Embedded Files
April 2025
Working principle of an SLS 3D printer
Similarly to an FDM printer, SLS printers build parts layer by layer by fusing thermoplastics using heat. Solid layers are built with a laser, which sinters the powder together, along with resistive and quartz-tube heaters which hold the powder just below the melting temp and for uniform temperature distribution, reducing warping. To build the powder layer for each lasing cycle, a common setup is to have a recoater move across the print bed after the platform descents, creating a uniform layer of powder. The Fuse 1/1+ use a counter-rotating roller recoater which generally improves uniformity and print quality over other styles over blade recoaters, which are more simple and economical. In the Fuse 1 series, linear motion is lead screw driven, and a static gear rack causes counterrotation to the direction of motion.
Fuse 1/1+ recoater roller counter-rotation mechanism. As the recoater moves left, the roller rotates clockwise which lifts the powder for an even layer. Source: Formlabs
Overview of Fuse 1/1+ architecture. Source: Formlabs
What is chatter?
As the recoater moves across the print bed, any vertical oscillations will result in ripples parallel to the recoater on the top powder layer. You'll notice these ripples in large flat parts which is undesirable in terms of aesthetics and functionality.
There are many causes to chatter, from mechanical slop all the way to how powder interacts and flows with the roller. These causes have unique visual characteristics which are huge clues in a root cause analysis.
Our problem
For each prototype of a printer, we print as much as possible, as difficult as possible to stress test our architecture and identify outstanding risks and problems to be resolved in future iteration cycles. During some prints with this prototype, it was observed that chatter was pretty bad. It certainly wasn't an architecture-killing problem, but would need to be resolved sooner or later to meet product-level print quality requirements.
An example of chatter in the next-gen printer. The recoater sweeps left to right in this image. Not the visible ridges are regularly spaced, indicative of a mechanical root cause. Chatter caused by powder interaction is generally more random. The period is a key insight into whether the source is related to the driving mechanism, the counter-rotation mechanism, or other mechanical compliance.
How to quantify chatter?
Before anything could be improved, the chatter first needed to be quantitatively assessed so progress could be tracked. The standardized test print is pretty simple; just a flat plate. For the purpose of iteration speed, stepped plates were also used, where each step corresponded to a different recoating speed. Another employee did an investigation into quantifying chatter, and found that standard surface roughness measurement devices were too noisy and couldn't isolate the chatter vs typical printed surface roughness. They devised a system using a Bridgeport mill with power feed and v-block clamps to hold a USB-connected dial indicator, jogging the dial indicator over the entire width. They also wrote a Python script to high-pass filter the result and translated the measurement into an average amplitude and period, which I adapted and improved so results would better align with visual inspection.
V-block dial indicator chatter measurement setup
What I did
In addition to the quantitative amplitude measurements, I used high-speed videos to observe the recoater movement, powder interactions with the roller, and flexing of motor mounts and gear rack mounts.
I repeated a ton of cycles of printing plates, measuring and recording their amplitude and frequency, attempting to analyze the causes (and getting a much better intuition each time), logging progress, and using the presumed root cause to make some mechanical tweaks.
Some changes were minor, both in complexity and cost but had significant improvement. An example of this is that I noticed we were using polymer bushings (ID tolerance +0.12/-0 mm). Switching to bronze bushings (ID tolerance +0.02/-0) gave less slop to the roller, which had an insanely good improvement already. Other minor changes were higher quality parts, such as lead screws with a better straightness tolerance.
Other changes started from a holistic inspection of the system. For example, investigating the lead screw motor mount, I hypothesized that the mounts being out of perpendicularity with the build plate could cause serious whip. I verified this through slow-motion video. A couple ways to solve this would be to improve the perpendicularity (for example, switching from bent sheet metal to machined), or to use a flexible shaft coupling between the motor and lead screw. The second option was preferred from a cost and lead-time standpoint. Thinking long-term at BOM cost effects, a new machined part would risk BOM cost target overruns. For prototyping, the part would be several days, whereas we had plenty of flexible shaft couplings around the office for me to try same-day.
Some improvements were found by trying crazy ideas. Almost all chatter testing was done with 50-70mm/s recoater speeds with chatter getting worse as speed increased. So, it was hypothesized that higher speeds = worse chatter. As early progress was made, I wanted to push up the speed. It was a pretty incredible change; at 70 mm/s the recoater sounded harsh and loud and produced terrible chatter but was much smoother and quieter at 130mm/s. Thinking from first principles, I hypothesized that 70mm/s was a resonant frequency somewhere in the system. As a second test for this, I ran the recoater at the same speed, but increasing the mass. Since the resonant frequency is proportional to the root of k/m, increasing the mass would decrease the resonant frequency, so I should expect similar results as the original recoater at higher speeds, which was proven through quantitative measurements.
A more mechanically complex change was to add an external linear rail to decouple the roller from lead screw straightness errors using an Oldham coupling. Essentially, an alternative to a standard anti-backlash nut allowing for a high degree of radial misalingment. Since linear rail straightness tolerances are typically far better than lead screws, it pretty much guaranteed that linear motion would be true. However, it was additional complexity for a machined parts, a more expensive lead screw nut, and an additional linear rail. After all, best part = no part.
Oldham coupling with built-in lead screw nut.
The aforementioned changes had the most significant impacts on reducing chatter. Other minor changes which improved chatter were:
Powder "fences" on the recoater roller shaft, which contained powder and had an extra benefit of less waste.
Switching from straight to helical rack and pinion for recoater counterrotation for improved gear meshing
Stiffening the gear rack mounting design, reducing vertical compliance (high-speed video was key for identifying this)
Results
This project was a 3-week sprint to resolve this issue before my internship ended. All in all, through repeated mechanical iterations and testing, I managed to reduce chatter to almost unnoticeable, while also increasing the recoating speed as a bonus, which had a great effect on print times (projected 1.5 hour average reduction on every print!) I learned a ton about linear motion systems and how to make them buttery smooth.
Final status of recoater when I finished this project. No visible or tangible chatter lines, and increased recoater speeds!
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