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ASME B1.22M-1985 (2006) — Gages and Gaging for MJ Series Metric Screw Threads
📏 ASME B1.22M-1985 (2006) is the gages-and-gaging standard for MJ series metric screw threads, providing the gage specifications and inspection practice needed to fulfill the provisions of the MJ product thread standard (ASME B1.21M). It was developed by Subcommittee B1.2 on Screw Thread Gages and Gaging, first published as ANSI B1.22-1978; this edition was approved as an American National Standard on September 26, 1985, issued December 31, 1986, and reaffirmed in 2006. The Standard deliberately contains no conformance criteria — the B1 Committee has assigned all levels of acceptability for screw threads to B1.3M. A considerable amount of new material covers the many gage and measuring-equipment options shown in B1.3M, and Mn/Mt gages are identified as NOT GO gages.
💡 A three-layer division is all you need: B1.21M defines what the MJ thread itself looks like; B1.22M defines which gages to use and how to measure; B1.3M defines how much error is acceptable. None of the three can be skipped.
1. Gage System and Marking Rules 📌
The MJ gage system follows the GO / NOT GO principle and, on top of it, separates functional size from individual elements. Key points:
- GO gages: control the maximum-material boundary (functional size), ensuring assemblability; for MJ, the GO end must accept the controlled root radius and the truncated internal minor diameter.
- NOT GO gages: control the minimum-material boundary, preventing the pitch/minor diameter from becoming too small for connection strength.
- Mn/Mt gages: explicitly designated NOT GO in this edition, used for minimum-material (pitch diameter) checks of MJ internal threads.
- Indicating gages: cone-contact and vee-contact probes read pitch diameter, taper, straightness, roundness, and equivalent lead and flank-angle errors.
⚠️ Marking trap: Mn/Mt gages were once misread as “GO” gages; this edition makes clear they are NOT GO. Continuing to use them as GO would pass oversized internal threads — correct your inspection procedures.
2. Functional Size and Individual Element Measurement 🎯
MJ acceptance is not just about pitch diameter but about the functional size — the equivalent effective size combining pitch diameter, lead, and flank-angle errors. The Standard presents several measurement approaches with these reading meanings:
| Gage / Method | What It Measures | Reading Meaning | Typical Use |
|---|---|---|---|
| GO thread plug/ring gage | Maximum-material functional size | Assembles to the specified depth | Assemblability (first check) |
| NOT GO plug/ring gage (incl. Mn/Mt) | Minimum-material pitch diameter | Must not pass more than the allowed turns | Prevents undersize threads |
| Cone-contact indicating gage | Functional size and pitch diameter size | Readings X = Y = Z combined | Taper, straightness, roundness |
| Vee-contact indicating gage | Equivalent effect of lead and flank angle | AD₂c cumulative, AD₂h resultant | Lead and angle differential analysis |
| Optical comparator | Thread profile, flank angle | Magnified projection comparison | Internal threads checked via cast replicas |
| Three-wire / micrometer method | Simple pitch diameter | Actual pitch diameter value | Process control and arbitration |
The cone- and vee-contact reading system corresponds to the “internal imperfect screw thread” and “internal perfect screw thread” analyses of Figs. 17A/17B: X is the minimum-material pitch-diameter reading (for taper, straightness, and roundness), Y is the reading used for lead differential analysis, Z is the maximum-material functional-size reading; combinations such as X−Z and (Y−Z)+taper/4 separate cumulative and individual element errors.
💡 Core idea: MJ thread errors are coupled — pitch diameter, lead, and flank angle interact. Functional-size gages answer “does it assemble”; element gages answer “where is the error.” Accept functionally first, then analyze element by element.
3. Optical Comparator Inspection and Cast Replicas 🔬
Para. 4.18.1 of the Standard specifies that the optical comparator magnifies and projects the thread profile on a screen for profile and flank-angle checks. Key points:
- Internal threads: inspected using cast replicas — the internal thread is copied into a raised specimen and viewed on the comparator, the standard way to project an internal profile.
- Lead-angle alignment: for the best profile image, the threaded item is positioned so that the light is aligned with the thread lead angle.
- Flank-angle correction: because the thread profile is defined in a plane containing the axis while the comparator views a normal section, a correction factor must be added to the measured flank angle.
🚨 The most common source of misjudgment: reading flank angles directly from the comparator without the lead-angle correction factor turns conforming threads into “out of tolerance” (or vice versa). Whenever measuring angles by projection, apply the Standard’s correction formula.
4. Acceptance System and Gage Management 🛠️
The Standard explicitly excludes conformance criteria; all levels of acceptability follow ASME B1.3M. In practice:
- 📋 Class correspondence: the product class (e.g., 4H5W4h6h, 6g/6H) determines gage size limits and the number of inspection turns; gage specifications must map one-to-one to product classes.
- 🔧 Gage calibration: thread gages must be re-verified regularly against setting plugs/masters for pitch diameter and profile; worn-beyond-limit gages are retired.
- 📏 Inspection environment: temperature and cleanliness strongly affect micron-level decisions; perform precision checks in controlled conditions.
- 🔄 Dispute handling: when gage results and three-wire measurements disagree, combine functional-size and element analysis to locate the error source.
✅ A complete MJ inspection scheme = functional GO/NOT GO gages (assemblability) + indicating gages (element analysis) + optical comparator (profile) + periodic calibration and B1.3M criteria.
5. Frequently Asked Questions ❓
❓ Q: What is the difference between B1.22M and B1.16M?
A: B1.16M is the American gaging-practice standard for conventional metric (M profile) threads; B1.22M is dedicated to the MJ series because the MJ external root radius and truncated internal minor diameter fall outside the coverage of B1.16 and need their own gage specifications.
A: B1.16M is the American gaging-practice standard for conventional metric (M profile) threads; B1.22M is dedicated to the MJ series because the MJ external root radius and truncated internal minor diameter fall outside the coverage of B1.16 and need their own gage specifications.
❓ Q: Why are internal threads inspected with cast replicas?
A: An internal thread cannot be projected onto the comparator screen the way an external thread can. A cast replica produces a raised specimen whose profile and flank angles can be viewed on the optical comparator — the Standard’s prescribed projection method for internal threads.
A: An internal thread cannot be projected onto the comparator screen the way an external thread can. A cast replica produces a raised specimen whose profile and flank angles can be viewed on the optical comparator — the Standard’s prescribed projection method for internal threads.
❓ Q: What is the difference between functional size and pitch diameter?
A: Pitch diameter is a single geometric parameter; functional size is the equivalent effective size that combines pitch diameter, lead, and flank-angle errors and directly corresponds to assemblability with a perfect mating part. MJ acceptance is based on functional size, with pitch diameter as one input.
A: Pitch diameter is a single geometric parameter; functional size is the equivalent effective size that combines pitch diameter, lead, and flank-angle errors and directly corresponds to assemblability with a perfect mating part. MJ acceptance is based on functional size, with pitch diameter as one input.
🔍 Q: Why are Mn/Mt gages considered NOT GO?
A: This edition explicitly identifies Mn/Mt gages as NOT GO, used to check the minimum-material condition (undersized pitch diameter) of internal threads. They do not perform a GO function; misusing them as GO would release nonconforming internal threads.
A: This edition explicitly identifies Mn/Mt gages as NOT GO, used to check the minimum-material condition (undersized pitch diameter) of internal threads. They do not perform a GO function; misusing them as GO would release nonconforming internal threads.
