From Part II I determined that I needed two cowl ventilators. One with a cowl having a .874 in. dia. and one with a cowl having a .712 in. dia. (rounded up from .711) I say .. “determined” .. meaning that this was ‘about’ the size that looked right.
The trunk is 1/2 the cowl dia. Since I want to use Evergreen tubing for the trunk extensions .. let’s see what is close.
.712/2 = .356 | Evergreeen #231 has a .344 O.D. and a .288 I.D.
.344 x 2 = .688
Engine Room Hatch Cowl Ventilator : Cowl 0.688 O.D. and Trunk 0.344 O.D./0.288 I.D.
The reason for including the I.D. of the tubing will be apparent in a bit.
We have to pause for a bit and talk about what to do next. We need to turn this primative into a Cowl Ventilator .. ok. Sure. We will be creating essentially a reducing elbow .. a tube that bends and gets smaller. A tube has walls .. and so will our model. That brings up .. how thick can/should the walls be?
I am going to be referring to Shapeways. This is the 3D printing service that I use so I am bound by the materials that they can print and the resolution of the printers they use. There are 3D printing services that can print in a greater resolution than what Shapeways can .. but that is outside the area of the hobbiest and into bumping shoulders with business. The greater cost reflects that. If you have recently won the lottery then go for it!
Shapeways has an assortment of Materials they can print your model in. That link will bring you to their Material Portfolio. You can navigate through the different materials where you can find the pros and cons of each.
For detail modeling though such as we will be doing with this Cowl Ventilator the choices (IMO .. everyone has one I know) .. your choice is limited to FUD (Frosted Ultra Detail) for a product you can use in your modeling. This has to do with the resolution, surface texture and such. They also have a wax suitable for casting that will work as the resolution is comparable to FUD – but for now let’s stick to something that you can pull from the box and paint and use for your modeling.
FUD/Frosted Ultra Detail has a Minimum Detail of .1mm/.0039 in. .. this is the minimum resolution of the 3D printer. The printer works much like a ink jet printer in that a print head squirts an acrylic polymer on a surface creating a single layer of your model. UV light hardens that layer and then the print table drops a fraction and the process repeats. The Minimum Wall thickness is .3mm/.0118 in. .. this is physically the smallest that the wall will support itself. The Materials Page for FUD goes into more detail .. for example an ‘unsupported wall’ .. one that is connected to other walls on less than two sides has a minimum 0.6mm thickness. There are other Guidelines dealing with supported and un-supported wires, minimum print size, the bounding box (maximum print dimensions) and so on.
Note: There is a $5 handling charge for each mesh printed along with a $6.50 Flat Rate Shipping here in the US. FUD itself costs $3.49 per . If we were to have one ventilator printed the handling and shipping would exceed the cost of the FUD itself. If you ordered 10 ea. you would be charged that $5 handling charge for each separate print so even before the materials cost you would be paying a $50 handling plus the $6.50 shipping. The answer is to duplicate the ventilator 9 times so there would be 10 in the mesh. That way you get charged for handling only once. In fact .. with anything printed at Shapeways this is a way to amortize the cost. One item and you are paying the full $5 handling charge. 10 items and now you are paying $.50 each. 100 and it drops to $.005 .. this is limited by the total cost since the material cost of FUD quickly rises at the number of pieces rises. Well duh! You also have to consider the maximum print size .. the bounding box .. that the printer can print. For FUD it is 5 x 7 x 6 inches. You print that much FUD and again .. you better have won the lottery!
I went into detail about FUD since the next step will be to create the cowl ventilator using that material so we need to determine what thickness to make the walls. FUD as I stated has a Minimum Wall Thickness of .3mm/.0118 in. .. but I am not going with that.
It WILL print but FUD is brittle enough that the minimum wall thickness is problematic. It is better to go with a thicker wall and thin just the visible edges. With that in mind I am going to go with a 0.020 in. wall thickness and at the end thin the visible edges of the cowl to the minimum .012 in.
Note: I am working in O scale. If I were to do this in N scale I would make an exception and use the .012 in. minimum wall thickness. Even that would be almost two inches full size but that can visually be hidden by carefully sanding the visible edges. Military Modelers have used this successfully for years to hide the oversize thickness of fenders on armored vehicles.