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Scratchbuilding pt. 3: Fundamental Structural Techniques

 

In previous instalments we covered materials, and tools - this time we'll cover some of the fundamental techniques needed for making scale objects out of styrene sheet.

In the next instalment we'll see how to turn basic shapes into recognisable things you might find on the roof rack of a 1:10 scaler or in a garage diorama, but first we need to make those shapes.

Through seven worked examples, the points I'd like to make about all the following will hopefully become apparent:

  • Usefulness & ubiquity of the box form;
  • Planning/Design;
  • Cutting;
  • Basic sanding, wet sanding & sanding for shape;
  • Bonding;
  • Clamping;
  • Reinforcing.

 Note on the plans: although it's theoretically possible to make these shapes based just on the plans, I'd suggest you refer to the dimensions in the text and measure out your own pieces, and just look to the plans as a "sanity check" :)

Plans for the examples can be downloaded in the bottom of this article.

 

Safety Note: some of the tools, materials & chemicals described in these articles have the capacity to cause damage or injury if used incorrectly. Read and pay attention to all warning labels & safety data sheets, take the required precautions, and have supervision if necessary.

You will need:

  • 0.5mm, 1mm & 2mm styrene sheet (if you use A4+ sheets & cut reasonably efficiently, quantities of 1, 2 and 3 respectively should be more than sufficient);
  • Liquid adhesive: EMA Plastic Weld, or bulk Dichloromethane + small jars to decant it in to + a brush to apply it;
  • Craft Knife;
  • 12" & 18" steel rules + pencil;
  • A3 cutting mat;
  • Wet & dry paper (e.g. 120, 400 & 1200 grits), Fairy Liquid to act as lubricant/surfactant when wet sanding, small block of wood;
  • Filler (e.g. Squadron White Putty);
  • Tape for clamping;
  • Regular "Poly" glue.

You'll also need to improvise some circular templates and formers, and some weights. I used:

  • can of tuna, Tamiya PS paint;
  • AA pen cell battery, marker pens;
  • Tins of beans.

Optional:

  • 2mm square strips (Evergreen # 164);
  • Small engineers square;
  • Dremel & sanding drum attachments;
  • light card (160gsm), string & pins if you want to draw your own ellipse (see later).

 

Sheet Thickness

Using styrene only as thick as it needs to be makes a lot of sense - it saves weight, and as styrene sheets get more expensive the thicker they get, costs are a bit lower too.

I started off a bit paranoid about how tough the scale bits and bodywork I made needed to be, so used 3mm sheet for structure, and 1.5mm for wider areas. In part, this was due to the apparent floppiness of large sheets of styrene, but it doesn't do that once cut into smaller pieces with other bits joined to it at angles.

The point is: I've moved down to 2mm and 1mm for the same functions, with a bit of bracing where necessary, and this is still tougher than it needs to be.

 

Cutting

Basically, it's always the same - mark where you need to cut, use a steel ruler and a sharp craft knife, and cut gently. Really, you don't need to do anymore than score the surface. Push down on the ruler by all means, but using a lot of force through the hand holding the knife means you can break the blade, and/or cause a slip - which is rarely a good thing when you're using a sharp edge.

There should be enough grip between the styrene sheet, cutting mat and ruler to hold things steady (provided you're not putting too much pressure on the knife), but when the sheet you're cutting pieces off gets smaller, the grip goes down. There's no hard and fast rule, but I would say when the area under then ruler is smaller than the width of the ruler itself (usually 25mm), you need to provide some extra grip in the form of scrap copier/printer paper between the ruler & styrene and between the styrene & mat to stop things twisting.

I did say that all you need to do is score the surface - that is true, but the number of times you have to do it will vary, depending on the thickness of the material, and the dimensions of the piece you're cutting off.

For example, with the size of the box top/base/sides/ends in the worked examples below (in 1mm sheet) all you need to do is score once, maybe twice, and there's enough leverage to easily snap the part off.

jr scratch 3 001

 

Scoring & snapping off 2mm wide strips from 2mm sheet (which I had to do for this instalment as I'd run out of 2mm strips at the time of writing, out of sequence, having just done most of the work needed for part 6) will never work as there's not enough leverage to snap it off without using tools (which would mark it) so instead I had to score it several times (between 7 and 12, depending on how much I was concentrating on doing it very gently).

