The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Author: Ed

Thing-O-Matic: Vent Fan and Charcoal Filter

Fan filter and 5 V dummy load

Hot ABS plastic gives off a characteristic ~~stink~~ ~~odor~~ ~~smell~~ aroma that’s hard on the nose and probably not particularly good for the lungs. Even in the basement, it seems like a Bad Idea to stink up the place, so I added an exhaust fan and charcoal filter to blot up the odor.

The key step is to add the fan provided with the TOM (which they recommend you don’t use!): outside the box, oriented backwards, and running on +5 V instead of +12 V. The general concept: free up some precious space inside the box, shove the exhaust through a filter, and do it with a gentle breeze rather than a mighty blast.

Although it’s not a part of this sub-project, the heatsink holds a 2 Ω 25 W resistor that serves as a 12.5 W dummy / minimum load on the +5 V supply to keep it within tolerance. Right now, the heatsink is just jammed between the screws, because I’m probably going to add a similar dummy load to the +12 V supply when I move to a stepper extruder.

In case you’re hypersensitive to overheated resistors: the heatsink runs at 65 °C, the resistor at 75 °C, and the specs give a permissible dissipation of 20 W. You could work it out…

Ersatz ATX connector

The first step is to route the 4-pin ATX power connector (which popped off the big connector block plugged into the Motherboard) out the left-rear hole in the acrylic floor under the XY stage. I don’t have a mating connector, so I conjured up something from the same square pins as I used in the Extruder power supply modification and some wire harvested from a dead ATX supply. The black heatshrink tubing holds the four wires and their pins in the proper configuration. Obviously, you want matching colored wires, because the “connector” isn’t polarized!

On the other end, a four-pin screw terminal block provides a convenient way to attach a variety of gadgetry. At last count, it serves the exhaust fan, +5 V dummy load, LED platform light, and a cooling fan. More details on those later…

Terminal Block

The fan frame required a small gouge to route the wire inward through the vent hole in the side of the TOM case:

Fan frame modification

Four nuts secure the fan to the frame. Fortunately, the fan’s motor housing sits on the exhaust end, so the filter material rests against the hub support and spider. Here’s what the whole arrangement looks like, with the filters pried away from the fan.

Fan and filter mounting

A trip to the local Big Box home warehouse produced a $10 20×25-inch activated charcoal air filter intended for a whole-house air conditioner. I now have a large plastic grid, a sheet of open-cell foam air filter, plus a generous supply of charcoal filter material. I cut a 4-3/4-inch strip from one side, chopped it into 4-3/4-inch squares (that’s 120 mm everywhere else in the world), trimmed off the corners, tucked two layers behind the TOM filter holder frame, and added four more nuts-and-washers.

The spare filter material goes in a sealed plastic bag, because activated carbon has a limited lifetime when exposed to free air. That’s what it does for a living: adsorb smelly molecules from passing air!

The final step is to close off all the TOM’s openings, thus restricting air flow through the case. This has the happy side effect of warming the build area and reducing drafts, both quite important in a wintery 50-ish °F basement. Taking pictures of clear acrylic sheet is essentially impossible, but you can see the front piece there and the paper seals around the filament spool there. I make no apology for the masking tape; after everything’s working, I’ll formalize the arrangements.

Incidentally, don’t get too secure with the front window, because the ABP pokes through the opening to disgorge finished parts. In fact, the front of the ABP whacks the window when the nozzle reaches the back of the ABP, so you don’t want a mechanical latch holding the window closed.

I’m thinking a magnetic latch is in order.

There’s enough leakage around the windows to keep the fan happy, although it sucks the last one closed. Those four square cable holes in the acrylic sheet between the upper and lower chambers provide the only air channels, so the exhaust fan probably doesn’t compete with the ATX supply’s cooling fan.

