[Elecraft] [K3} Help, hex key for split knob

[Jack Brabham]

Don,

Wrong answer, guess again.

The kit supplied allen wrench strips because it is a cheap, low quality tool
that can not support the required torque.  The required torque is higher
than necessary because of the overly tight, tapered, and/or incomplete
threading of the set screw hole in the knobs.

The concentric knobs are undoubtedly splitting because they are plastic
knobs that have the set screw threaded directly into the plastic.  That
design is a non-starter from a quality perspective and is bound to fail in
one way or another, sooner or later. 
  
The engineering dilemma with an all plastic knob is that if the set screw
threading isn't tight the set screw will strip out the plastic threads
before achieving enough force to secure the knob to the shaft.  On the flip
side if the threading is tight enough to allow securing the knob, it is also
tight enough to tend to split the plastic around the set screw hole via
radial pressure when torqued.  

This is why quality knobs are either all metal or have metal inserts for the
set screws.

Being an old gunsmith, I'm not intimidated by slotted screws and wouldn't
hesitate to swap out the allen drive set screws with slotted ones, and
probably will if these knobs ever need to be removed.  I realize that this
is a good solution only if one actually knows how to properly fit and
operate a slotted screw driver.

The "pinky" tight approach is OK if you are working with a "D" shaft that
provides positive indexing.  Otherwise, as in the round shaft K3 concentric
pots, the set screw has to be very tight to maintain the knob indexing.  


73 Jack KZ5A


-----Original Message-----
From: Don Wilhelm [mailto:w3fpr at embarqmail.com] 
Sent: Tuesday, May 11, 2010 5:39 PM
To: Jack Brabham
Cc: 'elecraft at mailman.qth.net'
Subject: Re: [Elecraft] [K3} Help, hex key for split knob

Jack,

If you stripped out the allen wrench, you were applying more force than 
is necessary, and excessive force may be the reason some of the newer 
knobs are cracking (that is just a guess).

73,
Don W3FPR

Jack Brabham wrote:
> Tim,
>
> I had a similar experience assembling #4169 a few weeks ago.
Unfortunately
> my stack of assorted allen wrenches didn't include any that small.
>
> My work around was to grind off the "stripped" portion of the cheesy allen
> wrench each time it stripped.  
>   
>