Tilted Cube (Test Print)

Will it print?

I decided to do an extreme test of the stepper-driver extruder and the volumetric math. In reality, most of this is testing Skeinforge.

Volumetric 5D Update

I contacted Enrique, creator and maintainer of Skeinforge, about my Volumetric 5D testing, and he liked enough to add it to the Dimension plugin of the latest beta of Skeinforge! (I’m guessing that it will be in version 40, but I’m not sure. It’s in the latest downloads available here.)

We had a slight disagreement about the exact method it should be implemented, but that’s really small potatoes. This is, as far as I’m concerned, a huge step in the right direction.

I’ve been making several different types of test prints, and I’m very happy with the volumetric math so far. I’m completely puzzled that I haven’t seen mention of this 15% volume loss in ABS printing until very recently, but it’s definitely the case. I suspect it may actually be a little more, like about 16% loss since I occasionally get very tiny gaps in solid horizontal surfaces. It’s very close, however.

Back to the Tilted Cube

That’s how it skeined. I had to set the Support Minimum Angle to 15 to get it to build support up near the edges of the cube. This wasn’t for support of vertical movement, as you usually use support for, but for horizontal movement of the cube since it’s on such a small base. It still moved somewhat and left on odd indention on a edge.

I didn’t thing that that shape would end up as a cube, but it did.

I didn’t get the whole print on video, but the end started seeming so fun I started to shoot video, and here’s what I got:

And, here’s the result:

Compared to the same STL file of a cube, normally oriented:

That’s with Layer Thickness at 0.3mm, Perimeter With over Thickness (ratio) of 2, giving a Perimieter Width of 0.6mm. Speed feed rate and flow rate were set to 40mm/s. I had Cool turned on with the Cool Type set to Slow Down and Minimum Layer Time was set to 20 seconds. That made for a very interesting situation as the top corner needed to take 20 seconds, as you can see in the video. I did not use a fan, but I plan on trying a fan with a funnel on it to aim the air more precisely.

I’m using the latest Dimension plugin that includes the volumetric math to scale the E value based on how thick the incoming filament is. This assumes that your firmware/software is configured so that an E change of 1 will result in precisely 1mm of filament entering the extruder. (If you’ve scaled your E_STEPS_PER_MM in the RepRap firmware, undo that first!) With Dimension, it assumes that your feed rate is the same as your flow rate. You really should always make those the same. If you find that you need that changed then something else needs adjusted, such as E_STEPS_PER_MM (RepRap) or axis … scale= in RepG for a makerbot. (My scale, or steps-per-mm, is 50.235478806907409 for a standard MakerBot Mk5 “pulley.”)

The biggest new feature of the new Dimension plugin is that it takes a Filament Diameter parameter. This needs to be precise, and measured before every skein, possibly every print, with a caliper. It also takes a Filament Packing Density, which should default to 0.85 for ABS and 1 for PLA.

You can see in the above photo that it left a gap on one side, but the opposite side was

Apparently, in Skeinforge, the Raft / Support Gap over Perimeter Extrusion Width (ratio) is assuming that the extruder will ooze plastic out over that gap. Since the ooze is pretty well under control now, I set this to 0. It still left a gap of one perimeter width (0.6mm) but that worked most of the time. I had one spot (the most horizontal of faces) where the support required a knife to remove, and it most likely due to some amount of drooping. Perhaps a setting of 0.1 might be better.

Here you can see the odd ripple on one edge where the upside-down pyramid moved horizontally while it was printing:

The raft pulled right off, and most of the support ripped off with a small screwdriver.

Conclusion

Printing with a stepper-based extruder is insanely better than with a DC motor. Adding volumetric math to skeinforge means that changes in the filament won’t require ugly changes to firmware, flow rate, or worse, feed rate. Support structures in Skeinforge obviously aren’t optimized to handle this type of overhang on all sides, but it’s an odd case to be sure.

Next Steps of Volumetric 5D

I’ve noticed that with 90º or sharper corners a little bit will flip up, and at first I assumed it was pulling it back up as it drug back across what it had already printed. However, which printing thinly sliced objectes (0.2mm for example) it’ll actually build up too much plastic. I had considered that the math might be off, but on horizontally flat surfaces and perimiters wil few curves or sharp corners the problem doesn’t show up.

I then realized that it really is going over that same place twice when it turns a corner. If you think of the filament pushing out and filling the space along the leading half-circle of the nozzle as it moves, as soon as it turns the half-circle overlaps with already placed plastic on the same layer, and on corners that overlap ends up getting pushed out to the corner as it vacates that space but holds down where the actual overlap occurs.

As you can see, the darker colored area is where there’s twice as much plastic deposited as needed. The sharper the corner, the worse it is. With a part with a lot of curves or sharp corners, especially if that’s on a small layer, you’ll see a buildup of too much plastic pretty quick.

I have an idea of how to fix it, but I’m almost out of battery. It’ll have to wit for the next post. :)

-Rob

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