Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Companion cube, with a slightly warped right corner:
Companion Cube – build detail
Now, those objects may have other problems, but two things work really well:
The first layer sticks like it was glued to the ABS film
The side walls build perfectly straight, without bulges or shrinkage
What’s important to me: this is dependable and repeatable.
It’s not yet a simple routine, because these objects were built while I was hacking away at the HBP + aluminum plate platform, some are on the old ABP + aluminum plate arrangement, and they’re not all first-attempt parts. However, given a proper setup, It. Just. Works.
Part of the process involves a very slow first-layer feed: about 10 mm/s. At that pace the molten ABS has enough time to bond with the layer on the plate, even around corners; much faster and it can pull free.
The Extruder runs at 210 °C, the HBP at 120 °C, feed is 40 mm/s, and traverse is around 50 mm/s.
It is yet to be seen if this lashup will remain stable, but the first indications seem pretty good.
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:
3.5
3.4
3.3
3.5
3.5
3.4
3.4
3.5
3.6
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 1
42
38
35
43
31
43
33
38
34
37
33
32
Plate 2
37
33
31
35
23
33
21
25
15
19
16
17
Plate 3
38
30
29
37
28
34
27
32
31
28
29
28
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)
The Skeinforge Outline plugin draws a rectangle around the first perimeter layer of an object. I use that single-width, single-layer extrusion to monitor the height of the nozzle above the build platform and the tilt of the plate. The Outline extrusion will either peel off separately or come off as the film peels away from the plate when I twist the object off.
These Outlines come from a variety of objects. The one in the lower left was a test case that I stopped after extruding only the Outline.
ABS coatings from aluminum build plates
I measure the Outline along each edge; larger objects provide three data points along each side of the build platform.
The good part of this is that it reports the build platform’s behavior during an actual extrusion, so you can keep an eye on whether it’s drifting out of alignment. The aluminum plates present a sufficiently flat surface that any variations will be due to a non-level HBP or an off-calibration Z-axis.
These numbers from around a large Outline told me that I should tweak the Z axis height down by 0.1 mm to increase the first layer thickness back to about 0.33 mm. The lower-right corner was slightly thicker because the wiper hit the Thermal Core insulation.
0.24
0.22
0.17
0.27
0.17
0.27
0.22
0.28
0.32
0.22
0.21
0.24
Given those values, I can tweak the leveling screws to adjust the platform tilt. What I don’t have at this point is any long-term record of how consistent my hacked HBP will be. But at least I’ve got numbers!
Printing ABS objects on an ABS film atop a heated aluminum plate works just about as perfectly as I could want, as witness those calibration objects. It turns out that the thickness of the ABS film makes a big difference in how well the first layer bonds to it.
I’m coating the plates with scrap ABS objects dissolved in MEK, because MEK seems to be less aggressively flammable than acetone. It smells horrible, though, and spreading a layer of toxic gunk with the consistency of honey can’t possibly be good for me. I use dead credit cards as spreaders and wonder if there’s a better way; a brush would clog up almost instantly.
The rough rule of thumb:
If the ABS layer isn’t obvious, then it’s too thin.
A Companion Cube growing out of a good pink film layer:
Companion Cube on ABS coated plate
Peeling a smaller Cube off a plate shows how well it bonds to clear film. Notice how the film peels off the build plate without leaving any residue except for a tear in the film covering the hollow underside of the Cube.
Companion Cube – Bottom surface with ABS coating
These outline extrusions show the effect of a too-thin film, where the extrusion simply peeled off the film. Where it’s thick enough, the extrusion is welded right to the surface. Intermediate thicknesses tend to rip on both sides of the extrusion.
ABS coatings from aluminum build plates
In round numbers, the perfectly formed film at the lower left is between 0.05 mm (the darker regions) and 0.09 mm (the deepest pink). The others range from 0.02 mm to 0.05 mm and are too thin for good bonding. Even the thickest film doesn’t add much to the first layer thickness.
The other part of the secret is extruding the first layer at 10 mm/s, which is 25% of the 40 mm/s I use for the rest of the object layers. The platform is at 120 °C, the Thermal Core at 210 °C, and the extrusion sticks like it’s welded… which, in fact, it is.
I think that a too-thin film cools the extrusion before it can bond with the film, while a just-right film melts slightly on contact. Extruding at 10 mm/s guarantees enough contact time for the filament to melt the film and cool down before the nozzle puts any tension on it: corners come out perfectly.
