Len Anderson wrote :
Not being a machinist I was hoping someone would come up
with the correct words to describe the splined whatsis so that
I could haunt a few mechanical parts suppliers for something
that would mate smoothly with it. I tried some copper tubing
(pressed over splines) with marginal success (several
diameters, easy to get at do-it-yourself stores for various
small water piping uses) but those bend rather easily. [it's not
too swift when one's home machine shop consists of a Big Lots
table top drill press...:-) ]
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Not knowing the equipment being discussed ,I could suggest the following
general method of extending a 6 -6.35 mm shaft . (6.35mm equals 1/4 inch)

Take 2 (if possible identical) non metallic knobs for the shaft diameter
concerned ,for example the push-on potmeter knobs found on almost any
VDU or older TV and BC radio sets , used to push onto splined potmeters .
Equipment from Asian origin is always metric ,hence shafts dias are 6 mm .
Fit the 2 knobs concentrically piggy back ,with either epoxy or 2 or 3
bolts /nuts either metallic or non-metallic (say nylon).

Push/fix 1 knob onto the splined shaft to be extended.
Fit tubular/solid extension shaft into other side.

If alignment with front panel opening is a problem , glue flexible
material (for example rubber ,dense foam or expanded polystyrene )in
between the 2 knobs . Of course all this provided there is enough
space in the enclosure.

This method is also very suitable for applications involving components
with a rotary shaft which should be kept free from ground ,like in
antenna matching units (tuners) .

If the to be driven shaft has a 1/4 inch diameter the mating knob must
suit this ,but the other knob can be for a 6mm dia shaft and final front
panel knob There is a lot of flexibility.

Frank GM0CSZ / KN6WH