A Report to the Cabal:
A tank turret without a gun is like a male cat without nuts: Not much use to
Time to build this turret a proper gun!
So, Jake set about turning, from some machine aluminum, a gun barrel master. He brought the finished work over here and we sat down and worked out a plan, a methodology, on how to create duplicate parts. Objectives included ease of production, minimal weight of the finished product, representation of a scale diameter bore throughout the length of the gun, and the ability of the gun to supports its own weight when affixed by trunnions at its extreme breech end without damage to gun or bearing points.
The significant challenge turned out to be the need to produce gun parts
that could support themselves against not only the one-gravity weight of the unit,
but also the high-G loads on the gun support structure that result from normal
vehicle acceleration, bouncing around, turret motion, and the shock forces
resulting from the inevitable collision of tank an/or gun with structures and other
tanks encountered on the field of battle.
We settled on gun parts that would feature an inner bore formed from a length of aluminum tube, serving as the scale bore element as well as an internal armature -- a strength element, surrounded by a resin gun proper. To achieve this part a mold was made from BJB TC-5050 silicon rubber. A thin walled aluminum tube would be encapsulated within a thin 'skin' of Alumilite 'regular' tan colored polyurethane casting resin.
Damn! We're good!
OK, the magic revealed:
Jake tripped in here the other day with his completed aluminum gun barrel master and wanted to use it to produce a rubber tool from which he could cast multiple resin parts. We figured that if we used a tube of Lexan to serve as the tool containment/mother-mold, we could not only minimize the amount of mold-making rubber needed but would also have the means of assuring perfect registration of the two mold halves each time they were assembled and placed back in the containment in preparation of manufacture of a new gun barrel piece.
And that's just how things worked out. (After an unimaginable, frightening, obscenity laden, tool throwing, snot-slinging battle extracting the cured rubber tool
from the containment -- that story too horrible to relate here).
Jake showing off the barrel master suspended within the Lexan tube containment. Note the wood plugs at each end. A one-sixteenth inch brass wire between each end of the barrel master and a wooden plug centered the master within the Lexan cylinder. The upper plug is temporarily removed as the mold making rubber is introduced into the cylinder -- once full, the upper plug is reinstalled.
I guestimated that sixteen ounces of BJB Enterprises TC-5050 mold making rubber would be enough to fill the containment. And we hit it right to the drop! (Sometimes, I'm not so sharp and I wind up wasting rubber -- not smart, that stuff ain't cheap!). It's a good practice to have another set of masters at hand, from which you'll need a tool, so that any excess rubber you mix up for the principle job can be dumped onto the secondary job. Waste not, want not!
During the pour of the rubber, the upper containment plug was removed to permit introduction of the rubber into the cylinder. After Jake had filled the containment vessel (the cylinder) with the rubber, the plug was inserted, the projecting one-sixteenth inch diameter pin projecting from the upper end of the barrel master run through the central hole in the plug to accept it, and the plug driven home within the top of the containment. The freshly poured tool was left to cure for twelve-hours, as the rubber changed state from a liquid to a solid.
Jake splitting the rubber tool into two halves with a sharp #11 X-Acto blade. With care you can do this without ever cutting the master with the blade: You insert the knife, exert opening pressure with fingers and by cramming in pieces of foam into the gash; the objective is to start the separation with the knife, but to complete the opening by tearing the tool, so by doing revealing the encapsulated master within. This not only saves the master from damage it also produces a very ragged interface between the two halves of the rubber tool -- a keying network that assures perfect registration of the two mold halves when reassembled. Note that care was taken not to make the slit a nice clean line.
With considerable effort (we finally worked it out by injecting air into holes drilled into the containment tube!) the rubber tool was pulled out of the Lexan tube. A sharp knife was then used to slit the tool in half, and the master was removed and put in storage for possible future use. Note that we have already cast a resin barrel part, its sprue still attached at the muzzle end.
Note the uneven separation flanges on the two mold halves, result of the knife cutting and tearing during the master removal process. This uneven flange pattern between the two tool halves assures exact registration of the two halves when assembled.
