O scale 24″ elbow

HO elbow
Since I originally created this elbow in HO scale it seems reasonable to discuss it a bit before re-creating it in O scale.

This is a solid elbow and I am not going to get into the actual process of creating it .. that’s simply application of CAD design. Instead I want to focus on the “why” of the dimensions – keeping in mind that these are mostly specific to the HO version.

This represents a Class 150 24-inch elbow.

  • Flange Diameter : 9.33 mm. This represents a flange measuring 9.33 x 87 = 811.71 mm / 32″. This was taken from this page. The bolt circle, bolt size and so on were also obtained in the same way.
  • Flange Thicknesss : The link shows the flange thickness to be 1.88″. In HO this would be 0.55 mm. I made mine 0.8 mm – the 0.25 mm difference is about 0.010″ .. mostly so the flange would not break during handling (FUD is brittle).
  • Stud Diameter : 5.6 mm. This stud fits into the I.D. of Evergreen #229 9/32″ tubing. This tubing has a nominal I.D. of 0.225″/5,7 mm. that .1 mm difference lets the stud slide easily into the tubing.
  • Stud Length : 2 mm. This is an arbitrary length to some extent – it has to be long enough to fit into the pipe to align fitting and pipe for gluing.
O elbow after scaling from HO
The reason I started with the HO elbow is simply that it can easily be scaled up to the O scale version. I mean .. why reinvent the wheel .. as it were.

The first thing to look at is what size tubing to use for the pipe. In O scale, Evergreen #236 1/2″ tubing is exactly the right size O.D. The I.D. is 0.444″ / 11,3 mm. That means the stud will need to be 11,2 mm dia. and can remain the same length (again .. it is arbitrary) or not. The Flange dia will be scaled up. The thickness will be adjusted as will the gap between the flanges.

let’s look at the numbers …

  • Flange Diameter : 16.93 mm. Punch those numbers though the calculator and we see that it is 31.9937″ scaled to full size … umm .. 32″ .. ok? So that is good to go.
  • Flange Thickness : 1.45 mm. That would be 2.74 ” full size. This needs to be thinned a bit. The full size flange is 1.88″ which would be 0.995 mm . ok .. 1 mm.
  • Stud Diameter : 10.16 mm .. needs to be increased to 11,2 mm.
  • Stud Length : scaling made it 3.63 mm. I said it was arbitrary – so I arbitrarily increased it to 3 mm long
O elbow
With all of the corrections this is what we have. It looks a bit different . not much .. but a different. The main thing that jumps out is the to scale flange thickness. The grove between the two flanges is still .1 mm wide and deep (minimum ‘engraved detail’ for Frosted Detail plastics) so it looks better. Finally the stud is larger than when scaled .. the ratio of O.D. to I.D. for Evergreen tubing changes as the tubing gets larger and is a bit longer as it seemed like a good idea.
The next thing .. which is very important unless you have $ like The Donald is to hollow out the part. Currently FD plastics cost you $3.49 Cm3 .. so for those of us who are not billionaires .. it is important. FD plastics require a minimum wall thickness of 0.3 mm .. and that isn’t enough. Minimum doesn’t equal ‘what works’ for two reasons .. 1) you will get errors trying to get a file to upload to Shapeways with a 0.3 mm wall thickness .. I suspect it has something to do with rounding errors in the software. Even bumping up that to 0.32 mm makes a difference .. and .. 2) it has to survive in a real world. A wall thickness of 0.3 mm is just to thin and is like trying to handle an eggshell made of rubber. For a curved object like these elbows 0.4 mm will work but I would bump the stud that sticks out up to 0.7 mm .. that has to fit inside the Evergreen tubing and will take some stress. All of this isn’t based on any scientific investigation but on “stuff I’ve learned” .. and “what works” .. so is subject to change as I do more printing.
Final Results?
I uploaded a single elbow to Shapeways:

Cost: FUD $7.33 – FXD $9.01 – From that you have to understand that $5 is for ‘handling’. If you have one elbow printed or 100 .. you have the $5 handling charge. I normally add a 10% markup so the cost for FUD would jump to $7.33 + $.73 = $8.06. Shipping is a flat rate $5. That means ordering ONE elbow in FUD would be $8.06 + $5 = $13.06 .. OUCH!!

The way to get around that is to order in quantity. If you had nine friends (optimistic) who each wanted 10 of these as you did and then you had me put 100 elbows into a mesh the price changes quite a bit. For FUD then you would have a cost of $238.00 + $5 shipping for a grand total of $243.00 .. which means that now the cost per part drops to $2.43 each

Is there a better way? Yep. Rework the elbow so it is in two parts .. like you would get in a plastic kit complete with locating pins/holes .. and use these as masters to make an RTV mold and then cast the things in resin.

There is a famous saying .. “There’s no such thing as a free lunch“. That still applies!