Tag: Improvements

Getting a good bond between the build platform and the first extrusion layer depends on the nozzle height above the platform: too high and it doesn’t stick, too low and the excess plastic ruffles up to ruin the next layer. I’ve been setting the height manually, but that’s tedious, fraught with error, and goes best with a cool platform that may change size when it heats up. Measuring the Outline or Skirt extrusion provides after-the-fact information about build platform alignment, but doesn’t ensure that the current print will work.

The best way to do this is to measure the actual height of the nozzle above the build platform immediately before starting the extrusion, with everything at operating temperature, then set the Z axis so that Z=0.0 puts the nozzle on the platform. Nophead put a height measurement station beside the build platform and I’ve built a tool length probe for my Sherline, so I’m not breaking new ground here.

The catch is that the switch must sit flat on a 120 °C platform, withstand being poked with a 220 °C nozzle, repel ABS, and trip with better than 0.1 mm repeatability. I don’t know that what I’ve done here meets all those criteria, but it’s a first step along the way.

The top picture shows a surplus SMD pushbutton switch with a metal actuator button mounted on a small steel strip. The gray epoxy blob to the front secures a brass tube that protrudes from the bottom into the middle socket head cap screw along the right edge of the platform.

[Update: The surplus place was likely Electronic Goldmine, but it seems they have no more. Sorry. Maybe glue a metal disk atop a plastic switch?]

The steel strip was an RF shield from a junked wireless network card. I bent the edge in a small sheet-metal brake to keep the whole thing rigid and, somewhat to my surprise, the strip remained as flat as I can measure throughout the adventure.

Drilling a good hole through sheet metal is easier when you clamp it between two sacrificial sheets and drill through the whole stack. That keeps the metal from warping and gives you a nice, circular hole; otherwise, you get a rumpled sheet with a triangular hole that’s good for nothing. You can see the bent edge sticking up on the left; the drill center is 4 mm from that side.

The brass tube in the hole and the bent edge constrain the switch to a known position relative to the underlying HBP. The tube fits snugly in the center bolt’s hex socket to set the XY position and the bent edge keeps the whole affair parallel to Y. That allows the upper plate to shift slightly in XY while the switch remains in the same location relative to the TOM’s XY home switches (the bent edge allows a bit of slop in X for the top plate’s hole tolerance)

The Z height, of course, depends only on the altitude and thickness of the top plate, which is exactly what’s being measured relative to the nozzle.

I built it in two stages: epoxy the brass tube, then mount the switch and a second tube as a strain relief around the wire. A layer of Kapton tape insulates the SMD switch terminals from the steel strip; the epoxy sticks well enough to the tape for my present purposes.

None of the dimensions are critical, although having the whole assembly narrow enough to stay out of the build area seems like a Good Idea. The center line of the platform bolts sits 4 mm inward of the plate edge, to give you an idea of the scale.

The right-side bulldog clamp holds it securely in place, with the wire from the switch threaded beside the platform and wrapped around a solderless lug on the front corner:

I think a tiny neodymium magnet embedded in the top plate would work just as well, although it’d tend to suck steel swarf out of the rest of the shop. This is not the right place for a random speck of grit!

The wire connects to a 24 inch CD-ROM audio cable fresh from the usual eBay supplier. I snipped off the end and added a resistor to resemble an MBI mechanical endstop switch: 10 KΩ to +5 V, switch connects the resistor to ground. No LED, alas.

Next, a dab of G-Code to poke the nozzle into the switch…

I’m using a removable aluminum plate atop a fixed plate on the HBP, but haven’t clamped the two together because I couldn’t figure out how to do it without an overly complex gadget.

It turned out to be easier than I expected, after I found a couple of bulldog clips in a drawer while looking for something else:

The clip on the right must be well toward the rear in order to clear the X axis limit switch and its cable. The clip on the left then goes near the front just for symmetry.

The nozzle can’t quite reach the left clip, but it can clobber the right one. I try to align the end of the clip with the middle of the bolt heads to keep them out of the build area.

