DEPARTMENT OF COMMERCE BUREAU OF STANDARDS George K. Burgess, Director REPORT OF THE NATIONAL SCREW THREAD COMMISSION (REVISED, 1924) MISCELLANEOUS PUBLICATIONS, BUREAU OF STANDARDS, No. 61 DEPARTMENT OF COMMERCE BUREAU OF STANDARDS GEORGE K. BURGESS, Director REPORT OF THE NATIONAL SCREW THREAD COMMISSION (REVISED, 1924) (AUTHORIZED BY CONGRESS, JULY 18, 1918, H. R. 10852) AS APPROVED AUGUST 19, 1924 FEBRUARY 11, 1925 MISCELLANEOUS PUBLICATIONS OF THE BUREAU OF STANDARDS No. 61 PRICE 35 CENTS Sold only by the Superintendent of Documents, Government Printing Office Washington, D. C. WASHINGTON GOVERNMENT PRINTING OFFICE 1925 PREFACE This report is the first revision of the progress report of the Na- tional Screw Thread Commission published in 1921. The material has been generally rearranged and subdivided into sections, all specifications relating to a given class of product being included in a single section. Among the more important changes made in the specifications as previously published are the following: The classes of fit have been renamed and renumbered; the tolerance on major diameter of screws of classes 3 and 4 has been made the same as for class 2; specifications for gages have been extensively revised, and the allowances and tolerances on fire-hose coupling threads have been revised to decrease the maximum permissible looseness. In general, any screw thread product which met the previous specifications will meet the revised specifications. New material added includes speci- fications for threading tools, recommended tool shapes, tap dimen- sions, and tap drill sizes; specifications for screw threads of special diameters, pitches and lengths of engagement; specifications for gages for fire-hose coupling threads; specifications for wood screws; and the appendixes, which embody information supplementing the specifications. Acknowledgment is made to the many individuals, firms, and. other organizations, whose cooperation and assistance have made possible the completion of this report. The thanks of the commission are especially due the manufacturers and users of screw-thread products, tools, and gages; the American Society of Mechanical Engineers; the Society of Automotive Engineers; the American Engineering Standards Committee and its sectional committees on screw threads and plain limit gages; The Federal Specifications Board; the National Board of Fire Underwriters; the United States Army; the United States Navy; and the Bureau of Standards. Attention is directed to the fact that in so far as the same ground is covered by this revised report of the commission and by report (B la—1924) of the American Engineering Standards Committee (Sec. II, Sec. Ill— 1, 2, 3, and 4, and Appendix 1, herein), the two reports are in substantial agreement. Criticisms and suggestions for the improvement of the report are invited and should be addressed to the National Screw Thread Com- mission, Bureau of Standards, Washington, D. C. ii APPROVAL BY THE COMMISSION AND TRANSMITTAL TO THE SECRETARIES OF WAR, NAVY, AND COMMERCE Hon. John W. Weeks, Secretary of War. Hon. Curtis D. Wilbur, Secretary of the Navy. Hon. Herbert Hoover, Secretary of Commerce. August 19, 1924. To the honorables the Secretary of War, the Secretary of the Navy, the Secretary of Commerce. The National Screw Thread Commission, having revised its Progress Report, dated January 4, 1921, herewith submits its report revised 1924, for your acceptance and approval, in accordance with Public Act No. 201 (H. R. 10852, 65th Cong.), approved July 18, 1918; as amended by Public Act No. 324 (H. R. 15495, 65th Cong.), approved March 3, 1919; Public Resolution No. 34 (H. J. 299, 66th Cong.), approved March 23, 1920; and Public Resolution No. 43 (H. J. 227, 67th Cong.), approved March 21, 1922. George K. Burgess, Chairman. E. C. Peck, Lieut. Colonel, U. S. A., J. 0. Johnson, Major, U. S. A., Appointed by the Secretary of War. M. A. Libbey, Commander, U. S. N., John B. Rhodes, Commander, U. S. N., Appointed by the Secretary the Navy. ' of F. O. Wells, Ralph E. Flanders, Appointed by the Secretary of Commerce from nominations by the American Society of Mechanical Engineers. Earle Buckingham, George S. Case, Appointed by the Secretary of Commerce from nominations by the Society of Automotive Engineers. in IV NATIONAL SCREW THREAD COMMISSION APPROVAL BY THE SECRETARIES OF WAR, NAVY, AND COMMERCE October 2, 1924. The attached report prepared by the National Screw Thread Commission, in accordance with the law establishing the commis- sion, Public Act No. 201 (H. R. 10852, 65th Cong.), amended by Pub- lic Act No. 324 (H. R. 15495, 65th Cong.), is hereby accepted and approved. John W. Weeks, Secretary of War. Curtis D. Wilbur, Secretary of the Navy. Herbert Hoover, Secretary of Commerce. 