The Semitruck is a rigid remote direct drive (RDD) extruder based on printed Cardan joints, square carbon fiber shaft, and a coupling that transfers rotational force through ball bearings that can freely roll along the four faces of the square shaft. It comes in two variants:

• OG Semitruck, where the vertical segment of the drive shaft engages with a fixed-position "slider gear" at the top of the printer and is free to move up and down, and

• Semitruck Prime, with a telescoping double Cardan shaft coupled directly to the motor at the top of the printer.

In either variant, the shaft drives an ultra-compact worm-based extruder on the toolhead. This makes it possible to achieve, simultaneously:

• Extremely low moving mass
• High extrusion speed
• High extrusion force
• Rapid high-precision pressure advance adjustments and retractions
• Near-zero backlash

The double Cardan shaft is net constant-velocity (CV), and is not subject to position-dependent offset of the shaft angle. The latter property sets it apart from most other RDD configurations, which rely on large reduction gear ratios along with well chosen motor positioning to minimize error.

The Semitruck has been operated at up to 90k mm/s² kinematic accelerations and 60 mm³/s flow. It can achieve pressure advance smooth times at least as low as 10 ms.

Like this project? Support it and my other work on Ko-fi or Patreon:

• https://ko-fi.com/daliasprints
• https://patreon.com/daliasprints

The OG Semitruck has been in public beta since May of 2025. It was successfully operated on the original build of the M45 delta from 2023 to early 2025, and was successfuly reproduced during private beta.

The Semitruck Prime has been operated on the original build of the M45 delta since March of 2025. It entered public beta in October of 2025.

The Prime does not supersede the OG, but makes different tradeoffs that might make it more or less suitable for different machine configurations and scalings. Both variants will be treated as first-class for the forseeable future.

For delta printers: room for mounting at the top center and clearance equal to Z travel range above the printer. The exact height constrtaints are explained below. If the printer has a closed top cutting/drilling a small hole (between 10 and 22 mm) may be needed for accommodate the shaft.

For fixed-gantry printers with bed moving down in Z: framing out a mount above the gantry at height roughly 75% of build width/height above the top of the toolhead.

Others: not recommended. While the Semitruck can be used with any kinematic system, the needed shaft length and framing above the printer will be disproportinate and probably unacceptable.

The toolhead side of the extruder has filament path and screw spacing designed to mount to a Slice Mosquito or Mellow NF Crazy heatsink/frame. Creating adapter plates for other mountings, or variants to the extruder housing, should not be hard, but they are not included with the official Semitruck distribution at this time.

The motor side of the extruder is designed to mount to the top of a 20-series T-slot or V-slot extrusion with M5 screws and T nuts. The slider gear should be centered directly above the worm gear shaft on the toolhead side when the toolhead is at the center of the bed. Off-center installation may work but increases the stress on the U-joints towards the edges of the bed and tightens the constraints on the shaft part lengths.

• LDO 42STH48-2804AC motor (1)
• M3 6 mm screws (4)
• 190-220 mm closed loop GT2 belt (1)
• M3 8 mm screw (1)
• M3 20 mm screws (4)
• M3 nut (5)
• M5 10 mm screw (3)
• M5 T nut (3)
• M5 washer (3)
• Printed emotor_mount.stl (1)
• Printed bearing_clip.stl (1)
• Printed pulley_60T.stl (1)

• 603ZZ (3x9x5 mm) bearings (8)
• 6702ZZ (15x21x4) bearings (2)
• M2.5 4 mm button head screws (4)
• M2.5 nuts (4)
• Printed slider_housing.stl (1)
• Printed slider_pulley.stl (1)
• Printed slider_insert.stl (4)

• Square 6 mm x 6 mm x round 4 mm ID carbon fiber tube (1)
• MR63 (3x6x2 mm) bearings (8)
• M2.5 6 mm screw (1)
• M2.5 nut (1)
• Printed ujoint_center.stl (2)
• Printed ujoint_arm.stl (3)
• Printed ujoint_3mmend.stl or ujoint_end.stl (1)

• Leadshine 42CM06 2.5A motor (1)
• M3 6 mm screws (4)
• (Machine-specific mounting platform)

• 603ZZ (3x9x5 mm) bearings (4)
• Square 6 mm x 6 mm outer by round 4 mm ID carbon fiber tube (1)
• Square 10 mm x 10 mm outer by square 8.5 mm x 8.5 mm inner carbon fiber tube (1)
• MR63 (3x6x2 mm) bearings (8)
• M2.5 6 mm screw (2)
• M2.5 nut (2)
• Printed telescope_housing.stl (1)
• Printed telescope_insert.stl (4)
• Printed telescope_stabilizer.stl (1)
• Printed ujoint_center.stl (2)
• Printed ujoint_arm.stl (1)
• Printed ujoint_big.stl (1)
• Printed ujoint_motorend.stl (1)
• Printed ujoint_3mmend.stl or ujoint_end.stl (1)

(Skip this section if you are building the original version using the discontinued NF Cannon gear set.)

