I posted some pics of my 3d printed 1:48 Ore bin chutes/gates the other day. Ray Dunakin asked if I could scale them up to 1:24. That .. CAN .. be done of course but I got to thinking about it and thought it would be worthwhile to show the problems/constraints etc. this process engenders. This will by the way be the most "geeky" part of this thread .. at least I hope so.

**-- Sketchup --**Ok. I'm not going to go anywhere near the "this is better than that" thing. I use Sketchup .. and will address the problems that can create .. and the workarounds in a bit. I am hoping the discussion can focus on the modeling while letting what software one uses be simply their choice.

Sketchup was created by @Last Software with version 1 released in 2000 as a modeling tool for “Architects, designers, and filmmakers”. The part about ‘Architects’ may have something to do with a couple of the problematic issues with the program – at least as far as it relates to those of us using the program to model scale objects.

**— Problem —**The program refuses to create a radius below approximately .018″ – which means the smallest hole you can create therefore is .036″. To give an idea what this means .. here is what that .036″ is in various scales:

- HO scale – 3.132″
- O scale – 1.738″
- 1:24 – 0.864″

Obviously, those of us who use Sketchup for scale modeling will have a problem with this.

This same restriction comes into play when re-sizing objects as you can get glitches where you have missing polygons when resizing curves. Sketchup can throw a fit sometimes when resizing a spherical surface.

So .. what to do?

One way would be to re-size a model up x 100 .. and then re-size back down x 100 when finished. This can work .. probably works *most* of the time but not all. You can still run into problems when that reduction re-size drops radii below that 0.018" radius.

**-- Pmm --**My own solution is to use what I term .. Pseudo Millimeter .. Pmm modeling. Ummmm. How to explain so it makes sense .. let’s try this …

Let’s take a cube which full-size is 1ft x 1ft x 1ft. We also want to punch a 1″ dia hole though it

- In HO scale this cube would be 0.138″ x 0.138″ x 0.138″ with a 0.012″ hole
- In O scale this cube would be 0.250″ x 0.250″ x 0.250″ with a 0.021″ hole
- n 1:24 scale this cube would be 0.5″ x 0.5″ x 0.5″ with a 0.042″ hole

Note that both HO scale and O scale the holes are smaller than 0.036″ dia. To model these cubes we would have to go through re-sizing up and then back once finished.

My Pmm method not only bypasses this problem it also allows working directly in scale .. Simply ..

I set the units to Decimal Inches but model as if I were working in mm.

sooo .. back to the cubes …

In HO the cube is 3.5 x 3.5 x 3.5 with a 0.3 hole

In O the cube is 6.35 x 6.35 x 6.35 with a 0.533 hole

In 1:24 the cube is 12.7 x 12.7 x 12.7 with a 1.07 hole

Ignore the ” .. indicating inches .. or simply uncheck the [ ] Display units format in Model Info.

Happy days .. that 0.036″ minimum hole dia is now .. ummm .. 0.00014″

In addition we can model directly .. as it were .. to scale. Example, for WSF the minimum wall thickness is .7mm .. so we can measure directly on the model to ensure that any all is equal to or greater than that .7 mm minimum. In reality .. I keep to a .8mm minimum for WSF and a .4mm for FUD (even though the stated minimum is .3mm) .. makes all the difference. I have found that using minimums causes unending pain to the modeler. Just my 2¢ – ignore it at your own peril.

- In HO that 0.138″ is 3.5mm (ok .. 3.5052 but .. that extra bit is not worth worrying about)
- In O that 0.250″ is 6.35mm
- n 1:24 that 0.5″ is 12.7mm

**-- Exporting for printing --**If I design a cube 25.4 on a side and export as a DAE file, I can then import it into MeshLab do what 3D stuff I need to do and export a STL file which will contain a cube 25.4 UNITS on a side. I upload to Shapeways and simply tell them that the model is in millimeters .. and the model is exactly the size I need it to be.