The machine in question is a 3D Systems Viper high resolution unit. I used a long exposure on a tripod to capture the actual laser beam tracing the part; the laser is a diode pumped solid state Nd:YVO4 100mW laser. For those of you unfamiliar with how SLA process works, it works like this:

1) A computer solid model is sliced into thin slices, between 0.002″ to 0.003″ thick. All the newer machines are 0.003″ or better in Z-resolution.

2) A laser then traces the cross section in a vat of photopolymer. Okay, by photo-polymer, I mean, a vat of very expensive goo – where the laser touches the goo, the goo turns into plastic. How expensive? I’ve heard that when it first came onto the market, resins were about $5000 – $6000 per gallon. They have since came down in price, but suffice to say it’s a very expensive proposition.

3) After each layer is formed, the platform in the tank drops by a build layer thickness and the goo flows over the part, readying the next layer to be drawn.

4) At the end of the build the platform gets raised again and the goo drains off the part, leaving you with a solid object.

These are very expensive machines – starting price of one of these is in the quarter-million dollar range, and the laser only lasts so many hours before it has to be replaced. Needless to say, unless you’re keeping the machine *very* busy, just the depreciation would eat you alive.

Our boss, who’s rarely wrong, always tell us that as design engineers, our time is the most valuable spent designing. “Whenever we need something, and need something quick, we just toss money at someone and have them bang it out and put it in a Fedex overnight box”.
My buddy Dave’s grandparents owns a laser engraver. They own a trophy engraving shop and laundromat down in Renton. It’s really wierd to think of a sweet old lady at the controls of an Epilog 20W CO2 laser system… but she doesreally good work. So the next time you need something engraved, check out the Puhich Dry Cleaners in Renton on 319 Main Ave. South.

I know what a 20W CO2 laser can do; it can do a lot more than mark plastic and burn through anodize layer on aluminum. There was a discussion thread on making lens cap holders, so I drew one up in Soldiworks real quick:

Here’s the test run in paper. So far so good, right?

Unfortunately, the laser engraver is running on Windows 98. It requires a firmware update before it can talk to anything past Win98, and there is always a risk of bricking a machine doing a firmware update. So we are stuck with a computer that works – abet a very slow one, with a parallel port printer connection. (I bet some of the folks I know had never seen one… they went the way of the dodo after USB became popular).

Complicating the problem is that the printer driver runs as a Corel Draw plugin. Corel Draw 8, to be exact. And even the earliest DXF that Solidworks can save the file in, the simple fillets on the drawings don’t quite come through – let alone the more complicated splines and polylines.

The example above worked okay, because the laser cut it out as a *Raster* art, instead of a vector art. But cutting in raster mode drastically drops the laser’s power output. And it’s not like you can run multiple passes over the same piece of PTFE either – the slow heat transfer of doing so just warps the plastic – and the results looked like someone tried cutting the material with a dull butter knife.

Obviously something like this, out of 3mm PMMA, will be a bit out of the question:

So my options are:

1) Try to mitigate the risk of the current laser cutter’s firmware upgrade (maybe see if I can do a hardware replacement of the logic board, upgrade the computer to something snazzy, then retry the Solidworks -> laser cutter workflow.

2) Pay someone like Pololu online to do the laser cutting for me. Essentially, someone else will be eating part of my lunch if these products go on sale. Might be okay if there’s only a few parts, but I’ll have to rethink the design a little bit.

Turns out, RedWolf airsoft out of Hong Kong will happily sell me a 30mm silencer for about $US10.00. Aluminum barrel, both ends with a machined aluminum plug. They even put a 14mm CW thread on one end and 14mm CCW threads on the other, so out of the box, the dang thing will fit on just about every single airsoft gun out there.

In Hong Kong, we have a saying that “the flour costs more than the bread”. The term originated from the housing bubble days where the value of the land gets bid up so quickly that older apartment buildings prices are being outstripped by the land value of neighbouring lots, but it also applies to engineering and business where by some form of competitive advantage (and economies of scale), someone can build a product cheaper than you can even start sourcing raw materials.

Got a cool little bookmark for all my troubles with the laser cutter though…

Tonight, the goal was to test the breadboarded prototype of the bullet flight sensor’s electronics. Remember – I am a mechanical engineer; this is a completely new foray into the world of electronics for me, aside from some simple “hook a solid state relay to a microprocessor and bit bang some code to turn on the rice cooker” projects. So even though this may seem like kindergarden EE stuff, it’s a fairly big leap for me, design wise; I’m no longer relying on the ability to clobber code and instead using discrete logic ICs and doing actual calculations and setting RC time constants, etc.

We begin with the breadboarded model:

And our setup in the lab:

On the bench is a trusty oscilloscope to look at the signals at different lines, a DC power supply set to 5V, 100mA current limit, and a signal generator. The signal generator won’t be used for this project here.

