Use any available glass tube of between 1.5 and 3 mm
diameter. (This can be drawn down from larger diameters by hand, but
requires a bunsen burner or gas torch.)
German "green band" capilliary tube is considered the best for this purpose and is
available in 100 mm lengths, it is 2 mm outside diameter, wall
thickness 0.436 mm, inside diameter 1.128 mm and thus the area of the
bore is 1 mm2 giving a total volume of 100 µl,
ie 1 µl per mm.
This tubing is available from laboratory supply houses
(visit them and explain your requirements as they normally will not
deal with individuals).
A laboratory clamp stand can be utilised with a fixed clamp
and a sliding one to grip the two ends of the capilliary. The lower
sliding clamp can have various weights hung on it and the drop height
adjusted using wooden blocks.
A description of the version I am making is on the
drop machine page along with links to other examples.
The melting of the glass is provided by a small coil of
NiCrome wire heated from a 12volt battery or a battery charger.
(A variable voltage supply is useful here).
A tungsten carbide scraper or planer blade can be used to
nick the surface of glass to provide a "notch" that facilitates
snapping. The point at which the nick is made can be located using a
"gauge wire" O.15 mm diameter, with end face ground at right angles
to the axis. This gauge is introduced from the open end. The gauge
wire will only enter until it meets it's own diameter inside the
taper, thus the nick can be made on the outside of the glass at that
point.
Initial polishing, and some shaping, can be acheived using
many items from the Dental Laboratory...Pumice powder...diamond
impregnated rubber discs (used in a Dremel Tool or other high speed
grinder).
Final polishing should be achieved using a flame.
This process may sound technical, but anyone with a modest
amount of skill and dedication can make top quality tips at a
fraction of the cost of buying "ready made" ones... The process is
also enjoyable and when completed you have the satisfaction of
knowing that it was "all your own work".
Revised... 23 October 2001
