Wednesday, September 29, 2021

How I designed and drew a Southwold 6-wheeled Cleminson coach using Tinkercad

 Very soon after acquiring my first 3D printer, I wanted to find a way of creating my own 3D models. Having found SketchUp to be tricky to use and FreeCAD to be baffling, I soon discovered TinkerCAD which was far better suited to the way I think in 3D. Unlike other 3D drawing packages in which drawings are made in 2D and then extruded into the third dimension, TinkerCAD uses as series of 3D shapes as 'tools'. By changing the dimensions of a shape and overlapping it with other shapes, quite complex drawings can be made. The really clever aspect of TinkerCAD is than any shape can become a 'hole' which can be used to take a hole-shaped bite out of another shape.

A cuboid shape and lettering turned into a 'hole'

The two shapes combined (by grouping)

In this article, I will take you through my design, drawing, printing and construction processes to hopefully demonstrate that if I (a muddlesome scratch-bash bodgeller) can do it, then I.m almost certain that you can too.


Step 1 - Find drawings and photos of the model

I prefer to use scale drawings but sometimes (eg my Schull and Skibbereen loco), I have to make do with photos. 

The Southwold Railway Trust produces a wonderful set of drawings of its rolling stock to 16mm:foot scale in a very reasonably priced booklet (£20UKP at time of writing). However, this only became available after I had started making this model and so I used the drawings printed in Vic Mitchell and Keith Smith's Branchline to Southwold.

After photocopying the drawing - increasing its size to fit on to a sheet of A4 paper, I then worked-out the scaling factor needed to convert the dimensions on the drawing to 15mm:foot scale needed for my model. This was done simply by measuring one of the 'known' dimensions on the drawing - in this case the distance between two wheelsets which the drawing gave as 14ft. On my enlarged drawing, this measured 82.5mm

In 15mm scale, 14ft would be 210mm and so the 82.5 on the drawing needs to be multiplied by 2.55 to give 210 (because 210 ÷ 82.5 = 2.545454).  So, any dimension on the drawing can now be converted to 15mm scale simply by multiplying it by 2.55.

Armed with a set of dimensions, I could now move on to the next stage - figuring out how to break the coach into printable sections.


Step 2 - Breaking down the model into sections

 I like to keep things simple and so I tend to print sections of my models as flat to the bed as possible. This might be because my first (very cheap) printer struggled to print vertical surfaces cleanly. I also find that small details such as rivets, hinges, strapping and beading generally print better on to a horizontal surface.

Clearly, the Southwold coach body was simply a rectangular box on a flat base. So this could be broken down into two ends and two sides. However, at 450mm the sides were longer than the width of my print bed which is a quite generous 300mm, so the sides needed to be split into two 225mm wide sections. My cheapo printer's printbed is only 200mm wide and so I would have had to divide it into three if I had used this.

Step 3 - Drawing the parts

As you can see, I split the side exactly in the middle as I felt it would be easier to disguise the join if it ran up the middle of the beading on the lower panel.

As the side doesn't have a tumblehome and the windows are square without corner fillets, it was a fairly straightforward job to draw the sides using the 'Box' tool for most of the parts and the semi-circular "Round roof" tool for the beading.

The ends of the passenger compartment came next. As with the sides, the majority of the shapes on the ends were drawn with the box tool, suitably redimensioned.

 The curved top for the roof involved a little bit of ingenuity, but fairly obvious when thought about. A large disk shape (ie a flattened cylinder) was positioned over the lowermost part of the end .........

This was then grouped with a larger cuboid 'hole' shape, suitably positioned so that when they were combined together, the hole would remove the unwanted part of the disk.

Next came the undercarriage. Once more this needed to be divided into two sections to fit on to the print bed. As I wanted flat surfaces beneath the undercarriage for the swivelling and sliding trucks, I placed a flattened box shape at each end, then built up the sides of the undercarriage around them.

The cross members will support the floor and also provide an anchor for the screws for the outer swivelling trucks. I also added strapping and rivets to the outside of the verticals, using photos of the coaches to ascertain where they were located. The tops of the verticals were given rebates to allow the floor sections to be aligned.