• HGX extruder parts set (v1 or v2, see below) (1)
• 3mm steel shaft, at least 35 mm in length, to be cut
• 0.5 module 2 start worm gear, 7mm outer diameter by 12 mm length
• F3-8M (3x8x3.5 mm) thrust bearings (4)
• M2.5 6 mm screws (2)
• M2.5 25 mm screws (3)
• M2.5 nuts (3)
• M3 10 mm button head screws (2)
• M3 nuts (2)
• MR63 (3x6x2 mm) bearings (2)
• Printed noncannonical_bottom.stl (1)
• Printed noncannonical_top.stl (1)
• Printed noncannonical_insert.stl (1)
• Printed noncannonical_insert_cover.stl (1)
• Printed wormwheel-hgxv1.stl (1)

(Skip this section if you will be using the HGX gears.)

• Mellow NF Cannon parts - set 2, without motor (1)
• F3-8M (3x8x3.5 mm) thrust bearings (2)
• F5-10M (5x10x4 mm) thrust bearings (2)
• M2.5 6 mm screws (2)
• M2.5 25 mm screws (3)
• M2.5 nuts (3)
• M3 10 mm button head screws (2)
• M3 nuts (2)
• MR63 (3x6x2 mm) bearings (2)
• Printed headcannon_bottom.stl (1)
• Printed headcannon_top.stl (1)
• Printed headcannon_insert.stl (1)
• Printed headcannon_insert_cover.stl (1)

• HGX v1 gear set: https://www.aliexpress.com/item/3256808173742215.html

• HGX v2 gear set: Not yet supported.

• 0.5 module 2 start worm gear, 7mm outer diameter by 12 mm length: https://www.aliexpress.com/item/2255800266497667.html or https://www.aliexpress.com/item/3256804727007575.html

• Mellow NF Cannon parts: NOT RECOMMENDED. These are discontinued, and may be changed/incompatible if re-issued.

• Carbon Fiber: https://windcatcherrc.com/product/carbon-fiber-sq-outer-with-rnd-inner-tube-6mm-x-6mm-x-4mm-x-1000mm/

• Bearings: https://www.avidrc.com/product/8/metric-bearings/

Pregenerated STL files are included with Semitruck release packages. To build STLs from the OpenSCAD source files, the supplied Makefile can be used in an environment with GNU make and the openscad command line interface available. The individual .scad files can also be processed manually in OpenSCAD, using the customizer UI to select which subpart to generate then exporting the STL.

If building the Noncannonical (HGX) version with printed worm wheel, the worm wheel should be printed with 0.14 mm layer height, 100% infill, and random seams. A seam that is aligned across layers may lead to periodic artifacts and should not be used. Printing this requires a well-tuned printer and may require some iteration with different settings depending on material shrinkage and other factors. The provided source and STL are designed to real physical dimensions, not any fudge factors for printing with a particular material.

The recommended material for the worm wheel is PET (not PETG). As the worm wheel and thrust bearings against it need lubrication, it should not be printed in materials incompatible with grease, such as ABS or ASA. POM or nylon may be options if PET is not available, but they have not been tested.

The toolhead-side insert parts should be printed with 0.1 mm layer height and 100% infill for best results. Everything else can be printed with 0.2 mm layer height, 1.0-1.2 mm shell thickness, and 20% infill.

The toolhead side top and bottom housing need minor supports for the nuts and bolt heads. No other parts need supports.

Primary development and testing was done with parts printed in PET.

The toolhead-side parts should ideally be printed in material with low to no creep, and the filament path insert especially should avoid PLA or other materials that soften at low temperatures, as it's possible to pull heat up into it when retracting, especially if using long retractions at end-of-print or when pausing to avoid oozing while idle.

For the rest of the parts, any rigid material is probably okay, even PLA.

In order for the extruder to reach the entire build volume, the lengths of the middle and vertical shaft segments must be chosen correctly.