First, I verified that the *new* sensor is working – the last one had a round put through it by accident:

Next, I verified that the 555IC is getting the power that it needs. Turns out that the power rails aren’t fully connected all the way. A bit of poking with an ohm-meter fixed that. Now I am ready to insert my test points:

And trip the break-beam sensor, with my gimpy fingers:

Orange line, or Ch1, is my sensor’s output. It goes from High to Low when the beam is broken. The turquoise line, or Ch2, is my 555′s trigger output. It goes from low to high when the input pulse is received. That’s a VERY promising sign.

My fat butter fingers can only move so fast through a 10mm opening, so the event scrolls off the oscilloscope’s screen. I tried dropping a small machine screw through the opening, but that actually prove to be much harder than expected (don’t laugh!). Frustrated, I finally came up with the following idea:

By flexing the rubber ducky antenna on one of my pocket wizards and getting it to spring through the break-beam sensor gap, I can generate a quick enough blip from the sensor:

Each major division is 5 milli-second on this setting, so the rubber ducky antenna is only in the beam’s path for about 10mS. My fingers can’t move *that* fast, for sure

Repeating the test again a few times, got me the same result:

Note that irregardless of the pulse length from the sensor, the 555′s output always sits at about 35ms. This is a litte bit off from the design goal of 40ms (1/250 second shutter speed, or sync speed on a 1.6x crop camera), but close enough for government work. I attribute the difference in component value tolerances on setting the RC constant.

Now the final test – does the output from the 555 trip the SCR to fire the strobe and pocket wizard?
(I selected an SCR instead of a cheaper / more common transistor. The SCR is rated to 400V, so even an older, high voltage “digital camera killer” flash will work on this sensor. )

And…

*drum roll please*

Nothing happened.

Turns out the same bug that bit me on the 555 timer bit me again. The top and bottom half of the power bus on this breadboard is not connected, and the SCR wasn’t grounded properly because of that. Now, plugging in a pocket wizard, this is what I get (with Ch2 now monitoring the anode of the SCR):

Interesting, it seems to add a bit of noise to the sensor output line. But the characteristic beep of the PW firing can be heard as the beam is broken. (Note that the sync voltage of the pocket wizard is only 3V or so).

Plugging in the 580EXii:

Again, some electronic noise on the sensor line, but we got what we need out of it – the clean voltage drop that triggers the monostable multivibrator.

And here’s the happy camera dork with his new toy (click link for video:)

Now that the circuit is verified working, I am okay with releasing the resources to order the acrylic for laser cutting to form the chassis, as well as starting PCB layout. Stay tuned…

(Leave a comment, with contact information, if you really want to know ).

-=- Terence

Note the “unusual” arrangement with the pocket wizard. The “hot shoe adapter” is actually plugged into the sensor to simulate a camera’s hotshoe firing the pocket wizard.

“Do you know how you tell the difference between a Chem student and a Physics student?”
“The Chem student washes his hands *BEFORE* he goes to the bathroom”.

There’s some truth in that statement, especially with regards to corrosive chemicals…

Tonight I decided to give the toner transfer photomachining process another go. The toner transfer material is from PulsarFX and purchased from DigiKey Corp along with the rest of my electronics components for the bullet flight sensor and … some other projects.

First, I did up the “photo mask” in Adobe Illustrator. Since the toner side is face-down on the etch surface, I do a “transform – flip” operation to mirror the text:

Top to bottom are: plain paper proof of the printout, plain paper proof of the inverted mask, and the actual mask itself, on the toner transfer paper. At a buck something per sheet, it’s advisable to do a plain paper proof every step along the way

The toner transfer paper works by heat and pressure. They sell a unit for doing this, but I am not about to spend a few hundred bucks on a special laminator. So I liberated the clothes iron and set it to linens, and tried my hand at ironing on the transfer:

Well, bummer. I had cleaned the brass piece with my Festool random orbital sander with 220 grit sand paper. Turns out a brillo pad works better.

If at first you don’t succeed, try, try again, and try harder:

My Dad would call this “paying tuition”.

After playing around with different methods of applying pressure, I finally got something good:

And a piece promptly flake off as I start blotting the workpiece dry. (the paper is dextrin coated, and releases after sitting in water a little bit, kinda like a water-slide decal).

After about 45 minutes in the ferric chloride (yeech!) etch tank, this is what I managed. Not acceptable, but at least I have an idea that the system works. Kinda. The etching is about 0.2mm deep. The design intent had been to sand to the black (raised) border, then fill the inside etched area with an ink, leaving the brass text raised against a black background. The piece would then be matted into a framed artwork along with the panorama being presented.

(For now, I’m just going to stick to my wax seal and signature, thank you very much.)

(This is “R&D”. Failures are not unexpected; what’s important is documenting how things failed, and learning from it.)