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New Thread Ground Micrometric Expanding Milling Spacers

New Thread Ground Micrometric Expanding Milling Spacers to pass final inspection and the batch being scrapped. New Thread Ground Micrometric An almost invariable query concerning such spacers is the question as to whether the fine threads can withstand the pressure used in tightening the cutters on to the milling machine Expanding Milling Spacers arbour. The significance of this matter in the minds of many engineers is probably influenced by visions of a mechanic wielding a 15 in. spanner and a heavy hammer in his endeavours to clamp TH E special steel used in the manufacture up the arbour assembly. In view of this Euco of the new Euco expanding milling spacers Tools has issued the following interesting is carefully heat treated to a Brinell of 590 analysis of the loads involved. and a tensile strength of 130 tons/sq. in., thus ensuring many years of continuous use without Loads Involved loss of accuracy. The case with a Euco S.1a, 1 in. dia. arbour is The calibrated scale, clearly seen in FIG. 1, as follows: major dia. of thread=l∙3125 in.; is machine divided in steps of 0∙0005 in. on pitch = 0∙025 in; T.P.I. = 40; max. tensile the British types, whilst the metric models are strength of material = 130 tons/sq. in. (=291,200 divided in steps of 0∙01 mm.; these divisions lb.); minor dia. = 1∙28 in. The resistance of the are comfortably open, being approximately micrometer threads to shear depends on the J in. apart. It will be noticed from the dimen­ surface where shear would be likely to take place sional data, that the narrowest spacer is ¼ in., as at A—C as shown in FIG. 4, then if P=pitch, and that the respective overall diameters are r=radius of thread root, it is evident from FIG. 4 slightly larger than those of the usual standard that spacing collars, in order to provide quick finger-tip control. CD=r sin 62½deg. K=P—2r sin 62½ deg. L=r—r cos 62½ deg. Adjustment The resistance of a single micrometer thread to Infinitely variable adjustment is effected by shear at this surface is therefore, turning the knurled conical outer sleeve, which π (Dm+2L)(P-2r sin 62½ deg.)f moves telescopically on the inner sleeve by means and remembering that for Whitworth form of the micrometer pitch thread. Tommy bar screws r=0∙13733 P and that cos 62½ deg.= holes and spanner flats are provided to enable 0∙4617 and sin 62½ deg. =0∙8870, then with N adjustments to be made under circumstances threads in engagement where it is impossible to reach with the fingers. Rs2=πNK(Dm+2L)f When two or more milling cutters are mounted Where Rs2=Resistance of micrometer threads on an arbour, as in straddle-milling operations, to shear, N=10 and L and K are constants etc., most of the setter's time is absorbed in (FIG. 2), spacing the various cutters accurately with the aid ...Rs2=π10(l∙28+0∙0037)(0∙025-(0∙00686× of shims, or even pieces of paper! The time taken 0∙8870))f to arrive at the right combination of "bits and t =3∙142×10×1∙2837×0∙019×291,200 pieces" will vary according to the number of ... Resistance of 10 micrometer threads to shear= shims and pieces of paper of different thick­ 223,250 lb. nesses the setter can lay his hands on. threads. In the case of Whitworth standard The figure thus obtained represents the pressure Setting usually involves the dismantling of the threads the screw would tend to break at the it would be necessary to apply in order to shear arbour support and arbour steadies, the removal section X— X or at the undercut as shown in the threads on the Type S.IA when only two- of the cutters and collars and the insertion, or FIG. 3.. Disregarding the undercut, the resistance thirds of the maximum thread engagement is in removal, of a shim. When all mating surfaces to tension at the section X— X is given by: use. have been carefully cleaned and the cutter spacing Although a perfect mating fit is afforded by the is more accurate, the support and steadies may be internal and external thread-grinding processes reassembled and production recommenced. employed in producing the threads, it is advisable Where Rsl=Resistance of core to shear, Dm= With a Euco micrometer spacer between each to reduce the figure obtained by 20 per cent in Minor Diameter, f =Maximum tensile strength. pair of cutters, however, it is only necessary to order to compensate for any minute and permis­ loosen the arbour locknut, and the adjustments sible play that may be present, the final value of may be made in a few seconds, without dis­ An Example Rs2 would therefore be 178,600 lb. Then as turbing the arbour support, or steadies. Rs2/Rsl=3∙2, this demonstrates that the resist­ Assuming a milling machine arbour to be made ance to shear of the micrometer spacer is con­ of 55-ton (123,200 lb.) per sq. in. nickel chrome Strength Formula siderably greater than the resistance to shear of steel, then in the case of a 1 in. dia. standard the milling machine arbour screw. In considering the strength of a screw thread milling machine arbour with the end screwed under tension in a static condition, it is necessary 7/8 in. B.S.F. (minor dia.=0∙7586 in.), the appli­ This machine is manufactured by Euco Tools to establish the following: (1) the resistance of cation of the above formulae would result in the Ltd., 30 Budge Row, London, E.C.2. the threads to shear at their base, or (2) the resist­ following: ance of the core to break at the bottom of the Dimensional Data BRITISH SCALE ...Resistance of core to shear=55,686 lb. Machin e divide d t o 0∙000 5 in. Very few setters willingly dismantle a set-up Widt h Bor e O/ D to make an adjustment when the work being pro­ No . (in. ) (in. ) (in. ) duced is "borderline" with regard to dimensional .. . . S. 1 1 1¾ limits. The inclination is to "let it go"; such .. . . S.1 a 1 13/16 1¾ circumstances often resulting in the work failing .. . . S. 2 ¼ 2 1/8 1¼ .. . . S.2 a 2 1/8 13/16 1¼ . . . . 2 3/8 S. 3 ¼ 1½ . . . . S.3 a 13/16 2 3/8 1½ .. .. S. 4 2 3 13/16 METRIC SCALE Machine divided to 0∙0 1 mm. Width Bore O/D No. (mm.) (mm.) (mm.) .. M. 5 .. 6∙ 3 1 6 33∙ 5 .. M.5 a 20∙ 5 1 6 33∙ 5 .. M. 7 .. 6∙ 3 2 2 41∙2 5 .. M.7 a 20∙ 5 2 2 41∙2 5 .. M. 1 .. 6∙ 3 2 7 50∙ 0 .. M.1 a 20∙ 5 2 7 50∙ 0 November 1946 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aircraft Engineering and Aerospace Technology Emerald Publishing

