How to Use an Optical Square


Everything that can be accomplished with a jig transit can be done with an optical square attached to the front of an alignment telescope. (See Fig. 13-28.) In the instrument illustrated, the pentaprism is inside the sphere. The use of the optical square will be described for such an instrument. The operation of an optical square in which the pentaprism is placed at a certain distance from the sphere is the same except that corrections in measurement must be made for this distance.


Example. Figure 14-5 shows two surfaces A and B which, on the plan, are shown as 7.000 in. and 4.000 in. respectively from the centerline. It is decided to establish a width plane 10.000 in. from the centerline. It is assumed that the work has been leveled.


To buck in with an optical square.

  1. With an alignment bracket, mount the optical square in one of the cup mounts to be used for the master line. Place it so that it is approximately level and the 90° line is aimed approximately toward the future position of the other target. (See Fig. 14-9.)

  2. Rotate the optical square in the bracket until the 90° line is aimed on the scale held at B. Level the tube with a striding level and read the scale at B.

  3. Aim at the scale at A and check the level. From the scale readings determine how far to move the cup mount.

  4. Continue to buck in on the readings at A and B in this manner until the correct position is attained. The optical micrometer is used to read the scales, but the final movements of the line of sight must be made with the cup mount because, when the mount is in its final position, the micrometer must read the number of thousandths of an inch desired in the offset. In this case, since the offset is 10.000, the micrometer must read zero.

  5. Finally aim in the direction of the other cup mount, and adjust the cup mount for line and height.

  6. Adjust the cup mount in which the instrument is mounted for height.

  7. Check the final position after the cup mounts are firmly bolted in place.


For very accurate work, the optical square should be reversed in the cup mount as shown by the dotted lines in Fig. 14-9. The readings should check. If they do not, a procedure for averaging out the error similar to that for the jig transit can be used, although with an optical square this is laborious. Most engineers believe that the optical square holds its adjustment so permanently that reversal is unnecessary.

When the final position is attained, a sub-master line can be set for a station plane at right angles to the master line by a direct sight through the square.

The Master Line. The cup mounts now permanently hold the master line. The cup mounts are supported where convenient, on the work, on a tooling bar permanently leveled and set in position, or on an instrument stand held firmly in position on the floor. Once the master line is established, all horizontal measurements should be made from it.


Station Planes. A station plane can be erected from the master line whenever desired either with a jig transit equipped with a telescope axle mirror or with an optical square. Both are supported by specially prepared positions along the master line. These may be brackets on the work, short tooling bars on the work, instrument stands, or long tooling bars on the floor. The station of a station plane is usually determined by measurements from a reference button or other device to reference buttons on the jig transit or optical square. Measurements are made with micrometer bars, tapes, or jig-bored holes in metal strips. An example of setting a sub-master line over a tooling bar is shown in Fig. 14-10. This illustrates the use of leveling, the use of the width plane, and the establishment of cup mounts. The sub-master line happens to slope in the figure.

Optical Squares for Station Planes. A station plane can be established with an optical square instead of a jig transit. The method is considered more accurate but more difficult. The procedure is as follows:

  1. Aim the master-line alignment telescope at its master target.

  2. Place a target in a cup mount, and position the assembly on the master line and at the correct station position.

  3. Mount a bracket on the cup-mount base, and install an optical square mounted on an alignment telescope.

  4. Focus both telescopes at infinity, and aim the optical-square telescope at the alignment telescope.

Excerpt From:

Kissam, Philip, Optical Tooling for Precise Manufacture and Alignment, New York, NY: McGRAW-HILL BOOK COMPANY, INC., 1962