Gluing

It's worth noting that using Plastic Weld (or DCM) is more like brazing than traditional gluing. If that isn't a helpful metaphor, let me say that most times you use a glue, you apply it to one or both surfaces, then bring them together.

Where the "glue" is purely a solvent, working on temporarily melting the parts, you need to hold the parts together, apply a little along the outside of the joint with a brush - and it'll get sucked into the joint by capillary action. Once the plastic hardens up again, the parts are stuck together.

jr scratch 3 002

 

Clamping

Where you're gluing one part to another under no stress, e.g. gluing one flat thing to another (like the first couple of upright pieces to the base of a box) then the joints seem to set very quickly.

However, where there's some irregularity (like putting the top on a box were the sides are not all perfectly level) or where you're attempting to put a skin on a round object, then the stress will cause the joint to pull apart & not set properly for quite a lot longer.

In these circumstances, the parts need to be clamped. By that I mean held together using some force - not necessarily using actual clamps.

The most useful thing to have is tape - 10mm Tamiya masking tape works very well. A flat surface (e.g. a bench) and improvised weights can also clamp things very effectively. Clothes pegs too (although I don't think I used them in these builds).

Actual model making clamps in all their various forms are useful, but really not a necessity ay first.

jr scratch 3 003 jr scratch 3 004 jr scratch 3 005

 

Sanding

No matter how lightly you score/cut styrene (see "Cutting", above), sometimes it just seems to want to get out of the way rather than cut, and forms a ridge up on the surface: most of the time you really need to sand these ridges off before proceeding with the part. Wet & dry paper wrapped around a small wooden block works very well for this.

jr scratch 3 006 jr scratch 3 007

 

Where you're dealing with a part finished object (e.g. you've made a box but some of the edges protrude more than they should, although a sanding block will work fine, another alternative is a sheet of wet & dry held on a flat surface & rub the box on that. Though in that case you do have to be aware that heat can built up to the melting point of the styrene quite quickly, especially with coarser grades of paper.

For final sanding (i.e. after you've fettled any edges & used filler), I'd suggest this is best done wet with a little bit of washing up detergent to break the surface tension - start with a rough grade of wet & dry paper to get the worst off (e.g. around 120 grit) with a back & forth motion (turn regularly so the sanding is even, then use a medium grade (c.400 grit) to get the worst off the scratches off (use an oval motion), then finish off with a finer grade (e.g. 1200).

The best place to this is right next to the sink, but consider getting a couple of sacrificial cloths and/or a chopping board to only be used for this purpose. Squeeze a tiny amount of washing up into a small dish - you only really ever need one a drop at a time.

jr scratch 3 008

 

Sanding for Shape

This is pretty much as above, but sometimes the corners need to be rounded: basically I'm talking about heavily radiusing some of the edges.

Having sanded each face flat and starting with your rough grade of wet & dry paper (e.g. 120) pick one edge and start with that at the top of the sheet, with the box near horizontal. As you draw the box down the sheet, rotate it so the edge is always fully in contact with the abrasive, but ends up nearly vertical at the bottom of the stroke.

Rotate the box through 180 degrees so it sands evenly, and repeat.

Repeat until the desired radius is achieved, then repeat for the other edges that need the same treatment. If you also need finish the points where radiused edges meet, this is best done very carefully with the wet & dry paper in one hand & the object in the other.

jr scratch 3 009

 

On to the worked examples:

#1: Basic Rectangular Box (thin walls, with internal bracing)

50mm x 35mm sounds about right for the key dimensions for this first box, so we can cut the top and bottom out that size from 1mm sheet.

To keep to those dimensions for the sides, we need to take account of the thickness of the material, simply enough 35-1-1 = 33, so the two side pieces have to be 50mm x 33mm.

The same maths applies for the ends, they need to be 33mm x 33mm.

jr scratch 3 010

 

I started construction by fixing one side to the base (see image 11). Next, one of the ends goes on (image 12) , then it's time to start on the bracing (image 13). Bear in mind that hand cut narrow strip cut is rarely perfectly square, so pick the best angle to go against the sides. The other thing to remember is that you need to leave space for the other bits that need to go in later. The basic dimensions are 48mm for the long bits (four needed) and 28mm for the short (8 required) - but be prepared to adjust as you go.

