INSTRUMENTS, MECHANICAL. The Bel7mrs Beam 111icrometer (Fig. 1) is a convenient instrument for mechanics desiring close measurements. The beam is provided with two heads—one fast, the other loose. The loose head is dovetailed to the beam, open on one side aml flush with the face of the beam, and is provided with a mierometer screw, having adjustment Set in the face of the loose head on an angle of 10°, and held in place by a thumb-nut on the reverse side, is a hardened stop, which, being angular on its sides and having no bearing on its bottom, will adjust itself in position. The beam is divided in half-inches by the insertion of steel pins, and the loose head is quiekly and ac curately set by bringing the stop in its face to bear against them, and when set is held in position by a and nut, which acts like a gib. Fig. 2 is a section of the micro meter screw, nut, and fastening device.
Limit Gauges for Round Iron.—These gauges (Figs. 3 and 4) are the outgrowth of the efforts of the Master Association to insure uniformity in the sizes of round bar-iron for United States standard bolts. The following table of dimensions for limit gauges is recommended : The, caliper gauges are drop-forged from tool-steel, and are hardened and ground exact to size. Accompanying each set is a standard cylindrical reference gauge, hardened and ground. for each separate end Measuring-Mach ines.—The Pratt & Whitney 12-in. standard is shown in Fig. 5. The screw is 50 threads per in., and has adjustments for compensation for wear in nut and shoulders. The index-circle is graduated to 400 divisions, giving subdivisions of of an in.; while, by estimation, this may be further subdivided to indicate one half or even one fourth this amount. Delicacy of contact bet ween the is obtained by the use of auxiliary jaws holding a small cylindrical gauge by the pressure of a light helical spring, which operates the sliding spindle. to which one of these auxiliary jaws are attached. The behavior of this sitive readily determines the uniformity of contact of the ing-faces at zero, and upon the gauge which is measured between them. An adjusting device fur the index-line is provided. to allow for slight variations of position of the at zero, or for any convenient reading on the Fig. fi shows a (mule by the ailkerson Machine Works, of Homer, N. V.
The screw has 16 threads to the in., and the wheel is graduated to read to in. by deci mals, 1111(1 also etc. The error of the screw is corrected by means of an adjustable piece fastened to the bed of the machine. The arm shown travels with the wheel, the lower end bearing against the eorreeting piece being held in contact by gravity. The upper end. prttjeel big forward, has a face on which may be graduated a vernier.
he Rogers- Itond ( 'out pa rahn..—From a lecture delivered nt the Franklin Institute in 1S8-1 by 31r. George M. I he head of the gauge department of the Pratt & Whitney CO.. who
was IIS.,(1Chli ed with Prof. Rogers in the design and enlist niet ion of the comparator, we abstract the following description : special features of the universal eompmnitor are, as its name implies, the variety of the methods employed and the range of work that can be done in com paring standards; each independent method, when carefully carried out, producing similar results, which serve to cheek or prove the com parisons. It includes a method for investigating the subdivisions of the standard by comparing each part of the total length with a constant or invariable quantity or dis tance." Referring to the illus trations (Figs. 7, 8. 9), the main features of its con struction are the follow ing : " A heavy cast-iron base, A, is mounted upon stone - capped brick piers, giving a permanent foun dation to the apparatus. Upon this base, and reach ing from end to end, are two heavy steel tubes, B and C, 3 in. in diameter, ground perfectly straight, and being true' when placed in the cen ters of a lathe, the object being to get a straight-line motion of the microscope-plate D, which slides freely on these true cylinders. Flexure of these cylindrical guides is overcome by lever-supports at the neutral points n and Fitted closely to these guides and outside of the range of motion of the microscope-plate D are two stops, E and F. one at each end, as shown in the figure. These stops are arranged to be adjusted at any desired position along the guides, and are securely held by clamping on the under side by the handles 0 and H. These stops are each provided with a pair of electromagnets. I and J, the poles of which do not quite come in contact with the armature seen at either end of the microscope-plate. Contact is made at K and L, which are hardened steel surfaces, tempered and polished, and placed as nearly as possible in the center of mass of the plate and of the stops. The magnets are intended to overcome the unequal pressure (Inc to ordinary contact, a rack and pinion be ing used to move the plate. The magnets are used to lock the microscope-plate at each end of the traverse between the stops. The use made of this sliding microscope-plate and the stops we shall see presently. Beyond the main base just described, and supported also on brick piers, is an auxiliary heavy cast-iron frame iv, which is provided with lateral and ver tical motion within the limits of zero, and of 8 and 10 in, respectively, for rough or ap proximate adjustment, and upon the top of this frame are two carriages, 0 and which slide from end to end, a distance of about 40 in, rpm] these sliding carriages are 'dared ta bles rand 7q, provided with means for minute adjustment, for !Didion lengthwise, sidewise, and for leveling, thus permitting the adjustment of a standard yard-bar quickly, and with out the necessity of its being touched with the hands after being placed upon the table until the work of comparison is completed.