¶ Pre-requisites, before you get too excited
Beware tip-forming is not supported with the OAMS. Trying to use it will lead to nothing but dissapointment, grief, and general despair.
In order to make the OAMS work with your printer, you will need to have some way to cut the filament before each filament swap.
In the past, other solutions, such as ERCF, have used methods like tip forming. Tuning tip forming has proved fraught with errors and a major source of headches and jams in the ERCF. Any change to hotend, length, or otherwise leads to new tuning of the tip forming parameters. These factors are the reasons bambulab printers include a filament cutter in their toolheads.
For Vorons, if using the Stealthburner, we can recommend the excellent filametrix mod, or the more recent (and its spiritual successor) filamATrix, which combines G2 and CW2 mods for the Stealthburner.
If using dragonburner, Chirpy has created a cutter designed for it.
If using an EVA style toolhead with an orbiter, this is a cutter designed for it.
If using XOL, xol-metrix is a solution for that toolhead.
If using the AT4, you can use KutteR (knight_rad.iant's) own, there are also options for the A4T-Cutter, and the CNC Crossbow.
Whatever you do, make sure whatever extruder you use DOES NOT have a pre-extruder filament pressure sensor. Below it will be explained why this setup is NOT recommended, here be dragons for those that choose to avoid this advice.
However, read this section thoroughly, and understand it:
Discord is not a nice to have option. If you have never used it, sign up for it and join our community, it is designed to give you the latest and best information on how to set up the FPS. It has information that might be missing from the documentation, as it is cutting edge or current, and waiting to be integrated. So please, join us
¶ Before You Begin: How OpenAMS Works (and Why You Need to Understand It)
If you’ve never set up a Multi-Material Unit (MMU) before — especially on a Voron — this is the section you absolutely must read before touching a wrench.
Bambu Lab’s AMS works because Bambu controls everything: the printers, the couplers, the tubing, even the filament. It’s the Mac ecosystem — closed, consistent, and polished.
OpenAMS on a Voron is more like the PC world. You can configure anything, but with that freedom comes troubleshooting. BLDC motors, encoder slip, PTFE tubing ID/OD choices — all of these variables are yours to manage. If you understand the system, you’ll get a rock-solid setup. If you skip this section, you’ll probably end up in #customer-support with bald filament and a motor full of dust.
¶ Stepper vs BLDC: The Mindshift
Stepper systems (ERCF, BoxTurtle, EMU) count steps. If the motor turns 200 steps, you know the filament advanced by 200 steps — unless it slipped.
BLDC systems (AMS and OpenAMS) don’t know their position. They rely on encoders that measure how far the filament goes, and HAL effect sensors that tell it if the motor is running and at what speed. If friction is high, the encoder happily reports rotation while the filament just sits there grinding into powder.
That’s why OpenAMS needs the Filament Pressure Sensor (FPS). It’s a HAL effect sensor that tells the entire system how much compression (in the filmanet) there is, and hence, in comparison to all other sensors, if the system is doing what it is supposed to do.
¶ The FPS: A messenger of reliability, and avoider of downstream problems.
The FPS is the filament pressure sensor. It is an integratal part of the OAMS system, and it is the thing that makes "following", syncrhonization between the BLDC and the toolhead extruder, possible. It also acts as a endstop switch to determine when the filament hits to main direct extruder filament gears.
¶ Filament Path Sensor (FPS) Behavior and Interpretation
¶ Overview
The Filament Path Sensor (FPS) is a critical safety and monitoring component in the AMS system. Its primary function is to detect whether filament is properly moving through the system toward the toolhead.
It is important to understand that FPS fault activation is not inherently a malfunction of the sensor itself. Instead, it provides diagnostic feedback on the state of the filament path.
¶ Early FPS Triggering
If the FPS slide triggers before the filament reaches the toolhead, this is not a flaw in the FPS. Instead, it indicates a discontinuity in the filament path, or the fact you are not using 3mm ID PTFE tubing, and that's the only thing that is going to make this thing work, which can lead to reliability issues across all systems, including BoxTurtle (BT), ArmoredTurtle (AT), EMU, ERCF, and similar setups.
¶ Mechanism of the Issue
- Filament snagging: During a tool change or swap, the filament may encounter a point of friction or obstruction along the path.
- AMS response: The AMS motor continues to drive filament, but the encoder detects that the filament itself is not advancing, or the FPS slide moves all the way to the closed (or full compresssion) position.