Repeat for the other end & other side.

The top is slightly different in that the braces needs to be glued in before the top goes on (see image 13) - slightly proud is better than below the surface as the top can always be clamped or taped down while it sets.

Trim or sand off any really obvious discrepancies, but this one doesn't need any filler as it doesn't have to be particularly neat.

 

jr scratch 3 011 jr scratch 3 012
jr scratch 3 013 jr scratch 3 014
jr scratch 3 015  

 

#2: Basic Rectangular Box (mixed thickness walls, no bracing)

Next up - making a box with thicker side walls, so you don't have to do any bracing. Less fiddly, but heavier overall - not that it matters much on one box but it can mount up on a number, especially if they're going to be joined together to make a much more complex structure like a truck body.

But I digress ... I think that overall dimensions of 80mm x 25mm x 25mm for this one would look about right. The base & top can be cut out that (80mm x 25mm) size from 1mm sheet, the ends 23mm high (25-1-1) by 25mm wide, and the sides 23mm high x 76mm long (80-2-2).

As always, finding the most common dimension(s) & cutting strips of that (i.e. 25mm from 1mm sheet, 23mm from 2mm - see image 16) then cutting required lengths off that (see image 17) saves a bit of time & improves accuracy.

 

Construction starts with one end glued to the base, then one side. The other side and the other end should be fitted together to allow for tweaking of the angles if required (see image 18). Assuming the levels are correct (if not, a bit of sanding should fix it), glue the top on & leave to set.

The standard of finish needs to be a bit better on this one, so put a thin layer of filler on any gaps (Image 19), leave to set overnight, and wet sand back to flat (image 20).

jr scratch 3 016 jr scratch 3 017
jr scratch 3 018 jr scratch 3 019
jr scratch 3 020  

 

#3: Basic Rectangular Box (mostly thick walls, mostly braced)

The third & final rectangular box for this instalment is just a variation on the previous two - a thin top because it can be, thick sides and base and bracing because a lot of the corners need will be radiused.

Basic dimensions are 50mm x 30mm x 30mm - we can start by cutting out the base & top (from 2mm and 1mm sheet respectively) to that 50mm x 30mm size. For the sides and end, start with a strip 27mm wide (30-2-1) & cut off two pieces 30mm long for the ends, and two 46mm long for the sides.

Glue together as for #1. Bracing dimensions (2mm square strip) are 48mm for the long lower edges (2 needed), 22mm for the short lowers (2 needed) and 25mm for the upright pieces (4 needed).

This will need a good surface finish (and invisible joints) so fill any problems and sand back.

This box has been constructed to have a flat top, but rounded edges on the base and sides. Refer back to "Sanding for Shape", near the start of this article.

jr scratch 3 021 jr scratch 3 022 jr scratch 3 023

 

#4: Basic Cylinder (low/wide)

Boxes don't have to be rectangular - they can had a round top & bottom (like a hat box). In which case they're really a cylinder ...

This one needs to be fairly low & wide - 88mm diameter by 34mm thick seem like decent enough dimensions, so we'll work from that.

You could use a compass cutter, but to be honest it's often easier to find something of a suitable diameter to draw around - the top lip of a can of tuna is 86mm, so by the time I've drawn a pencil line around it & cut the styrene sheet back to it (and put sides on), it'll be close enough.

Cut/dremel/sand two of the 86mm circles from 2mm styrene sheet for the front & back, then cut two 34mm wide strips from thin styrene sheet. These strips need to be at least 280mm to allow some margin for error (piD = 3.142 X 86mm = 270mm). You could use 0.25mm thick sheet, but the diameter they're going round is pretty wide so 0.5mm sheet is probably a better choice.

Start by rolling the first layer around something small & round (like an AA battery) to put a bit of a curve on it. Glue one end of the strip to one of the discs (image 27), and use a bit of masking tape to hold it while it sets (note use of a metal offcut to avoid possible staining from the cutting mat).

