There has been early feedback about hard starts or "crunchy" threading on the Compacts, and about how the threads look.
While it may feel or look wrong at first, most of this is an expected result of tradeoffs we chose on purpose: more thread engagement for strength and heat transfer, and formed threads over cut ones. I'm extremely confident it won't affect how your hotend works. Below is the why for each, and what we're doing about it.
The tightness comes down to a few things:
- A tighter-than-normal thread class
- A less generous lead-in chamfer on the threads
2b) The leading thread edge deforming during tumbling, before plating
The appearance comes from the manufacturing process. These threads are formed, not cut.
Thread classes
A lot of thought goes into threads, thread classes and fit. Nozzle and heatbreak interfaces need preload on the threads (literally stretching the material) to create the clamping pressure that seals against the heatblock, so molten filament goes out the nozzle and not out the threads.
We also want to maximize heat transfer into the nozzle. Heat gets into the nozzle through the face where it seats against the block, and through the threads along its length.
Thread classes define the sizes and tolerances of the internal and external threads relative to each other:
https://www.machiningdoctor.com/threadinfo/?tid=1015#classes
To maximize holding strength and heat transfer, you want a tighter fit with more material engaged in the thread. So we kept as much material as possible in the threaded interface. The tradeoff is that nozzles on the large end of their tolerance will meet more resistance, especially when the hotend is brand new and there are still small high points in the threading from the material and from the plating process.
Based on our experience and internal analysis, other manufacturers aim for the opposite end of the tolerance band and prioritize ease of assembly over outright strength. That means the stick-slip ("crunchy") feel will be more noticeable on Chube Compacts for the first few nozzle changes, until the high spots smooth out.
We can address this several ways in future batches. If the community would rather have the fit other hotends usually ship with, we can switch to an even larger oversize tap (we already use an oversize tap, and the threads would still be formed) with a larger pre-tap drill. That would let you thread a nozzle in with the lightest touch, and it would take the least torque to install and seal, with the tradeoff being that its more possible to strip out the nozzle with a more enthusiastic installer.
Thread appearance and formed threads
Forming an internal thread starts with a hole drilled larger than the thread's minor diameter. The tap then displaces material to create the major diameter and the final minor diameter (making the hole smaller again) at the same time. Nothing is cut away. Because the material is pushed into shape, the grain flow stays continuous and the thread surface is work-hardened, which makes it stronger than a cut thread.
As material flows up both flanks and meets at the crest, it can leave a small seam at the tip of the thread. This looks unusual next to cut threads, but it's normal for formed threads and isn't a sign of a bad thread.
More on formed vs. cut threads, with images to show what you're actually seeing when you look at the ends:
https://www.ultrasonic-resonators.org/design/threaded_fasteners/studs_bolts_threads.html
https://baer.tools/en/blog/thread-cutting-as-a-machining-production-process
Lead-in chamfer
A lead-in chamfer helps align a nozzle and start it in the thread.
As is standard practice, we added a lead-in chamfer to the Compact. But again, sticking with "more material, more better," we chose one that is less forgiving than people want once it's plated. We tested the interface with standard nozzles from Phaetus and Bondtech, plus the "Purple Nozzle" from Raccoon Works (RIP YURA, you will be missed). We included the Purple Nozzle specifically because it has no lead-in chamfer on the nozzle side. All of them threaded in successfully before post-processing.
Once plated, though, the chamfer is less generous than people expect, so nozzles with small lead-ins need better alignment to start. The next batch will get a larger chamfer, so long term this is a non-issue.
With the smaller chamfer, the thin leading edge of the thread also appears to have folded in during tumbling before plating. This is the one part that was entirely unintended.
The first few times you install a nozzle, there may be extra resistance until that edge gets pushed out of the way. It wears in and doesn't affect long-term durability.
"You yapped a bunch, so what are you doing about it?"
- Once we're done delivering Compacts, we'll review the feedback and decide whether to loosen the thread fit on future production.
- We're increasing the entry chamfer.
- We shipped roughly 50 to 100 units before adding a check-and-fit step for the nozzle entry. Anything shipped after the first 100 (we're well past that now) is unaffected. We now check with worst-case nozzles and proper thread gages before shipping, but threading nozzles may still take more effort than you're used to.
- We'll keep being transparent, earnest and honest about what we're doing and why.
Other updates
I'm about halfway through replying to the feedback form submissions.
We've shipped about 25% of Chube Compact preorders! Once preorders are done, stock will be available immediately.
We're also filling backorders for Air and Conduction units, with plenty of extra units already made and in stock.
I'll go into more detail in my feedback form replies, but making more of something is a lot easier than starting first-generation production, meaning we'll be able to keep things in stock far easier than *getting* them in stock in the first place.