New Thread Ground Micrometric Expanding Milling Spacers

Aircraft Engineering and Aerospace Technology , Volume 18 (11): 1 – Nov 1, 1946

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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0002-2667
DOI
10.1108/eb031442
Publisher site
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Abstract

to pass final inspection and the batch being scrapped. New Thread Ground Micrometric An almost invariable query concerning such spacers is the question as to whether the fine threads can withstand the pressure used in tightening the cutters on to the milling machine Expanding Milling Spacers arbour. The significance of this matter in the minds of many engineers is probably influenced by visions of a mechanic wielding a 15 in. spanner and a heavy hammer in his endeavours to clamp TH E special steel used in the manufacture up the arbour assembly. In view of this Euco of the new Euco expanding milling spacers Tools has issued the following interesting is carefully heat treated to a Brinell of 590 analysis of the loads involved. and a tensile strength of 130 tons/sq. in., thus ensuring many years of continuous use without Loads Involved loss of accuracy. The case with a Euco S.1a, 1 in. dia. arbour is The calibrated scale, clearly seen in FIG. 1, as follows: major dia. of thread=l∙3125 in.; is machine divided in steps of 0∙0005 in. on pitch = 0∙025 in; T.P.I. = 40; max. tensile the British types, whilst the metric models are strength of material = 130 tons/sq. in. (=291,200 divided in steps of 0∙01 mm.; these divisions lb.); minor dia. = 1∙28 in. The resistance of the are comfortably open, being approximately micrometer threads to shear depends on the J in. apart. It will be noticed from the dimen­ surface where shear would be likely to take place sional data, that the narrowest spacer is ¼ in., as at A—C as shown in FIG. 4, then if P=pitch, and that the respective overall diameters are r=radius of thread root, it is evident from FIG. 4 slightly larger than those of the usual standard that spacing collars, in order to provide quick finger-tip control. CD=r sin 62½deg. K=P—2r sin 62½ deg. L=r—r cos 62½ deg. Adjustment The resistance of a single micrometer thread to Infinitely variable adjustment is effected by shear at this surface is therefore, turning the knurled conical outer sleeve, which π (Dm+2L)(P-2r sin 62½ deg.)f moves telescopically on the inner sleeve by means and remembering that for Whitworth form of the micrometer pitch thread. Tommy bar screws r=0∙13733 P and that cos 62½ deg.= holes and spanner flats are provided to enable 0∙4617 and sin 62½ deg. =0∙8870, then with N adjustments to be made under circumstances threads in engagement where it is impossible to reach with the fingers. Rs2=πNK(Dm+2L)f When two or more milling cutters are mounted Where Rs2=Resistance of micrometer threads on an arbour, as in straddle-milling operations, to shear, N=10 and L and K are constants etc., most of the setter's time is absorbed in (FIG. 2), spacing the various cutters accurately with the aid ...Rs2=π10(l∙28+0∙0037)(0∙025-(0∙00686× of shims, or even pieces of paper! The time taken 0∙8870))f to arrive at the right combination of "bits and t =3∙142×10×1∙2837×0∙019×291,200 pieces" will vary according to the number of ... Resistance