17.6.
Engineering microscopes are optically assisted instruments for measuring the geometric dimensions and forms of small and medium sized technical parts. These present the enlarged view of the observed object area, either in its contours, or as a surface image. These are also used for referencing which can be accomplished by providing index lines on a transparent graticule inside the microscope tube, observed concurrently with the magnified image. The index lines guide in determining reference positions for specific surface elements. Index lines are also used for alignment and comparison with the observed part.
17.6.1.
Tool Maker’s Microscope.
The tool maker’s microscope is based on the principle of optics. The microscope consists of a heavy, hollow base ; accommodates the illuminating unit underneath ; and above this, on the top surface of the base, the work-table carriage is supported on balls and controlled by micrometer screws. Projecting up from the rear of the base is a column which carries the microscope unit and various interchangeable eyepieces. Light from the lamp at extreme right is collimated in the tube connecting the lamp to the centre of this instrument and is reflected as parallel beam by the prism at the end of the tube. On its way up this beam collects the image of the object to be inspected and this enters
the microscope being turned by another prism into the inclined eyepiece. Here the inspection may be made by eye in the normal way or the projection attachment may be fitted which turns the beam by mirrors to bring up the image on the back of the transluscent screen. The magnification obtained depends upon the objective of the microscope but instruments are commonly supplied with facilities to give magnification from 10 X to 100 X on the projection screen. In order to adopt the apparatus to deal with a wide variety of work, various attachments may be fitted to the worktable. Microscope may be revolved on its mounting and normally has a recessed hole for accommodating a disc of glass upon which flat work may be rested and a clamp attachment is available for clamping. Linear movements of the table are controlled by micrometer screws having a movement upto accuracy of 0.0025 mm. The table slide is held against the ends of the micrometer spindle by light spring pressure so that movements greater than the micrometer travel may be controlled by interposing a slip gauge of the required dimension between the spindle and the point where it bears against this slide. This method is preferable even for movements within the range of this micrometer, as it is positive and less liable to error.
For the most effective manipulation, magnified image of the work is compared through the eyepiece (or projected) superimposed on a prepared background engraved on a glass disc in the eyepiece. There are several inter-changeable eyepiece units. The protractor unit is prepared with radial and cross-setting lines. This may be rotated by a knurled screw for setting any of the line with the work or image, and readings of the protractor may be made to 1 minute by means of a scale which divides each degree into 60 parts. The protractor which is illuminated is read by the eye through small hole in projecting eyepiece. Means are provided for setting the work table axis correct relative to protactor for zero reading. The thread template unit has selected thread forms arranged round the glass disc in the eyepiece and this may be rotated to bring any desired thread form into position for comparison with its magnified image of the work.
In order to view screws along the helix angle of the thread, the whole of the column unit with the underside illuminating arrangement attached may be swung at the helix angle of the thread. The pivot upon which this column swings is near the base. Most of the uses of this instrument will be on work where the shape of profile is projected from below, but surface shapes occur such as the edge of a recess, where surrounding metal prevents light from passing across the profile. These may be illuminated from above by a special arrangement which then allows the profile to be received in the normal manner, except that the intensity of light will be reduced from that when received direct. The chief applications of the tool room microscope are as follows :
1. The determination of relative position of various points on work.
2. Measurement of angle by using a protactor eyepiece.
3. Comparison of thread forms with master profiles engraved in the eyepiece, measurement of pitch and effective diameter.
4. Comparison of an enlarged, projected image with a tracing fixed to the projecting screen.