First, work out the middle segment length. A good starting point is not to go over 45° at the U-joints. To satisfy this on a delta, the middle segment length should be at least √2 times the build radius. To satisfy this on a square build area, the middle segment length should be equal to one side of the square.

Next, determine the height the slider gear and motor should be mounted at. The vertical distance between the bottom of the slider and the top of the extruder housing on the toolhead (at max Z) must be at least 45 mm plus the length of the middle shaft segment.

Finally, the end shaft segment length may be worked out based on toolhead at minimum Z and maximal distance from the origin, but it is recommended not to compute it yet, instead waiting to cut the shaft until after installation is checked.

The middle shaft segment should be cut to exactly 32 mm shorter than the length determined above. Each U-joint arm end adds 16 mm to the length (measured between joint centers).

One U-joint arm end, both in the same orientation, should be pressed onto each end of the middle shaft segment. They can be attached permanently with epoxy if desired, but are designed such that press fit should be sufficient.

Press a single U-joint arm end onto the end shaft segment (not yet cut to length) and prepare the 5 mm shaft-clamp U-joint end that will attach to the toolhead side of the extruder by installing the M2.5 hardware on it.

Place a pair of MR63 bearings onto the pegs of one arm end, then slide them as a unit onto a U-joint center block until they snap into place. Then, rotate the center block and repeat with the other side of the joint. A screwdriver shaft can be inserted through the opening between the arms to hold the center block if pressing the second side of the joint on is popping the first side's bearings out of their snap-fit slots.

Repeat for the second U-joint.

The slider consists of a 4-part printed insert holding 8 bearings, an outer housing, and a printed GT2-tooth-profile pulley integrated with the cover for the housing.

Assemble half of the insert by pressing four 603ZZ bearings onto the pegs of one insert piece, then pressing a second insert piece's pegs into two of the bearings already on the first.

Repeat to assemble the second half of the insert, then press the two halves together.

Press the insert into the housing.

Press four M2.5 nuts into the hexagonal insets on the cover (integrated with the pulley), and attach it to the housing using the four M2.5 4 mm button head screws.

Place the two 6702ZZ bearings over the two ends of the slider assembly in preparation for slotting it into the motor mount assembly.

Attach the slider to the printed motor bracket with the closed loop timing belt wrapped around it by pressing the two 6702ZZ bearings into the slots. The belt should be captured. Press 4 M3 nuts into the slots beside the bearings, and use the printed bearing clips, each with two 20 mm M3 screws, to secure the bearings.

(Note: The upper clip is optional, since belt tension will suffice to secure the upper bearing. You may wish to omit it for ease of disassembly.)

Loosely attach the motor with the 6 mm M3 screws, making sure it can still slide freely for adjustment. Attach the printed 60T primary pulley to the motor shaft using an M3 nut and 8 mm M3 screw to secure the pinch coupler. Bring the timing belt around the pulley, pull the motor until the belt is no longer slack, and verify vertical alignment of the belt. If needed, loosen the clamp on the pulley and slide it up/down on the motor shaft to align with the slider-side pulley, then retighten. Pull the motor tight to tension the belt adequately, and, while holding it, tighten down the four motor screws.

The necessary parameters for determining shaft length and thereby mounting height are the length $z$ of Z travel range immediately below the motor and the maximum needed XY travel radius $r$ relative to the center point just below the motor. A third parameter $k$ is a constant determined by the geometry of the U-joint and telescopic joint parts, and has a value of 61 mm for the standard parts.

In order to allow for error in mounting position, it is recommended that the values $z$ and $r$ be padded by a few millimeters. However, the location of the motor-end U-joint arm on the motor shaft allows for some degree of adjustment, and motor shims may also be used if needed.

In terms of these parameters, the shaft length $h$ measured between U-joint centers is given by the formula:

$h = \frac{2k + z + \sqrt{4z^2 + 3r^2 + k^2 + 4kz}}{3}$

The length of the fully-extended shaft, measured between U-joint centers, is then $2h-k$.

The inner carbon fiber square tube needs to be cut to a length of $h-33$ mm.

The outer carbon fiber square tube needs to be cut to a length of $h-52$ mm.

The motor needs to be mounted such that, with the toolhead at maximum Z just below the motor, the shaft is at minimum length. This means the motor-side U-joint center should be exactly $h$ mm above the extruder-side U-joint center.

There is presently no standardized mounting setup for the Semitruck Prime motor.