- Filament damage: As a result, the filament can be ground down, creating a flat spot or “bald” section. This reduces traction and can cause future feeding errors.
¶ Correct Interpretation
The FPS should be treated as a diagnostic messenger:
- Message: “There is an issue along your filament path that needs attention.”
- Do not assume: “The FPS itself is broken or misaligned.”
Addressing the filament path discontinuity—such as smoothing bends, removing friction points, or adjusting PTFE tubing—is the proper solution to early FPS activation.
¶ Summary
- Early FPS triggering signals a problem upstream in the filament path, not a sensor malfunction.
- Ignoring the FPS warning can lead to filament grinding, reduced traction, and print failures.
- Proper maintenance and optimization of the filament path will ensure reliable AMS operation.
¶ PTFE Tubing and Filament Path Considerations
¶ Smoothness and Clearance
The PTFE tubing in the filament path must be perfectly smooth and unrestricted by the FPS case. Any interference can prevent the FPS slide from moving freely, which may lead to feeding issues.
Common causes of interference:
- Tubing bend radius too large – although in most setups this is not the issue.
- Overextruded FPS case – the case may create excess friction against the PTFE, restricting movement.
¶ Lubrication
Even if the FPS slide moves smoothly, applying a small amount of EP2 Lube (or a similar PTFE-compatible lubricant) to the tubing where it meets the FPS case can improve performance and reduce friction.
¶ Tubing Dimensions
-
Standard filament path: 3 mm ID / 4 mm OD PTFE tubing.
-
Special cases (e.g., Orbiter extruders): Some extruders accept only 2 mm ID / 4 mm OD PTFE. In these cases:
- A transition piece is required near the extruder to reduce from 3 mm ID to 2 mm ID.
- Without a smooth transition, filament can snag on the internal 1 mm step created by the size change.
- Provided STLs should ensure this transition is smooth.
- Bambu Lab couplers are not suitable for this purpose.
¶ FPS and AMS Operation
-
There is no hidden mechanism or “black magic” in AMS operation.
-
The AMS feeds filament until the FPS is triggered.
-
Once
ptfe_lengthis configured after calibration:- The FPS signal is ignored until ~100 mm before the filament enters the toolhead.
- At this point, the AMS slows feed speed to prevent slamming filament into the extruder gears.
- The FPS begins monitoring, and filament feeding stops when the FPS reaches the configured
upper_threshold(a value between 0 and 1 representing proximity to the buffer end).
¶ Summary
- Maintain smooth, unobstructed PTFE tubing.
- Use appropriate tubing diameters and transitions to avoid snags.
- Lubrication at FPS contact points can improve reliability.
- AMS and FPS operate in a predictable, calibrated manner; interruptions indicate path issues, not system failure.
¶ Toolhead Switches and FPS Interaction
¶ Overview
A common question in AMS setups is whether toolhead switches can replace or supplement the Filament Presence Sensor (FPS). In most cases, adding toolhead switches is not recommended.
¶ Why Toolhead Switches Are Not Recommended
- Additional Resistance – Adding a switch introduces extra friction and mechanical resistance in the filament path.
- Potential for Snagging – Extra components in the path increase the risk of the FPS slide catching or sticking.
Some users ask: “Why not use the toolhead switch instead of the FPS?” While this can work in stepper-driven follower systems, it is not suitable for AMS systems with BLDC motors.
¶ Differences Between Steppers and BLDC Motors
- Steppers: Stepper motors have precise positional feedback. A toolhead switch can detect filament presence reliably because the motor moves in known increments.
- BLDC Motors (AMS systems): BLDC motors do not have intrinsic positional feedback. They rotate continuously without knowing the exact distance traveled.
Implication: The AMS must rely on the FPS to determine the exact filament position at the extruder gears. “Close enough” is not sufficient—precise engagement is required to avoid grinding or jamming filament.
¶ Appropriate Use Cases for Toolhead Switches
-
Toolhead switches are functional on systems using stepper-driven followers, such as:
- BoxTurtle
- ArmoredTurtle
- Any custom system where steppers push the filament
-
In these systems, a switch can indicate filament presence and ensure proper tool engagement.
¶ Why FPS Is Critical in BLDC AMS Systems
-
The FPS ensures full engagement of filament with the extruder gears.
