Why is the First Half of My 3D Print Stretched Over the Z-Axis?

Finding that your 3D print is taller than it should be—specifically during the first half of the print—is a rare but frustrating occurrence. While "squashed" prints are common due to binding, "stretched" prints indicate that the Z-axis is moving further than the G-code intends. When this happens primarily at the bottom or middle of the print, it points to specific mechanical irregularities or firmware misconfigurations.

1. Incorrect Z-Steps (Steps per mm)

The most common reason a print is vertically distorted is an incorrect Z-steps/mm setting in the firmware. If your printer thinks it needs to rotate the motor more than it actually does to move 1mm, every layer will be slightly too tall.

• The Calculation: If you use a standard T8 lead screw with a 2mm pitch and 8mm lead, your Z-steps should typically be 400. If it is set higher (e.g., 410 or 800), the print will stretch.
• The Fix: Check your settings via the printer menu or by sending an M503 command. Calibrate your Z-axis using a digital caliper and a 20mm calibration cube.

2. Mechanical Slippage at the Coupler

It seems counterintuitive, but a loose coupler can sometimes cause a "stretched" effect. If the motor shaft spins but the lead screw "jumps" or catches late, the pressure of the X-gantry can sometimes cause the assembly to settle in a way that creates inconsistent layer heights.

• The Symptom: The stretch happens early because the weight of the gantry is at its lowest, putting the most pressure on the Z-coupler and the motor's internal bearings.
• The Fix: Ensure the grub screws on the flexible Z-coupler are tightened—one on the flat of the motor shaft and one on the lead screw.

3. "Faded" Mesh Bed Leveling or Z-Fade

If you use Auto Bed Leveling (ABL), Marlin and Klipper use a feature called "Fade Height." This feature gradually eliminates the bed level compensation over a set number of layers (usually 10mm).

• The Artifact: If the Fade Height is misconfigured or the bed is severely tilted, the printer will add extra Z-movement to compensate for the slope. As it "fades" out this compensation in the first half of the print, it can create a perceived stretching or compressed effect depending on the tilt direction.
• The Fix: Check your M420 Z command. If your fade height is set too low or too high, it can distort the lower half of the geometry.

Estimated Costs for Z-Axis Repair

Fixing Z-axis stretching usually requires calibration, but if the mechanical hardware is faulty, here are the expected replacement costs.

4. Slicer Scaling Errors

Always verify that you haven't accidentally applied a Non-Uniform Scale in your slicer (Cura, PrusaSlicer, or Bambu Studio). It is easy to accidentally pull the Z-handle on a model, stretching it to 110% while keeping X and Y at 100%.

5. Lead Screw "Whip" or Pitch Inconsistency

In rare cases, low-quality lead screws have inconsistent thread pitches. If the bottom half of the screw has a slightly different pitch than the top due to a manufacturing defect, the Z-movement will not be linear.

• The Test: Flip your lead screw upside down. If the "stretch" moves from the bottom of your prints to the top, the lead screw itself is defective and must be replaced.

Conclusion

Z-axis stretching in the first half of a print is almost always a result of firmware Z-steps being out of sync with your mechanical hardware. By calibrating your steps-per-mm and ensuring your Z-fade height isn't distorting the initial layers, you can return your prints to their intended dimensions. If the issue is localized to the bottom of the print, always double-check the tightness of your Z-coupler and the straightness of your lead screw.