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Gantry Backing Plates
Due to different thermal coefficients of linear expansion between steel (rails) and aluminum (extrusions), the Voron V2 gantry forms a sort of giant bimetal temperature sensor element. This causes a warping of the gantry as it heats up with increasing chamber temperature.
While exploring frame expansion behavior, a number of us were noticing empirical values several times what we'd predict simply due to linear expansion of the Z extrusions alone. Although plugging those empirical values in to the compensation code has given me great first layers, the difference from expectation was still bothering me. Maybe, I thought, there was some sort of warping happening? Checking out a table of thermal expansion coefficients, I saw that indeed aluminum expands quite a bit more than steel with temperature.
Bottom X rail increases frame thermal compensation value
After some discussion on Discord, it appeared that people running the BOM double X rail, with an MGN9 rail on the front and bottom of the X extrusion, by and large seemed to be needing higher compensation values than those who only had a single X rail on the front face of the extrusion.
So I tried loosening the screws on the bottom X rail of my V2.4 350, and re-ran the thermal expansion measurment. Wouldn't you know it, my observed compensation factor went from around 2.8 to about 1.5:
Bed mesh changes as predicted with frame temperature
To confirm, I decided to go ahead an loosen all the rails on both X and Y, heat soak the machine for a couple hours, and then tighten them down; this should ensure that the gantry is as flat as possible when at temperature, but will bow downwards when it is cold as the aluminum contracts more than the steel rail beneath.
And that's indeed what I saw. After hot-tightening the rails, I took a bed mesh while everything was still hot. I then allowed the printer to cool for a couple hours, reheated the bed, and took another mesh while the bed was hot but the frame was still cold.
What I saw was just what we'd predict, with the