Advertisements

Archive for January 9th, 2011

Thing-O-Matic / MK5 Extruder: Static Control

The Thing-O-Matic touches the plastic filament in three places:

  • Filament Drive Frame
  • Extruder Thermal Core
  • Automated Build Platform Belt

In each case, the plastic filament slides (or oozes) along another plastic surface, which is the classic way to generate a charge of static electricity. Think of a running a comb through your hair, rubbing a cat on a balloon, shuffling across a carpet in your fuzzy slippers, or pulling off an acrylic sweater.

In addition, the X and Y stepper motors each drive a rubber-ish timing belt around a plastic roller. Non-conductive belt on plastic pulley = static charge, with metal motor pulley collecting it on the motor shaft, thence to the motor frame. The motor shafts and frames do not connect to any of the motor conductors, because in most machines the stepper motors mount to a metal chassis. The Thing-O-Matic insulates its motors on plywood or plastic sheets with no conductive path to ground.

None of those metal parts has any provision to control a static charge accumulation, which means the charge will increase until one of two events transpires:

  • The charge reaches an equilibrium with leakage through the air
  • The potential reaches air’s breakdown voltage and arcs to an adjoining metal object closer to ground potential

The former situation may be tolerable (and is most likely during the humid summer months), but the latter causes those annoying random crashes and, sometimes, hardware failures. In round numbers, air’s breakdown voltage exceeds 1 kV / mm (25 kV / inch), which explains that blue-hot spark from your fingertip to the screw on the light switch.

I added drain wires to all of those locations, using wire stripped from an old ribbon cable. There’s no particular current involved, so thin wire will work just fine. Double it over a few times to fill the barrel of the solderless connectors, though, and use some heatshrink tubing for strain relief.

The ABP platform heat spreader underneath the belt looks like a huge (and completely isolated) capacitor plate with respect to the plastic accumulating atop the belt. The wire attaches to the far right rear of the spreader and trails off with all the other ABP cabling. Yes, those are the wooden side plates, not the acrylic ones, for a reason I’ll explain when I work through my embarrassment.

ABP Heat Spreader static drain

ABP Heat Spreader static drain

There’s no good way to attach a wire to the metal foil, so I used a dab of Wire Glue. The cured carbon-rich blob probably isn’t rated for protracted use at 125 °C, though, and perhaps a mechanical flange captured under one of the socket-head cap screws will be a better idea. This is a detail of the contact end; I threaded the wire through the solderless ring terminals for strain relief.

ABP static drain - detail

ABP static drain - detail

The Extruder DC motor has bolts passing entirely through the Filament Drive, so I captured a solderless connector under one head. After taking this picture, I realized that the lower motor bolt on the left side is a better location, as that one aims the connector’s open end up and to the right. Make it so.

Extruder motor static drain

Extruder motor static drain

The X axis stepper motor drain wire dunks down through a motor mount slot and follows the motor winding conductors out of the housing.

X Axis static drain

X Axis static drain

The Y axis stepper motor frame serves as the connection point for the Extruder Motor and X-axis drain wires, each secured under a separate motor mounting bolt. The third wire (with black and white heatstink tubing) snakes down through the left-front motor mounting slot in the acrylic sheet above the electronics bay.

Y Axis motor with static drains

Y Axis motor with static drains

The Z axis stepper has only metal-to-metal sliding contact, so it’s presumably free of static buildup. If you’re being fussy, ground that one, too.

The Extruder Thermal Core also requires a drain wire, but that one must also handle the fault current from a resistor failure that shorts the +12 V supply directly to the Thermal Core; I’ll discuss that situation separately in a few days.

The ABP and Y Axis drain wires join a hacked-together ground point secured to the metal case of the ATX power supply metal case. You could, of course, connect these to a DC common supply lead (any Black wire), but these are, by definition, non-current-carrying leads that ought not be mixed with the power distribution. The case is a known-good grounding point that’s bonded to the AC line’s earth-ground conductor, exactly where static charges want to go.

Static drain to ATX supply connector

Static drain to ATX supply connector

The connector is obviously from a cut-off Molex-style hard drive power cable with all four sockets wired together; I sacrificed a handful of Y-splitter power cables for another project a while ago. The pins are lengths of 12 AWG copper wire harvested from a length of Romex house wire, with the drain wires soldered to one end, then covered with heatstink tubing. This is a kludge, but a workable solution.

Although I think static discharge is a relatively minor contributor to the random crashes and failures, it’s easy enough to eliminate with no side effects… as long as you leave enough wire to reach the far end of the axis travel range.

Advertisements

, ,

19 Comments