Production of highly accurate decahedron prisms from hardened stainless steel is discussed. Prism is used to check angular alignment of mounting pads of strapdown inertial guidance system. Accuracies obtainable using recommended process and details of operation are described. Photographic illustration of production device is included

Abernathy, W. J.

Sealy, J. R.

English

NASA Technical Reports Server

September 1972	 B72-10597 
NASA TECH BRIEF 
Marshall Space Flight Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Precision Machining of Steel Decahedrons 
The problem: 
A decahedron prism used in checking angular align-
ment of the mounting pads of all the components of a 
strapdown guidance unit requires a high degree of 
accuracy. To provide this accuracy, the prism must be 
fabricated with great precision. However, the required 
prism tolerances of 0.5 second of arc could not be 
produced with existing equipment.
The solution: 
The required tolerance is obtained with positive-
reference precision machining. This method uses a 
precision flat surface plate for reference. 
How it's done: 
The equipment that provides this precision is shown 
in the figure. It consists of a surface plate, a base block. 
is
(continued overleaf( 
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	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
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an angle block, angle sides, angle stop, cube, and spacer. 
The decahedron prism is also shown in the figure. It is 
made from hardened stainless steel and has 10 facets 
polished to a high degree of accuracy and reflectivity. 
In positive-reference precision machining, each facet 
of the prism is lapped so that it is parallel to the surface 
plate to a tolerance of 50 nm (2x10 6 inch). The surface 
plate constructed from granite has an overall flatness of 
635 nm (25x10 6 inch). 
The prism is lapped with a diamond lap wheel which 
is adjusted to 508 nm (20x10 6 inch) accuracy with the 
three adjusting screws. At the beginning of the lapping 
procedure, the cube with the attached prism is removed 
from the angle block assembly and base block and placed 
directly onto the jig bore table. The first two opposite 
facets are lapped parallel with the cube which is 
resting directly on the jig bore table. The remaining 
facets are lapped with the prism and cube mounted 
again on the angle block. To prevent dust particles from 
adhering to the tooling and prism surfaces, the entire 
process is conducted in a dust-free atmosphere. Each 
facet angle is lapped at 310432.8"±0.5'.
The finished prism is measured for flatness with an air 
gauge which compares each facet with the reference 
surface plate. Finally, the prism angles are checked with 
an autocollimator and a mechanical polygon which have 
respective angle accuracies of 0.1' and 0.25", respect-
ively. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Code A&PS-TU 
Marshall Space Flight Center, Alabama 35812 
Reference: B72-10597 
Patent status: 
NASA has decided not to apply for a patent. 
Source: W. J. Abernathy and J. R. Sealy 
Marshall Space Flight Center 
(MFS-21361)
0 
B72-10597	 Category 07


Precision machining of steel decahedrons

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