-
Replacing the FPS with a switch could allow the BLDC motor to push filament before it reaches the correct position, risking:
- Filament grinding
- Hub motor jams
- Print failures
-
Adding a switch in the path only increases impedance and does not improve reliability. At best, it is redundant “belt-and-suspenders” hardware.
¶ Recommended Alternative Placement
If a switch is desired:
- Place it after the extruder, not before.
- In this position, the switch can confirm successful filament loading without interfering with FPS operation or AMS motor behavior.
¶ Summary
- Do not use toolhead switches as a replacement for the FPS in AMS systems.
- FPS is essential for BLDC AMS motors to ensure precise filament engagement.
- Toolhead switches may be used after the extruder for confirmation purposes only.
- Avoid adding extra components in the filament path to prevent friction, snags, and jams.
Here’s a beginner-friendly guide explaining how OrcaSlicer maps “colors” to toolchangers and how it relates to Klipper and MMUs:
¶ OrcaSlicer Multi-Material Mapping: Colors, Toolchangers, Klipper & MMUs Explained
¶ 1. What Are “Colors” in OrcaSlicer?
In OrcaSlicer, “colors” don’t just refer to visual colors. Each “color” in the software represents:
- A separate material or filament.
- A separate extruder/toolhead in multi-extruder printers.
- A separate virtual tool for MMU-style printers.
You’ll often see them as Color 0, Color 1, Color 2, etc.
¶ 2. Tool Mapping: How Colors Link to Extruders
When slicing for multi-material or multi-extruder prints:
- Color 0 → mapped to T0
- Color 1 → mapped to T1
- Color 2 → mapped to T2
- ...and so on.
In G-code, these are invoked as T0, T1, etc.
¶ 3. What Are Toolchangers?
A toolchanger is a printer that physically swaps toolheads (extruders). Each toolhead is usually fixed with a different material.
- Example: A Voron with the EVA toolchanger or Jubilee printer.
- Klipper manages the tools with commands like
T0,T1,T2, etc. - OrcaSlicer simply maps Color 0 → T0, and so on.
If you're using a toolchanger, each “color” in OrcaSlicer must match the right tool number in your Klipper config.
¶ 4. What About MMUs (Multi-Material Units)?
An MMU (like the Prusa MMU or ERCF) uses a single extruder but swaps filament between different materials.
-
Klipper treats this differently: It still uses
T0,T1, etc., but all tools are virtual and share the same hotend. -
The MMU loads/unloads filament automatically when the tool (color) changes.
-
In OrcaSlicer:
- Still assign Color 0, Color 1, etc.
- But all tools map to the same extruder (
extruder 0in Klipper), and the MMU handles the switching.
In your Klipper config for MMUs, you’ll often see all
T0–T4mapped to the same physical extruder.
¶ 5. How to Set It Up in OrcaSlicer (Step-by-Step)
¶ For a Toolchanger:
- Go to Printer Settings > Extruders.
- Add extruders: Extruder 0, 1, 2, etc.
- Map each color/material to a different extruder.
- Ensure G-code flavor is set to Klipper.
- When slicing a multi-color model, assign parts to Color 0 → T0, Color 1 → T1, etc.
¶ For MMU (like ERCF):
-
Still go to Printer Settings > Extruders.
-
Add multiple extruders (Color 0 to 4), but:
- All extruders use the same nozzle position.
- All extruders are set to share the same physical extruder.
-
Set tool change G-code to manage filament loading/unloading (
T{n}). -
Assign parts in the slicer to Color 0–4. The MMU system will handle the swaps.
¶ 6. Where Klipper Comes In
Klipper doesn't care about "colors"; it only cares about tool numbers (T0, T1, etc.).
- In your
printer.cfg, define each tool with the corresponding heater and stepper. - MMU setups require macro handling (
T0triggers a macro to load filament 0).
¶ 7. Tips for Beginners
- Each color = one material/tool, not just a visual difference.
- For MMUs, you don’t need multiple nozzles—just multiple materials.
- Keep your tool ordering consistent: Color 0 = primary material/tool.
- Test with simple two-color models before attempting complex ones.
- Watch your filament purging between swaps—OrcaSlicer has purge tower and waste block options.
