There is a sub-page to this one .. British O Scale 10″Flanged Elbows


I was asked by a Gentleman (name withheld unless he gives me permission to use his name) to create flanged elbows for a project he is working on. It shouldn’t have been a surprise to discover that the US, UK and Australia all use a different system. That means I need to figure out the dimensions I need before even starting to do CAD.

Pipe Sizing

Be careful when specifying Nominal Pipe Sizes. They could either be USA sizes, or BSP (British Standard Pipe) sizes which are similar to but NOT the same. Generally, when a thread specification is called out on a pipe, it will determine which system is needed. However, this is not an absolute. Many of the pipe sizes will support either the BSP or USA (NPT) threads.1 .. a quick look at the chart shows that the pipe is listed under “Nominal Pipe Sizein inches in both BSP and USA sizes. A couple of examples – 10 in Nominal Pipe Dia is 273 mm/10.75″ under both BSP and USA sizes. 12 in Nominal Pipe Dia is 323.9 mm/12.75″ under the BSP sizing and 323.8 mm/12.75″

Looking closer I find that the discrepancies is simply rounding of numbers – as far as we modelers are concerned.


For the most part Australian flanges (PN 20, PN 50 ….) are “designed to be interchangeable” with US flanges. I am only concerned with the US Class 150/PN 20. This is the lower end and the simplest for me to design a model for .. and since it is after all a model I can ignore everything else.

This is a work in progress .. for the moment .. some links
Diameters and bolt circles for standard ASME B16.5 flanges – 1/4 to 24 inches – Class 150 to 2500 This does not have the flange thickness and only goes up to a 24″ flange.
Steel Flange Dimensions We have the flange thickness in this chart. For 3-1/2″ and smaller pipe there are two numbers but since this is (for me) for scale modeling I simply go with the larger (has to do with the raised face I think)

British Standard Pipe Flanges Table D From what I can tell at a quick glance this is it’s on set of standards .. for “Working Steam Pressure up to 50 lbs PSI” .. an example being the flange for 10″ pipe has 8 bolts compared to 12 on the US flange for Class 150.

In the attached graphic I have highlighted the 10″ pipe & flanges for Class 150, BS-10 Tables D & E. The first and last one are so close that I think I can comfortably model either .. or I could say .. the difference is negligible. The BS-10 Table D is a different matter and would I suppose be obvious to someone with a ‘clue’. Happily that’s not me!

Evergreen #228 Tubing

I was asked to create flanged elbows to fit Evergreen #228 tubing …

  • .250″/6,3mm OD
  • .194″/4,9mm ID

The important numbers here are the OD – .250″/6,3mm. This is what we see and what will represent our pipe. The ID doesn’t come into it other than finding the correct size for a stud on the end of the flanged fitting that will fit the ID of the tubing.

One thing to note here is that Evergreen tubing is ‘Nominal”. I don’t know for sure but I would not be surprised if it were manufactured similar to how pasta is made with the hot plastic squeezed through a nozzle. My ‘solution’ to this is to run the appropriate drill bit through the tubing before gluing on a flanged fitting. The stud on the fitting will be slightly under the nominal ID of the tubing – running a drill bit through will insure a proper fit.

Well . .says you .. and what drill would that be? The nominal ID of Evergreen #228 tubing is .194″/4,9mm …

  • 13/64 = 0.203″/5.16mm – this would ream out the tubing 0.009″/0.23mm over the nominal ID. a bit loose for my taste but will work
  • #10 = 0.1935″/4.95mm – perfect
  • 4.9mm = well .. sure!

So .. what pipe is it then?

The client lives in the UK and models in British O scale which is 7mm to the foot or 1:45.5 so the pipe is .250/43.5=10.875". Nominal 10″ Pipe Dia is 10.75″ so using the #228 Evergreen tubing we would only be over a nominal 10* pipe by 1/8″.

