A discussion of how closely  a control rod can be duplicated is approached using the practical method of turning flats of the control rod.  The simple tolerance build up would indicate a number of greater than ±.005 as being the tolerance that could be held.  The Control Rod Rigger design  and practical considerations when using the tool has shown in test that the Control Rod Rigger can duplicate a control rod to within ±.005.  Mounting the control rod without binding requires the tool to be close to the center position of the rod bores.  This practical fact explains the accuracy of the Control Rod Rigger. 

How this simple test is performed:

An  AN-3 LH and RH 3/8-24 threaded control rod was positioned on the tool.   The rod was marked.  The rod was turned until binding was felt.  The amount of turn was noted and the amount of movement calculated.    At 30 degrees (1/2 flat) rotation some binding can be felt.  At 45 degrees it is difficult to force the rod onto the tool.  At 60 degrees (1 flat) the rod will not rotate any more. This test includes the thread play in a very poorly made control rod from a home-built aircraft.  I hope to use a high quality helicopter pitch link to repeat this test in the future. 

The calculations:

(1 inch/24 threads) = .0416 inch per revolution for one rod end. 
2x.0416= .0832 inch per revolution of a control rod. (LH and RH threads)
(22.5 degrees/360 degrees per revolution) x .0832 inch per revolution =  .0052
(45 degrees/360 degrees per revolution) x .0832 inch per revolution = .0104


One flat of rotation is 60 degrees, 1/2 flat is 30 degrees, which is more than the 22.5 degrees at which binding is felt.

High Precision tool ±.001

Repeating the above test using a prototype  High Precision Rod Rigger: 

Loading the tool  I set the loose lock nut with a point up and marked the rod.       

At less than ± 1/8 flat (3/8-24 control rod) it is difficult to unload the part.  At ± 1/4 flat you can not rotate the control rod. This included the thread play which would be less for a precision helicopter control rod.


The calculations:

(1 inch/24 threads) = .0416 inch per revolution for one rod end. 
2x.0416= .0832 inch per revolution of a control rod. (LH and RH threads)
1/8 flat = 60/8=7.5 degrees.  (7.5/360)x.0832=.0017