¶ 
How OrcaSlicer Maps “Colors” to Tools
In OrcaSlicer:
-
Each color you assign to a part of a model maps to a tool number (
T0,T1, etc.). -
For example:
- Color 0 →
T0 - Color 1 →
T1 - Color 2 →
T2 - Color 3 →
T3
- Color 0 →
These tool numbers are inserted directly into the generated G-code (e.g., T0, T1...), and Klipper runs the associated macros when it sees them.
¶ 
How OpenAMS Maps T0, T1, etc. to Filament Groups
OpenAMS defines a filament group as a specific physical spool slot in the AMS (Automatic Material System), like this:
[filament_group T0]
group: oams1-0
[filament_group T1]
group: oams1-1
[filament_group T2]
group: oams1-2
[filament_group T3]
group: oams1-3
That means:
T0maps to spool slot 0 on AMS 1 (oams1-0)T1→oams1-1T2→oams1-2T3→oams1-3
So when you select T2, the system will:
- Unload the currently loaded filament (if any)
- Load the filament from spool slot 2
- Purge the hotend
- Resume printing
¶ :tools: How Klipper Macros Handle This (Step-by-Step)
¶ 
1. Tool Macros: T0, T1, etc.
Each tool macro just calls a shared macro with the right filament group:
[gcode_macro T0]
gcode:
_TX GROUP=T0
_TX is the central toolchange handler. It’s where the logic lives.
¶ 
2. What _TX Does
[gcode_macro _TX]
This macro:
-
Checks if the right filament is already loaded.
-
If yes, it skips switching.
-
If not, it:
- Calls
SAFE_UNLOAD_FILAMENT - Loads the new filament via
OAMSM_LOAD_FILAMENT GROUP={GROUP} - Verifies filament movement with optional sensors
- Feeds the proper amount into the extruder
- Calls
CLEAN_NOZZLEto purge/clean before printing resumes
- Calls
¶ 
3. Safe Unload: SAFE_UNLOAD_FILAMENT
This macro:
- Pulls back filament using defined lengths
- Cuts it cleanly with
CUT_FILAMENT - Ensures proper retraction past extruder gears
- Triggers the unload sequence (
OAMSM_UNLOAD_FILAMENT) - Validates with sensors (if enabled)
¶ 
4. Filament Load: OAMSM_LOAD_FILAMENT
This command is part of the OpenAMS plugin and:
- Starts the motors in the AMS
- Feeds filament through PTFE tube to the extruder
- Stops once pressure is sensed via the FPS (Filament Pressure Sensor)
- Part A → Color 0 → G-code includes
T0 - Part B → Color 1 → G-code includes
T1
At print time:
T0triggers_TX GROUP=T0→ loadsoams1-0(white)T1triggers_TX GROUP=T1→ unloads current → loadsoams1-1(black)
In OpenAMS, the filament group feature enables seamless filament management by allowing multiple spool bays to be grouped together. This setup facilitates infinite spooling, ensuring uninterrupted printing by automatically switching to another spool within the same group when the current one runs out.([OpenAMS Documentation][1])
¶ How Infinite Spooling Works
-
Primary Spool Selection: When a print begins, the system selects the first available spool in the defined
filament_group.([OpenAMS Documentation][1]) -
Automatic Switching: If the active spool depletes during printing, OpenAMS automatically transitions to the next available spool within the same group, provided it matches the required filament type (e.g., same color and material).([OpenAMS Documentation][1])
-
Spool Replacement: You can replace an empty spool during an ongoing print. Once the current spool is exhausted, the system will revert to the newly loaded spool, maintaining continuous operation.([OpenAMS Documentation][1])
This functionality is configured in the Klipper firmware using the [filament_group] section, where you specify the associated AMS bays. For example:([OpenAMS Documentation][1])
[filament_group T0]
group: oams1-0, oams1-1
In this configuration, the system will first attempt to use the spool in oams1-0. If it's unavailable or runs out, it will switch to oams1-1.([OpenAMS Documentation][1])
¶ Behavior When No Spools Are Available
If all spools within the filament_group are depleted or unavailable, the printer will pause and prompt you to load a new spool. This pause prevents print failures due to filament exhaustion and allows you to resume printing once a new spool is loaded. ([Bambu Lab Community Forum][2], [Reddit][3])
¶ Practical Considerations
-
Filament Consistency: Ensure that all spools within a group contain filament of the same type and color to maintain print consistency.([Bambu Lab Community Forum][2])
-
Monitoring: Regularly check the status of your spools to anticipate when replacements will be needed, minimizing pauses during printing.