unchanged

Read forwards: turn to 0.5196 / 0.5212, grind to 0.4996 / 0.5012,
plate, and the part measures 0.5000 / 0.5020 at final inspection
whichever thickness inside the plater's window it comes back at.

The limits shown at a station are what the part must measure when it is inspected there, with that station's own work already done. That is why plating shows the print spec: by the time the part is inspected after plating, it is the finished size.

Threads

Plating on a thread flank moves pitch diameter by much more than it moves a plain diameter, because the coat lands on both flanks at an angle. For 60 degree UN and ISO threads the rule every plating shop works to is that pitch diameter shifts by four times the per-side coat thickness, which is what MIL-STD-870 and the Federal screw thread specification H28 give.

Shallower flanks shift more. The multiplier scales by the tangent of thirty degrees over the tangent of half the thread angle, and it comes off the angle stored on the thread row, so a thread that is not 60 degrees needs no special handling.

Masking one flank halves it. On the row itself you see the class-of-fit spec with the gauge bracket underneath, and the bracket turns blue at the stations where plating has moved it.

What the colors mean

• Black: the same value you typed at the final inspection. Nothing between here and there moves it.
• Blue: computed. Hovering shows the whole chain, station by station, that produced the number.
• Red, with an exclamation mark: the band came out inverted. See below.

When no size holds

There are two ways out and the tool will not choose between them: tighten the process range, or open the print tolerance. Either is a conversation with somebody, which is the point of finding out now rather than at first article.

What does not walk

GD&T, basic dimensions, surface finishes, notes and reference dimensions show the same requirement at every station. There is no sensible sense in which a position tolerance is different before plating, so nothing is invented for it.

Mixed units

A characteristic keeps its own unit and an operation's effect keeps its own. Conversion happens at full precision before the arithmetic, and rounding happens only when a value is displayed, so an inch-spec plating on a metric drawing does not accumulate rounding error across four stations.

• Next: the control plan
• Setting up effects and behaviors