January 17, 2026
Description
17.01.2026: added modified CORE ONE Loveboard mount with more spacing for screwnut movement
26.11.2025: Note: It could be possible that you need to tighten the Idler screws after the first 1-2 prints (depending on the used filament for the idler). The observed symptom is, that the filament undetection does not work when unloading filament from extruder. In that case tight the screws 1 or 2 additional revolutions.
CoreONE: Added Screwnut without Spring holding part, which avoids collision to the loveboard mount part. (modified loveboard mount available soon)
07.07.2025: IAF Idler v1.1+ now online. Improvements:
printed spring and joint to swivel durability
printability with 0.4mm Nozzle
improved sensitivity filament trigger with concurrent reliable unload filament detection
extended coil spring tension adjustment the idler works with preset coil spring tension from 0mm (coil spring not loaded) to approx 3mm applied preset tension by coil spring screws.
for PRUSA CORE One optimized screw-nut (thanks @brechelm_1476311)
recessed the leftmost screw head of the swivel screw to balance the weight on the lever use now M3x18mm or 20mm instead of 22mm.
Main-Plates are not changed in v1.1+, so you can keep v1.1 Main Plates.
IAF v1.1 model files can be found in subfolder.
IAF-Idler (Install And Forget) was started developed to get rid off the failure behaviours like:
Stopped extrusion midpoint and pulley grind filament (no more filament transport to Nozzle)
Fail detection as well as false positive detection and failed un-detection of filaments
Complex respectively Progressive Idler Tension rate, like a suspension for filament transport ideal to use strong filaments and flex filaments in the same print.
Enables to recover Filament transport even if a grinded hollow in the filament appears.
Tested down to 0.25mm hardens steel nose for CF Materials and using 0.15mm brass for non-abrasive Materials without clogging.
Filament detection trigger decoupled from the two strong Idler springs. No recalibration after idler adjustment.
Idler adjustment access from swivel side, opposite to the original design, adjust idler tension easily when print head is in park position.
Because of now the smooth part of the screws are inside the heat-break, which avoids to the screw threads to hook into the heat-break edges and causes errors.
Screw-Nut with M3 Lock Nuts which also supports the Idler springs, no more worries about damaged self tapped plastic thread.
Optimised filament path, to pass filament straight into the cooling tube, ideally if using bridle or stiff filament, avoids loading failures due to filament bumping onto the rim of the Cooling tube respectively onto the Main-Plate parts.
You can test this by yourself, take a piece of Filament like ABS, PETG or other strong and flexible Material, switch off your printer and push the filament into the extruder while Idler door is closed. Due to bearing position the filament tries to follow the pulley radius and didn't pass directly into the cooling tube. (see picture).
Using iaf-Idler v1.1 with that self pushing test, the filament passes directly into the cooling tube. (See pictures)
Known Issues and Todos:
Printed Spring durability:
The first stage printed spring is well developed but currently I don’t have yet experience for long-term durability, If there happens issues, I will release another existing Idler nut option which uses extra hardware instead of a printed spring (But that isn’t yet tested)
The documentation will be completed and corrected in the coming days, but I hope the current status will help for the beginning .
Please tell me all your suggestions and experience (good and bad), which I can use improvements.
For best results, use the uploaded 3mf file with my changend printing parameters for best printing results of the idler.
Tolerances are serious, especially over-extrusion isn’t good to achieve the tolerances needed.
Indicators for tolerance issues are, if your printed Idler has many visible retraction scars, if the screws holes are too small to fit the screws without force. Or if you need extensive force to assembly the idler lever. If so, reduce the extrusion multiplier and print the IAF-Idler again or try to use another filament.
Printing Parameters:
PrusaSlicer Preset 0.12mm Structural, 0.4mm Nozzle.
Reduce Printing Speed for bridges to 10mm/s or less.
Paint On Support for Main-plate like shown in the Picture
If using Material which tends to stringing like PETG, consider printing with „Complete individual objects“ Option.
Best experience for printing the Idler parts with using PC-CF, PA-CF, ASA-CF or PC-Blend filaments.
If your Print is based on the Parameters I uploaded with the 3mf file, then post processing, except removing support at the main-plate, is not necessary.
Proove the tolarances by putting all parts together:
the parts should fit exactly without applying much force.
