Tuesday, March 7, 2017

Post 3: Core X-Y Musings...

It's been 4 months since my last update Post 2: Improved Z Screw Bearing Blocks so time for an update. Things were going a little (ok, a lot) slow because I was getting caught up in analysis paralysis and over design-itis - primarily because of the hgh cost in both materials and time for the aluminum parts I planned to make. But recently I discovered Atomic Filament PETG-carbon fiber. This stuff is really nice, it prints beautifully, has great dimensional stability and print accuracy and is quite stiff. So, my new plan is to print all of the parts in this PETG-CF and get the printer operational. I can always come back and replace with machined parts if needed. This allwos me to do fast design and test print iterations (with PLA) and then a final print for the machine.

I also have a home for the printer so it is up off the floor–where it was really difficult to work on.
There it is squeezed between RazMaTazz (my Taz 4) and a Terk (a mini Kossel).

This makes working on it much easier. And the new plan is working. Here are some of the parts in PETG CF:

X and Y stepper mounts


Bearing mounts for ballscrews with angular contact bearings


Monday, March 6, 2017

Segment-less Delta Movement

This post comes from a post I originally made on the SeeMeCNC forum in December, 2015. I find myself referring to that post often and repeating it in multiple places so I'm adding it here where I can keep it updated.
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updated 3/6/2017

Segment-less Delta Movement
A little know secret to most delta (and Cartesian) printer owners is that path movement for all firmware except David Crocker's dc42 RepRapFirmware is actually broken up into short straight segments. In other-words, a curve is actually "drawn" as a series of short lines and not a smooth arc like you might expect.

These other firmwares are interpolating movements into short line segments so, in theory, there is a negative impact on print quality. There are other factors at play too though - hence the reason for "in theory". By it's very nature, 3D printing starting with an STL file sacrifices some resolution depending on how the original CAD model was converted into an STL (most profound is the # of triangles in the final model). Here is an example of how this works...

Imagine you have a CAD drawing of a 20mm diameter sphere. The CAD tool uses sophisticated math to calculate the geometry and display it. It looks like a perfect sphere on screen. Now, let's run that perfect sphere through the meshing process to create STL versions. To demonstrate I am using 5 different resolutions that generate a low number of triangles to a high number of triangles. When you download an STL from Thingiverse, you have no control over this, the original author made the resolution choice for you. And frankly, most of the folks uploading to the shared services really don't know how to create high quality meshes. Anyway, back to the example - here I've meshed to create a "sphere" with 48, 224, 960, 16128 and 99856 triangles (from left to right). You can see that the far left sphere is course and the one on the far right is reasonably smooth.

Now to take this one step further, imagine your slicer creating g-code from these STL models. To illustrate, I created an imaginary slicing plane 12mm above the base that is .02mm thick and took a cross section of the model at that plane. This is the red line you see inside each model. I projected these slices up above the models so you can see them clearer (black). The 48 triangle model has a slice with 8 sides - pretty low resolution. You can see as you move left to right, each slice has more line segments. These will result in a finer/higher resolution print all things being equal. But they are not! In addition to the slicer converting curves into short line segments, the firmware also imposes its own line segmentation on top of that - that is all of them EXCEPT dc42 RepRapFirmware. dc42 draws each point along the way completely tracing the original path it was given. Smoothie, Repetier, Marlin and the others actually break the path up into short line segments and draw those. This does not always match the original path exactly.

As you can see, if the STL had a low triangle count, you are going to get a course print whether or not your firmware does segment-less movements. Increasing the triangle count will improve the print quality until you reach the segmentation threshold of the firmware, at which point, increased # of triangles won't have an effect and might actually make things worse. That is except dc42, which will faithfully trace each of the tiny line segments exactly point by point.

But, there are other factors that come into play like stepper resolution (1.8° vs 0.9° steppers) and a host of others. Will segment-less moves make all your prints look fantastic? No. But if you design your own parts and optimize output for high quality meshes, I assert you will be able to see the difference. This difference is minute and impossible to photograph, I've tried for over a year. But, parts in hand 10 out of 10 test subjects will pick the part printed with segment-less moves as the best quality part.

I posted Musings on Impact of STL Triangle Count in Jan, 2016 if you'd like to get more info.


Friday, March 3, 2017

The "Tusk Fan Shroud"

I don't understand why many people feel they must blast their part, hotend and heated bed with lots of air. Consider this: blasting a large area with a lot of air can create more problems than it solves - problems like part warping, hot end temperature fluctuations, beds that can't reach and maintain higher temperatures and a host of others. A much better approach - particularly for common filaments like PLA, ABS, PETG, etc - is to direct the minimal amount of air as precisely as possible with laser focus.
I started experimenting and writing about this a few years ago. See Strategy #12 Be a Fanboy here: A Strategy for Successful (and Great) Prints for more details and examples of part warping due to too much air cooling. Here's a photo of an extreme warping example due to too much air:
Part on the left was cooled with a typical 25mm fan blasted at the part.
Part on the right cooled with my "soda straw" air duct (note the straws look far away, that is an illusion!)
This early soda straw air duct concept gave way to a slightly more sophisticated version using several soda straws - one on either side - and ultimately to a 3D printed or machined ring (Re: BerdAir coming soon...). I used this concept on all my deltas (8) until Berd-Air produced their bent aluminum tubing cooling ring. It precisely puts air where it's needed and it is also very low mass on the effector and very low profile so it solves multiple problems. But the air pump is noisy and large.
Recently I've refined my design to address the loudness issue by leveraging a normal 25mm fan or squirrel cage fan. I call this new version "the tusk" for obvious reasons (see photos below). The aluminum tusks allow positioning closer to the heater block for more precise air flow control.The photos below show it mounted on the effector of a SeeMeCNC Rostock MAX V3 with a soon-to-be released new Bondtech mini extruder I'm testing. 



The "air shell" is low profile and can be located up and out of the way or even designed into an effector or platform if necessary. I'm showing a version with a squirrel cage fan but a standard fan tilted at a slight angle also works well. The tusks are 4mm OD aluminum tubes with 1mm holes. I have a printable jig that is used to align and space the holes for drilling. Then you crimp off the end of the tube. This has the additional benefit of allowing the crimp to be used as a sight to align the holes. With this arrangement, I have 100% clearance, something the three "hovercraft fans" of the stock V3 don't allow.
The middle hole on each tusk is centered with the nozzle tip. The air is directed 1mm below the nozzle and can be aimed as necessary with the sight crimps. With this precise air flow, you will not introduce thermal stress in your part (see link above), nor will you inadvertently cool your heated bed (especially important at higher bed temperatures), nor will it cool your hot end - especially if you use the E3D V6 sock (which I highly recommend for this and other reasons).
I have done smoke tests with this to study the air flow and can verify that it precisely streams air as claimed. The best part is, even with a 25mm fan I rarely ned to run more than 50% fan speed. The even better thing is, with this form of directed air flow, you can print longer bridges in PLA and ABS since the cooling air is in the right place to "freeze" the filament as its being drawn across the bridge. And part warpage is non-existent. And thin walls and posts/pillars like those in my fly fishing reel prints that are very difficult to print without distortion using the nuclear blast cooling method print perfectly every time.

If you'd like to try the idea, I've put STL and STP files for the Tusk Fan Shroud and STL for the Tusk Drill Jig on my Google drive: https://drive.google.com/drive/folders/0BxntGMCn8PVKOTFwU3F4NzNvNG8