Phase 0 · Fundamentals · L1
The machine in four parts
Four subsystems, one job: put a controlled amount of melted plastic in a controlled place. Every calibration in this course is fixing one of the four.
This lesson covers motion systems (bedslinger vs CoreXY), the hotend and melt zone, direct-drive extruders, and the heated bed — plus which of the four causes which class of defect. If that's already yours, skip straight to Phase 1 →
You are about to tune three machines that fail in three different ways. An MK3S and a Core One printing the same file with the same filament will produce different defects, because one drags a heavy bed around and the other flings a light head around a fixed frame.1 If you can't say which of the four parts a defect came from, every calibration becomes trial and error. This lesson gives you the four buckets you'll sort every problem into for the rest of the course.
Strip away the enclosure, the touchscreen and the marketing and an FDM printer is four subsystems cooperating on a single sentence: put a controlled amount of melted plastic in a controlled place, about a hundred thousand times.
The four are motion, the hotend, the extruder, and the build plate. Two of them control place and two of them control amount. Hold onto that split — it is the fastest diagnostic you have.
1Two parts control place
Motion is the frame, rails, belts and stepper motors that decide where the nozzle tip is at every instant. The build plate is the reference surface all of that is measured from: a heated aluminium bed carrying a removable spring-steel sheet.
Motion is where the two Prusa families genuinely diverge, and it is not cosmetic.
| Bedslinger — MK3S, MK4S | CoreXY — Core One | |
|---|---|---|
| What moves on Y | the bed, plus your whole part | nothing — the head does X and Y |
| Moving mass | grows as the print grows | constant, and small |
| Motors | one per axis, X motor rides on the gantry | two fixed motors, crossed belts, both drive X and Y |
| First thing to give | acceleration and external-perimeter speed | gantry squareness, if the two belts aren't matched |
| Tell-tale defect | ghosting worse on tall thin parts | circles come out slightly oval |
That "moving mass grows" row is the whole reason the MK3S is the slow machine of your three. Halfway up a 200 g part it is accelerating 200 g of plastic back and forth on the Y axis, and every reversal rings the frame. The Core One accelerates the same light toolhead on layer one and layer four hundred.
On a bedslinger, X and Y are separate belts doing separate jobs — a loose X belt is an X problem. On CoreXY both belts contribute to both axes, so unequal tension pulls the gantry out of square. That's why Prusa's Core One procedure insists you tighten the two tensioner bolts evenly and alternately: it's what keeps the gantry square while you adjust.2 You'll do this for real in Lesson 6.
2Two parts control amount
The extruder is a geared motor that grips filament and pushes it. The hotend melts what arrives. All three of your printers are direct drive: the extruder sits on the moving toolhead, a few centimetres above the melt zone, rather than pushing filament down a long Bowden tube. That short path is why direct-drive machines handle flexible filament at all, and why they need much less retraction than Bowden machines.
Inside the hotend, four things stack up:
- Heatsink Kept cold by a fan. Filament arriving here must still be stiff — a stiff rod is what lets the extruder push at all.
- Heatbreak A deliberately thin-walled bridge between cold and hot. Its job is to make the transition abrupt, so the molten region stays short and predictable. When it stops doing that, softened filament creeps upward and jams the cold section — heat creep.
- Heater block Cartridge heater and thermistor. The thermistor reads block temperature, not plastic temperature — a distinction that matters the moment you print fast.
- Nozzle The 0.4 mm orifice everything is measured against. A worn or clone nozzle is not 0.4 mm, and will not reproduce anyone's numbers including your own.
The melt zone — the short heated stretch where solid becomes liquid — is the machine's real speed limit. Push filament through faster than it can absorb heat and the centre of the strand arrives still solid. It comes out under-pressure, matte and weak. That ceiling has a name and a number: max volumetric speed, in mm³/s, and you will measure yours in Lesson 15.
Practical flow ceiling on a 0.4 mm nozzle: MK3S ~11–13 mm³/s, MK4S ~22–28, Core One ~22–28. Same plastic, same nozzle diameter — roughly twice the throughput on the two newer machines, purely from how much heat the hotend can put into the strand per second. These are the working figures from your own tuning guide, to be replaced by measured ones in Phase 2.
