Everyone wants their prototypes to look polished, as opposed to looking like 3D-squirted weekend afterthoughts. The combination of Delrin and a Laser Cutter make this easy, especially if you learn a few tricks-of-the-trade that will make your assemply pop, both functionally and aesthetically.
Last time, we took a deep dive into fabbing parts with Delrin and a typical 40-watt laser cutter, and we discussed some of the constraints of the material. More recently, [Gerrit] gave us a close look at the material itself. It’s been about a year since our first post, but the list of tricks is far from complete.
If you’re just getting started in this domain, let me introduce you to two classic techniques for laser-cut prototypes: puzzle-piecing and the T-nut-slotting. While these techniques are tried-and-true, I hope, fearless reader, that they’ll leave you hungry for something cleaner, something more refined. If that’s the case, read on!
Rivets offer us a similar feature to screws: they’re great for fastening down two plates together. An added benefit to rivets is that their shaft offers a smoother surface for parts that need to rotate relative to each other. In quantity, rivets are comparable in price to screws, and they offer a different aesthetic appeal, possibly because we don’t see them as often in hobbyist projects. Rivets come in all sorts of shapes and sizes, but I prefer the standard, semi-tubular variant since their top and bottom protrude only slightly from the installed material. With semi-tubular rivets, the exposed bottom mushrooms into a characteristic button feature that we’ve seen on clothing and many-a-hinge in consumer goods. As a heads-up, specifying the rivet length takes a bit of prior foresight into the dimensions of the part. For a back-of-the-envelope calculation, rivets should stick out of the part by about 55% of the rivet diameter [PDF] before they’re crushed.
A word from the wise: rivets are permanent! Unlike screws, rivets are (barring fancy removal equipment) a one-time operation. When working with rivets, we must clearly keep the order-of-operations in mind when assembling our parts together. True story: I have scrapped many parts after riveting two pieces together in the wrong order.
Countersinking offers us two immediate benefits. First, it enables us to put screws into our designs that are flush with the plate. Second, it dramatically cleans up the part’s look and feel. No longer must our screws poke grotesquely from our parts, a pragmatic pimple of necessity.
On the practical side, flush screws invite us to squeeze together the clearances between parts, saving us space that those otherwise protruding screwheads so eagerly want to take away from us. In the example on the right, this robot wheel’s tight clearance to the body mandated a flathead screw solution. On the aesthetic side, countersinks are not commonly seen in laser cut prototypes. They make our audience think harder about the how in “how did they do that..” rather than jump the gun and cry “laser cutter!” on our project.
So how does one create such a cavity? Fortunately, Delrin takes to standard metal tooling like butter. For the tentacle mechanism vertebrae shown above, I created the feature with a miniature countersink cage. These tools were designed specifically for the aerospace industry for efficiently riveting planes together on the factory floor. Pro-tip: you can achieve the same effect with a traditional countersink cutter by setting your drill press’ hard-stop nut appropriately.
Do you need a thread feature capable of withstanding both heavy loads and repeated screwing and unscrewing? Then heat-set inserts may be for you! Like many other self-respecting thermoplastics, Delrin is a prime candidate for these brass nuggets.
Heat-set inserts do exactly what they sound like: take any thermoplastic with a hole cut to a predefined dimension and stake a hot metal insert into that hole. Let the result cool off and — poof — a resilient thread feature is now permanently embedded in your part! Vendors sell specific insertion tools, but we’ve found that our soldering irons (cranked up to 250ºC) and a pair of steady hands works just as well.
Arm yourself with 3 new techniques
Sometimes, finding new tricks is a matter of opening our eyes to the world around us. Just have a look around. Nearly every consumer product that surrounds us was fabricated in quantities that mandate techniques for mass production. To make these products, designers need repeatable, reliable techniques with predictable outcomes. Rivets, countersinks, and heated inserts aren’t anything new. They’re borrowed from this world. With some careful attention to detail, perhaps you’ll find a few more techniques that lend themselves to laser-cut Delrin. Let us know in the comments! Until then, keep rolling out those prototypes!