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Subject: Drums and travelling Drums
From: Danie Overbeek danieo@............
Date: Mon, 19 Jun 2000 11:50:15 +0000

You could call Cap a drum enthusiast after reading his most
interesting letter on making a drum.  By the same token you
could call me a "travelling drum enthusiast":

One can obtain the necessary displacement for successive lines
either by causing the penmotor to move in a path parallel to the
drum axis or by making the drum travel axially in front of a
stationary penmotor.  (See Martin G Murray's 1938 design on PSN
recently).  The second version is far simpler to construct and
gives one much freedom to experiment with different penmotors.
I still have the little drum assembly which I made for a drum
chronograph in 1957 during those heady Sputnik/Moonwatch days.

As for constructing a drum from scratch, there is no law which
states that one's chart speed must be 30 or 60 or whatever
mm/minute.  It is far simpler to find a thin walled round tube
tube of metal, plastic or even cardboard and fit two end
pieces.  Then one simply makes a scale for measuring the time
interval from the nearest time marker so as to fit in with
the chart speed, which is fixed by whatever drum diameter you
have and the motor speed.

Amateur constructors need not be daunted at the prospect of
having to buy expensive penmotors.  An adequate substitute
can be made, based on a loud speaker moving coil assembly.
The cone can provide the necessary restoring force or if one
is adventurous one can dispense with the cone and use an
external spring to provide the restoring force.   The somewhat
limited movement of the coil needs to be amplified and of
course transmitted to the writing stylus.  Model airplane
balsa is good for the linkage especially if one wants a penmotor
with a fast response.  Blobs of silicone rubber judicially
applied can serve as friction-and shake free bearings.
The experimenter only needs these simple freely available
materials and a little bit of imagination.  One also needs
to protect these home made devices from domestic accidents
such as the family cat "calibrating" the equipment in hot
pursuit of a mouse.

Cheers all

Danie Overbeek.


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Larry Cochrane <cochrane@..............>