Micrometer Gauge: Pratt, 1878

A micrometer clamps the work between a fixed abutment and the tip of a fine screw. Both faces must be smooth and very hard, but tempering a finished screw changes it: the heat expands the metal, part of the expansion stays, and the cooling is uneven, so a screw that was accurate before hardening often is not after. George M. Pratt of Middletown, Connecticut, patented the remedy in 1878, US 202,866: leave the screw untempered and give it a socket at its end for a separate tip of steel tempered as hard as possible. A worn tip is simply replaced. He also shaped the horseshoe frame widest at the middle and tapering towards the ends, so it is stiff where it most needs to resist springing but lighter overall.

متقدم

Workshop process

التعليمات

Frame

Make the horseshoe frame widest at the middle of the bow and tapering towards both ends, where the abutment and the screw bearings sit.

المواد لهذه الخطوة:

إطار الميكرومتر على شكل حدوة1 قطعة

Screw

Cut the micrometer screw in steel and leave it untempered. Form a socket in its measuring end.

المواد لهذه الخطوة:

برغي الميكرومتر1 قطعة

الأدوات المطلوبة:

مخرطة معادن

Tip

Make the tip separately, with a shank that fits the socket closely, and temper it as hard as possible.

المواد لهذه الخطوة:

رأس ميكرومتر مقسّى1 قطعة

الأدوات المطلوبة:

فرن المعالجة الحرارية

Assemble

Insert the tip's shank into the screw's socket. Fit the adjusting abutment screw in the other end of the frame; the work is measured between its flat face and the tip.

المواد لهذه الخطوة: