Step 74 : Conning Tower casting
Time to complete : 44 hours
Total time to date : 558 hours.
With all the little stuff pretty much done, it was time to get into making the larger parts, which includes the conning tower. The process for creating the conning tower castings is different that what has been covered so far with the smaller pieces. In theory, the exact same method could be used, but you would have to put a second mortgage on your house to afford the rubber molding material that would be needed. Instead, what is done is to expose 1/2 of the master at a time in order to make the mold for 1/2 at a time as well. The master is placed in what is called a parting board which is basically a board with a hole cut in it as close to the shape as the master as possible, with the excess gap filled in with Clay to make sure it is totally sealed from the other half. In the case of my tower, what I actually made is more of a "parting box". I built a box because I was then able to fill it with cat litter to act as a support for the master to assist in making sure that only 1/2 the master was exposed. Why cat litter ??....because it was the closest thing to sand that I had, as I figured something like sand would be a real good support to conform to the master......I was right, the cat little work excellently. Before placing my master in the parting box, I marked the dead centre of the master all around and then lined it with 1/4" masking tape. This tape is going to make an indentation in the mold which will show the exact centre. This is going to be important because when the casting is trimmed/sanded down to get rid of the excess fibreglass/cloth, a reference mark is going to be needed to know where that centre is. This tape line also acts as a real nice reference as to how much of the master to expose. Once the master is taped and placed in the parting box, it was ready for the rubber molding material application.
* Close look at the photo and the green masking tape line can be seen marking the centre of the master *
A thin coat of the rubber RTV is brushed over the master tower. This coat only needs to be thick enough to capture the fine details such as the rivets, weld lines, flood hole markings, etc. Once this thin coat, or "glove", is cured, 2 more coats of a thicker rubber material is applied to give the glove a little more support. I used stuff called Rebound 25 made by the same Co. that the Smooth Sil 920 rubber RTV product that I'm using. These materials were suggested to me by Kevin McLeod for their quality and ease of use, not to mention their availability here in Canada. Kevin was right on the mark as these materials are first rate......but by no means cheap. ($130/gal.). Once the Rebound 25 has cured, it was time to make what is called the hardback.
The hardback is simply a rigid support for the rubber mold. Since the rubber is obviously "bendy and floppy", there needs to be something to hold it in the proper shape, basically an exo-skeleton. I decided to cast a fibreglass hardback right over top of the Rebound 25. This technique worked very well. When the fibreglass hardback cured, I just peeled the rubber mold out of it very easily, no stick, and no release agent needed. Others have used plaster hardbacks, but I found the fibreglass one that I made much lighter and easier to work with. This will be REAL important when I make the hardback for my hull molds (8'9" long x 15" wide). Since the hardback is basically of the shape of the master, I would have to build stands for the hardbacks so that they can sit in the proper position to cast the tower parts.
These stands were of simple 2x4 and hardboard construction with a hole cut out for the hardback to fit into. I cut the hole just slightly smaller than the hardback. I forgot to mention that when I made the hardbacks, I put a 1/2" lip around the perimeter so that the hardback would rest on this lip in the stands. Once in the stands, the rubber molds encased in the hardbacks were very secure. The molds were now ready to cast some parts.
Casting the tower halves was a matter of applying two thin coats of fibreglass resin in the molds, letting the set up well before coats. These thin coats of resin are going to capture the details of the master without any fibreglass cloth distorting them. Once the two thin coats cured, two complete applications of resin/fibreglass cloth were applied. Once all was cured, the rubber mold was removed from the hardback and then easily peeled off the resin casting. The casting was then trimmed and sanded, using the now visible line from the tape as the reference. With the two halves made, they could now be assembled into the complete tower. Before this was done, all the flood holes were drilled/filed out.
As hard as I tried to make these two castings mate up PERFECTLY, ....they don't. A little bit of grinding and fiddling is required to make that happen. Once I was generally happy with how I had them fitting, they were "welded" together using a thin bead of the fibreglass resin while held together with clamps. Once this resin cured, I applied a layer of short hair fibreglass over the top and bottom of the tower. This layer was then sanded to a perfectly flat surface. Both of these areas are not crucial because the bottom surface is not going to be seen, and the top is going to be covered with a planking casting as well. The end result is a fibreglass tower that is impossible to tell that it was once two separate parts. I was pleased with how well the resin castings captured the fine rivet detail of the molds. I then added the tower wall enclosure, wind deflector, and grab irons to the tower, giving the whole thing a coat of primer. Since I was not happy with the look of my casted "square looking" grab irons, I decided to use bent brass rod instead. I like the look much better and it was very easy to do. The prototype tower is now ready for some planking castings for the deck surfaces before the application of the smaller detailed parts.
More to come.
"I don't have anything else planned for this afternoon." - Lt. Col J.O.E. Vandeleur
A Bridge Too Far (1977)