Glue the lower disc about half way round (image 28), then start on the top. Glue most of the way round & allow to set (image 29). Mark where the side strip needs to end, cut & then glue down.

Repeat for the second layer - making sure the joint is offset from the first layer (image 30). Fill & sand back the edges, including radiusing them. Obviously the technique described under "Sanding For Shape" will have to be adapted a little.

jr scratch 3 024 jr scratch 3 025 jr scratch 3 026
jr scratch 3 027  jr scratch 3 028  jr scratch 3 029
 jr scratch 3 030  jr scratch 3 031  jr scratch 3 032

 

#5: Basic Cylinder (tall/thin)

Next up, a narrower/taller cylinder. Final dimensions will be 88mm tall, 55mm outside diameter.
Construction is very much like one above, but the shape means it needs some internal bracing - not so much to stop it collapsing inwards, more to stop it bulging outwards. In this case a 0.25mm skin would be too thin, but 0.5mm doesn't bend so well into smaller diameters.

The ends & bracing are four identical discs cut from 2mm sheet, with an outside diameter of 53mm - which just happens to be what you get when you draw around the bottom of a can of Tamiya PS paint (image 33).

The sides are two 88mm X 175mm rectangles cut from 0. 5mm sheet - draw lines 1/3rd and 2/3rds of the way up the long sides of one of the panels as guides for the braces (image 34), then roll them around something suitable (a marker pen?) to give yourself a head start on the curve required.

Start by gluing one end to the first disc (using a bit of tape to hold it in place), then glue about halfway round (image 35).
At this point, the bracing discs need to go in (gluing halfway round), following the lines drawn on earlier (image 36).

The top should be inset by a couple of millimetres - glue that in halfway round too.

Apply regular Poly glue (so it won't evaporate immediately) to the unglued portions of the bracing discs, then hold the "skin" down with tape. Apply a little plastic weld/ DCM to the outer edges of the ends - just not all the way round, you need a bit of wiggle room later (image 37).

Once set, trim off the excess outer skin & glue down (image 38). You can see I made a bit of a hash of it here (image 39) - but it didn't matter so much as the second layer was yet to go on.

Start the gluing with one edge (image 40) & work round. Once you get about half way you should start thinking about the other edge - reduce it a little at a time until it's the correct length to butt up against itself.

The second skin will bulge in the middle at the point it meets itself, glue a short section at a time and hold down with tape (image 41).

Once set, fill the edges & joint (image 42). Sand back as above (image 43).

jr scratch 3 033 jr scratch 3 034 jr scratch 3 035
jr scratch 3 036 jr scratch 3 037 jr scratch 3 038
jr scratch 3 039 jr scratch 3 040  jr scratch 3 041
 jr scratch 3 042  jr scratch 3 043  

 


#6: Basic Oval/ Ellipse

I wanted to include something oval in this article so I could include drawing an ellipse (and show glossy, wood effect paint in a later instalment).

First, drawing the outline. You could just use the templates in the plans, but this is the method in case you want to draw your own:

  • Take a bit of thin card & mark lines half way across each side.
  • I thought an overall length (major axis) of 240mm and a width (minor axis) of 60mm would look about right - so marked two points (A and B) along the long axis, each 120mm from the centre -and two points (C and D) 30mm from the centre on the vertical axis.
  • We could use some trigonometry to work out where the focus points are - but it can be done a lot more simply. Measure half the major axis (240mm / 2 = 120mm) from point C to the AB line near point B, then from point C to the AB line near point A. These are the focus points of the ellipse - I labelled the new points F1 and F2 for clarity.
  • Next, take a length of string & put a pin though it and through to point B. Stretch the string along the AB line, and pin though the string and the point A. Keeping the pins through the string, move the pins to the F1 and F2 points - this will give a bit of slack in the string.
  • Stretching the string to the same degree as before, use a pencil to mark the upper side of the ellipse. It may take a few tries to get a nice arc, and depending on how thick and/or flexible the string is, it may not be possible to get that close to the pointy ends of the ellipse. Repeat for the bottom line, and take an average when it comes to cutting out your template.

