Saturday, June 1, 2013

Bins and Bin Roofs

Howdy gang.

     The thing about model silos is ultimately the 3rd dimension-----height------ and Merriwa relates greatly to this dimension. The bins are 104 ft tall or in model terms, 364 mm with the elevator tower being a further 120 mm, giving a total of 484 mm and that sits on a 12 mm base, so from base to top we have a total height of just under 500 mm. From a model point of view this is immense, and if placed in a layout, the layout does need the depth and length to ensure that the silo is not too overpowering. Honestly these things ARE overpowering and that is just a fact. The last trip up to Merriwa, that I was involved with comes to mind, and I remember as we reached the outskirts of Merriwa, over the rolling hills, we were greeted with this fantastic monolith being the S052 Merriwa Silo, like a magnificent welcoming beacon in the near distance, and we then knew that our journey was complete. Big, yes they are big.

     When I first started designing the Merriwa silo model I was pleasantly surprised that the bins measured 31 ft 6 in in total or 110.25 mm. Surprise, surprise, standard sewer pipe measures in at 110 mm in diameter, ----PERFECT----- we have a winner. So it was a simple matter of designing the silo to fit the commercially available conduit.

Elevation view of Merriwa S052 Silo

     The conduit was now cut over size, and each piece was faced on a facing sander, that was set up with an aluminium jig, to ensure that the pieces are a square as possible. then checked in an acrylic jig used like a go-no go jig, many pieces were rejected and reduced to fit the larger bins in the S016 and S024 kits (so they weren't wasted).

The sanding jig that was designed by my son Tim, which allows a lot of adjustment, for various conduit sizes. The jig allows for accurate face sanding of the conduit pieces.
The face sander with 110 mm conduit being machined, This system produces plenty of dust from the PVC conduit and a vacuum and face mask is necessary.

     Now that the conduit was selected, the steel strapping had to be addressed, as these were prominent features of this and other similar silos. The forces involved with loading and unloading the bins continually, puts a massive strain on these concrete bins, and so to reinforce the bins 6'' x 3/4'' steel strapping was placed on all of the bins in the stress areas of the bins, hence they are only located in the middle sections of the bins. There are 16 steel straps, roughly 6 ft apart.

      Evergreen 6'' x 1'' HO styrene strip was used to simulate these steel bands. Many packets of this material were needed, (3 bins x 16 pieces, 48 pieces ----10 pieces per pack----LOTS OF THE STUFF). Luckily each piece, was just a tad longer that the bin circumference, which was only required for the lone standing bin as the other two bins are attached to the elevator tower, and the band is not required where they join to the tower.

     Now, one problem to overcome was how to get the strips, in the correct position and square with the model. The first thing, where do we start? This was achieved by using a 5 meter staff to obtain a rough starting point, and measuring the material thickness and the gap between each band, then scale this information to the model.

The staff showing the height of the fist steel band on the silo. The starting point.

     Al Cutmore  (AR Kits) once showed me a method where you can use a vernier gauge as a scribe, to scribe a line in plastic, and that is what I employed on the bins. It is a slow and laborious task, but necessary to achieve a great result. The vernier is 300 mm long and the base of the bin is used as a datum point and the vernier is adjusted and then locked in position and all 3 bins are done at the same time. The scribe line is locate at the base of each strip and the styrene strip is butted up to the scribe line (slight ridge), keeping the styrene in line while running glue between the strip and the bin. micro adjustments were required to ensure that the strips line up. After doing this to three bins, it become very tiring, due to the fact that you need to get it correct the first time. next the bin roof.
The 110 mm conduit with styrene strips glued in place. The markings are for the location of the out loader in number 1 bin

     The bin roofs are an octagonal shape and and made up from layers of 1.5 mm acrylic. Just a note about acrylic/perspex etc. The 1.5 mm acrylic is a cast materiel and as such can vary from one corner of an 810 mm x 460 mm sheet to another. In some cases, it can be anywhere from 1.45 mm up to 1.75 mm. This can present problems with ant tab and hole joints that are used, as the hole is laser cut and is accurate, but the tab that fits into the hole can vary and in some cased requires sanding, so that it fits snugly. Forcing acrylic together when it is too big will result in cracking that can be extremely frustrating.
Merriwa Roof Plan showing octagonal roof and conveyor.
The components for 1 roof, the 1.5 mm acrylic is covered with brown protective paper. 
Pegs are used to hold the base and circular pieces together so that glue can be run into the joint. The 6 mm brass tube centres the 2 pieces,  the lamination process makes the base much more rigid.
The sanding block is a simple piece of 9 mm craftwood with sand paper glued to both sides, the uprights required sanding to fit into the tabs on the base.

So back to the roof construction. There are 18 acrylic parts for each roof. The basic octagonal shape, and circular piece slightly smaller that the inside diameter of the conduit. 8 risers, and 8 roof sections, designed to fit snugly together. The pieces are glues together with DCM (Di Chloride Methane), to hold in place, then sanded so that each joint is smooth and then hit with thin fast drying supa glue. The edges of each roof section are sanded extends at an angle slightly and need to be sanded flush with the base pieces, then filled and sanded clean and smooth. There is a 6 mm hole in the middle of the roof which caters for a brass tube that is attached to the top conveyor of the silo. Finally a small piece of acrylic is glued to the base of the roof over the hole as a stop for the brass tube.
The finished  roof ready to have the joints and edges sanded smooth, filled and sanded, ready for cladding, and drilling.

The bin roof sitting on top of the  conduit, the circular section is laser cut to fit with a small amount of movement for adjustment.
     The bins are then clad in corrugated iron, a small styrene drilling jig is used to pivot on the brass tube, and small pilot holes are located to allow the positioning of the roof vents. there are four of these vents on each roof and there are located equally on 4 of the roof sections. Not all bins have the same treatment. the stand alone bin has a stair section as does the bin next to it, which is recessed into the elevator tower. The bin on the other side of the elevator tower is also recessed, and has stairs and an access door leading into the elevator.

The roof showing the recess into the elevator, as well as the stairs going down to a door into the elevator.

The steel work at the top of the  roof is comprehensive, and is a feature of the silo. Access from the roof to the conveyor, is also provided via a ladder.

On the next installment, there will be more on the steel work and how it is modelled, to great effect. as well as the conveyor, and cladding the silo in general. until then, happy modelling.