Ender 3 V3 SE Extruder Not Feeding: Fixing the Freely Spinning Drive Gear

The black reduction gear turns, the motor appears to work, but no filament is moved: repairing a failed press fit with an M2 grub screw.

The Creality Ender-3 V3 SE is a surprisingly pleasant little printer. It is inexpensive, fast enough for most everyday work, and its mechanics are simple enough that most problems can still be understood by looking at the machine rather than by replacing an opaque, sealed module. This makes the following failure particularly confusing at first: the extruder motor turns, the small black motor gear turns, the large black reduction gear turns, but the filament is neither pushed towards the nozzle nor pulled back during retraction.

The usual suspects would be a clogged nozzle, insufficient pressure between the filament gears, a broken cable, a defective stepper driver, or ground-down filament. None of these explanations fits when the extruder has been opened and the actual filament drive wheel can be rotated freely while the large black gear on the same axis is held stationary. Those two parts are supposed to rotate as one unit.

In this case the original press fit between the steel shaft and the hobbed filament drive wheel had failed. The wheel was still captured strongly enough that it could not simply be pulled off the shaft, but it no longer transmitted appreciable torque. A small radial hole, an M2 thread, and a 5 mm long M2 grub screw turned the unreliable friction fit into a positive mechanical connection. The repaired extruder immediately worked normally again.

Symptom in one sentence: If the black extruder gear rotates but no filament moves, and the metal hobbed drive wheel can be turned independently of the large black gear, the press fit between drive wheel and shaft has failed.

• How the Ender 3 V3 SE extruder is supposed to work
• Confirming the failure before drilling anything
• Tools and materials
• Disassembling the print head
  • A short note about the glued connectors
• Drilling the hole for the grub screw
• Cutting the M2 thread
• Installing the M2x5 mm grub screw
• Reassembling the extruder
• Cleaning and lubrication while everything is open
• Recalibrating the nozzle height
• Conclusion: The pleasant side of inexpensive printers

How the Ender 3 V3 SE extruder is supposed to work

The reduction drive inside the print head is mechanically quite simple:

The large black gear and the hobbed metal wheel sit on the same shaft. There is no hidden grub screw in the original assembly. Instead, the metal wheel is coupled to the shaft by an interference or press fit. Under normal conditions the contact pressure between shaft and wheel creates enough friction to transmit the extrusion torque.

For a cylindrical press fit, the maximum transmissible torque can be estimated as

[ T_{\mathrm{max}} \approx 2\pi \mu p L r^2 ]

where $\mu$ is the coefficient of friction, $p$ is the contact pressure, $L$ is the axial length of the fit and $r$ is the shaft radius. Once wear, plastic deformation or insufficient original interference reduces the contact pressure, the transmissible torque falls. Alternatively, a sufficiently large overload can exceed the available friction torque and initiate slip, after which wear may rapidly worsen the fit. The wheel may nevertheless remain difficult or impossible to pull off because axial retention and reliable torque transmission are not quite the same thing.

This produces the failure mode seen here: everything visible from outside appears to rotate correctly, but the last metal wheel in the transmission silently slips on its own shaft.

Confirming the failure before drilling anything

The printer should first be switched off, unplugged and allowed to cool completely. Dont hurry and burn your fingers. Any remaining filament should be unloaded if this is still possible. After opening the extruder, the large black reduction gear can be held with one hand while the metal hobbed filament wheel on the same shaft is carefully turned with the other.

There must be no relative rotation between these parts. A tiny elastic movement caused by gear play is uninteresting; free rotation of the metal wheel around the shaft is the fault described here.

If the metal wheel remains rigidly connected to the large black gear, this repair is not applicable. In that case the nozzle, heat break, filament tension, idler gear, motor wiring and stepper driver should be examined instead. Drilling a healthy drive assembly would be a fairly funny but useless way of not fixing a clog.

Tools and materials

The following tools were used for the repair:

• a 1.5 mm HSS-Co drill bit,
• an M2 × 0.4 tap,
• one M2 × 5 mm grub screw,
• a small hex key for the grub screw,
• cutting fluid suitable for steel,
• a drill that can be operated at very low speed,
• a small vise or another rigid method of holding the part,
• a fine centre punch or another tool for marking the start of the hole,
• medium-strength thread-locking compound,
• normal tools for opening the print head and extruder.

A 1.5 mm cobalt drill worked for this particular repair. The conventional tapping-drill diameter for an M2 × 0.4 thread is commonly 1.6 mm. Using 1.5 mm produces a somewhat fuller thread, but it also increases the torque on the extremely small tap. Either size may therefore be reasonable depending on the available drill bits, the exact material and how accurately the hole can be produced. With a 1.5 mm hole, patience and generous chip clearing are especially important.

An M2 tap is small, brittle and impressively difficult to remove after it has broken inside a steel shaft. This is not the operation during which additional force improves the outcome.

