> 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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
