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Straight answers to machining and inspection questions

Last updated 10 August 2026

Every number on these pages is computed rather than typed, by the same engine behind the free calculators, so the page and the tool cannot disagree. Each answer starts with the answer.

They are here because the usual result for these questions is either a chart with no rules attached or a calculator that quietly gets the arithmetic wrong in the direction that ships bad parts.

Surface finish

• What is Ra 63 microinches in micrometers?Ra 63 microinches is Ra 1.6 micrometers. They are paired standard values on the preferred-number ladder rather than a conversion.
• What is Ra 125 microinches in micrometers?Ra 125 microinches is Ra 3.2 micrometers. Dividing by 39.37 gives 3.175, which is not a value any finish specification uses.
• What is Ra 50 microinches in micrometers?There is no metric partner for Ra 50, because 50 is not a preferred number. The nearest rung is 63, which pairs with 1.6.
• Is there a surface finish conversion chart for Ra in microinches and micrometers?The full ISO 4288 and ASME B46.1 preferred-number ladder, from Ra 500 microinches down to 1, with the quotient each rung would have given.
• What is Ra 1.6 micrometers in microinches?Ra 1.6 micrometers is Ra 63 microinches. Multiplying by 39.37 gives 63.0 here and stops working at every neighboring rung.

Converting a tolerance

• Why is my converted tolerance wider than the original?Because it was rounded to nearest. Rounding a lower limit down and an upper limit up adds tolerance the drawing never granted.
• How do I convert an inch tolerance to millimeters?Multiply by 25.4, then round the lower limit up and the upper limit down so the converted band stays inside the original.
• How many decimal places should a converted tolerance have?Enough that the converted band still fits inside the original. Three is usual for millimeters, four when the inch band is tighter than half a thousandth.
• How do I convert a millimeter tolerance to inches?Divide by 25.4, round the lower up and the upper down, and carry four decimals rather than three or the band collapses.

Allowances and walk-back

• What size do I machine a part to before plating?Subtract the plating buildup from both limits, doubled for a both-sided feature, and remember the plater quotes a range rather than a figure.
• Do I machine a bore bigger or smaller before plating?Bigger. Plating adds material either way, and inside a bore that makes the dimension smaller, so the hole has to start oversize.
• How much do I allow on a thread for plating?About four times the per-side coat thickness on pitch diameter for a 60 degree thread, not twice. The flank angle multiplies the shift.
• Does the four times rule work on an ACME thread?No. A 29 degree ACME flank shifts pitch diameter about nine times the per-side coat thickness, because the multiplier scales with the flank angle.
• How much does anodize add to a dimension?About half the stated coat thickness sits above the original surface, because anodizing converts base metal as it grows.
• How much stock do I leave for grinding?Around .005 on a diameter, or .008 if you want to be certain of cleaning up. That is .0025 to .004 per side.
• How much does a part grow in heat treat?It varies by alloy, process and grain direction, and it is not predictable from the drawing. Measure your own parts.
• What if the plating range is wider than my tolerance?Then no machined size works, and the calculator says so rather than returning limits. Tighten the range or open the tolerance.
• How much stock do I leave in a bore for grinding?The bore starts smaller, not larger. Grinding opens a hole, so the pre-grind size is under the print by twice the stock per side.
• How much do I open a bore before anodizing?Half the stated coat, doubled for sidedness, and the bore closes rather than grows. Both corrections at once, and both are commonly missed.
• How much do I allow on a tapped hole for plating?Four times the per-side coat on pitch diameter, and the hole starts larger rather than smaller. The thread rule and the bore reversal together.
• What costs me more tolerance, a thicker coating or a wider plating range?The range. Thickness only shifts the band; the width of the plater's window comes straight out of it.
• What are typical plating, anodize and grinding allowances?The per-side windows this engine models for each process, what each does to the part, and which of them state a total rather than a per-side figure.

Threads

• What is the pitch diameter of a 3/8-16 UNC-2A thread?Basic pitch diameter is .3344 and the class 2A limits are .3287 to .3331, because 2A carries an allowance that hangs the maximum below basic.
• What is the difference between a class 2A and a class 3A thread?3A is a quarter tighter and carries no allowance, so its maximum pitch diameter is the basic value rather than below it.
• What pitch diameter should a 1/4-20 tapped hole be?A class 2B tapped hole runs .2175 to .2223. The minimum is the basic pitch diameter exactly, because internal threads carry no allowance.
• What pitch diameter do I cut a 3/8-16 to before plating?Look up the 2A limits, then walk them back at four times the per-side coat. .3287 to .3331 becomes .3279 to .3311.
• Is there a pitch diameter table for UNC and UNF threads?Common UNC and UNF sizes in classes 2A, 3A, 2B and 3B, computed from the thread geometry rather than transcribed, with the derivation published beside it.

GD&T and ASME Y14.5

• Does perpendicularity need a datum?Yes. Perpendicularity, parallelism and angularity are relationships, so a frame carrying one with no datum reference is incomplete.
• Can a flatness callout reference a datum?No. Flatness is a form control, checked against the surface itself, so a datum reference on it is non-conformant.
• Can a runout tolerance carry an MMC modifier?No. Runout applies regardless of feature size, as do circularity, cylindricity and both profile controls.
• Was concentricity removed from ASME Y14.5?Yes, in the 2018 revision. Use position for coaxiality of a feature of size, or profile of a surface where the requirement is about the surface.
• Is the symmetry symbol still in ASME Y14.5?No. Y14.5-2018 removed it. Use position for a center plane between two features of size, or profile of a surface.
• Is a true position tolerance a round zone or a square one?Round, when the diameter symbol precedes the value. Checking it as X and Y deviation passes parts that are out of tolerance.
• What does it mean when a feature control frame has no material condition modifier?Regardless of feature size. Y14.5-2018 treats RFS as the default, so there is no symbol for it and no need to state it.
• Which GD&T symbols require a datum reference?Orientation, location and runout controls require one. Form controls take none. Profile is optional and depends on what is being located.
• Can a profile tolerance stand on its own with no datum?Yes. Profile is the one family where a datum is optional, because a profile can control form alone or form and location together.
• What does a position tolerance of zero at MMC mean?It is a real and legal callout. All the positional tolerance is bonus, earned as the feature departs from maximum material condition.
• Does a position tolerance always need the diameter symbol?No. Without it the zone is two parallel planes, which is what a slot or a width needs. With it the zone is a cylinder.
• When do I use LMC instead of MMC on a position tolerance?When the concern is wall thickness or minimum material, not assembly clearance. LMC protects the thin section; MMC protects the fit.

The calculators these come from

• Allowance calculator: plating, anodize, grinding and heat treat
• GD&T reference and feature control frame checker
• Tolerance and surface finish converter
• AS9102 first article report builder
• Reference data as JSON, free to use with attribution