December 9, 2016 at 12:17 pm #1369MicroTraxxKeymaster
Why Evergreen? : When we are designing a model to be printed one of the things that is important for me is to save as much money as possible. It is expensive in any case and combining the printed parts with a commercial product like Evergreen styrene makes sense – both from a cost standpoint and also for strength.
In this specific case we are creating pipe fittings as needed – elbows, flanges etc. The vast majority of the piping though will be straight pipe so using the readily available Evergreen tubing and rod just makes sense. Why Evergreen and not another brand or material? Ahh. Good question.
Connecting : The pipe fittings need a way to connect .. to glue .. to the material we will use to represent pipe. Simply gluing to the end of a tube, or rod will work in theory but in practice will be problematic as it would break off fairly easy. There is also a problem with alignment. There are a couple of ways to get around this:
- Stud: Create a stud on the fitting that will slip into tubing. Again, this allows alignment of fitting and pipe and provides a secure gluing surface.
- Socket: Create a socket in the fitting. A rod then slips into the cavity and then it automatically aligns the fitting and pipe. The socket also allows a sturdy gluing surface. Win win situation.
Theory vrs. reality. When designing these relatively small parts to be printed you have to constantly juggle what looks best against what will actually print. There are advantages and disadvantages to using a stud or socket to connect fittings and pipe.
Stud: Here is a 90° elbow that I made to fit 1/8″ Evergreen tubing (click the image for a larger version).
Evergreen 1/8″ tubing – Part No. 224 has a nominal O.D. of .125″/3,2 mm and a nominal I.D. of 0.69″/1,8 mm. Notice I used ‘nominal’ twice .. that was on purpose. I believe that the Evergreen styrene shapes are extruded from a die. That means that if I say that No. 224 tubing has an I.D. of 0.69″/1,8 mm that dimension is ‘nominal’ and can vary a few thousands. That is something that we just have to live with and take into account.
Famous last words and all that .. but if you look at the O.D. of the stud on the elbow it is 1,8 mm ..exactly the same as the nominal I.D. of the Evergreen tubing. This is an example of “lessons learned”. Since the I.D. is nominal it can vary a couple thousandths of an inch around that 1,8 mm so sometimes the elbow’s stud will slide into the tubing .. and sometimes I won’t. The answer is to drop that stud O.D. from 1,8 mm to 1,7 mm. That .1 mm is about .004″ and gives us enough clearance to overcome that ‘nominal thing’.
Take another look at the elbow. I show a hole 1,16 mm dia in the end of the stud. This is because the elbow is hollow. While a solid elbow would be much stronger .. it will cost a lot more also .. perhaps three times as much. Currently as I write this, FUD runs $3.49 cm3 .. so it makes a difference. Now the problem is that if I reduce the stud diameter from 1,8 mm to 1,7 mm I also have to reduce the hole dia. The minimum wall thickness for FUD is 0,3 mm. I made the one here 0.32 mm and while it worked it was really too thin and fragile. It really should be at least 0,4 mm at minimum and 0.7 mm if at all possible. If I do that .. using a 0,7 mm wall thickness then we would have a stud O.D. of 1,7 mm .. hole dia of 1,7 – 2x 0,7 mmm = 0.3 mm. That in turn causes a problem in that this hole acts as an ‘Escape hole’ for the support wax used during printing. Shapeways requires a minimum escape hole of 0.4 mm. Okaaay .. so now we run backwards a bit and subtract that 0,4 mm minimum escape hole from our 1,7 mm stud dia to get 1,3 mm .. so now our wall needs to be 0,65 mm.
Socket : Flipping over and creating a socket instead of a stud creates it’s own problems.
Here is an elbow again using the Evergreen 1/8″ tubing. I made the socket 3.4 mm dia so we have a 0,2 mm clearance for the nominal 3,2 mm tubing O.3. That is about .008″ .. perhaps a bit much but I did this for the Rusty Stumps Backwood Water Tank kit and figured erroring on ‘bit much’ would be better than not doing that and the tubing not side easily into the elbow.
The walls are 0,4 mmw which kicks the collar O.D. to 4,2 mm. The socket is 1,6 mm deep – which is about 1/16″ – you need SOME depth for the socket. There has to be a back wall to that socket so we end up with a collar width of 2 mm (just say 2 mm. That .032 mm excess is negligible .. an artifact of the CAD being OCD)
Point is .. just drawing up something doesn’t mean it can be printed. You have to constantly juggle the numbers.
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