All that functionality comes with a lot of data processing. The load cell is far from the end of it: Nextruders on the Original Prusa XL also feature an accelerometer to gauge movement and vibrations. There’s so much data for the XL’s print head that it has its own dedicated CPU on an internal mainboard called the “Dwarf” board. Prusa really took its time winding up before flying past those 8-bit boards of yesteryear.

One neat Dwarf board feature mentioned in Prusa’s second XL Update is two programmable buttons. Nothing jaw-dropping, to be sure, but convenient! Prusa says it’s been using the buttons to quickly extrude and retract, but that they can be programmed to do anything you want. There’s also a status light here, so you can tell at a glance if one of the many sensors has picked up on a problem with the Nextruder.

Although the XL is made up of multiple control boards with their own firmware — the XLBuddy board, Dwarf Nextruder board, and a board for the segmented heatbed — updating the firmware will be done all at once to keep things easy, Prusa revealed in its first XL update. What a relief.

We’d hazard a guess that the extra silicon here boded poorly for the production of Prusa XLs amid the ongoing global supply crisis (on top of confirmed supply difficulties for parts like Hall sensors). Průša even mentioned in the Nextruder livestream that the company has found it was bidding against itself for the same lot of components. The issue was repeated in a later blog article. The shortages culminated in repeated delays to the XL.

The difficulty in acquiring these parts is also the reason Prusa’s open-source policy has a caveat this time around, Průša said. Though the Original Prusa XL and Nextruder will be released open source, details won’t be made public until the supply chain crisis has subsided to prevent would-be clones from bidding on the same hardware.

Prusa Research has made a point of highlighting its new extruder’s gearbox — now in two different versions.

The first gearbox was a zero-backlash 20:1 ratio cycloidal system, and the company made no small fair about that zero-backlash. Unfortunately, in Prusa’s first development update, it said difficulties in large-scale production forced them to pivot from the planned gearbox and go with a 1:10 gear ratio planetary gearbox. The company owned that its new planetary gearbox has “a negligible backlash”, but attests to improved quality due to more consistent extrusion.

So what exactly are we missing with that zero-backlash gearbox gone? And why might it matter?

Simply put, “backlash” is the empty space between gear teeth. Some backlash is typically required in gears to account for manufacturing tolerances (how precise a given manufacturing process can be). Without backlash, a gear’s teeth may grind together during operation and create a myriad of issues including jams or damage. But there’s a trade: the space between teeth also allows for lost motion. When you’re trying to get as much torque as you can out of a small box, that’s no good.

There are other potential tradeoffs to cycloidal gearbox designs, though, and that might account for the improved performance with a planetary gearbox that Prusa is claiming. After the XL’s initial reveal, MihaiDesigns pointed out in his highly-detailed Prusa XL impressions video that vibrations could pose a problem for the cycloidal system and force users to print very slowly. Interestingly, Prusa mentioned speed in this update and how it’s still working towards finding the right balance. Hopefully the new gearbox has helped.

The Nextruder’s hot side has a custom “volcano-size” nozzle. Dodd noted that it doesn’t have the full volcano melt zone, however.

The nozzle was developed with E3D and bears a certain resemblance to the RapidChange Revo system, but Prusa has made clear that it’s different. Like the Revo, the Nextruder’s nozzle eliminates the need for hot-torquing — and hopefully clogging — but does so with a much longer filament guide on its single-piece heat break and nozzle, and threads into the heater block instead of past it.

It’s also different than the Revo in terms of swapping. The XL’s nozzle isn’t a “rapid-change” system, per se. Where the Revo’s nozzles are designed to toollessly unthread from the rest of the assembly, the Nextruder’s hot end assembly is held together with two screws, and the nozzles need a wrenching to separate from the heater block.

Normally, everything we find cool about the Nextruder’s nozzles would be diminished by the knowledge that we have to buy a bunch of new proprietary nozzles that will probably be limited in variety for some time (lookin’ at you, Revo) but Prusa has us covered. The company is offering a V6-style nozzle adapter, so all the hardened steel, ruby-tipped, or whatever exotic filament squeezer you’ve got in a drawer will work with the Nextruder, too.

Oh, there’s also a neat little LED nozzle light. How fun!

Some renewed effort has been put into monitoring potential heat creep in the Nextruder, with a second thermistor added to the heat break for extra input. It doesn’t seem like a totally necessary part but certainly is a useful monitoring and safety tool.

Dodd recited Prusa’s logic for the extra thermistor: “you only get it clogging in that heat break when the heat break is hot, so why not monitor it?”

During the Prusa Live livestream, it was pointed out that this new thermistor adds an extra failsafe in preventing thermal runaway. “If one fails we can find out that it failed,” Průša said, “because before we couldn’t.”

Beyond safety features, Dodd pointed out that a thermistor on the heat break will allow users to further tweak performance settings. If it’s vital for your material that the heat break remains totally cold, you’ll be able to see that it is. If you’d like a little pre-heating in the break to help run your prints faster you’ll be able to do that, too. This will likely be accomplished just by managing fan speeds, as Dodd also mentioned fan regulation as part of the new performance features and that, with a heat break thermistor, the fans will only be as loud as they need to be.

The Nextruder has a lot going on hardware-wise, and that could make modifications and maintenance an absolute nightmare if not handled correctly. Fortunately, Prusa has considered this and implemented some efficiencies and cable-management quirks to keep things tidy.

Probably the most noticeable, and most interesting, organizational solution Prusa has implemented is the Nextruder’s dedicated break-out board, the Dwarf board, that we mentioned previously. This might be obvious considering the extruder needs somewhere for its dedicated CPU, but it’s going to be a joy of a feature to have after the relative difficulty of detaching past Prusa hot ends. The board keeps things simple by connecting to the printer’s mainboard with a single cable, significantly simplifying disassembly and hopefully leaving our maintenance woes in the past.