The wire handles don’t quite touch the TOM’s case on the left side and have plenty of clearance on the right. They don’t get too hot after an hour’s worth of printing; clips with solid metal handles wouldn’t work well at all.

News flash: that’s almost the last of the pink plastic!

When I built the Wade-ScribbleJ filament tensioner, I used four of the stiffest springs available in the Little Box o’ Small Springs. They came without a pedigree, of course, and worked quite well. However, the filament would occasionally stop feeding, usually after an intense series of reversals, and it seemed more pressure on the filament was in order.

The 1.5 inch 4-40 screws limit the available length to no more than 12 mm and the tensioner must have at least 1 mm of free travel to accommodate filament thickness variations. Those springs had fairly dense coils and they were pretty much fully compressed.

They turned out to compress 9 mm with 2.5 pounds applied, for a spring constant of 1.2 N/mm or, for we metric-challenged Yanks, 7 lb/in. Some rummaging turned up my Brownell’s No. 71 Compression Gun Spring assortment and I found a spring that compressed 5 mm with 5 lb applied: 4.4 N/mm or 25 lb/in.

I know you’d love pix of that process, but I was already one hand shy of having enough to push the spring scale against the [4-40 screw + washer + spring + washer] over a metric ruler, then apply enough force to compress the spring  while reading the distance between the washers. Use your imagination, OK?

I sliced four 4.5 turn lengths from that spring with a Dremel cutoff wheel, cleaned up the ends a bit to get them all to about 13 mm, reassembled the tensioner, and cranked the screws to compress the springs down to 8 mm. The quartet now apply something like 25 lb = 110 N to the idler bearing. That’s about four times what it was before, so that filament should have no reason to slip, even under cough extreme duress.

Tomorrow: Applying some extreme duress…

So I finally got around to properly trimming & installing the silicone wiper to suit the aluminum build plates, then measuring where it sits in XYZ coordinates.

The notched upper edge (that’s not a shadow) more-or-less matches the nozzle shape, but I doubt that’s critical. The key part: make it short enough to miss the bottom of the insulation blanket around the Thermal Core, which is much thicker on my Extruder than yours.

From this angle you can see the nozzle just in front of the wiper. I used a low-profile bolt, although it’s still slightly higher than the top aluminum build plate.

This G-Code routine helped figure out all the parameters. Define the maximum Z height properly, then fiddle with everything else to center the nozzle on the platform, pause in front of the wiper, and stop at the splodge position.

(---- manual wipe and splodge alignment ----)
(MakerBot Thing-O-Matic with aluminum HBP)
(Tweaked for TOM 286)
(Ed Nisley - KE4ZNU - April 2011)
(-- set initial conditions)
G21		(set units to mm)
G90		(set positioning to absolute)
(-- home axes)
G162 Z F1500	(home Z to get nozzle out of danger zone)
G161 Y F4000	(retract Y to get X out of front opening)
G161 X F4000	(now safe to home X)
G92 X-53.0 Y-59.0	(set XY coordinate zeros)
G92 Z116.0    (set Z for HBP with aluminum sheet platform)
G0 X0 Y0 Z10	(pause at center to build confidence)
G4 P2000
(-- manual nozzle wipe)
G0 X56 Y-57.0 Z15	(move to front of cut-down wiper)
G0 Z5   		(down to wipe level)
G4 P4000		(Wait for filament to stop oozing)
G1 Y-40	F1000		(slowly wipe nozzle)
(-- manual splodge)
G0 X-52 Y-58  (to front left corner)
G1 Z0.50      (just over surface)

This becomes a chunk of the heavily tweaked start.gcode that Skeinforge folds into every sliced file.

Of course, just when I get this figured out, Skeinforge 40 eliminates the Outline plug-in, shuts off the extruder before zipping off to begin the Skirt (starting from the right side!), and prints the perimeter thread of the bottom layer first. More pondering is definitely in order…