While the filter doesn’t kill off all the stink, the TOM is a much better companion now…

2011-02-13
Thing-O-Matic: Platform Light

Platform light overview

The inside of a Thing-O-Matic gets pretty dark, particularly with the Lazy Susan spool parked on top, so I added a spot light to the Z stage.

The alternative seems to be LED strip lighting all over the inside, but my Parts Heap doesn’t have any of those yet and it did have a 10 mm white LED. The thing runs at 100 mA, so a 15 Ω 1/2 W resistor (to a +5V tap), a few snippets of heat-shrink tubing, and a blob of hot-melt glue did the trick.

Some sculpture armature wire that’s been kicking around for years holds the LED (wrap it around, add hot-melt glue) and doesn’t mind the occasional bump. I crimped the wire in a solderless connector and grabbed it in one of the Extruder Frame screws. It’s allegedly fatigue-proof, but it looks a lot like aluminum.

A bit more detail, with a Kapton-and-graph-paper belt (about which, more later) on the ABP:

Platform light detail

2011-02-12
Thing-O-Matic: Improved Timing Belt Clamps

The laser-cut plywood clamps holding the timing belts to the drive ribs slant diagonally across the rib + belt and secure one edge of the belt.

Belt clamp before modification

While this certainly works, it offended my sensibilities and is probably why the instructions call for that low-profile bolt.

Introducing the belt clamp to Mr Disk Sander provided just enough relief to clear the belt’s backing, while not making for a sloppy fit. In round numbers, if you barely trim off the plywood veneer it’ll be about right. Use an ordinary file if one of Mr Sander’s relatives doesn’t live in your shop.

Modified belt clamp

And then it works just like it should. If you were even fussier, you might chamfer the outer edges to allow the belt to lie flatter against the rib, but that’s in the nature of fine tuning. At least on my Thing-O-Matic, there’s plenty of air between a standard bolt head and the adjoining carrier rod.

Modified belt clamp in place

This is obviously not something you should dismantle your Thing-O-Matic for, but if you’re in the delightful position of facing that mountain of parts, this is perfect timing.

2011-02-11
Thing-O-Matic / MK5 Extruder: DC Motor Safety Lamp vs Fuse
The MK5 Extruder’s DC motor seems prone to a shorted-winding failure that reduces the DC resistance of (at least) one pole to (at best) a few ohms. The A3949 H-bridge driver has an upper limit of 2.8 A, but the failed winding jams too much current through the chip and eventually (instantly?) kills it stone cold dead.

Discussions on the Makerbot Wiki tended to favor fuses. My buddy Eks suggested putting an incandescent lamp in series with the motor leads, as described there, and that’s what I’ve done. That discussion is also informative.

It’s worth noting that the A3949 datasheet has this to say about overloads:
```
Output current rating may be limited by duty cycle,
ambient temperature, and heat sinking. Under any
set of conditions, DO NOT exceed the specified
IOUT or TJ.
```
So all this may be irrelevant: any transient overload could kill the driver chip stone cold dead, regardless of how clever you (think you) are.

Anyhow.

Yesterday I came across my Big Box of Fuses and said the obvious thing:

Let’s Find Out!

Note: that’s not the same as the Famous Last Words “Hold my beer. Watch this!”

I clipped the oscilloscope across a 1 Ω power resistor, set a 3 A bench power supply to 12.0 V, and connected a Device Under Test between the +12 V lead and the resistor:
- The #89 bulb from my TOM
- A Littelfuse 3AG 1 A fast-blow fuse (actually, two of ’em)
- A dead short
I used a 1 A fuse because that’s what I have. I strongly suspect a 1/2 A fuse would behave about the same way.

The oscilloscope trace starts at 0 V, jumps when the DUT contacts the resistor, and then settles at the final current. The 1 Ω resistor makes the vertical scale read directly in amps. Pay attention to the horizontal scale.

