We finally replaced the cover hinges on our Form 4 today.
Our machine was from the early batch affected by the hinge issue where the cover would gradually stop holding itself open. We were supposed to receive the replacement much earlier, but the reseller had apparently missed delivering it, and we too had not followed up after the initial replacement notice from Formlabs.
We finally received the replacement hinge kit from the reseller only about a month ago, and with the recent LPU issue taking priority, we did not get around to installing it until today.
For me, replacing it turned out to be much more engaging than expected. As a machine designer, getting the opportunity to actually open up a machine we use, look at some its mechanisms, understand some of the design decisions behind it, and appreciate the thought process that went into the machine was genuinely enjoyable.
Looking at the old hinge assembly, there are die casted aluminum arms, HF1012 one-way roller clutches along with a LEECO friction hinges.
The basic idea appears quite elegant: the one-way clutch permits motion in one direction (lifting of lid) while locking in the other (closing), while the friction hinge provides the controlled resistance needed to support the lid and after prespecified torque it allows closing of lid by allowing rotation of friction shaft.
Naturally, once the old hinge was out, the next question was:
Why did the old hinge fail?
At first glance, the replacement assembly looked remarkably similar to the original one: the same general aluminium hinge-arm design, the same HF1012 clutch format, and what appeared to be the same type of LEECO friction hinge.
So I started eliminating possibilities, my initial hypotheses were:
-
The friction hinge was slipping below its intended torque.
-
The one-way clutch itself was allowing rotation in both directions.
-
The clutch was no longer being held firmly enough inside the aluminium housing.
The friction hinge seemed fine, and the one-way clutch also behaved correctly when tested by itself — free in one direction and locked in the other. But when I loaded the complete hinge in the direction in which it was supposed to lock, the assembly still moved.
That was the part that caught my attention, so I marked the shaft, clutch and housing to see exactly what was moving.
The entire outer shell of the clutch was rotating inside the aluminium hinge arm.
With a little more force, I could even get the clutch to begin moving axially out of its bore.
That seems to explain in my case, at least mechanically, why this particular hinge could no longer hold the lid reliably: the one-way clutch could lock perfectly internally, but if its outer shell moves relative to the housing, the locking action is effectively bypassed.
The HF1012 has a nominal outer diameter of 14 mm, and using a vernier caliper I measured the clutch OD and housing bore at approximately 14–14.04 mm.
At a nominal level, those measurements are not broadly in the region expected for this type of press-fit arrangement, but they are not precise enough to establish the actual fit. NSK’s installation guidance for drawn-cup bearings recommends an N7 housing tolerance under normal conditions with a sufficiently rigid steel housing. For light-alloy housings, or steel housings thinner than 6 mm, NSK notes that the housing fit needs to be adjusted further, by roughly 0.013–0.025 mm relative to the normal recommendation, with the optimized value determined in the actual housing because the housing material and geometry affect the result.
A vernier measurement at this scale is nowhere near precise enough to confirm an N7 fit or the actual interference present. We are dealing with differences of only a few microns, while the measurement itself, the bore geometry and even how the bearing shell responds once pressed into the housing all become relevant.
So while the slipping of the clutch inside the housing is clear, the reason for it is much harder to pin down. Without accurately checking the bore diameter and roundness, the condition or deformation of the aluminium around the bore, the actual clutch dimensions in the assembled condition, assembly conditions and the mating surface condition, it would be difficult to say exactly why the fit was insufficient to prevent the clutch from moving.
And that is the part I would genuinely love to understand.
Formlabs has already identified this as a manufacturing issue affecting a set of early Form 4 printers and provided replacement hinge assemblies for the affected machines at no additional cost.
It would be valuable if someone from the Formlabs engineering team could share what they ultimately found to be the failure mechanism and what was changed in the corrected production parts.
Understanding why it happened and how it was corrected is often the most rewarding part.
A post on replacing a hinge may seem silly to some, but the general repair environment in India is a little different. Call us spoilt, or call us low-trust individuals, but the majority of replacements and repairs here tend to go through resellers or service partners.
In our case, it is a little like owning a machine, having a Master’s degree in machine design, and yet rarely getting the opportunity to actually tinker with, repair or tear down the machines you work with every day.
I think that is partly why we have always enjoyed machines such as the Prusa printers, where assembly itself gave you an understanding of how the machine worked.
Today felt pretty close to that with the Form 4, you come away with a new appreciation for the machine.