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

Tag: Thing-O-Matic

Using and tweaking a Makerbot Thing-O-Matic 3D printer

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
Thing-O-Matic / MK5 Extruder: Protecting the Thermocouple

The stock MK5 Extruder head assembly instructions suggest wrapping the thermocouple with Kapton tape before capturing it under the washer against the Thermal Core. Alas, as I’ve found, that doesn’t work well: the tape isn’t proof against mechanical forces applied to small objects and the thermocouple bead can punch through the tape to contact the Core.

This isn’t a problem until one of the heating resistors blows out and shorts the +12 V supply to the Thermal Core. The only ground path is through the thermocouple, which leads to the MAX6675 thermocouple interface chip, which generally results in a dead Extruder Controller. The third picture in that thread is chilling, isn’t it?

I cast my thermocouple into a brick of JB Industro Weld epoxy for both mechanical and electrical protection. The epoxy is rated for 500 °F (call it 260 °C), which is barely adequate for the job, but JB Weld is cheap & readily available. Note that this isn’t your really cheap garden-variety clear epoxy, which falls apart at much lower temperatures. That discussion suggests a higher-temperature epoxy from Omega, but I haven’t gone that route yet.

Anyhow, I converted three credit-card-thickness sale coupons from Staples into a brick-shaped mold around the thermocouple. The middle card has a slot for the thermocouple wire, which means the bead is positioned in free space in the middle of the opening.

Thermocouple positioned in mold

A close-up of the thermocouple bead:

Thermocouple positioned in mold – detail

I taped that assembly to another coupon, filled the mold with JB Weld, made sure everything was saturated, and gave it a day to cure. This view shows the brick after peeling off the top coupon, so you can see the cable slot:

Removing thermocouple from mold

A bit of filing and general cleanup made it presentable:

Finished thermocouple brick

A wrap of Kapton around the brick gives the Thermal Core washer something to grab onto:

Thermocouple in place – ceramic insulation jacket

The brick could be much smaller without any penalty. There’s no issue with excessive thermal mass here, however, because the Core itself has a 10-minute time constant, so the thermocouple has plenty of time to tag along.

The red wire in the upper-left corner connects the plate above the Thermal Core directly to the static drain ground point that leads to the ATX power supply case. In the event of a resistor failure that shorts the +12 V supply to the Thermal Core, the power supply should shut down. Whether that will actually happen, I cannot tell, but now a failed resistor won’t destroy the thermistor or the Extruder Controller.

The ceramic wool insulation (from a lifetime supply of furnace chamber lining; it’s rated for direct oil burner flame impingement) may seem excessive, but I wanted measurements from a well-insulated Thermal Core at reduced power: 40 W seems to do the trick.

However, the insulation on the bottom of the Core around the Nozzle tended to catch on the ABP’s silicone wiper. The next iteration used just the original MBI ceramic cloth insulation on the bottom, protected by Kapton tape, with ceramic wool around the rest of the Core. Much better!

2011-02-06
Thing-O-Matic: X Axis Motor/Belt Adjustment

The socket-head cap screws securing the ball bearings that ride on the left-side Y stage rod prevent the X axis motor from sliding rightward along its mounting slots. The rightmost position, as enforced by the SHCS heads, makes the belt far too tight.

This picture (cropped from one in the MBI Thing-O-Matic assembly instructions) shows the situation under the Y stage with the as-built components:

X axis motor mount – MBI image

This suggests that the ball bearing assembly was an afterthought, perhaps solving the same overconstrained rod problem as I fixed in the X axis stage. The as-built motor position pulls the X axis belt just slightly less taut than a banjo string, which isn’t a Good Thing.

The solution is to replace the four SHCS with pan-head screws to get a bit more clearance. Fortunately, my Parts Heap had some salvaged 3 mm screws of sufficient length, so I avoided a trip to the Big Box retailer. Rather than put everything together and discover the heads were still too tall, I ground them down so just the barest hint of the slot remained:

Modified pan-head screw

That provided enough clearance to make the X axis belt entirely slack, which means I probably didn’t have to grind the heads in the first place. In any event, the proper position looks more like this:

Adjusted X axis motor position

If you look very closely, you can see the marks from the original position near the middle of the slots. Here’s a blown-up and contrast-stretched view:

X axis motor mounting slot – detail

Those few millimeters make all the difference in the world: the belt is now decently tight, the motor responds well, and all is right with the world.

2011-02-05
Thing-O-Matic: X and Z Axis Rod Alignment
Many of the discussions in the Makerbot Operators Google Group involve bad prints due to “missing steps”, overheated stepper motors, and other motion-related maladies. The proposed cures generally don’t address the real problem, which has nothing to do with slipping belts, inadequate motor current, or general hygene.

The problem is rod alignment, which is not guaranteed by the laser-cut plywood frame.

The Thing-O-Matic guides all its moving parts with bronze bushings sliding on polished steel rods to ensure low friction and exceedingly long life. Unfortunately, you can easily assemble a TOM with X and Z (and sometimes Y) stages you can barely push by hand: I’ve done it!

