Too long didn't read.
Also, I'm not Brook Drumm, but I did put a link in my description which apparently posts it to the files and photos section. That's not my video, but it is Brook explaining high flow spiral printing.
I am drawn to the spiral vase like rocks are drawn to gravity. I regard it as the first and sometimes the only choice when approaching a new design, or for making an existing print design a little faster or a little stronger.
Most of my designs use spiral vase and I typically post some print settings to help ease the exquisite pain that is 3D printing. Occasionally.. only occasionally, I get something half-right or even dead-wrong. Thus follows the tedious task for running through my posts and correcting each one individually.. bugger that.
.. I don't change nozzles.. because of a deep and abiding disinterest. I have two Prusa MK3 printers with 0.4mm nozzles. With these nozzles I have achieved 0.4mm - 1.0mm wall thickness and that has suited my purposes. If you are looking to achieve even thicker walls during spiral vase printing, then swapping the nozzles would change the range available to you.
Line Width and Flow Rates
Flow rate is critical to most [all] of my designs. I got started with spiral printing after watching a YouTube video by Brook Drumm (Printrbot), in which he describes one of his client's techniques for printing prosthesis. It boils down to this; high temperatures and high flow rates to create thick, solid walls.
I use Cura for my slicing. Cura has two settings that will affect the flow rate, and if used together, they don't have the desired result. Setting the line width parameter will adjust the flow rate to achieve the width selected. Modifying the flow rate seems to adjust off of this value. What this means is that if I set 0.67mm line width and 160% flow, I get a spiralized wall that is roughly 1mm thick. If you are using Cura, I would suggest using line width as the driving parameter and only use flow rate to make minor adjustments.
Using line width, or an equivalent slicer setting, is likely the simplest way to achieve prints that have the correct outer dimensions. Cura adjusts the center of the nozzle based on the line width parameter, so if you ask for a 20mm OD cylinder, that's what you get.
If you do not have an equivalent slicer setting, or if this setting doesn't maintain the outer dimension of the part model for you, you still have flow or extrusion rate and scaling. In both scenarios, your slicer 'should' offset the nozzle position by half the nozzle diameter, so we start from there.
( Target OD - ( Wall Thickness - Nozzle Diameter ) ) / Target OD = Scaling %
With a little luck (I'm looking for feedback here) that should create a part with the correct outer dimensions.
Profile or profile line: The 2D line describing the outline of your part
Extrusion line: The 3D line described by the extruded plastic
The spiral is self actualizing, if you don't have a spiralizable.. spiralish.. spiral-fluid.. part, your slicer will immediately recognize that fact and destroy the part. This will typically be caused by one of two things.
Thing the first: Slicers still think and represent themselves in layers. Each layer must be a continuous profile. A circle, rather than an arc or a line. If you imagine yourself looking at a single layer of your part and you can identify two ends that don't connect to anything, the part needs some work if you want to spiralize through that layer.
Thing the second: Slicers are really good these days. If two lines of your profile are equal to or greater than two times the extrusion line width apart, you will get two distinct walls, hopefully completing your profile. If two lines of your profile overlap, the slicer recognizes the overlap (typically) and tries to skip that section. Skipping sections of profiles is really bad if you're shooting for continuous profiles.
Cura does let your profile lines get pretty close before it breaks the part, and we can use this to our advantage. As the extruder nozzle moves along the center of the near-overlapping profile lines, the extrusion lines overlap one another and weld that section of your profile together.
Your slicer doesn't know anything about your intentions. It's an adorable little puppy waiting for you - the center and lodestone of it's life - to tell it what to do.
Let's take a model that has profile lines brought near enough to weld extrusion lines together. If we scale it, you will run into issues at roughly 30-40% variance. You placed profile lines 0.4mm apart for a 0.4mm extrusion width so they will barely weld together. If you scale it up by 50%, your profile lines are now 0.6mm apart, leaving a 0.4mm gap between your walls.. so no welding.
Scaling down runs into our intersecting profile line issue mentioned in part design. The slicer will start skipping parts of our profile, since it recognizes that the lines intersect, but it doesn't know how we want to to resolve the intersection.
You can either alter the model in the design program, or increase/decrease your extrusion line width in accordance with your scale.
Model Scale = 150%
Extrusion Width Modifier = 150%