Archive for January 13th, 2012

Thing-O-Matic: HBP Rebuild

The basic problem with the heater on the Heated Build Platform is that the SMD pads must both make electrical contact to the Molex-style connector and withstand mechanical stress from the dangling wires & cables as the platform moves along the X and Y axes. Rather than replace the entire heater, I attached pigtail leads to the PCB, anchored those leads to the wood platform under the heater, and routed the cables through the deck under the Y axis stage a bit differently.

However, attaching pigtail leads to the PCB poses a problem, because ordinary electronic hookup wire has thermoplastic insulation that melts or deforms at temperatures well under my usual 110 °C platform heat setting; shorting the heater wires would be a Very Bad Thing.

Some concerted rummaging in the Big Box o’ Multiconductor Cable turned up a hank of Teflon-insulated shielded two-wire cable that, as nearly as I can tell, has pure silver conductors and shield braid: the ends were tarnished like silver and there’s nary a trace of copper in the fresh cuts. It must be military surplus and, based on a vague recollection, was most likely cough salvaged by my father, who worked as an avionics tech at Olmstead AFB in the mid-60s. Ya gotta have stuff, right?

[Update: Alas, it’s not pure silver, as shown in the comments.]

The general idea is to scuff up the shiny PCB surface enough to anchor blobs of JB Industro Weld epoxy that surround brass tubes holding the cables. A pair of tubes secure each cable and provide strain relief; the cable is free to move, but not by very much. The thermistor cable has a long arch that will, I hope, keep the cable at the platform temperature and reduce its cooling effect on the thermistor:

Thermistor rewiring - heat cure

Thermistor rewiring - heat cure

The alligator clips connect to a bench power supply that delivered 4 V @ 2 A = 8 W that heated the PCB to about 40 °C in the rather chilly Basement Laboratory and encouraged the epoxy to cure in less time than forever.

The final result looked like this, with Anderson Powerpoles now attached to the heater cable:

Rewired HBP

Rewired HBP

The 24 AWG conductors in the cable may seem scanty for 6 A of heater current, but, hey, they’re silver.

The three-pin connector on the end of the thermistor cable is a pure kludge, built from a 4-pin header to match the CD-ROM audio pinout on the new cable from the Extruder Controller. I kept the default pinout on this end to provide some protection against plugging it in backwards:

Kludged HBP thermistor connector

Kludged HBP thermistor connector

With all that in hand, I screwed the PCB to the aluminum sub-plate, bolted it to the plywood platform, and stuck the cables onto the platform with adhesive clamps:

Rewired HBP - front

Rewired HBP - front

Reaming out the hole between the red and black Powerpole shells provided just enough room for an M3 screw to anchor them to the HBP: they won’t flop around under acceleration.

The thermistor cable exits to the left, the rest to the right, and I’m unhappy with the overall routing. I added a small bumper (made from bent steel shim stock) to keep the thermistor cable out of the gap between the Y axis stage and the left side wall:

Y Axis gap filler

Y Axis gap filler

So, yes, it works, but it sure ain’t elegant.

The first object was the revised platform level test pattern:

Rewired HBP with level test pattern

Rewired HBP with level test pattern

The platform is holding level within ±0.05 mm across build plates 1 and 2, somewhat better than before. On the other paw, the whole thing doesn’t have many hours on it…

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