Welcome back to part II in this ensemble of techniques with laser-cut Delrin. Thanks for many of the great insights along the way in the comments. In this guide, I’d like to go over some of the more immediate kinks that come to mind when getting started with this material.
Sourcing Delrin Sheets
When it comes to shopping, there are a variety of suppliers to choose from, but there are a few key words and thoughts to keep in mind.
First, Delrin, is the “brand name” that refers to the Acetal homopolymer. Variants may also be labeled, acetal or acetal homopolymer. Delrin’s natural color is a soft white, but dyes can take it into a range of other colors. Black and white are, by far, the most common, though.
In the previous guide, all of the examples were cut from a small range of sheet thicknesses (0.0625[in], 0.09375[in], and .125[in]) sourced from OnlineMetals. As the thickness of the sheet increases, the tolerances on the thickness rating will also become more loose. You might buy a .125[in] plate and find it to be .124[in] in some places and .126[in] in others. If you purchase a .250[in] sheet, however, you’ll find that it may vary as much as .126[in] oversize though!
Buy it Flat
Despite McMaster-Carr being my go-to solution for one-off prototypes where rapid build iterations trump BOM cost, I don’t recommend purchasing Delrin from them as their sheets don’t have a flatness rating and often gets shipped bent in (oddly sized) boxes. (Seriously, has anyone else gotten a few oddly-sized parts in a gigantic McMaster-box before?)
Extruded Delrin has internal stresses built up inside of the sheet. There are a variety of reasons why this could be the case, but my biggest hunch is that the extrusion process at the factory results in different parts of the sheets solidifying at different times as the sheet cools, possibly causing some parts of the sheet to tighten from the cooling before other gooier sections have yet to finish cooling. What this means for you is that as your part gets lased out of the sheet, you’re, in a sense, relieving that stress. As a result, the part that you cut–especially for thin sheets–may come out of the laser cutter slightly warped.
Assuming that the sheet put into the laser cutter was flat in the first place, then you can guarantee that features like holes and pockets will be the correct relative distance to each other when the part is spread out flat. To flatten out parts in their final application, I’ve come up with two solutions.
Because the relative location of the holes is precise, you can bolt plates together with standoff to flatten them out.
This is the method that I used in the GameCube-bot-2’s chassis. With multiple standoffs though the width and length of each piece the chassis as a whole pulls all sheets flat.
Alternatively, you can also embed holes for spring pins and sandwich two plates together such that the pins hold both plates flat relative to each other and cancel their warping. Spring pins embedded in Delrin can also serve a dual purpose in that you can “double-up” your Delrin plates for thicker geometry. Below, I doubled-up on some eighth-inch plates to better accommodate a flanged bearing.
The Dreaded Taper
Alas, for CO2 lasers in the 40-to-60-watt class, we’re pretty-much stuck with this feature appearing on the edges of all our parts. By “taper,” I’m referring to the beveled edges that appear on Delrin (and some other) laser-cut parts. This beveled edge is a characteristic of the way the laser cut lases through material.
If we imagine the beam focusing into a small dot in the same way we focus the sun’s light through a magnifying glass to burn paper, it’s not too difficult to imagine the “coned” shape that the collimated light takes on as it exits the lens. beyond the point of focus, this cone fans out into another cone. This “double-cone” moves through our part cutting material. Depending on where we focus the lens prior to cutting will determine the shape of the lens–unless we cut our part in multiple passes with different focal points per pass. In the spirit of repeatability, I’ll opt for an alternative workaround.
Getting around this issue entirely is nearly impossible, so it’s usually easier to design around it. Press-fit parts that rest completely inside the lased pocket, such as bearings, are usually experiencing a tighter squeeze on one end, rather than the other. Hence, they’re likely to pop out. To get around this issue, you can either opt for their flanged counterparts or sandwich two Delrin plates together with opposing beveled edges to better hold the part inside.
Delrin is well-know for being chemically-inert–so much so that it’s nearly impossible to find a glue that bonds Delrin to another surface! While there’s certainly an opportunity to try melting it together in this case, I’ll say, for now, that gluing Delrin is out-of-the-window of possibilities for reliable techniques–especially in models that need to function in some way besides looking nice.
(Re)Sizing Holes with Tight Tolerances
Sometimes, holes need to be just the right size to accommodate the tight tolerances of metal-to-Delrin press-fits. When that’s the case, we can capitalize on the laser cutter’s “stronger parts” and touch up the rest manually. In this case, the laser cutter excels at achieving tight tolerances between relative dimensions, such as the spacing between holes. On the flip-side, the laser cutter doesn’t excel at nailing the exact dimension of those holes with tight tolerances. Consequently, we can undersize our holes, lase them out, and ultimately bring the holes to their final dimension with the properly sized drill bit or reamer. It’s a small bit of extra work, but it’s a guarantee that certain holes can meet tighter tolerance constraints.
Store it Flat
Delrin tends to warp under its own weight. If stored on end, it will develop a bend over time that makes it very difficult to cut evenly in a laser cutter since the beam is focused at a fixed distance from the lens. To keep problems with gravity out of your Delrin side-projects, I’d recommend storing sheets flat.
Only the Beginning
Thanks for tuning in for this two-part Delrin special. Don’t forget that while these techniques might lay out a foundation, there’s certainly many more undocumented tricks worth trying with this material–and that’s where you come in! If you make any discoveries on your own, be sure to keep us posted in the comments. As a community, we can keep refining our knowledge-base as we keep rolling out the projects.