I am going to skip ahead for a bit here. We are not ready to submit the file to Shapeways but I want to point out something important. Look at the screen to the left. I told Shapeways I was uploading test_export.stl .. and look then at the drop-down box under “Unit of Measure”.
The choices are – “Meters” .. “Inches” .. and .. “Millimeters”. This is an IMPORTANT detail.
STL files do NOT include units of measure. They are units. Let’s say you create a cube full scale and it is 12-in x 12-in x 12-in in Sketchup. You then export this file as a STL file. It is in UNITS. What I mean is that in the STL file .. it is 12 UNITS. Not inches, miles, meters, yards, rods, millimeters, fathoms .. whatever. UNITS. That is why Shapeways is asking me the Unit of Measure!
This is where we can use this to our advantage.
Let’s use that 12-in x 12-in x 12-in block. You save this as “Full Scale” to your hard-drive. Suppose then you want to print this in O scale and also in HO scale. In Sketchup you have the ability to scale objects. It’s really easy .. you simply use the Tape Measure Tool – measure a known length – and then type the NEW dimension in the ‘Length’ box and hit enter.
You can run into two different problems simply doing this – both connected to the Shapeways minimum radius thing. (1) If after you re-size your block – let’s suppose you want to create a stud on one face. If you re-sized the block to 1:48 then it only measures .25-in x .25-in x .25-in. That’s fine .. but .. suppose you want that stud to be 1 scale inch in dia? Sorry. You can’t as the radius would be below Sketchup’s minimum. Sure .. you can simply drop back to the saved full-scale version .. create the stud and then re-size that new object. The problem there is you are reverting back to the original. What if you have made changes? The other problem (2) is that Sketchup can get confused trying to reduce things like curves and intersections when they are below that magic 0.018-in radius.
What to do .. what to do?
The answer is .. don’t scale it to O scale or HO scale or whatever.
Take a look back at that 12-in x 12-in x 12-in cube. In O scale that is .. as I said .. .25-in x .25-in x .25-in.
Convert that to Millimeters. .25-in is 6.35-mm. Now .. that would be (In O scale) a cube measuring 6.35-mm x 6.35-mm x 6.35-mm .. right?
Take a copy of your full-scale cube. (ALWAYS use a copy, never the original). Scale one 12-in side to 6.35-INCHES.
The object now measures 6.35-in x 6.35-in x 6.35-in. If you need to make a stud .. make a hole .. round the corners .. it is plenty large enough that you will never run into the minimum radius problem.
When you export as a STL file it will be .. remember .. UNITs .. 6.35 x 6.35 x 6.35 UNITS. When you upload to Shapeways you simply tell it that the file is in MILLIMETERS!!! You then get an exact scale copy.
If you need the object in another scale you simply take the original full-scale version .. copy it .. figure what that 12-in dimension is in your new scale in millimeters .. and then scale your copy as INCHES to that mm number.