MIL-STD-I20 12 December 1950 W/CHG. NOTICE #1 9 SEPT 1963 MILITARY STANDARD GAGE INSPECTION UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON : 1951 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D. C. - Price $1.00 MIL-STD-120 12 December 1950 MUNITIONS BOARD STANDARD AGENCY DEPARTMENT OF DEFENSE WASHINGTON, D. C. 12 DECEMBER 1950 Gage Inspection MIL-STD-120 1. This standard has been approved by the Departments of the Army, the Navy, and the Air Force for the purpose of establishing uniform practices throughout the Military Services for the care and use of inspection gages and special measuring devices used in connection with the dimensional control of all products and equipment procured by the U. S. Armed Forces. 2. The Munitions Board Standards Agency approved this standard for printing and inclusion in the MIL series of standards on 12 December 1950. 3. In accordance with established procedure, the Munitions Board Stand- ards Agency has designated the Ordnance Corps, the Bureau of Ordnance, and the Air Force, respectively, as joint custodians of this standard. 4. When repeated deviations from this standard are required by a depart- ment or technical service, a report shall be submitted to the Munitions Board Standards Agency by the activity concerned explaining the reason for the deviation. II MIL-STD-120 12 December 1950 CONTENTS Paragraph Page Paragraph Page 1 PURPOSE AND SCOPE-------- 1 3.2.1 Steel rule--------------------------- 6 1.1 General---------------------- 1 3.2.2 Hook rule--------------------------- 6 1.1.1 Scope------------------------ 1 3.2.3 Steel rule with holder------------ 6 1.1.2 Purpose--------------------- 1 3.2.4 Rule depth gage------------------- 7 1.1.3 Classification of gages and 3.2.5 Adjustable steel square--------- 7 measuring equipment-------- 1 3.2.6 Combination set and combina- 1.1.3.1 Length standards----------------- 1 tion square---------------------- 7 1.1.3.2 Master gages---------------------- 1 4 PRECISION MEASURING 1.1.3.3 Inspection gages------------------ 1 EQUIPMENT-------------- 9 1.1.3.4 Manufacturer’s gages----------- 1 4.1 General------------------ 9 1.1.3.5 Nonprecision measuring equip- 4.2 Length standards----------------- 9 ment--------------------- 1 4.2.1 Line-graduated standards------ 9 1.1.3.6 Precision measuring equip- 4.2.2 Precision gage blocks----------- 9 ment--------------------- 1 4.2.3 End measuring rods------------- 9 1.1.3.7 Comparators--------------- 1 4.2.4 Master disks----------------------- 10 1.1.3.8 Optical comparators and 4.2.5 Wires and rolls-------------------- 10 gages---------------------- 1 4.2.6 Solid angle standards and pre- 2 DIMENSIONS AND TOL- cisions squares----------------- 10 ERANCES---------------- 2 4.3 Scale and vernier---------------- 11 2.1 General-------------------- 2 4.3.1 Vernier caliper-------------------- 11 2.2 Definitions--------------------- 2 4.3.1.1 Directions for reading the ver- 2.2.1 Dimension---------------------- 2 nier caliper---------------------- 11 2.2.2 Nominal size----------------------- 2 4.3.2 Vernier depth gage--------------- 12 2.2.3 Basic size--------------------------- 2 4.3.3 Vernier height gage------------- 12 2.2.4 Design size------------------------- 2 4.3.4 Gear tooth vernier caliper----- 12 2.2.5 Actual size------------------------- 2 4.3.5 Vernier bevel protractor-------- 13 2.2.6 Limits of size----------------------- 2 4.3.6 Directions for reading the ver- 2.2.7 Fit------------------------- 2 nier protractor----------------- 13 2.2.8 Tolerance------------------ 2 4.4 Micrometers--------------- 16 2.2.9 Allowance---------------------- 2 4.4.1 Micrometer caliper--------------- 16 2.3 Tolerances----------------------- 2 4.4.1.1 Directions for reading a mi- 2.3.1 Unilateral tolerances------------ 2 crometer caliper--------------- 18 2.3.2 Bilateral tolerance--------------- 2 4.4.1.2 Directions for reading the ver- 2.3.3 Gage tolerances------------------ 2 nier micrometer caliper------ 19 2.3.3.1 Tolerances on plain gages------- 3 4.4.2 Screw thread micrometer cal- 2.3.3.2 Tolerances for thread gages------- 4 iper-------------------------- 19 2.3.4 Gage wear allowance------------- 4 4.4.3 Inside micrometers-------------- 20 2.3.5 Relation of gage limits to part 4.4.4 Micrometer depth gage--------- 20 limits----------------------- 4 4.4.5 Supermicrometer--------------- 21 2.3.6 Disputed rejections-------------- 5 4.5 Measuring machines------------ 22 3 NONPRECISION MEASUR- 4.5.1 Universal measuring machine---- 22 ING EQUIPMENT----------- 6 4.5.2 Special measuring machines----- 23 3.1 General---------------------- 6 4.6 Comparators---------------------- 23 3.2 Description----------------- 6 4.6.1 Vertical comparators----------- 23 III MlL-STD-120 12 December 1950 