Prusa Mk2.5 Bed Flattening

My beloved Prusa is about to turn 4 years old. It has given me lots of years of good service with only a bit of maintenance and some minor upgrades. It is time to show it a little more love. While re-greasing all the bearings I decided to flatten the bed. Normally the bed warp is compensated for by the bed level probe. That is great, but it means the bottoms of your prints can be warped. The Mk3 printers have some simple upgrade using nylon lock nuts. The older printers are designed differently, so that isn’t an option. I do precision alignment professionally, so this should be easy.

First up, I used an octoprint plugin to see how flat my bed really is.

Gross. It is over 1.5mm off on the one corner. To fix this I am going to shim. The situation looks like this. Removable bed on top, heated bed under that, a standoff, then the metal frame. The heated bed and standoff are really tightly screwed together, the standoff was assembled to the metal frame when I built it.

I have brass washers from the hardware store. They turned out to be quite flat and 0.5mm thick. I will be slipping these in between the metal frame and the standoff one corner at a time, then re-testing my flatness.

I can’t drop my high spot, so I have to raise up the low spot to meet it. After adding shims one at a time to the low spots I got really confused. 1 washer added to the front left, made it go down. Also, the back looks flatter than before. I added a washer to the front right because I thought the scale was maybe reversed. It made the back worse!?! I am so confused.

I kept adding and removing washers and not understanding what was happening. I don’t know how it does these calculations and builds the maps, but I am completely confused. After an hour I found another plugin that is designed for the Mk3 nylock upgrade. I don’t have as many adjustment points, but it was still helpful.

According to this I was high on the right, ok in the center and front left, and low in the back left. I started adding shims to the whole setup based on this. Instead of slipping one in at a time I had to take the bed off to get to the center. I used super glue to hold the washers in place so they wouldn’t fall out during assembly.

This worked a lot better. My back right is still a little high, but I don’t have enough shims to continue. I might pick this up later and I can sand down the washers to be thinner if needed. For now though, this is pretty flat. I will avoid the far corner if possible. The other program shows it as being all high. I guess I don’t understand how it calculates 0. Maybe that was my main problem.

Did that do anything, or did I waste 2 hours of time? After all I have been printing for 4 years without it, and that is what a bed level sensor is for. I think it helped. Large prints come out flatter, and I can see that the z stages aren’t doing much compensation as they put down the first layer. For a little time and a modest cost in shims, this was an upgrade I wish I had done ages ago.

By the way, if you do this, 1ea 0.5mm washer was ok with the 8mm screw normally used. If you put in 2, you need a 10mm long screw. The fronts can be socket head cap screws, but the backs have to be button heads to clear the frame. Might want to pick some of those up if you plan on doing this job. I used brass washers for number 4 screws. Your mileage may vary.

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