Small-Scale Fab: Embrace the CAD Mindset
We’ve done some renovating at the old Dr. Torq homestead over the last year, so I’ve talked to quite a few tradespeople. Invariably, people ask me what I do. I tell them I design and write about physical computing gadgets.
The conversation often turns to 3D printing and several of the contractors, told me that they had 3D printers at home. Enthusiastically, they’d go on about printing all their cool Star Wars helmets and miniature Transformers figures. Members of local maker spaces and robotics clubs dig 3D printing too, although they tend to like more techie-specific items like enclosures, cases, little brackets and technology related knick-knacks.
Hooray! 3D printing is now mainstream.
Well, sort of.
You likely won’t find a bracket to mount a recycled can-opener motor to your Steampunk automaton project, on Thingiverse. Being able to download pre-made models is certainly fun and useful. In my mind, designing and building your own CAD models are the missing links needed to seriously push 3D printing beyond the hype and make it go truly mainstream.
This time we’ll explore some of the basic principles, you’ll want to think about as you start to design your own parts for 3D printing with FreeCAD.
2D vs. 3D Modeling
There are two fundamental ways to model parts in FreeCAD, draw a 2D pattern then either extrude it or rotate it around an axis. Or, grab a 3D object, like a cube and attach it to other objects using Boolean operations until you have a finished part. I frequently use a combination of building 2D shapes into 3D objects and then hack them together with other 3D objects.
Not only can you “add” parts together, but you can also “subtract” objects in your model. Say I start with two 4x1x1/4 inch cubes at right angles, to form a bracket. They are made into one object by “joining” at two of their edges. Then, two 1/4 inch diameter by 1/4 inch long cylinders are placed on each of the legs and “subtracted” (make a cut of two objects) to form mounting holes.
Adding and subtracting objects takes a little getting used to. If you want a hole using traditional fabrication techniques, you’d clamp the part down and use a drill to bore through the material. In 3D printing, you get a hole by just “not” printing any filament, where you want the hole.
Adding and subtracting objects in the FreeCAD model is awesome, compared to manually welding two pieces of flat stock together to form a bracket, then drilling out four holes. That’s if you have a welding machine and some kind of drill. Naturally, plastic 3D brackets aren’t as strong as their equivalently-sized metal cousins. There are always trade-offs in engineering, which is another fundamental principle, by the way.
Production efficiency begins to figure into projects too. What if you were making a 10-foot-long bracket with twenty 1/8-inch diameter holes, in two rows on each leg? That would be trivial to design in FreeCAD and print on a printer and moderately time-consuming to set up and execute on the drill press.
What if you need 3/16 inch instead of 1/4 inch holes in your bracket? Simply change the model to 3/16 inch holes. That is not easily done with manual fabrication techniques.
Consistency, Accuracy and Repeatability
Readers are surely familiar with traditional drilling, cutting and milling techniques for making prints. The process removes material to produce parts of a particular shape and dimension. There is also much manual measuring and marking, as well as positioning of parts between operations. Hand tools are frequently used for adjustments and fitting. Although skilled craftsmen can make remarkably precise parts, the new DIY small-scale fabricator might not have the experience, patience or time to turn out exacting tolerances.
Making standard, consistent and aesthetically pleasing parts quickly becomes a challenge. Revisions and repeatability are problematic as well.
Modern, desktop-level 3D printers, like the Prusa MK3S can hold tolerances of a couple (.002 inch or .003 inch) thousandths of an inch. That is plenty good enough for designing something like a recycled electric can-opener motor bracket. They can also print the same part over and over until you run out of filament.
In the beginning, one principle you’ll certainly overlook is the matter of clearances.
With the excitement of such accurate dimensioning on parts, you might forget to allow for proper clearances. For example, if you have a 1/4-inch (.250 inch diameter) bolt that needs to go through a hole, make sure that hole is .250 inch diameter plus a little bit more, like maybe .010 inch or .015 inch.
That way the bolt will fit through the whole without interference. Figure in your clearances for corners, tops to enclosures, sliding parts and so on.