Note what that does not mean. Your MK4S and Core One share an MK4S-class hotend, so their melting capacity is comparable — but the MK4S is a bedslinger dragging your part around on Y while the Core One flings a light head around a fixed gantry. Same flow ceiling, different motion ceiling. Their filament profiles will look similar; their speed and acceleration profiles will not.
3Where the machine ends and the profile begins
Here is the idea the whole course is built on. Every filament calibration you will run in Phase 2 measures filament behaviour plus machine behaviour, mixed together, with no way to separate them afterwards.
# belt is 12% loose
corners bulge → lower PA
walls measure thin → raise EM
circles are oval → ...?
You have now encoded a
loose belt into a
filament profile.# belt set to spec, logged
corners bulge → lower PA
walls measure thin → raise EM
circles are round
Every number now describes
the filament, and travels
with the spool.This is not a philosophical point. A profile tuned around a mechanical fault stops working the moment you fix the fault — and it never worked on your other two printers at all. Phase 1 exists to turn the machine into a constant, so Phase 2 can treat the filament as the only variable.
Two features complicate the picture, and both belong to the machine rather than the filament, even though they get tuned per filament:
- Input Shaper cancels the frame resonances that cause ghosting, so the printer can run much higher acceleration for the same surface finish. Your MK4S and Core One have it; the MK3S does not.1 Note what it does not do: it compensates for resonance, it does not tighten a belt. Prusa are explicit that belt adjustment and general maintenance come first, because those mechanical factors limit how much Input Shaper can help.1
- The bed probe differs by family and this changes the entire first-layer procedure. The MK3S uses a PINDA inductive probe, which senses the steel sheet — not the nozzle. The MK4 family has a loadcell in the heatsink that detects the nozzle physically touching the sheet, so first-layer height is measured automatically before every print and no manual calibration is needed.3 Two different procedures, in Lesson 7.
4Sort the symptom
Click each symptom. Which of the four parts owns it — and is it a place problem or an amount problem?
- Faint repeated echoes downstream of every sharp cornerMotion
- Walls go matte, thin and weak above a certain speedHotend
- First layer lines are round and separated, and lift at the cornersBuild plate
- A rhythmic click, and a flat chewed patch on the filamentExtruder
- On the Core One, circles print consistently ovalMotion
5Check yourself
Answer from memory. Pulling these out of your own head is what makes them stick.
Write down what your three machines actually are
Open a file called fleet.md in this folder and fill in one row per printer. Read the
values off the machine — the firmware version from the LCD, the nozzle from what you last installed, the
sheet from what's on the bed right now. Don't copy them from the guide.
| Printer | Firmware ver | Kinematics | Probe | Nozzle | Sheet | Last hardware change |
| MK3S | ? | bedslinger | PINDA | 0.4 | ? | ? |
| MK4S | ? | bedslinger | loadcell | 0.4 | ? | ? |
| Core One | ? | CoreXY | loadcell | 0.4 | ? | ? |Then add one sentence per machine: "The first thing that will limit quality on this printer is ___, because ___."
→ Paste it back to me and I'll tell you which of your three predictions I'd bet against.
What you can now say
- Four subsystems, two jobs. Motion and build plate control place; extruder and hotend control amount. Every defect belongs to one of the four.
- Bedslingers and CoreXY fail differently. Growing moving mass versus gantry squareness — which is why one profile can't serve both.
- The melt zone is the real speed limit, and it is a different number on each of my three machines even with the same nozzle.
- Filament calibration measures filament plus machine. That's the whole argument for doing Phase 1 first.
- Input Shaper compensates for resonance, not for slop. Belts come first.
§References
- Prusa Research. Input Shaper (CORE One, MK4/S, MK3.9/S, MK3.5/S, XL, MINI/+). Which printers have Input Shaper, what it cancels, and that belt adjustment and maintenance limit its effectiveness. Link.
- Prusa Research. Adjusting belt tension (CORE One). The instruction to tighten both tensioner bolts evenly and alternately so the gantry stays square during adjustment. Link.
- Prusa Research. Loadcell (MK4/S, MK3.9/S, XL). The loadcell sits in the hotend heatsink and detects nozzle-to-sheet contact, so manual first layer calibration is not needed. Link.
Lesson L1 of the prusa-profile-tuning course · all lessons · The Calibration Card · glossary