The Ammonit

By Harald Werner
07 Mar 2001

Surfacing

Materials: polyester filler (car putty), polyester netting, gloves, spattles, acrylic glass, orbital jigsaw

Before starting the surfacing work, read carefully the "instructions for use" of polyester filler. You should work in an empty, well-ventilated room.
In car-assessory shops various polyester fillers are available. Even large supermarkets in Germany offer bargains. Regular filler without glassfibre will do.
Building-trade shops carrying different variety of polyester netting (this is normally used for plastering works at houses). Best for our intentions is the narrow-meshed; and two square meters will do nicely. Before mixing the filler with the hardener, you should cut the polyester netting into pieces of around 5 on 10 cm. When using the filler I recommend wearing gloves. You should work swiftly when you mix the polyester with the hardener. This applies also to the surfacing work. Do not mix too much filler at the first time, because it hardens in 4 or 5 minutes. Put some of the cutted polyester netting on the snail´s skeleton; perhaps you´d better begin at the bottom. Just apply the filler with enough pressure that it squeezes through the polyester-mash and the wire netting. Then press some more of the polyester-mash pieces into the filler: some of this pieces should enter only partly into the polyester filler, so that you get a better connection when you apply the next mixture. But just give the filler time to harden.

First of all, fix the mounting bases for the drivers. I made these from acrylic glass, which I cut with a jigsaw: the base for the low-midrange driver is a ring-disk,that was made out of a 1 cm thick acrylic pane (available in sanitary shops). Consider the mounting measurements of the driver to get the inner diameter, and add 4 cm to that to get the outer diameter. So, the ring-disk is 2 cm broad (but this can vary to the driver you intend to use). The base for the tweeter is made from the same material, but the shape should be more of an inverted "U", 1 cm bigger than the cutout diameter of the tweeter, and with a hole to let the magnet and the connetion terminal through. Here, I recommend drilling several little holes under the magnet hole (but not within the cutout diameter). This is to squeeze polyester filler through them to get a better connection.

The ultimate connection for the base of the low-midrange driver is made later, when you carefully drill the holes for the dowels, which hold the screws for the driver.

By fitting in this base apply the filler as described, press it through the double front part of the wire skeleton, apply quickly some polyester-mash pieces from the inside of the tube, apply here some more filler and smooth it with a spattle. Similar you procede with the base for the tweeter. Ensure that you keep the bases always clean.
Now you can give the final design to your snail: The walls of the snail should be at least 2 cm thick. No polyester netting is used anymore, but you can use the polyester filler with glass fibres. Nevertheless, the standard filler is easier to form.
The design depends on your taste, probably a rough outside like a fossil does look very nice as well. If you would like to have a smooth surface, you should fill all indentations and level all bulges. First you can use a rasp, than a coarse-grained sanding sheet. At the end you use more and more higher grained sanding paper.

Observe the safety instructions for the use of polyester filler!

For the design of the surrounding of the tweeter, I used the disassembled front plate.

Base
Material: sand, cement, strong iron grating (3 to 4 mm), small wooden slabs, angular iron fastenings, trowel

The size of the base should be a little bit broader than the snail and long enough to carry the crossover (if you intend to buy stands from any manufacturer or blacksmith, build your base to match the size of the stand´s upper plate).
The cast for the base can be built with small wooden slabs and agular fastenings. Now the snail can be fastened to the reinforcement, which is placed at the center of the cast. The reinforcement should be made of very strong wire. Bent the ends down by 90°, so that it stand by itself. The frequency crossover can be placed in a small box under the reinforcement. The drivers´cables can pass into the box through a small hole. The 4 strong wires which leave the bottom of the snail should be wound around the reinforcement several times. The snail should stand by itself now.
Now you can cast concrete to make the base. Make sure that the reinforcement is about 3 cm away from the cast on every side. Cut more of the polyester netting into pieces of 5 x 10 cm.

Mix the concrete: dry sand and the cement are mixed in the ratio 3 : 1, then, under stirring, water is added until the concrete has a viscous consistence. A bucket of 20 l will be enough for one base. Pour a layer of 1 cm into the cast and stick some of the polyester netting pieces in it. Then pour another layer and add more of the polyester pieces (they should stick in the concrete in all directions, this will reinforce the concrete even better). The final top layer should be done without polyester. Stir the concrete carefully with a small stick to clear it of air bubbles. Fill the cast up again and smooth the surface of the concrete with a trowel. Keep the concrete moist for 24 hours. After 2 days carefully remove the cast, which can be used again.
The full hardening process of the concrete takes one month. Meanwhile, preparations for the paintwork can be done: small indents can be filled with polyester filler and bulges can be levelled. To obtain a particularly smooth surface, you can use a filler aerosol (available in car assessory shops).

After one month, 4 holes can be drilled from every corner of the frequency crossover box into the polyester hull of the snail (utmost care! Make sure that you drill in the right direction). With dowels and screws you can strengthen the snail to make it "bomb-proof".

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