Disassembling the print head

The photographs for this article have been taken during the assembly process, beginning with the repaired gear set and ending with the complete print head. Disassembly is simply the same sequence read backwards.

The exact order is not especially complicated:

• The printer is switched off, unplugged, and cooled down.
• The front print-head cover is removed.
• The touch probe on the left side of the print head is removed.
• The extruder and stepper assembly is detached from the print-head base by removing the four mounting screws.
• One tried to disconnect the connectors on the small local print-head PCB. Won’t work.
• The extruder housing is opened. The spring-loaded lever should be held while doing this so that neither the spring nor the idler gear attempts to explore the workshop floor.
• The drive gears and their shafts are removed while their orientation is noted.

It is worth photographing every layer before lifting the next component out. There are not many parts, but the orientation of the spring, lever, bearing, and paired filament gears is much easier to preserve with one useful photograph than with ten minutes of later speculation.

A short note about the glued connectors

Several connectors on the local PCB in the print head are secured with hot glue. The probable intention is perfectly reasonable: a printer head accelerates continuously, and an unsecured connector should not gradually shake itself loose. From the perspective of somebody trying to service the machine, however, burying tiny service connectors under glue remains a small act of mechanical hostility.

The glue could be peeled or cut away carefully while holding the connector body. The wires themselves must not be used as handles. A modest amount of swearing is technically appropriate here. The best approach in my personal opinion is: Leave all cables connected and make sure the parts don’t pull on them.

Drilling the hole for the grub screw

The purpose of the modification is to add a radial screw through the side of the hobbed drive wheel or its hub so that the screw positively locks the wheel to the steel shaft. The finished grub screw must remain flush with or below the outer surface; otherwise it may rub against the extruder housing or block the gearwheel.

The position of the hole should be selected before drilling. It has to satisfy four conditions:

• there must be sufficient metal around the hole for an M2 thread,
• the drill must meet the steel shaft approximately radially,
• the grub screw must not intersect the hobbed filament-contact region,
• and the installed screw must have clearance from the surrounding housing and gears.

The part should be removed from the printer before machining. One will never be able to apply the proper force there. The assembly is held rigidly in a vise with soft jaws or protective packing. The plastic gear teeth must not be crushed, and the part should not be clamped by the fine hobbed surface.

A small start mark is made with a centre punch. On a curved metal surface this matters: a 1.5 mm drill will otherwise wander towards whichever location is least convenient.

Cutting fluid is applied both to the drill and to the workpiece. The hole is drilled at very low speed with light, controlled feed. HSS-Co is suitable for the steel shaft, but cobalt in the name does not make a tiny drill immune to overheating or side loading. The drill should be advanced only a short distance at a time and then withdrawn to clear the chips. More cutting fluid is added as required (peck-drilling).

The drill is aimed towards the centre line of the shaft. In this repair the hole was drilled from the side of the metal drive gear into the shaft so that the later screw could form a positive connection between both parts. It is not necessary to continue blindly through the opposite side of the shaft as indicated by my photographs. Removing more material than required only weakens a very small component.

The drill should do the cutting. If appreciable bending force is required, the speed, alignment, sharpness, or chip evacuation is wrong.

Cutting the M2 thread

After drilling, cutting fluid is applied again and the M2 tap is started exactly square to the hole. Initial alignment and especially fixing the gearwheel as well as the shaft is critical because the tap is too small to correct a visibly crooked start later.

The thread is cut using the familiar forward-and-back sequence:

• 1/2 turn forward
• 1/4 turn backward to break the chip
• 1/2 turn forward
• 1/4 turn backward to break the chip
• repeat

The fractions are not sacred, the important point is that the tap is not driven continuously into accumulating chips. When one turns backwards one feels the chips breaking. After several cycles it should be backed out farther, cleaned, lubricated again and restarted carefully in the existing thread. Particularly when a 1.5 mm pilot hole is used, no attempt should be made to force the tap through a sudden increase in resistance. Side loading must be avoided. An M2 tap can tolerate the cutting torque for which it was made, but using it as a lever will leave a solid (if deep enough) but pretty expensive interlock mechanism inside the hole.

Once the required depth has been reached, the tap is removed and all chips are cleaned from the gear, shaft, bearing surfaces, and surrounding work area. The screw should be tested before thread locker is applied. It must enter cleanly, lock the drive wheel to the shaft, and finish flush or slightly recessed.

Installing the M2x5 mm grub screw

The M2 × 5 mm grub screw is inserted and tightened carefully. Its job is to prevent relative rotation between the hobbed wheel and the shaft. It does not need to be tightened with heroic torque. Stripping the new M2 thread would convert the repair into a slightly more advanced repair.

Before the final installation, the screw and threaded hole are degreased. A small amount of medium-strength thread-locking compound is then applied, and the screw is reinstalled. Excess compound is removed before it can migrate into a bearing, onto a plastic gear, or into the filament path.

