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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.

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.

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.

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.

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