Why a Gearless Extruder Works for 3 mm Filament

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While it is accepted dogma that one should always gear down a stepper motor to push 3 mm filamnet through an extruder - we think otherwise.

We start with principle calculations of filament strength. We know that it is possible to push 3 mm filaments strongly into the melt zone of an extruder, to push through the operative non-Newtonian fluid physics - of partially molten plastic being very viscous and thus hard to move. Read more - [1].

The maximum push force that an extruder can exert on a filament, at the limit of zero bending of the filament - would be equivalent to its tensile strength - about 5000 psi. [2]

3 mm filament thus has a 0.01*5000 psi maximum pull allowed on it, where 0.01 square inches is the cross sectional area of a 3 mm filament. This takes for 50 lb - which sounds about right. Let's say that compressive strength equals compressive strength. So at max, the max thoretical push is 50 lb.

How much force does a 72 oz in stepper motor, like the one we use, give? For an MK7 Drive Gear like we use - it is about 20 lb, see last link.

But the drive limit of filament is not determined by the filament thickness, but by how much of the filament the teeth of a drive gear actually grab. This determines when the filament would strip. If the teeth dig into the entire filament, the full 'grab' up to the compressive strength - of about 50 lb - would happen. This can never happen in practice - as grabbing all of the filament with the drive gear teeth means that you broke through the filament - you cannot feed this way.

So the answer is to make the teeth significantly less deep than the filament thickness. For the MK7 drive gear, that depth is about 0.7 mm according to the tech drawing in the last link. Let's assume 1 mm for simplicity. If only 1 mm of a 3 mm filament is grabbed, then the maximum theoretical push is about 17 lb. Thus, a 20 lb drive force of the gear appears quite adequate to drive the gear, right at the limit of it stripping.

This indicates that a geardown on such a system is completely unnecessary.

Point simple: A strong Nema 17 stepper (72 oz in or higher) without any geardown is sufficient to drive 3 mm filament.

So the force requirements are completely adequate to be provided by an un-geared 72 oz in stepper motor. This is exactly what we are observing with the D3D Flexible Gearless Extruder. Flexible Gearless squared: it works with both 1.75 and 3 mm, and works great with flexible materials by design.

In fact, using any more force in counterproductive, as it can easily lead to stripping of the filament.

So why do people use geardowns on 3 mm extruders? It seems largely dogma and bad design. How did this dogma arise in the first place? If the reader has insights, please let me know. The only thing I can say is that the idlers may have not been tensioned sufficiently. We use a roller skate bearing for an idler. But so did Lulzbot in its earlier printers.

The main culprit could be the widespread adoption of the E3D v6 heat sinks, which have 43 mm of heat sink distance, while our extruder has 13 mm of heat sink distance. This makes it particularly hard to drive by design (long filament path after extruder gear), and it appears that most people mistakenly take to a geardown to address a fundamental design problem. MK8 extruders also have a short 13 mm distance, though, but none of them are designed for 3 mm filament - so the mystery stands.

A geardown does make sense if your stepper motor is a smaller pancake one, which does not have sufficient drive strength (under 20 lb at the gear teeth), and which may allow for a lower overall extruder weight. And it may make some limited sense on a 12v (instead of 24v) stepper voltage system

But speaking from a lifetime design and industrial robustness perspective of OSE - geardown makes less sense than using a stronger motor. The design becomes much simpler, parts count is lower, and the wearable geardown is eliminated.