Following a Master Model Maker

By George Elder and Bruce Gordon

Part 2

Making Pom-Pom guns

The bodies of these "grow" out of the end of an often rectangular or circular bar that is usually made of plastic or dense resin. The bar is locked in place, and the end is carved, filed and added to as the base platform, gun supports, and finally guns are formed. The British 40mm octuple mount is a daunting challenge, standing a mere 1.8 mm tall. The trick is to get the barrels to form when metal or resin is placed in the mold. If the elements are too close together or too fine, the casting material will not fill in the void spaces of the mold. The mounting is so tiny that good pictures are nearly impossible.

It seems that making the holes for the wires representing the barrels is also a problem, and I was not told exactly how this is managed. Dave uses magnifying tools, and has drill bits that are little larger than hairs. Indeed, the Neptun 20mm quad German mount has some of the finest guns to be seen, but rendering AA barrels in exact 1/1250 scale is impossible because of the casting problems involved. It is difficult to get metal or resin to flow into tiny mold openings -- even under very high pressure gradients (vacuum or centrifugally induced). The large picture below is blown up perhaps 40 times. The barrels are actually less than 0.18 mm thick, but at this magnification, everything becomes blurry. The material properties of the alloys or resin being used become very important here. It is interesting to watch the process unfold, and I am sure that the stage will soon be reached where the barrels cannot be made finer and still cast. I am hoping that Alain or one of the other recognized experts in this area spells out the limitations of what can be achieved. I have no idea what they are...

Comments by Alain Picouet:

In resin casting there are no limitations to get the finest parts from the mold itself if one uses appropriate products and processes. The print capability is unimaginable and unlimited. Just consider that some can get a perfect cast of a natural flower with petals and pistils! The limits are rather in the caster himself to be skillful and imaginative enough to make the mold concept successful -- and this is the trick, not to say “secret” of fabrication.

Decisive also is the chemical resin choice: much more difficult to work, depending on many parameters. That being said, only your experience will limit what you can get. Here again the right resin, well processed, has no technical limits. But for frail parts (generally latticed ones) they will be thicker, or metal needles reinforced at casting, just as alloy-cast models are thicker and not in scale (generally small barrels, masts, rails). Resin can cast one hair, but not so with alloy casting - don't expect one to do what the other can do, and vice-versa. Technically, all is possible with resin properties, but the worktime cost for it makes it impractical, and to reach one hair size in casting you could lose your own last hair as the price to pay for it... ;-)

Vacuum high pressure does not exist in model casting if correct production process are used, above all because it is question of dispressure involved, not pressure. Physics law in dispressure makes it impossible to achieve more than 1 kilo per cubic centimeter, even where a total vacuum is obtained. You could not get "twice" a total air empty! Empty is empty! There is no air anymore in a confined volume at 1 kilo dispressure. So 1 kilo dispressure isn't high. A little more about that technique is to know that in physics, a perfect vacuum is impossible to get on earth atmosphere surface except by the very high technology such as NASA laboratories. But there is no need of that since only about -750 grams dispressure is enough for a model maker. If not, your resin isn't the right one and you will get poor results. I must warn users that that sort of armoured dispressure caisson has a risk of implosion. Indeed, addition of suction power in it for each cubic centimeter contained in the caisson volume (1 kilo/cm) and glass parts for some are too feeble to resist the 500 kilos possible maximum of your dispressor machine (almost as much as your car weighs!) If the caisson would come to break "your nose," watching operations in it you could be "vacuumed in the box" with serious injury! (Same effect as brutal airplane dispressure for passengers expulsed outside through portholes...)

Your Pom-Pom barrels’ scale level George are not so bad. If you consider the one/one scale of the real guns this would give a diameter for them in 1250 scale of about 0,12 mm thick. Your master of 0,18 mm isn't so far of what human eye could see in so small and makes no difference! Even the thickness difference of 0,06 mm too big would be that of the paint covering thickness!

There is no point doing gun holes for anything smaller than 15 inches main artillery in 1250 scale since one couldn't see them. To drill a gun hole at our "micro-scale" use a tender material rather than copper thread, or too hard a material. Silicone will do the rest, and don't forget that in such tiny work in 1250 the principle is that it's the part going to the tool, not the tool going to the part, as in bigger scales of modeling.

Thank you George for your interesting and positive animations you provide in your postings, in appreciation of all you bring to the Board. Their whole support they expressed could not say best.

Alain