In the US O scale is 1/4″ to the foot or 1:48 so the pipe is .250/48=12". Nominal 12″ Pipe Dia is 12.75″ which means our tubing would be 3/4″ undersized. Honestly .. I doubt that anyone other than someone with aggressive OCD would notice. JMO.

British Pipe Flange Dimensions per Table BS-1- Table-D – Full Size

In the link there are four tables 2 for different working pressures: The first table,”Up to 50 lbs per Sq. Inch”. is the one I am going with as it has the smallest flange. Since I am making this up on the fly .. why not?

From the table we get the following flange dimensions for nominal 10″ pipe:

  • OD of Pipe: 10-3/4″/273mm
  • Dia of Flange: 16″/406.4mm
  • No. of bolts: 8
  • Bolt Circle Dia: 14″/355.6mm
  • Dia. of bolt: 3/4″/19mm
  • Thickness: 5/8″/15.875mm

Finally a 3/4″ bolt has a 1-1/8″/28.6mm hex head about .46″/11.9mm high.

Flange Dimensions – British O Scale

  • OD of Pipe: .247″/6.28mm (the Evergreen tubing is oversized by .003″/.08mm)
  • Dia of Flange: 0.368″/9.34mm
  • No. of bolts: 8
  • Bolt Circle Dia: 0.322″/8.17mm
  • Dia. of bolt: 0.017″/0.44mm
  • Thickness: 0.014″/0.365mm

Finally the bolt has a 0.029″/0.73mm hex head about 0.011″/0.27mm high. (I bumped this to 0.29mm later)

Pseudo mm

The following is for those who are doing CAD work, specifically using Sketchup to create scale models.

My CAD software is Sketchup and for the most part it works fine for everything I need. The one problem I have had is that the program can have problems with small numbers. It was after all originally and is marketed for architects. When circles and arcs fall below 0.018″ it can get ‘wonky’. The usual method to resolve this is to say model at 100x size and when finished with the drawing resize down 100x. I use a variation of this .. working at a larger scale which works for me.

  1. Let’s use a simple cube as a model – one that is .1″ x .1″ x .1″. If we were wanting to poke holes through this cube in our drawing … or add bolt heads we will run into problems below the aforementioned 0.018″. The solution is my Pseudo mm .. PMM.
  2. The first thing is to set/verify that I have my Sketchup set to Decimal Inches … Window–>Model Info. Ensure that the Format is Decimal and Inches. I also set the Precision to 0.000 and finally turn off the “Display units format”
  3. Back to the cube. Convert those Imperial measurements (inches) to mm. Now it will measure 2.54mm x 2.54mm x 2.54mm
  4. Now in the Sketchup drawing we create the cube 25.4 x 25.4 x 25.4 .. we are still have Units set to Decimal Inches but we have the Units Format turned off so all we see are the numbers. This is our PMM .. it ‘looks’ like we are working in mm but actually it is inches. We are therefore working at 25.4 x the actual size and we can model happily away without any problems with that 0.018″ issue.
  5. Finally .. we do not have to convert back. The file is exported as a STL to upload to Shapeways or to a slicer if you are printing it yourself. A STL file does not contain units of measure .. only the numbers so the STL file just has 2.54 2.54 2.54 .. so .. you upload the file to Shapeways or send to your slicer and you simply tell the surface or your slicer that you are working in mm. Not only does this solve the small radii problem in Sketchup but your file will be printed exactly to what you wish

Flange Dim -> CAD (1:43.5)

Here are the various dimensions moved from text to CAD.

Note the 4.8mm stud dia. The stud is part of the fitting and slides into the tubing ID. I made it .1mm smaller than the nominal 4.9mm ID of the #228 Evergreen tubing. Theoretically it should fit fine .. but as I said the Evergreen dimensions are nominal and why I suggested reaming the tube. The plastic that Shapeways uses .. the Fine Detail plastic is brittle. Forcing the stud into the tube will lead to pieces snapping off .. ask me how I know this.