The telescopic coupling consists of a 4-part printed insert holding 4 603ZZ bearings and a printed outer housing. Assemble the insert in a cross shape by pressing two 603ZZ bearings onto the pegs of one insert piece, then pressing a second insert piece's pegs into two of the bearings already on the first. Repeat to assemble the second half of the insert, then press the two halves together, and press the insert into the housing.

Onto one end of the smaller carbon fiber tube, press fit the printed stabilizer and verify that it fits into the larger carbon fiber tube without any significant friction. If it does not, shave the outside of the stablizer down until it does.

Push the end of the smaller tube without the stabilizer on it through the telescopic coupling from the side with the square 10 mm socket. The bearings should roll on it as you push it through. Press fit a printed U-joint arm onto the exposed end of the tube so that it cannot inadvertently slide back out.

Press fit the larger tube into the square 10 mm socket on the telescopic coupling, letting the end of the smaller tube with the stabilizer go inside the larger tube. Then, press fit the U-joint arm with the 10 mm socket on the other end of the larger shaft, ensuring that both U-joint arms are in the same orientation (coplanar).

Assemble the U-joint centers and motor/extruder side arms onto the ends as in the OG Semitruck instructions.

First, make sure you have the appropriate set of printed parts -- Noncannonical to use the HGX hob; Headcannon if you have and will be using the discontinued Mellow NF Cannon gears.

First check that the filament path has clearance for filament. A 2 mm diameter screwdriver shaft should fit through it with some friction, and clear out any roughness or stringing that might be in the way.

Begin by assembling the filament path and idler bearing insert. Place two MR63 bearings in the cutouts of the printed insert and slide the printed insert cover over them. Place M3 nuts in the two hexagonal cutouts in the cover, and insert and tighten 10 mm M3 button head screws from the other side.

Before continuing, ensure that the filament path insert can fit into the rectangular holes in the printed housing top and bottom parts. Clean up any "elephant foot" or other print issues that might keep it from fitting later.

Now begin with the hob and worm wheel. If you are using the HGX hob, you will need to separate it from the spur gear that came attached to it. This can be done non-destructively for the HGX version 1. Insert a thin flat tool such as a putty knife or razor blade between the plastic gear and the filament hob, and lightly pry back and forth until they can be pulled apart by hand. Alternatively, you may apply pressure to the plastic pins through the hob to push the gear out. Once the hob is separated, press the printed worm wheel onto it.

If using the NF Cannon gears, the gear on one of the two hobs already serves as a worm wheel matching the worm. To check which one, hold the gear and hob vertically with the axis running left/right, and look at the slant of the teeth. The correct one is the one where the teeth slant upward to the left and downward to the right.

Now, insert one of the needle bearings from the HGX or Cannon set in the hob, and put the included shafts, 3 mm diameter and approximately 15.8 mm in length, through the center. Next, assemble the F3-8M thrust bearings on the shaft. The races with the larger inner diameter, which are loose and spin freely on the shaft, belong on the inside next to the worm wheel and filament hob, with the grooves facing outward. The balls go next, and the races with the smaller diameter go on the outside, with the grooves facing inward towards the balls. They should "snap" onto the shaft and not fall off easily. Before putting them on, apply a small amount of grease.

If using the HGX, the worm gear must be pressed onto the 3 mm shaft. This can be done without any specialized tooling beyond a hammer, 4 mm washer, and small socket. Place the worm on top of the socket with one or more washers in between, and hammer the end of the shaft until 3-3.5 mm of it extend out the other end of the worm.

If you are using the NF Cannon gears, insert the 5 mm diameter shaft through the worm with 4-4.5 mm extending out the threaded side and tighten the set screw on the worm sleeve.

Now, assemble the thrust bearings on the shaft. The process is the same for either version. This time, the smaller ID races go on first, next to the worm. The fit should be very tight, and may also require a press or hammering using the same technique as above. The balls and outer races will be loose until installation; for now, just grease them.

Begin by pressing 3 M2.5 nuts into the holes in the bottom part of the housing. Turn it upright, and place the hob/worm wheel assembly with the bearings in the halfround cutouts, with the hob on the side facing the rectangular hole and the worm wheel on the side facing the round hole.

Before continuing, insert the 6 mm M2.5 button head screws in the two holes beside the rectangular filament path hole. Depending on which variant you are building, they may be difficult to insert or remove later.

Next, place the filament path insert in the rectangular hole.

