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a good method of making primaries
Hello All
I made the primary last night and the results were good so I though that
I would describe the method.
I first printed a spiral on paper as required, in this case 10 turns
with 10mm inter tube spacing, made of 8mm hollow copper tube. I got a
spiral spring used for bending this copper tube at the hardware store
here in Germany for a couple of $, plus 10m of tube for around $20.
I pre-bent the spiral, starting in the centre, and working around - the
start of the spiral was marked on the paper and I took care to keep the
start of the tube in this position. The tube work hardened after a
couple of errors early on, so I put it under a blow torch for a couple
of minutes. After forming the spiral I then made four supporting plates
out of perspex 5mm thick with holes spaced along the top edge (20 deg
sloping) at the spacing of the spiral.
I then "wound" the spacers onto the spiral, feeding one more turn of the
spiral into the next hole on the spacer at each turn - this was a bit
fiddly, but I now have a spiral which, much to my surprise considering
the mistakes early on, looks as if the spacing is absolutely equal all
round. The advantage of using holes in the spacers rather than grooves
open at the top is that the whole structure is pretty rigid when
finished. The holes are 8,5mm dia for 8mm tube.
For anyone trying this for the first time, I would recommend more
spacers, perhaps 12, placed 90 degrees to each other in a row of 3 and
each with perhaps four holes (for four turns of the spiral). Viewed from
above each quadrant would then look as below :
spacer 1 spacer 2 spacer 3
-----------------------------------O----------------------
spiral turn 1 O
-----------------------------------O----------------------
spiral turn 2 O
-----------------------------------O-----O-----------
sprial turn 3 O O
-----------------------------------O-----O------------
spiral turn 4 O
------------------------------------------O----O----------
sprial turn 5 etc O O
------------------------------------------O----O-----------
O
etc. O
The reason for this is that with a row of ten holes in the support, it
becones increasingly difficult (unless the holes are quite large) to
thread the spiral into the supports. If the holes are large, the whole
structure is very flexible, so using three supports with four holes at
each quadrant gives a rigid structure and easy assembly. A bit of
throught would be required when threading to get the order right. A
disadvantage would be that the system has another degree of freedom -
but if the primary is sitting on a flat surface this should be no
problem.
I hope this is of help to someone out there
Happy (and safe) Coiling
Guy