Nuts should fit inside openings with moderate force and without model deformation
Screws should fit exactly through holes without a big clearance
coil screws should fit easily into screwnut
Consider the PRUSA Nextruder Assembly manual also!
Nextruder MMU Assembly:
https://help.prusa3d.com/guide/9a-mk4-s-mk3-9-s-nextruder-mod_683106#683464
Nextruder Non-MMU Assembly:
https://help.prusa3d.com/guide/5-nextruder-assembly_434014
Insert two M3 Lock Nuts into the edged openings
Push and rotate the two Idler springs into the round screw-nut openings, the springs should be inserted 3mm. Check that both springs are identical deep and parallel pushed into the screw-nut.
Insert the two M3 Lock Nuts into the corresponding holes.
Put both half together
Place the Spacer and fix with one M3x22mm Screw
Prepare the second M3x22 Screw
** Picture **
Insert the two M3 Nuts into the B-Part of the Idler Lever.
Place the first bearing on the inner side of the B-Part and use it as a guide for inserting the first Bearing axle. Repeat with the second bearing.
**picture**
Push the A-Part onto the bearing axles and gently push the two half together. Fix with two M3x6 Screws. Take care that Screws and Nuts don’t looking outside the lever dimensions.
Check that the bearings can rotate freely.
Check the surfaces again that the shape of both parts fits.
Insert the spacer tube in its opening
Check freely running bearings, mark bearings by pen to be sure the rotation
** Picture **
Use a Needle-nose pliers to insert the 3x1mm inside the trigger.
The trigger is design to be able to remove the magnet without destroying the trigger. This is useful, because on my experience, the detection characteristic is different based on magnet orientation.
Hall sensors are South pole sensitive, thats the reason why the south pole of the magnet should face towards filament, then the “On” detection has a wider range.
If the north pole of the magnet is faced to the filament, than the sensor will detect later and more steep, so “On” range is narrow and Off range is wider., which is not useful for different materials
To find out which the south pole is, you can use a magnetic compass, the side facing to the compass north is south of the magnet.
(For newer Iphone there are also Apps available. Search for „PoleDetector“)
Without compass, you can continue and check orientation once calibration is done.
For this, use the LCD and navigate to “Info”, then select “Sensor Info” and check “Filament Sensor”. Move the idler to left, if the values lowers for “On” detection and raises for “Off” detection, then the orientation is correct.
If it is vice-verse I suggest to remove the Swivel-Nut screw and remove the swivel, then remove the three PG-Cover screws and lift PG-Cover, PG Gear and PG Wheel by pulling the main plate until you have a gap and can remove the sensor trigger.
Then use Needle-nose pliers and change the magnet orientation in the sensor trigger. Mount all parts again (take care, that the printed spring of the Idler Nut fits into the swivel) Repeat the filament sensor calibration again.
Put the three Parts as shown in the picture together and fix with one M3x18(20)mm Screw.
**Picture**
Take care that the printed spring of the Idler Nut passes into the swivel.
Important: The printed spring is designed to correspond with the idler lever, do not apply for to the spring without lever, this may cause irresible damage to the printed spring
To set the nut into swivel, fix the screws completely and loose the screws for min. 2 revolution again.
check the free movement of the three parts!
carefully remove printing support, remove scars and seams and check that the backside of the Main-plate is smooth.
Check if screws can be inserted easily.
O-Ring: the Main-Plate is capable to use the O-ring, check if all seams are removed before inserting.
But, personally I didn’t got a reasonable point why the o-ring needs to be used, especially the non-MMU doesn’t use o-ring, o-ring is only used with MMU.
The O-ring produces a lot of friction, even if lubricate and heats up the Extruder Motor additional.
Using the O-Ring I observed only unintended behaviours, so I don’t use the O-Ring.
Check if the Main-Plate and the filament hole blocking pin fits correctly into the Heartbreak before starting with assembly.
Check filament path! If there are any scars or seams, try to remove with a small drill of screwdriver.
Insert the prepared Idler Nut/Swivel/Trigger Assembly into the Heartbreak.
Take two M3x35 Screws and insert them into the Idler nut and the Filament inlet mount.
