Prepping for 3D Printing

So What’s this about?
I was asked about the steps necessary to take a CAD drawing to getting your hands on a 3D printed object. I am by NO MEANS an expert on this .. but .. I have successfully had this done. I’m going to go over what I’ve learned and how I’ve done this. Like anything else there are other ways .. these are those that I know.
3D Application Needed
The first thing you need is the 3D object you will want printed. You can download files for free. You can Google “Free 3D Models” and find plenty. Most of us though will want to ‘roll our own’. For that you need software that will allow you to create your 3D object and then export it in a format that you can upload to your 3D printing service. In my case that is Shapeways. They have a page showing supported applications. Here they list some 47 or so apps along with things like what format can they export in along with notes. If you use one of these then ultimately you will need to export the finished object to one of the file types that Shapeways needs.

I use Sketchup. I export to a .STL file – which I am comfortable with using.

STL (Standard Tessellation Language) is a file format native to the stereolithography CAD software created by 3D Systems. This file format is supported by many other software packages; it is widely used for rapid prototyping and computer-aided manufacturing. STL files describe only the surface geometry of a three dimensional object without any representation of color, texture or other common CAD model attributes. The STL format specifies both ASCII and binary representations. Binary files are more common, since they are more compact.

While Sketchup does not support the .STL format there is a work-around for that and I will talk about that in a bit.

Sketchup’s Limits
Using Sketchup you run into a problem if you try to design a part the actual size in modeling scales. When drawing a circle, it has to have a radius greater then 0.018 in. If you attempt to draw a circle 0.017 in in dia you get an error. This is important as one inch in O scale (1:48) is 0.028 in. The answer is simply to create the object larger .. and I have a work-around so you don’t even have to do that much math.
Full size or what?
The first thing that comes to mind is to create the object full size. That works two ways. (1) Usually full size means that a small dia hole (radius) can be created with out causing Sketchup to flip out – and (2) It is MUCH easier to create the object full size. Think about it. In my example I will use a 6-in Gate Valve I created. I created mine mostly from things like looking up the dimensions for a Schedule 40 pipe (think standard). Those dimensions gave me the number of bolts, bolt circle dia, bolt size etc. That is basically the same as if you had a Gate Valve sitting on the floor and you are taking measurements directly from it. You then simply create your virtual model using the same measurements. Nice and easy.

To the left is a Gate Valve. On a side note – there is a reason behind the madness. That short stub will fit the inside of Evergreen 1/8″ dia tubing. The idea is to make a separate flange with a hole in the center that would fit over the Evergreen tubing. You could then pipe your industry .. slide the flange over the tubing, slide the tubing over the valve stud .. align everything and cement.

Note that I designed this full size. The 11-in flage dia is what is specifed for Schedule 40 pipe. Now .. look at the one bolt head I show with a 0.758-in dimension. That calculates to around 0.0158-in in 1:48 – giving us around a 0.008-in radius. WAY under the 0.018-in minimum radius for Sketchup. Creating the valve then full-scale just MAKES sense.

Shapeways Unit of Measure
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.

Ok. We have managed to get our virtual object designed and have multitudes of ‘Happy Feet’. What next?

We got to check our design for various errors. I’ll put that in Part II .. this page is getting long!


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