The barrel tool, outfitted with its aluminum tube, slid back into its containment, a standpipe inserted, ready to receive a pour of polyurethane resin. A cast gun barrel part is at the bottom of the picture. The internal aluminum barrel liner is vital. Without it the barrel would weight twice as much and the piece, without this stiffening element encapsulated within would eventually sag like a limp dick over time if subject to any significant heat. And, of course, the barrel liner also produces a scalelike bore for those detail freaks who what to peak into the muzzle end of the thing.
Before buttoning the two-piece rubber tool, I sprayed in a coat of silicon oil spray to inhibit contact of the liquid polyurethane casting resin and the cured silicon rubber. Over time there is a chemical change induced onto the rubber through contact with polyurethane resin -- typically you get about twenty to fifty castings per tool, the number varies owing to rubber/resin chemistry, degree of undercuts in the tool, and temperature of the work (thicker cross-sectioned parts generate more heat during the cure cycle). A length of aluminum tube is inserted in one of the tool halves, and the tool assembled and slide into its containment.
Here I'm using the excellent Alumilite 'regular' polyurethane casting resin. A fifty-fifty mix of resin to hardener. You mix it either by weight or volume. Once mixed you have under a minute to transfer it to the tools cavities and to then get it into the pressure pot and down to thirty PSI, about one-atmosphere. This casting resin is wonderful stuff, the same material used to slush cast (Roto-cast, for you purists!) the tank turret test article discussed earlier.
Decades ago Phil Kordich welded up three nice, purpose build, pressure pots for me. I've been using them all these years without incident -- a tribute to the mans work and insistence on over-engineering every aspect of this important shop tool. Normally the pot sits sideways. But, with this gun-barrel job I was compelled to mount the shorter of the three pots vertically so it would contain the gun barrel tool with the sprue opening standing up.
The prepared tool was placed in the pot, resin mixed up and quickly poured into its sprue hole, the pots gasket and door set in place, the four securing nuts spun down to make a tight seal, and the pot pressurized to thirty PSI.
The pressurization of the tool and contents acts to crush any air bubbles that might still be clinging to the cavity walls of the tool or gases formed during the curing process of the resin. By hitting the work with one-atmosphere of pressure, such bubbles are reduced in size, to the point where they are crushed, dissolved, back into the solution, where the gas remains until it changes state to a hard plastic. Pressure casting, like this, is done to achieve pinhole free resin parts.
Opening up the pot to extract the work. You can make out the Alumilite resin, which turns a creamy white color when cured, within the standpipe and as spillage that collected atop the rubber tool within the Lexan containment.
As you can see the pot construction is basic: a section of ten inch diameter steel tube with a circular end welded at one end; four door securing studs welded to the outside of the pot and projecting an inch or so past the open end of the pot; and a quarter inch taped hole to accepted a pressure gauge, relief valve and stop-valve. The lid (or door) of the pot is simply a piece of sheet steel with four holes punched in it to pass the studs. Note that the lip of the opening is beveled, this done to insure a nice tight squeeze between gasket and door.
If poorly built or improperly operated the pressure pot can easily become a shrapnel spewing bomb. Phil assured robustness of the machine by hydrostatically testing it to one-hundred PSI. After successful completion of that test he outfitted it with a thirty PSI relief valve. So tested and outfitted, this pot is a safe to operate and indispensable tool in the shop.
After about twenty-minutes in the pot, the pressures was vented, the lid popped off, the rubber tool slipped out of its containment, opened up and the cast resin model part pulled out. Note that the brass tube standpipe has already been pulled away from the sprue. All that is needed to prepare the cast resin piece for use to cut away the quarter inch of excess barrel-liner from the breech and muzzle ends, shave off the sprue, and sand away the slight amount of flash at the seam-line. Of course the part has to be scrubbed with 0000 steel wool soaked in lacquer thinner to cut away the mold release that still clings to the part before the first shot of primer goes down.
Another look at the cast resin part and the tool elements needed to create it. The entire weight of the cast resin (which includes the encapsulated aluminum tube barrel-liner within) is under two ounces! This is important as the r/c tank turret trunnion and elevation mechanisms won't be subject to unreasonable loads as the tank bounces around either in pursuit, or running away from, other r/c combatant model tanks. That gun barrel, if too heavy, with its very long moment arm, can upset a tank traveling on an incline, so weight of the barrel is a very important consideration for those who operate these type r/c vehicles. Ain't that right, Jake!?