Slip Tip Dart
--- Supergun
One of the most critical
aspects of spear fishing is the dart on the end of the spear shaft. This
essential component must anchor in the delicate flesh of the tuna. Most
people are aware that 'sushi' is very soft. It is a difficult task to design
an instrument that will stop a fish the size and power of a small automobile
when all that the line is holding in is 'sushi'. Many blue water hunters
have found that detachable toggle 'slip tips' are the most reliable points
for powerful soft bodied fish. This is a debatable design aspect of spear
fishing.
For my large, experimental
blue water Supergun, I chose to increase the size of the slip tip dart
to give it more holding power. As the spear shaft is 7/16" stainless, the
tip had to be proportionately large. The dimensions of this tip are: Length
= 8.5" (21.6 cm), diameter = 0.57" (1.45 cm).
This slip tip is the classic
WALLY POTTS design which fits onto a 7 degree tapered spear shaft. This
precision instrument was hand-made for me by the precision machinist and
tool maker, "Bud" Platten (of Carrillo Industries) who donated his spare
time to it's fabrication. It is the finest aspect of this system. (Thank
you "Bud"!) I chose to use titanium for the material the slip tip is made
from because of it's toughness, corrosion resistance and light weight.
This material was much more difficult to obtain and work with than stainless
steel. One of my design considerations was to reduce the mass of the tip.
As the forces of the shot are great I was concerned that the inertia of
a heavy massive tip, could cause it to deploy prematurely. There are still
many unknown aspects to what happens to a spear shaft and tip when the
spear gun is fired. It is believed that premature deployment has occurred
with some heavier tips. As Supergun has more power than other spear guns,
I was rightfully concerned.
Supergun uses two
slide rings behind the slip tip dart. One holds the dart onto the shaft
with a length of stainless cable and the other slide ring bridle holds
the spearshaft onto the track and attaches the spear shaft to the float
rig via the shooting line.
The stop for the slip
tip dart slide ring is two opposing stainless steel screws that are threaded
into the spear shaft behind the tip.
To make the tapered
interior conical fit (yet deployable) of the slip tip dart match the spear
shaft, the dart had to be machined on a lathe. A descending diameter and
depth drill holes were calculated which provided the necessary step tapered
hole in the titanium dart blank. A 7 degree taper angle was chosen, which
is the classic WALLY POTTS tried and true design. After the drilling, a
7 degree tapered reamer (mill bit) was inserted to 'clean-up' the inside
of the tapered hole. See the image of this reamer.
This tapered reamer (mill
bit) had to be ground down to the appropriate diameter of the spear shaft
prior to inserting into the step drilled hole. Thanks again "Bud" Platten,
master machinist and tool maker.
One of the machining
aspects we discovered was that the lathe turning of the adapter end of
the spear shaft had to be done carefully. If the cutting edge of the carbide
turning insert was not positioned exactly at the diameter of the stainless
steel shaft, the resultant conical adapter would not have a 7 degree angle.
In other words, the cutting tooth edge must start exactly at the widest
point of shaft diameter during cutting. If the cutting tooth starts below
or above this widest point, the resultant cone will cause the tip to wobble
on the spear shaft (adapter)!
A considerable amount
of care and effort had to be made to produce the machined components for
this experimental extreme spear gun.
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