Theatre Design & Technology - Winter 1980 - 12

cially turned steel plugs having 'kIf and 'Xs" diameter shoulders. The strut thus served as an accurate jig for drilling the
remaining floor holes.
4) The precise alignment of the overhead grid with the floor
holes was again achieved by aligning the end holes of each of
the struts with the corresponding floor holes. This was facilitated by using specially turned plugs in the end strut holes.
These had a centered hole large enough for the cords of
plumb bobs that were then carefully centered over special 'kIf
plugs that were pressed into the corresponding floor holes.
The plugs had conical points on top for greater accuracy and
a shoulder to facilitate driving the pin into the tightly fitting
floor holes.

finish bored, then inside chucked, parted, and the individual
parts faced to length. Finally, the groove was cut using a
hand-shaped form-tool having approximately 'Xs" radius. By
using the cross slide stop and again by inside chucking, acceptable dimensional accuracy and concentricity were
achieved.
The outer sleeve was cut to length from 2.000" 0.0. X 'fa"
wall mechanical tubing. The inner sleeve was turned down
from 1 'kIf N.P.S. schedule 80 pipe. This was less expensive
and more readily obtainable than the nearest available standard size of mechanical tubing. The rings were braze welded
to the sleeves (Figs. 8, 9).

Rail Cores and Rail Assembly
Lathe Work
Most theatre shops will not be equipped to produce identical precision parts in such relatively large numbers. Thus, the
following notes may suggest ways in which aspects of this
project may be facilitated. For instance, it is likely that some
special tooling will be required to reduce manufacturing time
per piece to tolerable limits. The lathe accessories we found
necessary beyond the basic bits, tool holders, chucks, etc.
were: a bell head live center to cover the range from approximately 1 '/.," 1.0. to 2'fa" 1.0. tubing, a carriage stop, a cross
slide stop and a parting tool and holder. The cost of this
equipment was approximately $130.00 (1978). The general
procedure that we found necessary was to perform all of a
specific operation on all parts of a given type before changing
the set-up. Although this obviously resulted in some loss of
both accuracy and time compared with what would have been
possible with a turret head equipped lathe, the alternate procedure of entirely completing each piece before removing it
from the chuck would have required far too long for tool
changes, etc. However, by using the procedure described together with carriage and cross slide tops, satisfactory tolerances were achieved.

Stanchion Plugs
The shoulders of these pieces were turned in face-to-face
pairs from 2" long rough-cut blanks of 2%" diameter cold
rolled bar stock then drilled and parted.

Sleeve Ring Assemblies
The sleeve rings were also turned in pairs but in this case
from rough-cut blanks of 2%" 0.0. x 1%" 1.0. mechanical
tubing which had been turned down to 2.325" 0.0. By supporting the tubing from the inside on both the chuck and tailstock ends, the slight eccentricity present in the tubing could
be eliminated. The pair blanks were then outside chucked and

:1:
8'12

FIG llA
STRUT AYOUT
(N'S)

jIWlIH.. EY 8-12 STRUl

f---+--161- ----f-+_~

Obviously, close dimensional control of the overall length
from one rail sleeve end to the other was critical. The possibility of cumulative error meant that overall sleeve center distances had to be held to approximately ± '!a/' or better. Satisfactory precision was obtained together with sufficiently rapid
assembly by a number of techniques.
1) Since the holes in the wood rail frame set the basic distance between the metal rail end cores, the former were jig
drilled.
2) Likewise, all the holes in the rail cores themselves were
drilled with a specially built "box jig" (Figs. 11, 12). The rail
cores were drilled in end to end pairs. This was done to allow
the most efficient use of a 2%" hole saw to create an accurate
seat for the pipe sleeve (Fig. 11 A). Thus, not only did the jig
provide accurate location for the eight pairs of 7/'6" diameter
bolt holes in each core piece, but also for the '/.," diameter
hole saw pilot drill. A drill press was temporarily modified with
an auxiliary countershaft to provide a sufficiently low speed
(140 RPM) for the hole saw. After the hole saw cut through
the strut side walls, the two sections were then sawed apart to
give two complete rail cores ready to be welded to the 2" pipe
sleeve.
3) The process of welding the pipe sleeve to the strut end
was simplified by drilling a %" hole through the center of each
of the pipe sleeves. One could then pass a length of %"
threaded rod through the sleeve and the length of the rail core
to hold these parts together during welding. The rail assembly
bolt holes were located close to the inside edges of the B-12
strut. By using square nuts the %" flat head socket head cap
screws could be securely tightened without requiring a
wrench to hold the nuts thereby simplifying final length adjustment.
4) The mounting holes in the strut were made slightly oversize to allow for slight adjustment in overall length. Final adjustment of the overall length could then be made in a simple
gauging fixture consisting of a length of strut and two carefully
set angle brackets.
Figure 12 Rail drilling
rig, open, ready to receive a length of B-12
strut. The jig is long
enough to drill two rail
core pieces at once.
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