Building the USS Missouri – Part 3

Back to Parts 1 or 2

The real build………

Building started in earnest in early 2014 with the forward support point and keel again, but in restored Meccano. The keel is essentially a large ‘I’ beam comprising a sandwich of braced girders spaced at ½ inch apart, with angle girders top and bottom, and capped at intervals with 1½ inch wide rigid plates or similar.

The keel was assembled on a piece of straight timber, itself secured to a long aluminium straight edge, to ensure it was straight and without twist. As the bottom surface of the keel was not an uninterrupted flat plane (bolt heads, doublers etc.) I used the bottom row of holes as the build datum. To ensure this datum line was parallel to the wood I used 1 inch pulleys on axles as spacers as seen in the pictures.

The triangular frames were each assembled on a jig to ensure conformity, with doublers as appropriate.

Getting tooling to the awkwardly located nuts and bolts was a nightmare at times. In tight spots, hex drive Meccano bolts were sometimes used, or Meccano slotted bolts which are even smaller but difficult to get something in the slot. The Meccano hex nuts are slightly smaller than the square ones for more clearance though pretty poorly made. In extreme cases I used Meccano set-screws which have the smallest head of all.

To gauge whether any twist was developing in the structure, long rods were inserted at intervals into the keel and fed through the frame cross members. Viewed from some distance at one end, if the rods were not all upright and in a line then something was off.

The pin joint arrangement to attach the bow section was one at the bottom and two at the top, and for the stern section was two at the bottom and one at the top.

Each pin goes through a multiple interlocking ‘finger’ arrangement like a woodworking bridle joint. The forward lower pin joint tapers back along the keel to gradually feed in the load to the keel.

Due to the inexact nature of Meccano, each pin will not contact all the joint’s ‘fingers’ to share the load evenly. In reality, only two pairs of interlocking fingers take the load, but if the load is such that the metal goes plastic or the finger strips move, then the others will come to bear to take up load. In this way, there is multiple redundancy at each joint. The high compression members are ‘C’ section, with the flanges supporting each other at intervals to resist buckling.

The three ‘sockets’ to accept the supports are 1 inch deep, surrounded by strips and plates. The supports bear up against the lower edges of a stack flat trunnions and strips, which form the upper ‘cap’ of each socket. I think I put a small piece of MDF as a buffer between the support and the cap to protect the Meccano and even out the pressure - I can't actually remember!

The supports themselves are 12mm coach bolts with the heads removed, and the top inch filed down on the sides to give a rectangular cross section. These insert into the sockets with enough depth to prevent the model ‘jumping off’ the supports when going over a bump in the car.

Soon after assembling the centre section structure it became necessary to make the stand to support the model, as the centre section build would need to be done ‘in-situ’ on its stand. The stand is a pine construction – screwed and bolted together – and varnished.

The previous picture also shows the 'castered' trolley I had to make to move the model from storeroom to lounge and back.  Each of the 4 feet are adjustable in height by hand, as the weight bears onto a washer and wing nut arrangement. The three supports go right through the thickness of the wood, each with an upper nut and washer taking the weight, and a lower nut in a counter-bored recess to hide it. This arrangement is therefore also height adjustable.

The bow and stern structures followed the designs previously sketched. With all three sections connected, the true size of the model became evident, as was the amount of work still to come. The whole arrangement was so rigid – and light enough at this stage – such that pressing down on the prow raised the stern end up off its supports with no discernible deflection. This picture illustrates why building it in sections was essential. There is no way it would even leave the room as a whole.

The installation of the two upper connection pins at the forward join needed to be easy to insert, but secreted from view. There is a mechanism which drives the two pins into place at the same time.

Turning a thumbwheel on the ‘non-public’ side operates this mechanism. Tri-axles were used in places to prevent slippage. The connection pins themselves have a tapered nose to aid insertion.

The three main turrets were going to be the most challenging of the mechanisms, so these were tackled next. The first working arrangement is seen here at one of our club meetings (note the temporary crank handles for functioning the turret for this early display).

Although it did work, it was neither elegant nor compact enough to be used in the other two locations. However, in my professional life I found that provided a solution of any kind can be found, it was unlikely to be either the only one or the best one. The mere fact that a solution is possible means it can be used as a fall-back, and efforts can be concentrated on finding other – better – ways of achieving the same thing. So other concepts were tried until at version 4, I was satisfied. The workings of the turrets will be covered in Part 4.

Next in were the propeller shafts, without propellers. There are 4, in two contra-rotating pairs. The inner two are 5 bladed, and the outer two are 4 bladed. This was to prevent a hydraulic resonance being generated which is more detectable by the enemy.

The contra-rotation was to cancel out the torque effect which would otherwise generate a list when in motion. The drive to the propellers came later.

Each shaft is supported by two bearings, and each bearing is mounted such that it can ‘gimble’. This principle is used throughout the mechanisms where possible, to allow freedom of movement, and to only constrain those degrees of freedom that need to be constrained for correct function. This prevents binding and gives some tolerance to deflections and distortions.

In fact, all of the bearings for gears, lay shafts, drive shafts etc. throughout the ship are (where possible) brass or thick plastic components. A shaft rotating in just a strip hole will wear a groove, increase friction, and cause gear mesh clearances to increase.

Many of these support bearings are simply collars supported on the tail end of a bolt. The collar is wound onto the bolt as much as possible but still leaving the shaft free to turn. The 'sloppy' nature of the fit on the bolt allows for freedom in most axes whilst still retaining the position of the shaft.

Composite picture of bearing examples

One of the problems with all the mechanisms is that no Meccano rod is dead straight! Gimbled bearings mitigate this, but can’t be done everywhere. I did manually straighten out many rods ready for use, but after a period of time (several months) the rods would have a slight curve in them again which could cause binding. So something which operated freely when built might bind later if the bearing arrangement didn’t accommodate this. This ‘shape memory’ was not unexpected, as the act of straightening using a vice and a hammer leaves residual stresses in the rod, which over time even out causing the rod to take up the least stressed condition, i.e. straighter than at first but no longer dead straight.

There are two anchors at the prow. Both anchor chains wind onto a common drum with a separator between the two windings. The end flanges of the drum are gears, only used because they are a larger diameter than a bush wheel.

Because the chains had to pass freely up a slope to the deck without snagging, they are guided through a pair of smooth sided channels formed by using thick plastic ‘5 close hole’ components before rolling over a pair of idler sprockets and on toward the prow (where they temporarily fed through the angle girder slots until later in the build). The anchors themselves are oversized to help keep the chains in tension. There is enough chain to let them down about 12 inches.

The two rudders are driven by a pair of 133 toothed gears meshing with a common 1 inch pinion. The pinion shaft is driven by a large bevel gear, and the drive train then continues forward. At this stage the rudders fitted were illustrative only.

 Back to Parts 1 or 2. Continued in Parts 4, 5, 6, 7, 89