Abstract

We made two very steep parabolas, one in quartz and one in a water-soluble salt. We well know that our use of Grandfather's pantograph is as out of date in the Eighties as the dinosaur; but that is what we have and it is simple to set up. The results speak for themselves, as they are adequate.

© 1980 Optical Society of America

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Figures (12)

1
1

A cylindrical quartz blank firmly cemented to a steel mandrel is rotated by a precision bearing assembly. It can be seen in the center of the photograph. The vertical spindle with its diamond wheel is to the left.

2
2

A closeup of the now formed quartz blank and the rim of diamond that ground it.

3
3

The shape was controlled by using a 6:7 heavy accurate pantograph. The six-times-size cam is to the left with a white border to show its shape. The follower, which is resting on it and to the right of center, must be six times the diameter of the diamond wheel. If sufficient care is taken the aspheric quartz may be polished direct from the machine to an accuracy of 0.0001-in.

4
4

A handheld flexible metal strip faced with polishing material is shown in position. The steel mandrel has been removed from the pantograph's workhead and put into a lathe for this polishing operation.

5
5

The asphere is shown on a precise X-Y coordinate measuring engine. Note the steel mandrel in the precision V-block. Since quartz is not easily indented, a sharp point may be used on the measuring stylus (accuracies to 0.000010 in.). In this case it was easier, since only one was needed, to correct the curve slightly by lapping and not to attempt to make all machine constants perfect.

6
6

A closeup of quartz asphere and measuring stylus.

7
7

A different solution is required to make the same asphere in a very brittle water-soluble material. A portable fixture was made consisting of bronze bushings, small ball-bearing thrusts and V-blocks. This was mounted in an instrument maker's lathe, and calibrated dial indicators were used to control manually the position of the very sharp diamond with a definite radius. By knowing the X-Y coordinates the tool was fed, over and in, while the work rotated.

8
8

This top view shows tool and work. Note the ridges in the crystal resulting from the over-and-in movement of the tool. These are later smoothed out with a flexible band coated with thin abrasive paper.

9
9

The portable fixture is moved to a second measuring engine.

10
10

A closer view shows the ⅛-in. diam ball on the end of the measuring stylus. This prevents indentation of the soft crystalline material. The possible accuracy with this method is 0.00001 in.

11
11

To make both ends polished and flat, the asphere was embedded in plaster inside a quartz ring. The polishing was achieved directly with a 320-grit-grind from a Blanchard grinder.

12
12

The finished product.

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