Why shouldn’t some things be 3D-printed?

How many objects in your home are made of only one material? Very few, and they’ll be simple things like spoons, pots, plates — things that don’t move or flex in a significant way. Different materials have different properties, so complicated objects tend to incorporate a wider range of materials to meet all of a design’s requirements. This is just as true for 3D printed objects; a spool of plastic can only ever perform as plastic. Although designing a 100%-printed object is a neat challenge, it often means that the designer has sacrificed the sturdiness, functionality, and longevity of the final object in return for convenience. Think of a plastic bolt that is printed in the default vertical orientation; put even a small amount of tension on it, and it will delaminate eventually. A waste of time and resources.

Instead, we should design our objects to use traditionally-manufactured fasteners and mechanisms (the RepRap community calls them vitamins, parts of a 3D printer that aren’t printed) to connect the 3D printed elements. By doing this, our objects will last longer, work better, and look and feel more complete and professional.

Assembling a kit of vitamins is completely affordable, the list below shows what’s in mine. I’ve grabbed product images directly from their sellers on AliExpress, which I hope they will forgive; I bought my stuff from them after all, so I’ve linked to their product pages in case you want to buy from them too.

What’s in my vitamin kit?

1. Parts I still print

Although many things shouldn’t be printed, some things actually make sense.

Washers. Print them in whatever dimensions you need, it only takes two minutes.

Print them in whatever dimensions you need, it only takes two minutes. Registration pins. Design them intelligently and print them horizontally.

Pivot/hinge pins. Actually, I just use lengths of raw filament.

2020-08-05: Since I wrote this article, I’ve replaced all printed pins with bamboo skewers in my designs.

2. Keystone kits (US$52 total)

Steel M3 socket head screws (US$12.20 for 90 pcs = $0.14 each)

AliExpress product page

The socket head screw is gorgeous and usable because it can be tightened with a hex key or with your fingers/pliers. I have settled on the M3 size for my vitamin kit because a) it’s the size my printer uses and b) it’s a very practical size, neither too small nor too large. This kit comes with 9 different screw lengths, but I have used the 25 mm length the most.

Steel M3 grub screws (US$9.85 for 180 pcs = $0.05 each)

AliExpress product page

A grub screw is a screw without a head; the screwdriver slot is cut into the shaft so that the entire screw can be sunk below the surface. You can see these in use on the shafts of your printer’s stepper motors.

Nylon M3 stand-offs, screws, etc. (US$6.07 for 180 pcs = $0.03 each)

AliExpress product page

I use nylon fasteners for two applications in my designs. First, to hold circuit boards and other electronics, because nylon is a great insulator. Second, to fasten light-duty objects that will experience vibrations, because the nylon fasteners noticeably bite into each other. The nylon stand-offs are particularly useful as spacers between different elements of a design.

Steel springs (US$13.28 for 200 pcs = $0.07 each)

AliExpress product page

Springs printed from plastic just aren’t as good as metal springs, no competition. Buying a kit means that I can always find matching springs when I need more than one for a project.

3. Nuts and inserts (US$18 total)

Steel M3 hex nuts (US$1.17 for 100 pcs = $0.01 each)

AliExpress product page

Steel M3 wing nuts (US$6.98 for 50 pcs = $0.14 each)

AliExpress product page

Steel M3 2020 nuts (US$7.96 for 100 pcs = $0.08 each)

AliExpress product page

Also look for thumb-nuts, which are round knobs with a thread going through them. They’re far prettier than wing-nuts.

These nuts are meant to be used on 2020-profile extruded aluminum bars, but they’re perfect for use as captive nuts when set into hollows on the surface of a print.

Brass M3 knurled insert (US$1.99 for 100 pcs = $0.02 each)

AliExpress product page

A threaded insert is a kind of nut that is designed to fit inside a hole. Knurling on its outside face cuts into the walls of the hole, holding it in place. This turns any printed hole into a smooth, infinitely reusable screw mounting point, and it makes a huge difference to the feel and usability of an enclosure. You typically fit a threaded insert into a hole by using the tip of a hot soldering iron to push it into the plastic, but I think this method is weak. It is better to mount the insert so that there is a retaining collar in front of it. Fastening a screw into the insert pulls it against the collar, bracing the insert and giving the screw something to pull on.

Optional assorted vitamins

Self-adhesive rubber feet (US$2.18 for 20 pcs = $0.11 each)

AliExpress product page

Put these on the bottom of an enclosure to hide screw holes and give it a non-slip base.

Steel M3 all-thread rod (US$12.74 for 5 pcs = $2.55 each)

AliExpress product page

Some all-thread rod lets you make fasteners of arbitrary length.

Small ball bearings (US$1.34 for 10 pcs = $0.13 each)

AliExpress product page

Ball bearings are the most common type in general designs, and small bearings are the most versatile. Do note the crazy low price of these things. If you’re designing something that needs precision, like a CNC table, replace these with good bearings of known provenance later.

Bamboo kebab skewers (USD$6.00 for 85 pcs = $0.07 each)

AliExpress product page

Bamboo skewers are suuuuuuuuper useful. They’re very strong, they’re dimensionally consistent, and they’re easy to trim. This makes them appropriate for use as strap posts, locating pins, through-pins, hinge pins, and many other applications. If a design uses 3 mm filament as a pin, it can be meaningfully replaced with a 3 mm bamboo skewer. You can get these in a small range of diameters and lengths too.

Neodymium (rare earth) magnets (AU$20.00 for 100 pcs = $0.20 each)

AliExpress product page

I’ve slowly standardised on a few sizes of magnets:

3 x 1 mm disc magnets for very small things, like putting into hex bit holders to retain screwdriver heads and stuff.

6 x 2 mm disc magnets for regular use, things like box closures and fridge magnets.

8 x 5 mm disc magnets for heavy use, things like magnetic clamps, jigs, tool hangers, and so on.

I haven’t found a use for square or rectangular magnets over disc magnets. Circles are nice and clean to print. Buy a hundred magnets at a time so that you can experiment freely. However, note that the quality and dimensional accuracy of magnets is a little varied from AliExpress. I have bought 8 x 5 mm magnets from AliExpress ($0.20 each) and from an Australian supplier ($1.35 each) and the pricier magnets are bang-on the nominal dimensions and significantly more powerful. The cheaper magnets are about 0.5 mm shorter and their pull force is weaker. I would say that it’s good to have cheap magnets for prototyping and perhaps selling, and to order good magnets if you need them. It’s a good idea to design your prints with a little pinhole behind the magnets (or with a through-hole for the magnets’ bores) so that you can push the magnet out without destroying the print.