Alvin Ramoutar

Designing multi-part assemblies, you expect a certain amount of assembly AND disassembly.

When the material is from a FDM printer like PLA, assembly and disassembly can weaken and eventually break parts, very common with hinges, clips, and matted faces where you use fasteners like screws to secure two or more parts together.

As part of a shooting gallery build I’m working on, there’s some design work needed for the target housing which contains both the target at rest, along with other components (motors, rack and pinions, lighting, micro switches). Other than the exposed target face, all other components will be contained within a box to keep them safe – however should one of these components fail like a stripped gear or stuck switch, I want the entire back to come off.

In terms of choice of inserts, I’ve went with INCLY 440PCS Threaded Inserts M2 M2.5 M3 M4 M5 M6 Brass Heat Set Inserts for Plastic Parts Metric Knurled Nuts Assortment Kit for 3D Printing Components Injection Molding – they seemed the most affordable at the time.

What’s nice is they have a size chart that breaks down the dimension of each insert:

I’m planning to use M3’s for the back cover attachment to the housing frame.

Now comes the question – how big should I make the hole for the insert?

I’m planning to use a soldering iron – put it on the tip, heat it up, then push it gently into the part.

However, too small a hole and it will deform the part. Too large a hole and the knurling will have nothing to grab onto, making a weak insert that eventually rounds the plastic or just falls out.

When using PLA, it’s important to note that any kind of hole tends to be 0.1 – 0.2 mm smaller as the plastic shrinks when it cools.

If you’re using ABS or Nylon those tend to shrink significantly more. Filament makers also don’t provide a ‘shrinkage coefficient’ that you can use to calculate this.

For the material I have on hand, one of the first prints tends to be a 1cm³ cube, I then use a caliper to measure how much shrinkage there is (typically for each cardinal direction X/Y/Z, then average).

Using this average shrink coefficient along with the smallest diameter of a M3 insert as seen in the chart above, I’ll size my holes 3.8mm exactly – this accounts for the above shrinkage where it matters most, at the knurling.

Construction lines show the difference in size, I don’t consider whatever material that will be displaced from the ~0.4-0.5mm to be significant (after factoring how much will fill the knurling grooves), excess I’ll just sand away. At the end of the day these are functional parts not models so they don’t need to be super pretty.

Continuing with the Shooting for charity – building the Hunger Games project, we’ve got a frame to design.

Re-focusing on project tenets

…and how they apply to the frame:

BE FUN

It should feel like a carnival game, that means colourful art/design, places to hang rewards/toys/plushies, angled header banner, LED lighting, the works. Also, fun is best had with friends, so the frame should be large enough to allow ~3 players to participate at once.

BE REUSABLE

Perhaps the most applicable tenet, the frame needs to disassemble into parts no larger than 6″ folding table (hint for next tenet), each part should also support several iterations of reassembly so no wood screws for major frame components. Expecting lots of bolts and (t) nuts. This also factors into the build material itself, it should be accessible (lots of Pine!) and simple enough to shape and replace in the frame.

BE FRIENDLY

To setup staff, game masters and players alike.
Setup staff in the sense that parts are small and light enough that anyone can assemble with minimal power tools – hence the 6′ folding table requirement for storage but also as the game base – the intent is the frame will literally sit on 2 side-by-side folding tables.
Game masters in the sense that darts should be kept in the game as much as possible (soft, foam-core back board material maybe?), helps if there’s an enclosed area to collect the darts (the area above the folding tables!).
And finally Players in the sense of accessibility, other than the 2 folding tables above for the frame, we’ll have another 2 several feet in front of the game for players to stand/sit behind and rest their arms on, hold darts, etc.

The first frame draft

_{red/maroon => supports, contains a loosely defined dimension, standard Pine but can be any dimension (using primarily 1×4/2×4/2×6).
cream/peach => key frame, strongly defined, all Pine.
gray => back boards, strongly defined and made of plyboard or some other cheap 1/4″ material. Holes not made yet.}

Couple of key notes:

• The top and bottom are open, with the top expected to have a black drape over it with LED lighting hanging from it and the bottom against the surface of the 6′ folding tables
• Triangular supports on the top are deliberately made obtuse to angle the 1×12′ banner facing down towards participants – with average height of a folding table being 3′ and the game volume being 12x3x4′, the total height is a little less than 8′ with the banner
• Backboard (and sides) are sliced, comprised of a total of 6 3×4′ panels, these will have foam core artwork adhered to the front and are to slide into the frame for assembly
• Red holders on each side for A/V (speakers)

Not really visualized well here is 2 2×4’s keeping the top frame bar straight mounted at 1/3rd and 2/3rd’s point of the base along with an additional channel at the front bottom for stuffies (part for the aesthetic, other part as awards for small kids).

Just for wood, estimating ~$300 CAD with an extra $100 for fasteners (bolts are not going to be cheap).

Next steps for the team are to finalize the design then start scavenging local Home Depot for non-scoliosis boards, perhaps the biggest challenge of the frame build yet…