Rotary gaps

I have built 5 different rotary gaps using the same technique for each. 
None have blown up and all have functioned perfectly. The largest one 
has 24 rotating pins on a 13" rotor. The next has 12 rotating pins on a 
11" rotor. the next is 4 rotating pins on a 8.5" rotor. and the last two 
have 4 pins on 7" rotors. 
Each break has 4 stationary pins constructed as follows: .75" aluminum 
rod is cut to lemgth. The rods are drilled and tapped in one end for 
1/4-20 screws and copper 1/4-20 machine screws are inserted for the 
attachment conductors. 1" from the top I drill a 9/64 hole to accept the 
1/4 copper rod which is used as the stationary pin. A 7/64" hole is 
drilled 90 degreees from this hole and is tapped for the 1/4-28 set 
screws which hold the 1/4" rod in place. The .75" aluminum rods are then 
mounted in 1" acrylic blocks which can then be correctly mounted and 
positioned on the base.
The rotors are made from 1/2" or 9/16" linen or canvas filled phenolic. 
This material is available from a local plastics distributor, Ain 
Plastics, in random pieces for about $2.00/lb. I only use a band saw 
and a drill press to fabricate the rotors. I first find the center of the 
piece and drill the hole for the shaft. Using this hole for the center I 
then cut out the rotor on the band saw. The holes for the 1/4" copper rod 
contacts are then drilled 1/2" in from the edge of the rotor and spaced 
as required. I use 1/4-28 set screws in holes perpendicular to the edge 
of the rotor to retain the copper rod contact pins. After all the holes 
are drilled I verify that the rotor is balanced. Only one rotor was not 
balanced and it was necessary to drill a few holes in the periphery of 
the disk to bring it into balance. I use arbors from ACO or Damman 
Hardware stores to mount the rotors directly to the motor shaft. The 
screw end of the arbors are usually about 3" long so I mount a ball 
bearing on a bracket to support the end of the arbor.
I generally run my breaks at 1800 rpm but I have used the 13" 24 pin 
rotor on a 1/2 hp universal motor and have run it close to 7000rpm. Since 
I only use neon transformers I haven't seen any benefit in running over 
synchronous speeds of either 1800 or 3600 rpm. I drive the small rotor 
with a 1/15 hp 1800 rpm synchronous motor. I have also driven all of the 
rotors with other motors of 1/4, 1/3 and 1/2 hp both synchronous and 
As I said I have never had an accident and these things are easy to make 
with only a drill press and a band saw.
Skip Greiner