A small contribution to the COVID effort
My girlfriend teaches at a local college which has what they call an "innovation and creativity center." This is like a hackerspace for college students: they have a small laser cutter, a bank of FDM machines, the standard stuff. Its purpose is to give students access to resources for product design, rapid fabrication, and other resources that students might employ throughout their studies. When COVID hit and they sent most of the students home, a small group of faculty and remaining student workers started turning that equipment to produce PPE for local hospitals and other communities in need.
So now they're making face shields, with a throughput of a few hundred units a week. They're making a couple different models from the many that exist now (the fire department has stated a preference for one model, the hospitals like a different one) but which are of the same general construction. This consists of an FDM1 or resin-cast frame, which holds an elastic piece in back for securement and a piece of transparent PET sheet that comprises the shield. The latter piece is laser-cut both for general shape and for holes to mate the mounting studs projecting from the frame.
Assembled facemasks. Note the mounting of the shield material to the frame.
One problem they were having is that the PET sheet they've sourced comes on giant rolls, and the material needs to be rough-cut down to fit on the 16x12" bed of the laser cutter. The rolls are 48" wide, so this is easily enough cut down to either dimension, but the difficulty comes in making a transverse cut across 48" of material that's both reasonably square and also close to the 10" dimension required by the design. Some slop is forgivable, since each shield is cut entirely from the interior of a piece and none of the rough cuts forms a finished edge. But it was still proving awkward and cumbersome to measure and square this by hand while managing the not-insignificant bulk of the roll itself. This had become a bottleneck in their production process, so I decided to build a fixture to help.
Design
Design requirements were pretty straightforward, the fixture needed to provide three things:
- Some means of retaining the spool and allowing it to unreel in slightly more elegant fashion than just flopping around on the work surface. If this can help prevent its getting scratched, so much the better.
- Some form of stop, square to the axial dimension of the reel and against which the working end of the material can be abutted and made fast.
- Some sort of guide, rigidly fixed at the desired distance of ten inches from the stop, along which a cutting tool may be worked.
While this allowed a lot of flexibility in implementation, a challenge was the timing. This was late March, when Amazon was still quoting lead times of 4-6 weeks on "nonessential" items, hardware and home improvement stores were basically closed and I myself was trying not to leave the house. Here's what I made do with parts (mostly) on hand:
Assembly
The finished fixture
The base is cut from some 3/4" subfloor plywood I had lying around. This was plenty rigid but a little more warped than was reasonable for the application, so I counterbored from the top and tied into a couple lengths of steel strut-channel, which got it straightened enough. With the limited materials I had to work with, this construction is going to be a bit rough.
The reel is stood-off on plywood that the innovation center was kind enough to laser for me, sandwiching some scrap 1x1 for rigidity and tie-in to the base. This plywood is slotted to accommodate a length2 of 3/8" all-thread, on which the ID of the spool sits.
The cutting guide is formed by the T-slot aluminum frame you see swung up toward the image foreground. This is corner-braced internally but also tied to more plates of laser-cut plywood you see up top. Edge-to-edge the frame is 10" wide,3 and serves to measure this dimension in the direction toward the spool from the plywood seen screwed into the base, to which the hinges are affixed4 and which serves as the material stop. When the frame swings down, it then defines a transverse line 10" from the stop. It can be toggle-clamped to hold the material in place, allowing the operator both hands free to manage the cutting tool.
Lessons learned
Altogether this was a very edifying experience, for a couple of different reasons.
From a systems perspective, this was a great object lesson in limiting features to scope of project. The engineer in me looks at requirement (1) above and immediately starts picturing endcaps that mate the ID of the spool and ride on sleeve bearings about the axial shaft such that the spool is balanced around its CoG and spins freely and etc. etc. That part of me is absolutely horrified at the thought of the spool just rough-riding over a threaded rod like a roll of toilet paper5. Yet, it turns out, that works just fine. Any attempt to build a more elaborate mechinism would have taken longer to design and fabricate, imperiled ourselves of a re-roll if something didn't fit or otherwise work, and been more likely to fail "in the field."
Not the most elegant way to hold a spool, but it works.
Similarly, I spent a lot of time thinking about requirement (3) and whether it would be easiest to build a hot-wire cutter versus a straight, hook, or rotary blade, what type of guide would best support same, what kind of clearance or cut-out might be required to accomodate the tool in the bed, etc. etc. before shelving all of that and just building a straight-edge. I figured that, if a captive tool were that important, I'd hear about it. In point of fact, the operators have made do with a manual knife just fine; the straight-edge was all that was required.
There's a lot of talk in engineering circles about the perils of overengineering, YAGNI, project scope, etc. For all that talk, actually being able to define and hit targets is still largely a matter of good judgement and experience and something even seasoned engineers often fail at. Being able to take something from concept to delivery and see that it succesfully meets a need is a rare pleasure and one that keeps me in these fields.
From a personal perspective, it was also very gratifying to be able to execute with the limited parts and supplies I had on hand. For years (at least when it comes to small parts like fasteners and toggle clamps), my policy has been to buy 25-100% more than I need for the current project "in case I need 'em for something else later." This doesn't always feel like the right thing to do, what with the additional clutter it brings and the alternative convenience of modern supply chains. Most of my personal projects spend a long time in what we'll call the "concept/pre-procurement phase." Being forced by circumstance out of that and directly into execution with just the stock I'd accumulated "in case I need it" was, again, a very gratifying and singular experience.
Notes
- I'm not going to engage the debate of that FDM isn't a great process for this (that's obvious). With some finishing work they do make great positives for the poured-silicone resin-casting molds, and there's been talk of tooling up an injection molding machine. Plus, when we've still got nurses wearing trashbags, it seems like any number of masks coming off the FDM line are better than letting it sit idle. ↩
- actually two lengths joined by a coupling nut in the middle. Like I said, this construction is rough ↩
- As close as I could get it with a friend's cut-off bandsaw. My friend has a mill, too, but the extra accuracy didn't seem worth it given the possibility of deflection. In any case, it's within a few hundredths, plenty accurate for the application. ↩
- The hinge attachments are slotted to allow for truing the frame against the stops, as well as translating the frame vercially to accommodate different material thickness or protective underlays. ↩
- with apologies for possibly overspending on the COVID zeitgeist ↩