Take the Screw Nut and screw in the two M3x35 screws. Important: Initially screw in the screws where the ends exceeding approx 0-3 revolution to set a initial spring tension. Unwind the screws until the screw ends are even to the Lock Nuts. To verify the distance between Lock Nut part and heat-break filament inlet should be between 11.00mm and 12.00mm if swivel is not locked
Insert the idler lever to the set screw axle and place the main-plate onto the heat-break. Insert the PG-Pulley Wheel and place the geared frame as described in the Prusa Assembly manual. Mount the PG cover and fix with the screws.
To set the Idler assembly, close the idler lever and swivel and move the complete idler by pushing the idler screw nut to the left a few times!
Perform PG Gear calibration first.
Note: With a new Main-Plate it could be at the beginning possible that pushing filament into extruder needs some force. This will be better after some filament loading alternatively, take a piece of strong filament e.g. PC-CF and strongly push it into extruder a few times. Now the extruder should load filament easily.
Calibrate filament sensor. Start calibration with a common PLA or PETG filament piece. Don’t worry if the calibration fails in the first try, that’s normal if there is a larger change of the sensor detection. I suggest always to perform at least two successful calibration cycles. The sensor is now calibrated even for stronger materials and also for flex material!
Optionally, you can verify sensor detection with other materials, but it isn’t necessary. Important: It can also happen, that calibration with flex filament fails, that is caused by the kind how the calibration works, but don’t worry if successfully calibrated with common filament like PLA or PETG, flex detection works while printing as intended.
Checking idler pressure:
You can now load a common filament PLA/PETG/ABS/PC… (no Flex material) to the nozzle and check if the filament pressure is adequate, for this load filament to nozzle with the LCD UI then press the „purge more“ button (the extruder motor now remains ON, without purge more, the extruder motor will be switched off.)
Remove the PTFE Tube from Heartbreak and carefully try to pull the filament. If the filament can be removed easily, then you can put more tension to the Idler by screwing the idler screws.
If you unwind the two screws by 3 revolutions, it should be possible to pull out the filament. Then the Idler tension should be okay.
Set back the idler screws 2-3 revolutions to the desired position, where the filament can’t be pulled out.
If you are using Rewind spools or if you have longer PTFE filament Tubes or many curves, you maybe need some more idler tension. The iaf-Idler v.1.1 is capable to produce the following working torques: PA natural: up to 24N, PLA: up to 18N.
In my Setup I use Rewind spools the PTFE tubes for filament is up to 2m long, where 800mm are used from MMU to Extruder. The Filament path has two 90° curves and one 180° U-Turn with some smaller curves until it reaches the Extruder. (4/2.5mm PTFE tubes) This can be seen as good working environment.
Optionally: Extreme idler tension and grinding hollow recovery test.
Load a filament to nozzle.
Remove the PTFE Tube from Extruder Inlet.
Select „Purge More“ to manually extrude filament.
Block filament movement on Extruder Inlet with your fingers.
Block filament movement until you feel extruder slipping over filament.
Release the filament.
If Idler tension is set correctly, the extruder remains to extrude filament through the Nozzle.
If you printed the Idler with the original Idler or other Idler mods, I suggest to print the IAF-Idler now using IAF-Idler again.
Happy frustration-free printing!!
Within development some versions shows worthful options to continue, like a more spacesaving option where it is possible to more open the Idler lever door or a Pro Version, that uses some extra Hardware. BUT for this I need some more help to fund material and parts.
To achieve the quality of this model, I have invested countless hours over the past few months in development and testing, not to mention the cost of all the printed idlers and main-plates. Until now: Approx. 1.5kg of printed designs, that equals approx 100 prints (approx 15g for a set) but it feels more.
After dozens of different idler designs and constructions (some very different to the original design), this model was tested with the highest reliability and filament range I ever had without adjustments and failures.
I’m totally happy with the result of my effort, the IAF Idler overcomes my goals I had for that Idler and additional brings some useful add-ones like accessible screws from left respectively for PRUSA XL by Front.
I hope the IAF-Idler will help many other users getting also rid off the failures like me, then the efforts was worth it.
Especially testing special filaments especially different Flex is expensive and till now the development did cost a lot due to needed spaceports, filament for models and test etc.
It would therefore be very grateful if you would or can express your satisfaction by a small donation, Thanks in advance!
I want here to give the persons which helped me already with some donation in my previous design, so I was able to pay for spare parts and material for this model.
License:
Creative Commons — Attribution — Noncommercial
7
Thorsten Lachmann