Note: the string I used wasn't really suitable (too thick and stretchy) so I had to repeat the process to get an oval of the dimensions stated at the start of this section. Note also my error in measurement in the photo sequence. Also refer to http://www.mathopenref.com/printellipse1.html - this is the same method, but with more diagrams :)

jr scratch 3 044 jr scratch 3 045
jr scratch 3 046 jr scratch 3 047
jr scratch 3 048 jr scratch 3 049

 

Cut three of the shapes out from 2mm sheet, then form a slight curve on them by bending them around something round - I used the tuna can again. Glue all three together and leave to set - I used a pencil under each end, a small block of wood over the middle, and three tins of beans to weight it all down.

Heavily radius the top edges. The underside edges should have even more material removed, especially at the points. I also made one end a bit more blunt. There's a lot of material to remove, and while it's theoretically possible to do it all with wet & dry paper, I was glad to have a Dremel to do the bulk of the work.

jr scratch 3 050 jr scratch 3 051 
 jr scratch 3 052  jr scratch 3 053
 jr scratch 3 054  jr scratch 3 055

 

#7: Layers of Material

I've included this to illustrate a point: sometimes a box shape doesn't have a void (or it's not practical to make it that way), and the best way to achieve a solid "box" is by gluing layers together.

Note: this would have had greater impact it I hadn't just shown the same thing in the section above - just another effect of writing things out of sequence (I wrote this section first & still wanted to included, even if it had become a bit redundant).

I've used https://upload.wikimedia.org/wikipedia/commons/thumb/3/30/Plan_palette-europe.svg/2000px-Plan_palette-europe.svg.png for the dimensions - and looking at that, the height of the 1:1 planks that make up the top and bottom layers of a Euro spec pallet is 22 (mm). Now we could achieve a pretty accurate 1:10 scale height by adding a 0.25mm layer to a 2mm piece (2.25mm vs "correct" 2.2mm), but I'd suggest that is going to pernickity lengths, and that just using 2mm sheet is accurate enough.

jr scratch 3 056

 

Looking at the gap between the top & bottom, the 78mm 1:1 dimension can be accurately enough represented by four layers of 2mm sheet (8mm vs a "correct" 1:10 scale 7.8mm)- this is good as it means we just have to cut a number of 10mm and 14mm wide strips from a single sheet.

My part list then:

  • Top: two 10mm x 120mm strips, three 14mm x 120mm strips, plus three 14mm x 80mm strips;
  • Bottom: two 10mm x 120mm strips, one 14mm x 120mm strip;
  • spacers: front & back rows: 14mm x 10mm (so six pillars X four layers: 24 pieces; middle row: 14mm x 14mm (three X four = 12).

Some rudimentary maths later, it's apparent that this requires three 10mm wide strips and three 14mm wide strips from an "A4" sheet (324mm on the long side). Note: 5 minutes of juggling shows that it can't be done in less than six strips, although who knows if more time spent on the maths would make it more efficient in terms of material use ... though not necessarily time.

jr scratch 3 057

 

Sand off any ridges on the strip edges, and cut to the required lengths. Using the grid marked on the cutting mat and/or an engineer's square, glue the five long strips to the shorter ones to make up the top layer (images 58, 59, 60).

Make the spacers with four layers of each size of the small pieces. There's always some variation in sizes, and using a square against a flat surface will ensure three of the six faces are level (image 62).
Once the glue has set, trim/dremel/sand the worst of any discrepancies off the spacers (image 63). Remember, it's only a pallet & a certain roughness of finish is not out of place.

Glue the appropriate spacers at each end of the bottom runners, and the remaining spacers to the central crossmember on the underside of the top (image 64). Glue the top section to the bottom planks to complete the pallet (image 65).

jr scratch 3 058 jr scratch 3 059 
 jr scratch 3 060  jr scratch 3 061
 jr scratch 3 062  jr scratch 3 063
 jr scratch 3 064  jr scratch 3 065

 

Downloads:

Plan 1 (right-click and choose "Save link as..")

Plan 2 (right-click and choose "Save link as..")

 

Next Time ...

I know some people dislike cliff-hangers, but this article is long enough as it is, and I think I've given you enough homework to be getting on with ...

Next time we'll cover adding simple details to turn these basic shapes into recognisable scale objects - although I think some of them will be easy to guess already ;)

 ________________________
Written by TB member Jonny Retro

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