Why Draw Out Your Designs?
For many years, I never formally drew out my designs, in spite of extensive drafting experience in my younger days. My low-volume desktop and one-off garage projects were always fast moving and I just wanted to get on with the build.
These days, you can’t make parts using CAD without a model. Where do you get a model? One place is a parts site like Thingiverse.
The other option is… you guessed it… design your own parts with FreeCAD. Remember, that new desktop technology like 3D milling or laser cutting (commonly called CAM for computer-aided manufacturing) also requires a model as input. If you want to do an analysis on your parts, you’ll need a model there too.
What’s cool now is that modern consumer-grade laptops can handle all those things, from the software side. FreeCAD modeling works great even with something as complicated as a car transmission.
The slicers (like the Prusa slicer) used for making the G-Code for a printer, are quick and effortless. Doing thermal or finite-element-modeling on a part isn’t any big deal either, on modern, off-the-shelf computing hardware.
The models you create can also be saved, reused and shared. They are just files you email or copy to other people. I couldn’t see the point of keeping paper drawings of my designs, in a file cabinet, back in the day. You couldn’t do anything with them.
Now, you can take parts, like a standard bolt or your newly designed recycled can-opener motor bracket and insert it into a new project. No need to recreate everything again, from scratch. Or, you might send a part to a colleague and ask for upgrade suggestions or to have them print it.
Let’s not forget too that FreeCAD has extensive dimensioning capabilities to document a design. Back in the old days, who wanted to dimension drawings? Nobody.
Now, you just place them where you want them to appear on your drawing. The physical measurements are part of the model and are reflected in the drawing dimensions. Makes for nice, professional and informational graphics, in presentations. Just ship your drawing over and use it in LibreOffice.
Finding Your CAD Mindset
Getting into the CAD mindset is pretty easy. Forget about Thingiverse trinkets and start pondering what you can make to fix something around the house.
One of my first useful projects was to repair my wife’s favorite pepper grinder. The base was cylindrical and had a big triangular chunk broken out. It was her favorite color, red and replacement parts simply weren’t available.
I ordered some red PLA filament, with the closest color and modeled the part in FreeCAD. I used the 2D Sketcher workbench to make an outline of the cross-section of the base.
Then, I rotated the sketch around the base center line. I used a $35 dial caliper for measuring. The design needed a little locating notch on the inside so I eyeballed a square of about the right size. With supports, the print took about an hour, using a .3 mm layer height. After the print was finished I popped the part off the print bed and it fit on the pepper grinder without any modifications.
The design used the FreeCAD sketcher workbench, to rotate a sketch into a 3D part. I next placed a smaller cylinder inside the rotated base and subtracted it out.
A square object and “subtracted” out the notch. Finally, I printed the part and replaced the broken pepper grinder base.
It took a few days until my wife noticed the new part. It wasn’t an exact match for the color or contour. It had layer lines.
But, my wife said it was good enough and appreciated my effort. It’s also good enough when friends and family come over, doubling as a great conversation piece.
Other easy-around-the-house parts include TPU (flexible filament) feet for step ladders and my Workmate bench. A solid shower drain cover (another sketch rotation object with two bolt holes) to replace the old 1960’s model drain/lever cover. I used PETG filament to survive any hot water. There were also handles to fix the broken ones on the Workmate.
Designing parts with FreeCAD on a modern laptop has only really been practical in the last couple of years. Before that Open Source (non-proprietary commercial) CAD software and the hardware just wasn’t up to the job.
Now that 3D printing is “fairly” easy to use, opportunities abound for enterprising DIY designers and desktop engineers to create their own custom, configurable parts and turn them into real everyday practical items.
There are lots of YouTube videos and courses out on the web on using FreeCAD. Study up, get a decent notebook and start thinking about how you would put together your own replacement pepper grinder base or step-ladder feet models.
If you are just starting out in FreeCAD don’t try to be too tricky right off the bat. Begin with a simple part, keep at it and your skills will come as you learn. It will take a little time. Being able to design your own parts will definitely be worth the effort.