The minimum detail you can get with this plastic is 0.1mm .. so the 0.27mm height should be fine. The minimum supported wall thickness is 0.3mm so the 0.365mm flange thickness would be problematic if it were just a thin flange – Shapeways will kick it to the curb. The minimum unsupported wall thickness is 0.6mm … but I will address that later (and .. trying to hit that ‘minimum‘ often causes problems. I suspect it has to do with internal rounding of numbers at Shapeways. It is always better to exceed the minimum even if only be a small amount).

CAD – First Run

Using the data collected, let’s brew up some CAD. I’m going to talk about what I am doing here .. the good, the bad and the ugly.

At this point in the design .. this IS NOT meant to be anything like the final design. I am stepping through the design process .. some of the steps/errors would have been bypassed during the design but I wanted to show them … so ..

  • 6.3mm : The bit extending off of the back of the flange. This is the same OD as Evergreen #228 tubing and could represent part of a Tee, elbow etc. The 0.75mm wall thickness is a bit over twice the minimum wall thickness and combined with that it is a cylinder should be plenty strong.
  • 4.8mm rear : One problem with this ID is where it ends .. even with the back of the flange. A stress point is located at that position
  • 4.8mm front : This being the stud dia and made to be .1mm smaller than the nominal ID of the #228 tubing. Again .. the small 0.365mm area between the stud and the rear portion could be an area of stress and subject to snapping at this point. Just something to be aware of during design.
  • 3.6mm : This is the ID of the stud which gives a .6mm wall thickness. Again .. double the minium wall for this material and as it is part of a cylinder a bit stronger. This IS the point that can break off if the stud is forced into the #228 tubing without reaming with the drill bit.
  • 2.54mm : This is simply the length of the stud. The 2.54mm or .1″ should be plenty to align the fitting to the tube for gluing
  • 0.365mm : The flange thickness. Like I said earlier although this meets the guidelines for “min Supported Wall Thickness of 0.3mm” it fails the “min Unsupported Wall Thickness of 0.6mm” – the flange being .. well .. unsupported.

Intermission – Design Stuff – Details

I have put Hex bolt heads on the flange which are 0.73mm across the flat. The minimum detail is 0.1mm so figure that is like .. 7 ‘blobs’ of plastic to create that bolt head (the process uses a laser to harden a bit of polymer). If you look at it under a Macro lens it will be a .. blob vaguely hexagonal. Viewing with normal eyes it will look like a hexagon. Shapeways provides two detail plastics suitable for small models .. their Smooth Fine Detail and Smoothest Fine Detail. From what I remember, the X,Y for both is the same. The difference is the Z axis .. and if I remember correctly the Smooth Fine Detail with have a 25 micron layer height while the Smoothest Fine Detail a 16 micron layer height (something like 0.001″ and 0.0006″ or 0.025mm and 0.016mm respectively). The downside being the increased price for the finer detail. Personal resin printers are on par with at least the Smooth Fine Detail from Shapeways.

Intermission – Design Stuff – Flanges and Friends

I am trying to replicate the “look” of a flanged fitting such as an elbow bolted to a flanged pipe (Evergreen tubing). That means both flanges .. the one on the fitting and the one (appears) on the pipe (tubing). There should be a visual separation between the flanges.

Here we have both flanges separated by a .1mm x .1mm groove. This is as small .. minimum detail . as will print. Once the pipe is painted a dark wash will highlight this to visual separate the flanges.

Now .. the ‘wall’ is 0.83mm which exceeds the minimum unsupported wall thickness of 0.6mm .. and looking straight down I matched this where the two cylinders meet which will reduce the stress point.

This then should print fine without getting errors from Shapeways, be strong enough for ease of handling and fitting and modeling. I will use this as the ‘flanged portion’ of any fitting .. be it a Tee, or Elbow or anything else.

  1. MDmetric – BSP/USA pipe O.D. comparison chart[]
  2. Jain Steel Tube Co.. British Standard Pipe Flanges Table-D[]

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