My intent with the modified HBP and removable aluminum build plate: get a stable, repeatable first layer height. That is where it all starts…
The thin(ner) aluminum plate clamped to the Heater establishes the overall platform alignment. The G-Code routine at the bottom probes the height at nine locations across the plate, with me shoving a taper gage under the nozzle at each spot and writing down what I read. The accuracy seems to be around ±0.05 mm (pretty much / sorta kinda), based on the spread-out scale on the gage and a good feel for when the gage touches the nozzle.
The results in mm above the sub-platform, after leveling the HBP with the adjusting bolts:
The 3.6 mm in the front right comes from the wiper hitting the Thermal Core insulation. I must trim that thing a bit!
Apart from that, it’s as flat and level as you could possibly want.
Measuring identical Outline extrusions with a max Z height = 116.5 mm and a 0.33 mm layer thickness on the three build plates produces these numbers, with units of 0.01 mm to save some typing:
Plate 2 may have trapped a bit of grit underneath the near edge; sometimes the dissolved ABS oozes around the edge or down a bolt head clearance hole to the underside, despite my best efforts.
Apart from that, they’re about as flat and level as you could possibly want from a loose plate sitting atop a moving platform. While the difference between a 0.43 and a 0.31 mm layer is visibly obvious, I don’t know how to get the plate any more level than that… at least without an entirely different and much heavier mechanical structure.
The real question comes down to repeatability: will the platform behave the same way under each extrusion, day in and day out, without requiring height adjustment for every object?
I know a bit about tool height probing, but it’s not clear attaching a Z-minimum limit switch to the side of the HBP, perhaps under the Heater, would track the top surface of the plate with sufficient accuracy. I don’t like touching the nozzle to the build plate itself, because either one may have an insulating layer of ABS or a bit of grit or whatever that would prevent electrical contact. Ditto for optical sensing, which depends on not having any snot hanging from the nozzle.
It’s definitely true that the platform height depends strongly on the HBP temperature. As nearly as I can tell, the build platform rises by about 0.5 mm as the Heater stabilizes the plates at 120 °C.
More numbers to follow, as they accumulate.
The height probing routine, in which you must set a suitable Z height for your very own machine:
(Measure surface flatness) (MakerBot Thing-O-Matic with ABP and aluminum plate) (Tweaked for TOM 286) (Ed Nisley - KE4ZNU - Mar 2011) (-- The usual setup --) 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 Z116.5 (set Z for HBP with aluminum sheet platform) G0 X0 Y0 Z5.0 (-- Begin probing --) G1 Z1.0 (center) G4 P9000 G0 Z5.0 G0 X-40.0 (left center) G1 Z1.0 G4 P9000 G0 Z5.0 G0 Y-50.0 (left front) G1 Z1.0 G4 P9000 G0 Z5.0 G0 X0.0 (mid front) G1 Z1.0 G4 P9000 G0 Z5.0 G0 X40.0 (right front) G1 Z1.0 G4 P9000 G0 Z5.0 G0 Y0.0 (right center) G1 Z1.0 G4 P9000 G0 Z5.0 G0 Y50.0 (right rear) G1 Z1.0 G4 P9000 G0 Z5.0 G0 X0.0 (mid rear) G1 Z1.0 G4 P9000 G0 Z5.0 G0 X-40.0 (left rear) G1 Z1.0 G4 P9000 G0 Z5.0 G0 X0.0 Y0.0 (center again) G1 Z1.0 G4 P9000 (G0 Z5)