The symptoms involve the actual position gradually departing from the commanded position: a G0 X10 Y20 command might actually put the extrusion nozzle at X=9.9 Y=20. That error produces a small offset along the X axis that gets worse on successive layers and eventually causes the object to resemble the Leaning Tower of Pisa. The TOM can’t correct the error, because it doesn’t know where the stage actually stops after a command: the steppers run open-loop.

This can’t be due to a slipping belt, because a toothed timing belt can only skip by multiples of the tooth pitch: a one-tooth “skip” means a 2 mm positioning error that’s immediately obvious. In any event, if the belts are that loose, you have other problems.

It could be a loose belt drive pulley on the motor shaft, but that will produce random offsets in both directions as the setscrew gradually chews a slot around the motor shaft. If that’s the symptom, fix it now because you won’t be able to get that pulley off after the setscrew finishes raising a burr around the shaft.

The errors generally happen in the X direction because the X stage slides on two rods, each of which is fixed at four places: both ends of the Y stage and both ends of the X stage. The tech term for this is “overconstrained”: two points determine a line, but here we have a line that must pass through four points.

If the rod-to-rod spacing in those four places isn’t exactly equal, then the X stage bushings will bind on the rods. Alas, tolerance creep in the plywood and maybe a bit of off-center sanding when you fitted the bushings into the plywood can produce exactly that situation.

The Y stage doesn’t have this problem, because the right side rides on bushings and the left side rides on three ball bearings, making it not so sensitive to
horizontal misalignments.

Diagnosing this in an assembled Thing-O-Matic presents a major nuisance, but is well worth the effort. Release the X stage drive belt by loosening the X axis motor bolts (or, if you haven’t modified those bolts, by dismounting the idler pulley, which means extracting the whole XY assembly from the TOM and taking it apart) so the carriage can slide without moving the belt and turning the motor.

If you can move the X stage back and forth along the entire length of its travel by pushing gently with one fingertip, it’s all good. Most likely, you must apply far more force than that, as was the case in my TOM after I first assembled it: moving the X stage required quite a shove and it definitely didn’t slide freely.

Fixing this is straightforward, at least with the entire X and Y assembly out of the TOM. There are two steps:
- Align the X stage bushings so the rods move freely
- Align the Y stage mounting points to match the X stage spacing
To begin…

Remove the X stage from the Y stage, then remove the base plate so you can see the inside end of all the bushings. Slide each rod out of one bushing, then try to slide it back. I predict it’ll look something like this:

Misaligned X Axis bushing

The rod wants to avoid the hole in the left bushing. Orbit it around in the right-side bushing until it’s well centered on the left bushing. You want it to look like this when it approaches that bushing:

Aligned X Axis bushing

When it’s properly aligned, slide it in. You should then be able to bat the rod back and forth with your fingertips; if it doesn’t slide freely, slide it out of one bushing, apply more wiggly jiggly action, and get it aligned. Bat the rod back and forth a few times to get a feel for free motion, then repeat for the other rod.

About lubrication: the bronze bushings are self-lubricating, but a bit of oil won’t do any harm. Machine oil is good, cooking oil is bad, butter is terrible. If the rods feel nice and slippery, it’s fine.

When both rods slide freely, pop the X stage back into the Y stage. This is actually possible with both rods in the X stage, although now that you know what you’re looking for, you can slide them out, put the X stage inside, then slide the rods back in again. Remember, you want free rod motion within the X stage itself.

With the rod ends captured in the Y stage, put the front end caps on to hold that rod in place. Slide the X carriage to the right end of its travel (hold the loose rear rod!), then push the rear rod out of the one end piece by pushing it into the plywood while supporting the X carriage. Most likely, the rod will go spung a fraction of a millimeter horizontally as it exits the end piece (you control the vertical offset by supporting the carriage).

That’s the rod’s way of telling you that the end hole is in the wrong position. If the rod slides easily in and out of that hole, then it’s all good. If it doesn’t, then sand the offending side of the hole until the rod slides easily into the hole.

I wrapped a length of sandpaper around a brass tube so the sandpaper formed a cylinder nearly the same diameter as the rod, which prevents sanding a notch into the plywood that makes things worse. Remove wood from the side of the hole, not the top or bottom:

Adjusting rod hole position

When the rod slides freely into the hole with the X stage at that end, slide the stage to the other end and repeat the process. You must do both ends of travel to get all four constraining points lined up properly.

Recheck the rod fit at both ends of travel, then install the end caps.

The X stage should now slide back and forth with just light finger pressure.

If you overdo the sanding, shim the loose side of the hole with aluminum foil and a dab of adhesive. If the rod rattles around, that’s bad; add an all-around shim and put a very thin slice of foam under the end cap to calm it down.

Verily, it is far better to sand a little and check a lot!

You can apply the same process to the Z axis stage and rods. Remove the bolts holding the motor to the top plate, then verify that:
- The Z stage freely slides up and down the rods
- The rods align with their mounting holes with the Z stage each end
Sand the holes for one of the rods to make that answer come out right, too.

Those bronze bushings work wonderfully well, but only when the rods are
exactly parallel and properly spaced.

[This post is a revised, corrected, and expanded version of a comment I posted on the MBO group.]
2011-02-04