1924 REPORT OF THE NATIONAL SCREW THREAD COMMISSION (Authorized by Congress, July 18, 1918, H. R. 10852) AS APPROVED AUGUST 19, 1924 CONTENTS Page Preface n Section I. Introduction 4 1. Historical 4 2. Authorization 5 (a) Commission authorized by Congress 5 (b) Life of commission extended by Congress 6 3. Organization of the commission 6 (a) Preliminary meeting 6 (6) Members 7 (c) Officers 8 (d) Committees 8 (e) Later committees 1 8 (/) Personnel on European trip 1 9 1 (g) Present organization 9 (/i) General procedure 10 4. Arrangement of report 1 11 5. General 12 (a) Strict interchangeability 12 (b) Need of definite specifications 12 Section II. Terminology 13 1. Definitions 13 (a) Terms relating to screw threads 13 (b) Terms relating to classification and toler- ances 14 2. Symbols 16 3. Illustrations showing terminology 18 Section III. Screw threads for bolts, nuts, commercial tapped holes, etc.. 18 1. National form of thread 18 (a) Specifications 18 (b) Illustration 18 2. Thread series 19 (a) National coarse-thread series 19 (b) National fine-thread series 20 3. Classification and tolerances 21 (a) General specifications 22 (b) Classification of fits 23 4. Tables of dimensions 37 1 New material not included in the progress report of Jan. 4, 1921. 1 2 NATIONAL SCREW THREAD COMMISSION Section III. Screw threads for bolts, etc. —Continued. Page 5. Specifications for threading tools 1 53 (a) Form of tools for producing screws 53 (b) Taps 58 (c) Tap-drill sizes 63 6. Gages 1 67 (a) Fundamentals 67 (b) Gaging practices and types of gages 69 (c) Specifications for gages 73 Section IV. Screw threads of special diameters, pitches, and lengths of engagement 1 88 1. Form of thread 88 2. Thread series 88 3. Classification and tolerances 88 (a) General specifications 88 (6) Classification of fits 90 4. Specifications for threading tools 101 (a) Form of tools for producing screws 101 (b) Taps 101 (c) TapdriUs 102 5. Gages 103 (a) Specifications for gages 103 Section V. National hose-coupling and fire-hose coupling threads 106 1. Form of thread 108 2. Thread series 108 (a) National hose-coupling threads 108 (b) National fire-hose coupling threads 109 3. Allowances and tolerances 109 4. Tables of dimensions 112 5. Gages 1 113 (a) Gages for national fire-hose coupling threads. 1 13 Section VI. National pipe threads 114 1. Form of thread 115 (a) Specifications 115 (6) Illustration 115 2. Symbols ... 116 3. Thread series 116 (a) National (American Briggs') taper pipe threads 116 (b) National straight pipe threads 120 (c) National locknut threads 122 4. Tables of pipe dimensions 124 5. Threading tools 1 126 (a) Taps 126 lb) Tap driUs 126 6. Gages 128 (a) Classification of gages 128 (b) Gaging practices 130 (c) Specifications for gages 130 Section VII. Wood screws 1 139 1. General specifications 139 2. Thread series 141 1 New material not included in the progress report of Jan. 4, 1921. 1924 REPORT 3 Section VII. Wood screws—Continued. Page 3. Tolerances on length 141 (a) Flat and oval-head screws 141 (&) Round-head screws 141 4. Standard sizes of wood screws 142 (a) Steel screws 142 (6) Brass screws 142 Appendix 1. Derivation of tolerances 1 145 1. Pitch diameter tolerances 145 (a) Tolerances for fastening screws 145 (6) Tolerances for screw threads of special diam- eters, pitches, and lengths of engage- ment 145 2. Relation of lead and angle errors to pitch diameter tolerances 146 (a) Diameter equivalent of lead error 146 (6) Diameter equivalent of angle error 146 Appendix 2. Wire methods of measurement of pitch diameter 1 147 1. Size of wires 147 2. Specification for wires 149 3. Methods of measuring and using wires 149 4. Measurement of pitch diameter of national straight threads 150 5. Measurement of pitch diameter of national taper threads 151 6. Measurement of pitch diameter of thread ring gages_ 153 Appendix 3. Control of accuracy of thread elements in the production of threaded product 1 156 1. Fundamental factors 156 (a) Tool controlled by lead screw 156 (b) Self-leading threading tool 157 2. Cutting of screw threads 157 (a) Single-point tool 157 (b) Thread chaser 158 (c) Tap or die 158 (d) MiUing cutter 159 (e) Threading hob 159 3. Rolling of screw threads 160 (a) Threading roll 160 (b) Thread rolling dies 160 4. Finishing of screw threads 161 (a) Grinding 162 (6) Lapping 162 Appendix 4. Design and construction of gages 163 1. Material 168 2. Plug thread gages 163 (a) Type 163 (6) Design of "go" thread plug gages 163 (c) Design of " not go " thread plug gages 164 3. Ring thread gages 164 (a) Type 164 (&) Design of "go" thread ring gages 164 (c) Design of "not go" thread ring gages 164 1 New material not included in the progress report of Jan. 4, 1921. 4 NATIONAL SCREW THREAD COMMISSION Appendix 4. Design and construction of gages—Continued. p3ge 4. Plain plug gages 164 5. Plain ring gages 164 6. Plain snap gages 164 Appendix 5.