Finally, place the outer race of the worm thrust bearing in the circular hole, being sure the groove is facing upward. Apply some grease and place the ball cage and worm assembly on top, with the long side of the shaft pointing upward. Grease the other ball cage and place it on top, followed by the final thrust bearing outer race, with the groove facing downward.

Line up the top of the housing and press it closed on top of the bearings and filament path. Insert three 25 mm M2.5 screws to secure it to the bottom part.

With the screws tightened, there should be no vertical play in the worm. If this is not the case, due to manufacturing variance in the worm, printing inaccuracy, or other factors, it will be necessary to shim the top side, either between the worm and thrust bearing, or between the thrust bearing and housing, with a suitably sized washer or printed part. It is critical that the worm be well-constrained vertically; otherwise the extruder will have catastrophic backlash and print quality will be very poor.

It should be difficult to turn the worm by hand, but possible by gripping the shaft with tools or a drill chuck, or with the printed U-joint arm end. At this point you can test feeding filament through manually by turning the worm.

Move the toolhead out of the way well below maximum Z, and insert the end shaft segment up through the bottom of the slider. The fit should be somewhat tight, turning the bearings inside the slider as it's moved. Raise the shaft until the joint at the lower end is higher than the top of the toolhead, then bring it down onto the worm gear shaft and clamp it in place by tightening the screw.

The HGX hob has a measured pitch circumference of 56.94 mm, and the worm gear reduction ratio is 36:2 (18:1). The motor-side pulley gives a 24:60 (2:5) increase in speed. If using Klipper, these values can be entered directly without having to do any math:

[extruder]
rotation_distance: 56.94
gear_ratio: 24:60, 36:2

To convert to E-steps-per-mm for other firmware, some math is needed. Applying the gear ratios, this is 7.90833 mm of filament advance per rotation of the E motor, or 25.2898 full steps per mm. At 16 microsteps per step, that comes to 404.636 E-steps-per-mm.

The Cannon hob has a pitch circumference documented by Mellow as 59.08 mm, and the worm gear reduction ratio is 39:2 (19.5:1). The motor-side pulley gives a 24:60 (2:5) increase in speed. If using Klipper, these values can be entered directly without having to do any math:

[extruder]
rotation_distance: 59.08
gear_ratio: 24:60, 39:2

To convert to E-steps-per-mm for other firmware, some math is needed. Applying the gear ratios, this is 7.57435 mm of filament advance per rotation of the E motor, or 26.4049 full steps per mm. At 16 microsteps per step, that comes to 422.478 E-steps-per-mm.

Polarity of the E motor step direction pin will vary depending on motor wiring. It may be determined experimentally, or set directly to whichever polarity is known to make the motor spin clockwise (viewed from the output side).

Additional recommended starting points for settings with Klipper:

[extruder]
max_extrude_only_velocity: 45
max_extrude_only_accel: 4000
microsteps: 64
pressure_advance: 0.036
pressure_advance_smooth_time: 0.010

[firmware_retraction]
retract_length: 0.20
# Speeds set way above limit so M220 won't slow down retracts
retract_speed: 200
unretract_speed: 200

[tmc5160 extruder]
run_current: 2.8
stealthchop_threshold: 0
interpolate: false

The above TMC configuration should be adapted to your particular drivers. If they do not support the full current for the motor, it may be necessary to run at lower retraction acceleration and/or increase the pressure advance smooth time window.

Once the Semitruck is mounted and configured, start off with manual motion tests through your printer's UI to check that the shaft slides freely up and down the slider and does not bind. Motion at low Z height (upper shaft extended down as far as possible) is the hardest case, and should be checked at the edges of the build area starting with short moves, so that binding can be caught before it breaks anything.

The usual fault mode, if it does break, is explosion of the U-joints, which are low enough in mass that they should not pose severe danger, but the bearings can easily be lost if this happens.

If binding is occurring, it may be necessary to mount the motor higher and use a longer middle shaft segment, but first check that the slider gear is not too tight on the shaft. This can occur as a result of printing defects, and can usually be remedied by scraping/sanding the inside of the slider housing (or reprinting it with very minor horizontal offset to compensate) so that the bearings are not squeezed as tightly against the shaft. It's also possible to run the system restricted to a smaller build radius until the problem is resolved.

Before actual printing, extrusion rate should also be checked to ensure that the motor is turning in the right direction and that the correct amount of filament is being advanced. This can be done though standard methods -- marking a known length of filament going into the extruder, commanding extrusion of the same length, and verifying that the marked spot ends at the point of entry to the extruder.