[ARC5] Part 6743 - tuning cranks and flexible drive cables for the ARC-5 sets.

[Mike Hanz]

On 4/8/2011 2:24 AM, Leslie Smith wrote:
> Tuning cranks and drive cables.
>
> http://aafradio.org/garajmahal/flex_drive_cables.html
>
> Mike Hanz has published a page giving details for the Bendix flexible
> drive used in various sets, including what we call "command"/ARC-5 sets.
>
> I am trying to understand these drawings, and the fabrication of these
> cables.  As I understand the drawing, the thread used to fix the
> flexible cable on the ARA, SCR-274 and ARC-5 receiver families is 1/2
> inch x28 threads per inch.  I would like to recieve confirmation that I
> understand this correctly.  (The other alternative is 27 tpi, and Mike
> seems to discount this for the "command" sets.)

I should probably have clarified the narrative a bit more, Leslie.  The 
Bendix style section that comprises the first part of the web page is 
the larger flex cable that one finds in WWII and earlier aircraft, not 
that used on the command sets.  Note that the threads are all 5/8" in 
diameter.  You can ignore the first section if you are only interested 
in the command sets, which starts at "Later ARC Flex Drive Tuning Cables 
(the "miniatures")".  In the drawing below that, the knurled nut shows a 
thread of 1/2-27.

> I would like to find a detail drawing for the tuning crank, ARC-part
> number 6743.  Can any-one on this list tell me if a drawing of this part
> exists, and if "yes", where I can find this drawing?

Since I just finished restoring a 6743 that had serious corrosion 
issues, I'll add a quick sketch to the bottom of the web page when I get 
back this afternoon.  I have never seen an official version.

> In the absence of a drawing, I turn to the drawings and photos from the
> web-page above, "flex_drive_cables".  A drawing is given of the cable
> assembly, "ARC-5 drive cable details" and that drawing (3rd from bottom
> of page) shows a female splined end, part 6788.  I believe this slips
> over the male spline on the receiver.  This is the end of the 3 gang
> tuning capacitor.
>
> Is my understanding correct?

That is correct.

> Assuming the detail in the paragraph above is correct, can anyone give a
> source for a female splined end?

I have never found a source for these, at least not from current 
manufacturers.  There are some moldable materials that might be used to 
make some, using  the end from the receiver variable capacitor as the 
internal pattern, but I have never pursued it very far because I have 
found what I needed at various times.  There was a member of this list 
who was looking into using a wiggle broach at his work to make some, but 
I don't know the status of that at the moment.

> Turning to other items in the Mike Hanz' drawing ("ARC-5 drive cable
> ..." consider part number 1167.  This looks suspiciously like a locking
> device that might hold a tuning crank in place.  I believe the internal
> thread in this locking device is 1/2 inch 28 threads per inch.
>
> Is my understanding correct?

No - it is 1/2-27.  All of those threads mentioned on that page are what 
is referred to as "special" threads and taps are available if at 
somewhat higher prices.  Here are two I bought just recently on sale 
from MSC Industrial Supply (mscdirect.com), but I think their normal 
prices are almost double that:

04844221     1/2-27 HSS HAND PLUG TAP      $17.28
66556168     1/2-27 HSS HAND BOTTOM TAP     $15.28

Most of these special taps are Chinese imports and need some dressing of 
the tip to make them work well, but using them in aluminum isn't too 
bad.  Note that a tapped sleeve is needed to approach a knurled nut if 
you want a clean start and well finished thread.  Otherwise you tend to 
get a tapered thread.

73,
Mike  KC4TOS