Category: Machine Shop

After building a few large objects (about which more later) and verifying that the Z-minimum switch remains stable and delivers useful results, I cobbled up a test script that simplifies measuring the build platform tilt and extrusion thickness for a given switch height value.

The pattern looks about like this, although the G-Code script below connects the top of the X so it hangs together better.

The extrusion uses my defaults for the first layer: 0.33 mm layer thickness, 2.0 w/t, 10 mm/s, 0.66 rev/min. Those are 20% of the normal extrusion speeds: 50 mm/s and 3.3 rev/min.

I built all the objects without adjusting the plate tilt to determine if all three plates produced the same result: they do! The results showed a consistent tilt, with the rear left corner high by 0.10 mm and the front right corner low by 0.10 mm. Those are all rubbery numbers, with accuracy based on measuring a filament, but they’re reasonably consistent.

The Z-min switch set the middle of the platform to about 0.35 mm, making the corners around 0.25 mm and 0.45 mm. That led to slightly too much plastic (plate too high) and slightly too little (plate too low), respectively, but the objects printed quite well.

Running the G-Code below under those conditions produced these numbers:

The numbers come from two measurements on each side of the outer square at about the 1/4 and 3/4 points, plus four measurements near the middle of the X. I didn’t average multiple measurements, some are definitely off due to error, and your mileage will vary.

Those thicknesses aren’t quite the same as I’d been seeing around printed objects, but if you squint you can see the tilt. There’s one missing number; that part of the X broke off and vanished somewhere between the Basement Laboratory and my upstairs desk.

I tightened the rear left bolt 1/6 turn (about 0.08 mm) to lower that corner and raise the opposite one, then stuck a sheet of 8-mil (0.008 inch = 0.021 mm) shimstock under the upper plate to see how the switch handled an abrupt change in plate height:

Another text X extrusion produced these thicknesses:

It’s obviously tilted even worse than before, which means the shimstock has the odd nick, bend, or bit of grit. What’s important, however, is that the extrusion thickness remains pretty close to normal, despite having the build surface 0.22 mm higher than usual. Without the Z-min switch on the plate, the extrusion thickness would suddenly decrease by 0.21 mm… and that’s enough to wreck a print!

Removing the shimstock and running another test extrusion produced about what I expected:

Overall, the thickness now lies within 0.05 mm of the 0.33 mm average. My measurement accuracy simply isn’t good enough to get any better than that.

The evidence so far suggests the platform tilt remains reasonably constant, even as the overall height varies, so I think a Z-minimum switch on the build platform should compensate for the changes that affect first layer extrusion thickness.

What that means: slice an STL into G-Code and fire the Thing-O-Matic!

This Level Test.gcode file, which is basically my start.gcode and end.gcode laminated around the snippet that draws the boxed-X pattern, lives in ~/ReplicatorG/scripts/calibration and thus appears on RepG’s pulldown menu:

(---- start.gcode begins ----)
(MakerBot Thing-O-Matic with aluminum HBP and Z-min platform switch)
(Tweaked for TOM 286 - Ruttmeister MK5 stepper extruder mod)
(Ed Nisley - KE4ZNU - May 2011)
(- set initial conditions -)
G21		(set units to mm)
G90		(set positioning to absolute)
(- begin heating -)
M104 S210 T0	(extruder head)
M109 S120 T0	(HBP)
(- coarse home axes -)
G162 Z F1000	(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 Z117.0	(set XYZ coordinate zeros)
(- fine home axes)
G0 X-51 Y-57 Z115 F400	(back off switches)
G161 Y F200
G161 X F200
G162 Z F200
G92 X-53.0 Y-59.0 Z117.0	(re-set XYZ coordinate zeros)
(- manual nozzle wipe)
G0 X0 Y0 Z10	    (pause at center to build confidence)
G4 P500
G0 X40 Y-57.0 Z10	(move to front, avoid wiper blade)
G0 X56            (to wipe station)
G0 Z6.0           (down to wipe level)
M6 T0			        (wait for temperature settling)
G1 Y-40	F1000		  (slowly wipe nozzle)
(- home Z downward to platform switch)
G0 X55.9 Y8 Z3	      (get over build platform switch)
G161 Z0 F50	          (home downward very slowly)
(-----------------------------------------------)
(- Set the Z height based on the switch height  )
G92 X55.7 Y8 Z1.45
(-----------------------------------------------)
G0 Z6.0			          (back off switch to wipe level)
(- start extruder and re-wipe)
G0 X56 Y-40     (set up for wipe from rear)
G1 Y-57.0 F1000 (wipe to front)
M108 R2.0	      (set stepper extruder speed)
M101		        (Extruder on, forward)
G4 P4000  	    (take up slack, get pressure)
M103		        (Extruder off)
G4 P4000  	    (Wait for filament to stop oozing)
G1 Y-40	F1000		(slowly wipe nozzle again)
G0 X0           (get away from wiper blade)
(- manual splodge)
G0 X0 Y-58		  (to front center)
G0 Z0.5 		    (just over surface)
M108 R2.0	      (set stepper extruder speed)
M101            (start extruder)
G4 P2000        (build up a turd)
(---- start.gcode ends ----)
(--------------------------)
(- print thread around platform)
( Speed as in Raft plugin for first layer)
( Print continuously to make it hang together while measuring)
M108 R0.66				(set stepper extruder speed)
G1 X-45 Y-45 Z0.33 F600	(to front left corner)
G1 Y45
G1 X45
G1 Y-45
G1 X-45					(return to front left)
G1 X40 Y40				(diagonal to rear right)
G1 X-40					(to rear left)
G1 X45 Y-45				(diagonal to front right)
(--------------------------)
(---- end.gcode starts ----)
(Tweaked for TOM 286)
(Ed Nisley - KE4ZNU - May 2011)
(- inhale filament blob)
M102		(Extruder on, reverse)
(- turn off heaters)
M104 S0 T0 	(extruder head)
M109 S0 T0 	(ABP)
(- move to eject position)
G162 Z F1500	(home Z to get nozzle away from object)
M103		(Extruder off)
G0 X0		(center X axis)
G0 Y40	(move Y stage forward)
(---- end.gcode ends ----)

Things are looking good!

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!