 A lot of these finer details were envisioned during the drawing process and later refined when other sections of the model, such as the floor panels, could be test-fitted on screen before being finalised.

 The rivet heads were dome shapes, resized and rotated until they were vertically aligned, ......

..... raised above the workplane by the required amount.

..... and then moved into the desired position.

The floor was split into three sections. I decided that this would help to reinforce the joint between the two halves of the undercarriage and the sides. On reflection, I could have made the sides asymmetrical so that the joint between them didn't coincide with the joint between the two halves of the undercarriage. 

The hole is to allow access to the bolt holding the swivelling truck in place and I decided only to represent the floorboards on the balcony part of the floor as the interior would be largely unseen.

The planking was done by inverting a roof shape, then  stretching it to beyond the width of the floor, turning it into a hole, raising it above the workplane by 1.5mm, duplicating it five times and then grouping it with the floor to make the grooves.

The middle section of the floor was left plain. I used plasticard for the floors on my first model of the coach but someone pointed out to me that the cost of the filament needed to print the floor was far less than the cost of a similar piece of 2mm thick plasticard.

The trucks were then tackled. The outer two trucks were identical and drawn from a series of box shapes grouped together.

 The indented curve in the end of the linkage arm was made by grouping a cylindrical hole with the arm

The curved end was made by taking a semi circular bite out of a box shape, turning this into a hole .....

....... moving it over the end of the arm

and then grouping it with the arm shape.

Three pivot supports were then drawn. The two domed strips will be added to the top of one truck to allow it to wobble from side to side to give a crude form of compensation. The circular support is designed to keep the other out truck level.

The centre truck was then drawn. It took several goes to get the dimensions of the linkage arms right. It was drawn in the same way as the outer trucks.

The W-irons were drawn next. Although such things appear to be quite daunting they are actually little more than a series of shapes combined together.

The springs were probably the most difficult shapes to represent. I eventually decided to use the "Ring" shaped tool and, after squashing and stretching the rings to the desired shape and size, used three box shaped holes to trim the segments into shape.

The increasingly sized segments were then moved into place over the rest of the W-iron

A 3.5mm diameter hole was then made in the underside of the axle box to take the ends of the Bachmann 21.5mm diameter wheelsets.

The bench seats for the interior were then drawn. I decided slatted seats would be more interesting than the original plain benches, particularly as 3D printing produces this type of part far more easily than could be achieved by hand.

The roof was tackled next. On the first coach, I used strips of plasticard but decided to make use of the printer's capabilities to produce a preformed curved roof.

A large cylinder was sliced with a box shape to produce a dome shaped piece of the required diameter.

The dome shape was then duplicated, turned into a hole and grouped with the original piece to make an arched roof.

Sides were then added to the roof using long thin box shapes. This would form the centre section of the roof.

For the ends of the roof an arched section was added to one end.

The arched section was also used to create ribs for the inside of the roof.

The steps were created with three box shapes with their ends rounded using a box hole with a rounded corner removed.

Firstly a box shape the right size for one of the steps was drawn. A box and a cylinder were also placed on the workplane beside it.

 The cylinder was then turned into a hole shape and positioned over one corner of the box.

 These two shapes were then grouped and the new shape turned into a hole.


 This hole was then duplicated and flipped horizontally, before the two holes were positioned over the corners of the step shape.


 All three were then grouped to produce a step with rounded corners.

Retaining brackets were then drawn with a vertical box joined to a horizontal box with bevelled edges.

The linkages for the Cleminson trucks were drawn. The longer pins were found to be necessary to allow the centre truck to rise and fall as it passed over undulations in the track.

And finally, works plates were produced - although the small painstakingly applied lettering proved to be too tiny for my printer to reproduce. However, the plates help to disguise and to reinforce the joint between the two halves of the undercarriage.

So, designing and drawing my own rolling stock has not proven to be such a daunting prospect as I envisaged. I really enjoy the challenge of mentally deconstructing a model into its constituent components, drawing the parts and then, after printing, putting them back together again in reality.


Steps 4 and 5

Part 2 will show how the parts are printed out and assembled.

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