of 10 micrometer threads to shear= shims and pieces of paper of different thick­ 223,250 lb. nesses the setter can lay his hands on. threads. In the case of Whitworth standard The figure thus obtained represents the pressure Setting usually involves the dismantling of the threads the screw would tend to break at the it would be necessary to apply in order to shear arbour support and arbour steadies, the removal section X— X or at the undercut as shown in the threads on the Type S.IA when only two- of the cutters and collars and the insertion, or FIG. 3.. Disregarding the undercut, the resistance thirds of the maximum thread engagement is in removal, of a shim. When all mating surfaces to tension at the section X— X is given by: use. have been carefully cleaned and the cutter spacing Although a perfect mating fit is afforded by the is more accurate, the support and steadies may be internal and external thread-grinding processes reassembled and production recommenced. employed in producing the threads, it is advisable Where Rsl=Resistance of core to shear, Dm= With a Euco micrometer spacer between each to reduce the figure obtained by 20 per cent in Minor Diameter, f =Maximum tensile strength. pair of cutters, however, it is only necessary to order to compensate for any minute and permis­ loosen the arbour locknut, and the adjustments sible play that may be present, the final value of may be made in a few seconds, without dis­ An Example Rs2 would therefore be 178,600 lb. Then as turbing the arbour support, or steadies. Rs2/Rsl=3∙2, this demonstrates that the resist­ Assuming a milling machine arbour to be made ance to shear of the micrometer spacer is con­ of 55-ton (123,200 lb.) per sq. in. nickel chrome Strength Formula siderably greater than the resistance to shear of steel, then in the case of a 1 in. dia. standard the milling machine arbour screw. In considering the strength of a screw thread milling machine arbour with the end screwed under tension in a static condition, it is necessary 7/8 in. B.S.F. (minor dia.=0∙7586 in.), the appli­ This machine is manufactured by Euco Tools to establish the following: (1) the resistance of cation of the above formulae would result in the Ltd., 30 Budge Row, London, E.C.2. the threads to shear at their base, or (2) the resist­ following: ance of the core to break at the bottom of the Dimensional Data BRITISH SCALE ...Resistance of core to shear=55,686 lb. Machin e divide d t o 0∙000 5 in. Very few setters willingly dismantle a set-up Widt h Bor e O/ D to make an adjustment when the work being pro­ No . (in. ) (in. ) (in. ) duced is "borderline" with regard to dimensional .. . . S. 1 1 1¾ limits. The inclination is to "let it go"; such .. . . S.1 a 1 13/16 1¾ circumstances often resulting in the work failing .. . . S. 2 ¼ 2 1/8 1¼ .. . . S.2 a 2 1/8 13/16 1¼ . . . . 2 3/8 S. 3 ¼ 1½ . . . . S.3 a 13/16 2 3/8 1½ .. .. S. 4 2 3 13/16 METRIC SCALE Machine divided to 0∙0 1 mm. Width Bore O/D No. (mm.) (mm.) (mm.) .. M. 5 .. 6∙ 3 1 6 33∙ 5 .. M.5 a 20∙ 5 1 6 33∙ 5 .. M. 7 .. 6∙ 3 2 2 41∙2 5 .. M.7 a 20∙ 5 2 2 41∙2 5 .. M. 1 .. 6∙ 3 2 7 50∙ 0 .. M.1 a 20∙ 5 2 7 50∙ 0 November 1946

Journal

Aircraft Engineering and Aerospace TechnologyEmerald Publishing

Published: Nov 1, 1946

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