I will not describe you here the principle of cutting polystyrene wings cores using a hot wire... All that is already on the Web and in particular on the excellent site devoted to hand launched gliders (formula F3K) that I advise you to visit.
I will just explain you how, starting from an existing arc (you will find description and some photographs here) and from beautiful gauges, to manage to make a perfect, automatic cutting, without need of assistance! The whole at a ridiculous price...
For the feeding of the wire I use a 12 V 4 A transformer directly connected to a triac variator (standard halogenous lamp variator ). The variator is inserted between the 220 V and the primary of the transformer. I know, it is not very orthodoxe to put an inductive load on a variator, but it works fine... Other option: have a look on S. Jaquemard site; one finds wonders there...
By the way, small easy way for pretty good gauges : use Formica (plating material for cheap furniture...). It cuts out very well and one obtains a perfect completion after sandpapering. The wire slips idealy on it and you'll get dreaming cores!
Let us return to the machine:
Let us take the case of a rectangular wing, the machine shall :
- start cutting at the same time on the two sides,
- advance at the same speed and without jolts on the two sides at the same time,
- leave well parallel at the trailing edge...
Nothing simpler, it is enough to:
- three strings ( roast string which has the merit not to melt with heat...),
- three pulleys (hurdy-gurdies wheels of toys or sections of wood logs in which you will have engraved a throat without much criterion of precision),
- some screws being used as axes for the pulleys,
- aluminium angle to fix the pulleys
- a weight of one kilo approximately (for example, a can, it is ideal...)
On your board, fix the pulleys as indicated on the diagram. I advise you to fix the first pulley on a small aluminum square allowing to regulate its position according to the length of the wing to be cut out. Not to load the diagram, I did not make appear the board, nor the square...
The first string is used to hang the arc, by its tensioner, at the ceiling. The arc should not be vertical (contrary to the diagram...) but it must rest on the gauges, with, approximately, a 20° angle to the vertical. The arc should not be too light; it is the weight which makes him follow the gauges. A wooden arc is fine.
Fix, by a simple node, a good length of string directly on the wire, on both sides of the gauges; then make circulate the string of left around the first pulley (vertical axis), around the second (vertical axis) and finally around the third (horizontal axis), downwards.
The right-hand side string, passes only on the third pulley. To be precise, it is tangenting the throat of the second pulley.
Without connecting the feeding, put the cutting wire in contact with the polystyrene bread on leading edge, tighten wire, tie two wire together and fix the weight.
Switch on the current, the wire (now hot) will cut the polystyrene
Under your wondered eyes everything is done alone!
All that it is quite beautiful, but does it work for a tapered wing ? No panic...
It will be necessary to :
- leave your computer solve " the equation "...
- invest in a fourth pulley,
- find a wood rule...
assume C, is the width of the wing (the cord) to the root,
assume c, is the cord at end of wing
There is always the following relation in a triangle:
c/a = C/A or if you prefer,
a = (c/C)xA
The wood rule is screwed on the section of the cutting table. It can thus rock until touching the ground by an end.
Let us take for example the cutting of a left wing.
The root side of the foam bread must be put in the alignment of the right-hand side pulley.
The left pulley must be put to be in the alignment of the gauge of tip of wing.
The position of the 2-intermediate-pulleys system is calculated
from the formula...
. root: C = 20 cm
. salmon: C = 15 cm
. distance from articulation to corde root: To = 150 cm
==> a = A.(c/C) = 150 X (15/20) = 112,5 cm
The strings are attached on the rule, respecting strictly the distances to the articulation... Not to be bored, simplest is to grip them using a spring clip for example.
It is necessary to ballast the rule a little bit (on the photograph, one sees an elegant system based upon a supermarket bag containing a 800 grammes can ). One puts to the juice and I guaranteed a perfect cutting with an output of the wire parallel with the trailing edge
should you want more details on the pulleys, click on the picture ...
Have a look to this photo, and you'll see how parallel is the wire to the trailing edge ...
The core is perfect :
When the cords ratio is to big (e.g: delta wings ...) it is usefull to use a "fixed point" method.
You need only one gauge, the root side one. You fix the cuuting wire at one end living a rotation liberty degree.
To cut the wing core, simply follow the gauge, and you will get marvellous foamies !