An inside micrometer accuracy chart must identify the instrument family and model, because different bore gauges can have different measuring methods, ranges and maximum permissible errors. This table covers selected metric Mitutoyo Series 368 mechanical Holtest internal micrometers. It lists order number, measuring range, measuring method, graduation and the manufacturer’s stated accuracy for that range group. The values are instrument specifications, not part tolerances and not a substitute for calibration records or measurement uncertainty evaluation.

Inside Micrometer Accuracy Chart

Accuracy, graduation and range

Graduation describes the scale increment displayed or read by the instrument. Accuracy is the manufacturer’s stated limit for indication error under the applicable product specification. The two values are different: a fine graduation does not mean the instrument is accurate to one graduation. In this catalog, Series 368 models through 12 mm use 0.001 mm graduation, while larger models use 0.005 mm. Stated accuracy changes across range groups from ±2 µm on the smallest models to ±6 µm on the largest listed group.

Each order number maps to a specific measurement range. For example, model 368-001 covers 2 to 2.5 mm and has a stated ±2 µm accuracy. Model 368-171 covers 62 to 75 mm and is listed in the ±4 µm group. Do not copy the accuracy from a neighboring range to an unlisted instrument. Some nominal ranges in the catalog overlap at their boundaries; use the exact model designation and manufacturer range, rather than inferring which gauge should be selected from the chart alone.

The catalog identifies two-point methods for the 2–3 mm and 3–6 mm ranges, then three-point measurement from 6–12 mm and above. This range-specific method information matters because contact geometry and self-centering behavior differ. This inside-bore reference is distinct from the separate Series 293 outside micrometer page. Other bore gauge families may use different accuracy definitions and calibration methods.

How to use the chart

Read the instrument’s complete model number, confirm the bore dimension falls within its range, and then check the matching catalog entry. If the model is 368-001, for instance, the chart shows 2–2.5 mm, 0.001 mm graduation and ±2 µm accuracy. That tells you the published instrument accuracy limit; it does not say whether a measured bore conforms to its drawing tolerance. Use the appropriate setting ring or master, follow the zero-setting procedure and apply the measurement technique specified for the instrument.

For a quality decision, consider calibration status, traceability, temperature, cleanliness, alignment, contact force, bore geometry, operator repeatability and the required decision rule. A bore may have taper, ovality or lobing that a single reading cannot characterize. Take readings at the locations and orientations required by the drawing or inspection plan. If an error limit is close to the part tolerance, assess the complete measurement process rather than relying on the instrument specification alone.

Common mistakes and limits

Do not confuse the gauge’s accuracy with its graduation or with a calibration certificate result. Do not use this table as an allowable bore tolerance; that belongs to the engineering drawing and governing product specification. The cataloged values apply only to the exact Series 368 models and source edition. The table does not extend to other Mitutoyo series, other brands or other measuring methods.

The manufacturer’s current instructions and calibration procedure control. Confirm the actual order number, condition and accessories, especially for deep holes or when an extension rod is fitted. Do not assume an extended configuration has the same performance unless the manufacturer says so. Store the instrument cleanly, protect the contacts and verify zero with the specified standard before use. For acceptance or audit work, retain calibration evidence and record the measurement method with the result.

Source

Selected specifications are from Mitutoyo’s Holtest/Borematic Series 368 catalog. Confirm the latest specification for the exact order number before inspection use.