TABLE OF CONTENTS NPL NOTES ON SCREW GAUGES ...........................................................................................................................1 FOREWORD ..................................................................................................................................................1 PART I: INTRODUCTION...........................................................................................................................................2 1 SCOPE ..............................................................................................................................................2 2 DEFINITIONS..................................................................................................................................3 2.1 General definitions for screws ............................................................................................3 2.2 Definitions for parallel screw threads .................................................................................4 2.3 Definitions relating to taper screw threads .........................................................................5 3 ELEMENTS OF A SCREW THREAD AND THEIR ERRORS .....................................................5 3.1 Pitch diameter .....................................................................................................................5 3.2 Pitch....................................................................................................................................6 3.3 Error in pitch in relation to pitch diameter..........................................................................7 3.4 Errors in flank angles in relation to pitch diameter.............................................................8 3.5 Radii at crest and root .........................................................................................................9 3.6 Summary.............................................................................................................................9 4 GAUGING PARALLEL SCREW THREADS...............................................................................10 4.1 Gauges for external threads ..............................................................................................10 4.2 Gauges for internal threads ...............................................................................................10 4.3 Length of gauge ................................................................................................................10 4.4 Thread form of gauge .......................................................................................................10 5 PREPARATORY FEATURES OF GAUGES................................................................................11 5.1 Gauge blanks ....................................................................................................................11 5.2 Hardness and stabilization ................................................................................................11 5.3 Centre of plugs; end faces of rings ...................................................................................11 5.4 End threading....................................................................................................................12 PART II: PARALLEL SCREW THREADS ..................................................................................................13 6 CONDITIONS OF MEASUREMENT...........................................................................................13 6.1 Temperature......................................................................................................................13 6.2 Units..................................................................................................................................13 7 MECHANICAL MEASUREMENTS OF PARALLEL SCREW PLUG GAUGES ......................13 7.1 Major diameter..................................................................................................................13 7.2 Pitch diameter ...................................................................................................................14 7.3 Minor diameter .................................................................................................................18 7.4 Pitch..................................................................................................................................19 7.4.1 "Drunkenness" ..................................................................................................................21 7.5 Thread form and flank angles ...........................................................................................22 8 FORMULAE AND CORRECTIONS FOR USE IN THE MEASUREMENT OF THE PITCH DIAMETER OF PARALLEL SCREW PLUG GAUGES BY MEANS OF SMALL CYLINDERS ..................................................................................................................................23 8.1 Formula for calculating pitch diameter E of thread with symmetrical flank angles .........23 8.2 Determination of rake correction, c ..................................................................................25 8.3 Correction e for the elastic compression of cylinders.......................................................27 8.4 Consideration of rake and compression corrections .........................................................28 8.5 Measurement of pitch diameter using cylinders other than best size................................29 8.6 Measurement of thread angle using cylinders of different sizes.......................................31 9 MECHANICAL MEASUREMENTS OF PARALLEL SCREW RING GAUGES.......................32 9.1 Minor diameter .................................................................................................................32 9.2 Pitch diameter ...................................................................................................................33 9.2.1 Displacement method .......................................................................................................33 9.2.2 Radial method...................................................................................................................35 9.2.3 Horizontal diametral comparators.....................................................................................38 9.3 Pitch..................................................................................................................................39 9.4 Thread form and angle......................................................................................................39 10 INSPECTION OF PARALLEL SCREW RING GAUGES WITH CHECK PLUGS ....................40 11 TESTING OF SCREW CALLIPER GAUGES ..............................................................................41 12 OPTICAL PROJECTION APPARATUS.......................................................................................42 12.1 Horizontal projector..........................................................................................................43 12.2 Examination of thread form..............................................................................................44 12.3 Modern projectors.............................................................................................................45 12.4 Measurement of flank angles............................................................................................47 12.4.1 Improved protractor and throwover method of flank angle measurement........................48 PART III: TAPER SCREW THREADS..........................................................................................................50 13 SCOPE ............................................................................................................................................50 14 DEFINITIONS FOR TAPER SCREW THREADS, SEE FIG. 28 .................................................50 15 GAUGING TAPER SCREW THREADS ON COMPONENTS....................................................51 16 MEASUREMENT OF TAPER SCREW PLUG GAUGES............................................................52 16.1 Elements to be measured ..................................................................................................52 16.2 Measurement of pitch, major, and minor diameters .........................................................52 16.2.1 "Spotting off" to locate the specified plane.......................................................................52 16.2.2 Measurement of pitch diameter in a floating micrometer machine...................................53 16.2.3 NPL/Matrix radial measuring machine.............................................................................55 16.2.4 NPL double sine bar machine...........................................................................................55 16.3 Measurement of taper .......................................................................................................58 16.3.1 Floating micrometer machine ...........................................................................................58 16.3.2 Double sine bar machine...................................................................................................58 16.3.3 Minor diameter: major diameter ......................................................................................58 16.4 Measurement of pitch .......................................................................................................58