Paragraph Page Paragraph Page 4.6.2 Electric comparators--------------- 24 5.3 Ring gages----------------------------- 37 4.7 Optical instruments and com- 5.3.1 Plain ring gages--------------------- 37 parators------------------ 25 5.3.2 Twin ring gages--------------------- 37 4.7.1 Optical flats-------------------------- 25 5.3.3 Progressive ring gages------------ 37 4.7.2 Optical projection machines---- 25 5.3.4 Taper ring gages-------------------- 38 4.7.3 Tool maker’s microscope----- 27 5.3.5 Thread ring gages------------------ 38 4.7.4 Optical dividing head-------------- 27 5.3.6 Taper threaded pipe ring gages- 38 4.8 Auxiliary equipment-------------- 27 5.3.7 Taper plain pipe ring gages---- 39 4.8.1 Sine bars and plates--------------- 27 5.3.8 Spline ring gages------------------- 39 4.8.2 Dial indicators----------------------- 27 5.4 Triroll gages--------------------------- 39 4.8.3 Surface laces------------------------- 30 5.4.1 Threaded pipe triroll gages----- 39 4.8.4 Tool maker’s flat-------------------- 30 5.4.2 Taper plain pipe triroll gages---- 40 4.8.5 Hardened steel square------------ 31 5.5 Snap gages---------------------------- 40 4.8.6 Pipe thread checking block------ 32 5.5.1 Snap gage construction------------ 40 5 TYPES OF GAGE--------------- 33 5.5.2 Adjustable length gages---------- 40 5.1 General----------------------- 33 5.5.3 Combination ring and snap 5.1.1 Scope------------------------------- 33 gages----------------------------- 40 5.1.1.1 Standard gage blanks------------- 33 5.5.4 Thread snap gages----------------- 40 5.1.1.2 Standard screw thread gages--- 33 5.5.4.1 Roll thread snap gages------------ 40 5.1.2 “Go” gages----------------------------- 33 5.5.4.2 Flat-anvil thread snap gages-- 41 5.1.3 “Not Go” gages---------------------- 33 5.5.4.3 Single-point thread snap 5.1.4 Acceptance check gages----------- 33 gages---------------------------- 42 5.1.5 Wear limit check gages----------- 33 5.5.4.4 Segment roll thread snap 5.1.6 Master setting gages-------------- 33 gages------------------------- 42 5.2 Plug gages----------------------------- 33 5.6 Miscellaneous and special 5.2.1 Plain cylindrical plug gages----- 33 gages---------------------------- 43 5.2.1.1 Standard designs------------------- 33 5.6.1 Flush pin gages---------------------- 43 5.2.1.2 Cylindrical plug gage, single- 5.6.2 Spanner gages----------------------- 43 end, solid---------------------------- 34 5.6.3 Fixture gages------------------------- 43 5.2.1.3 Cylindrical plug gage, single- 5.6.4 Concentricity gages---------------- 44 end, progressive------------------ 34 5.6.5 Template or profile gages------- 44 5.2.1.4 Cylindrical plug gage, double- 5.6.6 Functional gages-------------------- 44 end------------------------- 34 5.6.7 Chamber gages---------------------- 45 5.2.1.5 Cylindrical plug gage, replace- 5.7 Air gages------------------------------- 46 able-------------------------------- 34 5.7.1 Pressure type air gage---------- 46 5.2.1.6 Cylindrical plug gage, revers- 5.7.2 Flow type air gage------------------ 46 ible----------------------------- 34 5.7.3 Air snap gage----------------------- 47 5.2.2 Plain taper plug gages----------- 35 5.8 Bore inspection devices----------- 47 5.2.3 Thread plug gages----------------- 35 5.8.1 Star gazes----------------------------- 47 5.2.4 Taper threaded pipe plug 5.8.2 Dial indicator bore gages------- 47 gages---------------------------- 35 5.8.3 Horoscopes------------------------- 49 5.2.5 Taper plain pipe plug gages----- 35 5.8.4 Mirror and lamp devices------- 49 5.2.6 Spline plug gages------------------ 36 6 CARE, USE, AND MAIN- 5.2.6.1 Involute spline plug gages------ 36 TENANCE------------------------- 50 5.2.6.2 Straight-sided spline plug gage- 36 6.1 General---------------------------- 50 5.2.7 Alinement plug gages------------- 36 6.2 Surveillance of gages-------------- 50 5.2.8 Graduated plug gages------------ 37 6.2.1 Identification marking----------- 50 5.2.9 Flat plug gages---------------------- 37 6.2.2 Loan of gages to contractors--- 50 5.2.10 Miscellaneous plug gages--------- 37 6.2.3 Cleaning--------------------------- 50 IV MlL-STD-120 12 December 1950 Paragraph Page Paragraph Page 6.2.4 Preservation procedure----------- 50 7.2 Surface roughness measure- 6.2.5 Maintenance and control pro- ment----------------------- 65 cedures------------------------- 50 7.2.1 Tracer type instruments-------- 65 6.2.5.1 Gage records-------------------------- 50 7.2.2 Profilometer----------------------- 65 6.2.5.2 Time limit checking system------ 51 7.2.2.1 Tracer unit---------------------------- 65 6.2.5.3 Use of checker master setting 7.2.2.2 Microinch meter--------------------- 65 gages------------------------- 51 7.2.2.3 Mototrace----------------------------