GD&T & Metrology Calculators

GD&T and metrology calculators turn drawing language into inspection numbers: position deviation from basic dimensions, limit sizes for hole-shaft pairs, ISO 286 bands on linear size, and roughness parameter conversion between Ra, Rz, and RMS.

True Position applies the standard relationship between tolerance zone, basics, and measured coordinates. Fit Calculator works clearance and interference classes. ISO 286 Tolerance looks up grade and deviation for a nominal diameter. Surface Roughness Converter maps finish symbols to numeric values suppliers report.

Datum targets on castings and forgings are often machined in a second operation. True position on machined features references those finished datums, not the as-cast surface.

One hole out of position can scrap a housing. Bonus tolerance under MMC is easy to misapply when the modifier is buried in the feature control frame. Handbook ISO lookups slow down a incoming inspection queue.

European prints quote Rz. North American prints quote Ra. The same polished face gets two numbers. Conversion avoids rejecting good finish or accepting bad finish.

MMC bonus on a hole pattern can salvage parts that look out of position until you apply departure from MMC. LMC on a shaft works the same way in reverse. The true position tool documents which modifiers it supports.

CMM alignment errors look like part deviation until you verify the setup. True position is only as good as the datum features that established the part coordinate system.

Casting and forging datums are often machined in a second operation. Position on machined features references finished datums, not the as-cast surface.

Threaded holes with position callouts still need pitch-diameter inspection separate from cylindrical fit math on a plain bore.

Paint thickness gages read differently than stylus roughness on the same panel. Convert finish parameters when cosmetic rejects cite Ra but the supplier measured Rz.

ISO 286 grades on a legacy print may not match ANSI fit callouts on the same drawing sheet. Read the title block carefully here first before you pick fit versus ISO tool.

• Position deviation from CMM or layout data
• MMC and LMC limits on a hole-shaft pair
• ISO tolerance grade for a shaft or bore callout
• Ra versus Rz on a supplier certification

True position compares actual feature location to basic dimensions within the stated zone. Fit and ISO tools map nominal size and grade to deviation limits per ISO 286 and related product geometry standards. Roughness conversion uses standard parameter relationships for equivalent finish.

Each form documents whether MMC bonus is included and which fit table it references. Enter measured values in the same datum setup the official report used.

Profile and position on the same face are separate requirements. A part can pass position and fail finish. Convert roughness before you argue with the supplier cert.

Roughness conversion maps Ra, Rz, and RMS using standard parameter relationships. Supplier certs and drawing symbols must refer to the same parameter before you accept or reject finish.

• Match the print callout to the right tool
• Align basics and measurements to the same datum frame
• Select fit class or ISO grade before comparing limits
• Convert supplier Rz when the drawing shows Ra only

Use during tolerance allocation on new prints, CMM report review, incoming inspection, and supplier disputes on fit or finish. Single-feature checks and fit screening are the sweet spot.

Full datum reference frame analysis and composite position belong in CMM software and qualified interpretation. ISO calculator output is not legal metrology certification.

Supplier material certificates list chemistry, not position. Fit tools still decide whether bore and shaft from that heat will assemble without hand work.

Legacy coordinate paper may list basics in a table while CAD exports a different origin. Align the datum story before entering measured values.

Several dimensions closing in a fixture stack are in Fixture & Tooling Calculators. Minimum bend radius on a formed flange is under Sheet Metal Calculators.

• Design review on tolerance schemes
• Inspection sign-off on position and fit
• Finish comparison on mixed Ra/Rz reports
• Not for full DRF analysis without CMM support

Trace every symbol on the print to the edition your quality manual cites. Customer and supplier GPS editions differ more often than teams expect on international supply chains.

• ASME Y14.5 on many North American prints
• ISO 1101 and ISO GPS on international drawings
• ISO 286 for linear size and fit bands
• ISO 1302 and ISO 4287 for texture symbols and Ra
• Customer-specific GPS addenda when the title block cites a corporate standard

Gather the print and the measurement report together. Mixed datums cause most arguments with the CMM output. First-article inspection often blends hand-gauge and CMM data on the same feature.

• Feature control frame with tolerance and modifier
• Basic dimensions from the approved datum frame
• Measured coordinates from the same setup as the report
• Nominal size and fit or ISO grade for limit math