The other part of the puzzle requires an absolutely level build platform at a constant height from the nozzle. The platform leveling described there helps, but it’s a hassle to get everything set up.
This should go without saying, but I got it wrong the first time: protect those little pins that drive the ABP belt motor if you’re not using them, particularly if you have a grounded aluminum build plate a few millimeters away.
I used a pair of jumpers, side by side, that did not connect the two pins, because they were close at hand when I first shorted the build plate to the pin carrying +12 V. A jumper that connects the pins will blow the motor control MOSFET on the Extruder Controller if you (or the G-Code) should happen to ever turn it on by mistake, so you need two.
Protected motor terminals on HBP
Better, just put some heatshrink tubing over both pins.
Heatshrink on ABP motor terminals
You could print up a little dummy plug for the thing…
The first aluminum build plates had to fit around the gimcrackery atop my tweaked ABP: two solderless grounding lugs and a lump of Wire Glue. The new HBP setup put the grounding lug below the fixed plate and did away with the lump, so the removable plate could have five holes and a wiper cutout without any fancy trimming.
I’d squared up three plates and machined only two for the ABP, so I had one plate that just needed drilling. Rather than machining two new plates, I filled the cutouts on the old plates with JB Industro Weld epoxy, flycut the excess, and drilled new holes.
Flycut and drilled epoxy fill
This was straightforward manual CNC: get the plate square on the table, touch off the plate edges, and then drill the holes in two steps.
If those thin epoxy webs break off the outside of the holes, it’s not the end of the world: the plates won’t go anywhere because they’re indexed by the holes on the other side.
Memo to Self: Next time, make a fixture to hold the plates relative to a starting hole and eliminate all the tedious alignment steps.
An aluminum plate coated with ABS provides a very flat, very adhesive build platform, but that kludge atop the ABP didn’t provide enough stability or adjustability. I decided to scrap the ABP and modify the HBP to use the same removable plates.
The ABP kludge involved simply resting an aluminum plate atop the Heater PCB, which is ordinary PCB material with heat applied to only one surface and, on my ABP, has developed a pronounced warp. I decided to clamp the Heater to a thinner aluminum plate, ignoring the fact that PCB material has a much higher coefficient of thermal expansion than aluminum. Although the heater PCB may want to distort, I’m counting on the aluminum to maintain a flat upper surface.
HBP Heater and aluminum sub-platform adjuster
Those bolt heads sit in a 0.2 mm recess that lowers them just enough to be flush with the surface of the removable plate.
Then the removable plate sits atop the flat plate atop the heater: the two plates should snuggle together in Z and I think there’s no need to clamp them together. The holes have slight clearance around the bolt heads, making the plate a drop-in fit.
HBP with aluminum plates
This view shows the small notch filed in the front left corner of the removable plate. I use a small scraper blade to pry the hot top plate loose after a build session, grab it with pliers, and carry it away for cooling. The Sherline mill’s tooling plate provides a wonderful cold surface and the ABS gives off a distinct snap when it cools enough to detach from the aluminum plate.
Separation notch in aluminum build plate
Although it’s not automated, swapping plates isn’t a tedious affair. The real delay comes from heating a cold plate to operating temperature again, which requires about five minutes.
The silicone wiper holds the removable plate against the bolt heads, providing some stability in the XY plane. There’s no need for precise indexing.
HBP Heater and sub-platform with wiper
The Heater normally attaches to the HBP with six bolts, which severely overconstrains the surface. Here, three (center left, front+rear right) bolts clamp the heater to the sub-plate and three (front+rear left, center right) extend through the HBP plywood to nuts epoxied to the bottom surface.
Six matching springs from my Parts Heap support the whole affair, with the three on the clamping bolts being more compressed by the nuts below the Heater PCB. They come without pedigree and nearly anything that fits should work; it’s not like they must support an engine block.
The M3 bolts have a 0.5 mm thread pitch, so one turn changes the plate height by 0.5 mm and 1/6 turn (which is easy to make with a hexagonal wrench) changes it by 0.08 mm. The threads catch on the plate and Heater, so I may saw off some longer partially threaded bolts to get a smooth cylinder through the holes.
I did the initial adjustment on the surface plate with the entire XY stage assembly up on parallel blocks. Those blocks really should be under the Y guide rods inserted in the bearings, but this was enough to get a good first approximation to a level surface.
HBP initial height adjustment
And then it went back into the Thing-O-Matic…
I thought of the X Rod Follower while I had this all apart, but after putting it together, I wasn’t going to build the follower just to tear the stage down right away.
The Smell of Molten Projects in the Morning 