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

[Tom W8JI]

> 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.

Jack,

We used tens of thousands of knobs at Prime, Heathkit, and 
other places. Plastic knobs without inserts were generally 
OK except for rare cases. All-plastic knobs without inserts 
historically have been used in everything from TV's and 
radios to cars and everything else around us.

The set screws in my K3's knobs are a cup point style. That 
is not a good holding screw, but it should be OK for a small 
knob if the shaft isn't too hard. It would be great on 
plastic shafts, or even a very soft metal. It is not a screw 
of preference for a plastic knob, especially on a hard 
shaft. Since the knob is pretty well set in one place and 
not frequently removed, and since it is a plastic knob 
without a metal insert and tight clearances, it should 
probably be a sharp pointed setscrew like a cone point.

It would take very minimal torque to securely lock a sharp 
cone point.

Cupped screws mainly lock on the raised edges that run 
parallel with the circumference of the shaft. This means 
they "slip" with the sharp edge in line with the force, like 
a saw cutting across a round shaft. They also spread the 
pressure over a larger area. This all means a great deal 
less holding power, especially with  hard shaft.

A cone pointed screw would put all the pressure on a needle 
point, and even with a hard shaft dig in pretty deep with 
minimal pressure. They are far better on non-insert knobs. 
With metal insert knobs they could make it difficult to 
remove the knob because they would divot the metal and not 
allow easy removal, so a cone point or even flat point and 
higher torque is better with a insert knob.

The problem is the screw has to turn freely, has to have the 
right tip, and can't be torqued down on. It really should 
just take "pinky pressure" or less. This isn't the rear 
axle-shaft nut in a 1960's Chrysler or AMC, it is a tiny 
knob on an easy to turn shaft.

73 Tom