Machining allowance: working the print tolerance back through the process
What the calculator is doing
A drawing gives the finished part. It almost never gives the size the machinist has to leave before plating, before grinding or before heat treat, and on a print that says nothing more than "all dimensions after plating" the shop is left to work it out. The arithmetic is not hard, but there are three places to get it wrong and two of them look right afterwards.
The first is sidedness. A both-sided diameter moves by twice the per-side figure, so a coat quoted at two tenths per side closes a bore by four. The second is direction: plating adds material either way, but that makes an outside diameter bigger and a bore smaller, so the correction on a bore goes the opposite way to the correction on a journal. The third is the range, and it is the one that costs the most.
Why a process range costs you tolerance
A plater quotes a window rather than a number, because a plating line holds a range and not a figure. The part has to be in specification whichever thickness comes back, so the band the machinist gets to work in is narrower than the band on the print by the width of that window, times the sidedness factor.
A window of .0002 to .0005 per side on a both-sided diameter takes .0006 out of the tolerance before anyone has cut anything. On a print with a .002 band that is nearly a third of it, gone, and it never appears anywhere as a number. A deterministic operation, where the two ends of the window are equal, shifts the band without narrowing it at all. The difference between those two cases is the single most useful thing on this page.
Threads are the expensive one
A coating sits normal to the thread flank, and on a 60 degree thread that moves pitch diameter about four times the per-side thickness rather than twice. Two tenths of plating takes eight tenths out of pitch diameter. The rule appears in MIL-STD-870 and in Federal Screw Thread Specification H28, and it is the number most often left out of a hand calculation, because the person doing it has already correctly doubled for sidedness and has no reason to suspect there is another factor of two waiting.
The factor scales with the flank angle rather than being fixed at four. A 29 degree ACME thread comes out near nine times, a 55 degree Whitworth near four and a half. Shallower flanks move further per unit of thickness.
Anodize builds up half
An anodize call-out specifies the total coat, but anodizing converts base metal as it grows rather than only adding to the surface, so roughly half the stated thickness ends up above the original profile. Walking an anodize specification back the way you would walk a plating specification leaves twice the correction the part needed. The resulting number is wrong and entirely plausible, which is why it survives review.
Grind stock and heat treat
Grinding runs the same arithmetic in the opposite direction: the part arrives at the grinder larger than it leaves, so the upstream limits are bigger rather than smaller. The commonly quoted figure is five thousandths on a diameter, or eight if you are worried about cleanup, which is two and a half or four tenths per side. How much you actually need depends on how far out of round the part came in, which is why it is worth entering as a range and seeing what the range costs.
Heat treat is the one nobody can give you a number for. Parts have been measured moving ten thousandths in one axis, tool steels move more than stable alloys, and grain direction changes the answer on the same part. It also does not behave like a coating: a part scales, so the bore and the outside diameter move the same way as each other rather than opposite ways. The calculator asks which way rather than deriving it, because the geometry genuinely does not settle it.
When the answer is that it cannot be done
Sometimes the walk returns a lower limit above the upper one. That is not a rounding artifact and it is not the calculator giving up. It means the process window is wider than the tolerance it has to fit inside, so there is no size that can be left at that station which is in specification for every outcome the process can produce. The two ways out are to tighten the range with the outside process or to open the tolerance with the customer, and both are much cheaper to discover before the part is cut than at final inspection.
Doing it for a whole part
Checking one characteristic is what this page is for. A real part has forty of them through five operations, every one needs the same walk, and the walk has to be redone when the drawing revises. That is what ctrlPlanner does: balloon the print once, set what each operation does once, and every station sees the limits it has to hold instead of the finished part number. The control plan and the AS9102 forms come out of data already entered rather than being typed a second time.
Common questions
How much should I leave for plating?
As much as the plater's window says, times two for a both-sided diameter, and about four times for a 60 degree thread. Ask the coating shop for the range they actually hold rather than the nominal on the specification, because it is the width of that range that decides how much tolerance you lose.
Is the plating thickness per side or on the diameter?
Platers quote per side. The dimension moves by twice that on a both-sided feature, which is where a lot of the confusion starts. This calculator takes the per-side figure and does the doubling.
How much stock do I leave for grinding?
The usual answer is .005 on the diameter, or .008 if you want to be sure of cleaning up. That is .0025 to .004 per side. How much you need depends on how far out of round the part is coming in, which for a heat-treated part is often more than for bar stock.
Why does my thread gauge fail after plating when the diameter was fine?
Because pitch diameter moves about four times the per-side coat thickness on a 60 degree thread, not twice. A correction that was right for the major diameter is a quarter of what the pitch diameter needed.
Does anodize need the same allowance as plating?
No. An anodize call-out is total coat thickness and only about half of it sits above the original surface, so the correction is roughly half what the same number would be for plating.
Does anything leave my machine?
No. There is no server component and nothing is transmitted. The page runs in your browser, and closing the tab is the end of it.
More about ctrlPlanner · Walking a whole plan back · Free AS9102 report builder