3D printing Interior of ball turret for B-17 G Fearless Fosdick...

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Night Fighter Nut

Master Sergeant
Nov 3, 2009
Plano Texas
As I continue to work with this FreeCad, I'm slowly improving and figuring out shortcuts. Last night I spent the evening working on the traverse mechanism. It's crude but with some refinements it might come out ok. What do you guys think?
turret progress 5.jpg

I found that as long as an assembly is active, you can draw and buildup any flat surface in any plain.

Thanks for following.

Night Fighter Nut

Master Sergeant
Nov 3, 2009
Plano Texas
Ok, time for me to close this up. At this point, I have printed all that I can for the ball turret. What I will do now is walk you through the process I go through when I am developing and printing a part. In this case the seat.

The first thing I do is research as much as possible the subject I want to make. I try to determine, to the best of my ability, the size of the part I need. This is not the actual size on the real thing but the size of what I will need that will work. In the case of the seat, I looked at several photos and tried to determine how wide and how high it is. I did this by measuring the door width on the model and what would be visible through the window. Then I started creating it in FreeCAD on the XY plain.


When all dimensional variables are solved, the drawing turns green as you can see above. From here I extrude the drawing giving it a third dimension.


I chose a surface on the now 3d seat to now create the back of the seat. The curve was made to match the circumference of the inside of the ball. The process for making a curved back part was to start with a curve then extrude alone a curved path instead of a straight one.


The drawing for this path is perpendicular to the surface I'm drawing away from and is curved the match, again, the inner circumference of the ball... on the YZ plain.


The result of the two drawings looks like this. Now with the drawing fully defined and extruded, its time to turn this file into an STL file.


For this part you can't just save the file as is. You have to export it into an STL file so that the slicing software can interpret it.


The object must be fully highlighted as seen about before it can be saved as an STL file. Now I open a slicer program and figure out how best to situate the object in order for it to print properly.


Since there is nothing really fancy about this drawing I decide to print it as you see above and proceed to slice the object into something the printer can recognize.


The printer has its own slicing software so I had to convert what I had to what my particular printer would recognize. I also wanted to add the trunnions at the same time. The total time to print is just over an hour.


This is what it looks like after its been sliced. The file is then plugged into the printer and printing starts. Below is how it came out after it was cleaned up and set in the sun for a few hours to finish curing.


A test fit of the trunnions found that they fit well enough. Snug but not too tight. I'm still able to rotate the ball on the trunnions without too much interference.


I found that the seat was a little large. Still I can just sand it down to what I need.


I printed extra items just incase I break one or two. The parts are a little flimsy, and dare I say fiddly.


A couple of quick notes. I mentioned earlier about adding a special oil (PTFE) onto the film in the vat before adding the resin. I found that this same oil added to the surface of the print face helps to remove parts once printing is completed. Only like a drop spread over the entire plate surface... just a hint of oil. The light exposed resin still sticks but it seems easier for me to remove the part.

The other note is to remember that making a part to scale isn't always a good idea. Very thin resin parts are brittle.

I will be scratch building other fiddly bits inside the ball but for all intents and purposes the ball interior and immediate exterior is completed. I will need to scratch build the rest of the support system. Once the ball is finished, I'll add a few pictures here. I will now go back to the other thread and start updating it. My goal is to try to finish this plane by 11 November. Not sure if I'll make it but I will try.

Thanks for following.
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