The large black gear is then held while an attempt is made to rotate the metal hobbed wheel by hand. There should now be no relative movement. The complete gear assembly should still rotate smoothly without binding, eccentric resistance, or contact between the grub screw and housing.

Adhesives such as two-component epoxy or cylindrical retaining compound might also restore the connection, provided that they can enter the damaged fit and bond to clean metal. Spot welding would introduce considerably more heat and distortion risk into a tiny assembly. The grub-screw repair has the pleasant property that torque transmission no longer depends on a microscopic adhesive layer or on the remains of the original interference fit.

Reassembling the extruder

The following image sequence shows the reassembly. It can also be read from bottom to top when disassembling another machine.

Here one sees the repaired drive gear assembly. The radial M2 hole is visible, while the installed grub screw is recessed and may not be visible in the photograph.

The repaired drive gear and the opposing idler gear are returned to their original positions. The spring is then inserted so that it tensions the two gears through the extruder lever. The hobbed regions of both gears have to meet the filament path at the same height.

The motor-side bearing is placed on the stepper motor plane. The shaft must be inserted into this bearing.

The open extruder is placed on the stepper motor and secured with the two black screws visible outside the housing. Before closing the housing, the lever should move freely and the spring should produce a sensible clamping force between the filament gears. The extruder housing is then closed. The screws should be started by hand and tightened evenly. Small screws in plastic and aluminium do not benefit from maximal wrist torque, and a correctly fitting hex key is much cheaper than grinding access into a cover after destroying a screw head (which has actually happened on this printer).

The complete assembly is mounted to the print-head base with its four screws. Cables must follow their original routing and must not be trapped between the extruder, motor, and mounting plate. Also make sure the PTFE tube fits into the appropriate place.

The touch probe is mounted again on the left side. Depending on terminology and replacement parts this may be referred to as CR Touch. Its bracket must sit flat and its cable must be routed clear of the fan and moving parts.

Finally the front housing is mounted again.

The ground-open section visible on this particular front housing on some of the photographs is of course not original and is unrelated to the grub-screw repair; it resulted from an earlier service operation involving a destroyed screw head.

Cleaning and lubrication while everything is open

Once the print head has already been dismantled, one should always use the chance to remove all the accumulated filament dust, grease contaminated with particles and fan debris with the same care as the useful components.

The hobbed filament surfaces should be cleaned with a stiff brush or a fine tool, so that compacted filament particles no longer fill the teeth. Metal chips from drilling and tapping must be removed completely. The fans, air passages, and accessible surfaces can also be cleaned before the housing is closed.

A small amount of plastic-compatible grease may be applied to the reduction gear teeth, appropriate shaft bearing surfaces and the lever pivot. Grease should be used sparingly. The hobbed filament-contact surfaces, filament path and the nozzle, must remain clean and dry, you do not want to contaminate them!

The gear train should be turned by hand after lubrication. It must move smoothly, and the spring-loaded lever must return reliably.

Recalibrating the nozzle height

Removing and reinstalling the extruder and the touch probe can change the mechanical relationship between probe, nozzle, and print bed by a small amount. A small amount is entirely sufficient to ruin the first layer. The nozzle should therefore be cleaned, the build plate should be clean, and the printer’s automatic nozzle-height or Z-offset calibration (which is surprisingly easy thanks to the strain gauge below the bed) should be run again. The bed mesh or automatic bed-level procedure should also be repeated. A first-layer test should be observed closely before a larger print is started.

The grub-screw modification does not alter the reduction ratio or the effective diameter of the filament drive wheel. The extruder steps-per-millimetre should therefore normally remain unchanged. Nozzle height is the calibration that must not simply be trusted after the probe and complete extruder assembly have been removed.

Conclusion: The pleasant side of inexpensive printers

The failed press fit is not an especially glorious design detail. A factory-installed grub screw, a keyed shaft, or a better controlled interference fit would have prevented the problem. Nevertheless, this repair also demonstrates one of the genuinely pleasant properties of many inexpensive Chinese 3D printers: the machine is not a sealed appliance that has to be discarded because one small mechanical interface has failed.

The extruder can be opened with ordinary tools. The gears can be inspected directly. Standard drills, taps, screws and thread-locking compound can be used to create a stronger connection than the original one. Even the mildly infuriating hot-glued connectors are understandable once vibration is considered, and they can still be serviced without vendor-only equipment.

Cheap does not necessarily mean disposable. Often the contrary. And it also does not have to mean low quality - even the electrical connections are crimped properly in those devices. In this case a 1.5 mm hole and an M2 grub screw were enough to turn a printer that moved every extruder component except the one touching the filament back into a fully working machine.

This article is tagged: Tutorial, Hardware, Mechanics, Machining, 3D printing, Repair