First, the lamp:

Type 89 Lamp

The peak current hits 4.5 A before the bulb lights up and limits the current to about 600 mA in the steady state. The supply’s current limiter doesn’t seem to come into play: the bulb wrestles the current under 3 A before the supply notices what’s going on. Indeed, it’s under 3 A in 2 ms and below 1 A in 20 ms.

Next, the fuse:

Littelfuse 3AG 1A Fast – 50 ms

The peak current starts off-scale high, well in excess of 7A, drops to the power supply’s 3 A limit, then falls to zero when the fuse blows 76 ms later.

Finally, the dead short:

Bare 1 ohm resistor

I changed the vertical scale to capture the initial peak, which tops out just under 10 A, obviously not limited by the power supply. The supply eventually clamps the current to 3 A and, because there’s no fuse, the current just sits there.

So…

The lamp does a much better job of protecting the H-bridge chip than the fuse:
- The peak current is lower
- It cuts off sooner
- And the sustained current falls well within the chip’s limit
The TOM does not have a current-limited +12 V supply, which means a nominally “protective” fuse will conduct whatever current the failing motor’s winding will permit until it eventually blows. The time-to-blow depends on the fault current: if the winding fails at, say, 6 Ω the fuse will last much longer while it passes 2 A than with the 3 A you see here.

Here’s an example of how that works. The first time I tapped the fuse to the resistor, I flinched and it fell off:

Littelfuse 3AG 1A Fast – 20 ms

That’s indistinguishable from a blown fuse, but the same fuse subsequently produced this result (another fuse died to produce the first fuse picture):

Littelfuse 3AG 1A Fast – 100 ms

Moral of the story: a 1 A fuse can pass 3 A for 80 ms and live to tell the tale!

Of course, I knew how this would work out: Eks didn’t accumulate 100+ patents during his career by not knowing what he was doing…

[Update: It works just like it should! Bacon saving in full effect!]
2011-02-10
Thing-O-Matic: Cable Clampage

The snarl of wires, cables, and filaments inside a Thing-O-Matic is a wonder to behold. A few cable clamps can tidy it up and reduce the chance that a loose wire will snag on a moving stage.

It’s probably a Good Idea to keep the thermocouple cable out of the bundle with the stepper cable, but, other than that, a few clamps inside the body work fine:

Cable clamp inside body

There’s another clamp inside the right-front corner that corrals the ABP cabling.

Atop the body, a clamp keeps the Z axis cable and Extruder motor wires under control. This was before I added Powerpoles and the Safety Lamp into the DC motor cable.

Cable clamp atop body

A little clamp immobilizes the thermocouple cable near the Thermal Core. The fat red wire across the top is the Thermal Core static drain and ground connection.

Thermocouple cable clamp

These clamps have an adhesive backing, which means you don’t have to drill holes and lose screws under the bench, and it’s not the end of the world should you stick one in the wrong spot.

2011-02-09
Thing-O-Matic: Rod End Cap Tweakage

The Y axis rods seem to be a bit too long for the overall case size; they stuck out the better part of 2 mm.

Y axis rod protrusion

I applied a 3/8-inch Forstner bit to the inside of the rod end caps to make a slightly-too-deep recess, then shimmed the hole with some cardboard to make the answer come out right.

Recessed Y-axis rod caps

The Z axis rods were just barely too long, but I did the same thing to those caps.

The X axis rods were fine!

2011-02-08
Thing-O-Matic: Nut Anchoring

The next time you take your Thing-O-Matic apart, epoxy the damn nuts in place so you’re not going crazy trying to manipulate them.

Inside the ends of the Y axis stage, which makes removing the X axis rod covers trivially easy:

X axis rod cover nuts

Inside the front and back body panels, which makes removing the Y axis rod covers trivially easy:

Y axis rod cover nuts

That’s in addition to applying tape inside the panels at all the most-likely-to-be-removed T-nut locations, of course. I’m loathe to epoxy those nuts in place, but I could overcome that reluctance after bringing a few more of the things to heel under the bench…

2011-02-07