Why bed leveling?


Just a quick sanity-check question… All this bed levelling activity… Why? Surely if the bed were mounted on two extruded rails and fixed without springs the bed would be level and never change?

Just askin…

Not necessarily. What if your extruder rails are not exactly flat? as in, where they screw in and attach to other parts, if the holes are off by a bit. remember, you are dealing with .4 or less mm accuracy.

If the bed is off by much it will cause uneven layer adhesion. Bad Prints.

Bed leveling is very important.

With that said, there is a degree of unevenness that you can get away with. It’s called being true, not exactly level.
The more true you can get the better the things look. Not just in 3D printing.

Example, you can get away with 2mm height difference in CNC world, but 3D printing requires a bit more accuracy in that 0.?mm range.

It’s all relative. You really don’t need it in most cases, but if you want to geek out, or you have a specific problem with Z, you can get tighter tolerances with auto levelling. That’s the theory anyway, there’s plenty that can still go wrong.

Auto Leveling I researched quite a bit. In reality I found that it’s not necessary to get great results.
Once I get my bed as level as it needs to be, I rarely ever touch it again.

If my first layers are laying down properly, the rest just works.

Unless Auto Leveling is already built into your printer, I think it’s a frigidity waste of time.

I am on the fence about about it. 6 printers, none use it. Once the bed is level (provided your bed is fairly planar), you don’t need it. So for me it is put in the work one time to get things real nice, then only ever mess with it if I do something stupid and knock it out of wack (twice a year on each printer on average).

You have to be careful if your bed is way out of wack bed leveling will produce funky parts, even with fade turned on.

I have (at work) a lulzbot Taz 5. When I level its bed it usually stays good for about 6 months unless it gets bumped or I have to work hard to remove a print from the bed. Most of the time print stick nicely. And lay down evenly. The corners are usually a little off but usually not enough to cause problems. Usually.

Comparing it to my prusa with a bed level probe. Night and day difference. The bed isn’t mounted on springs so it never gets adjusted. The probe just checks at the beginning of each job. And I get a perfectly level first layer every time. Love it time saver I think and stress relief.

But I don’t know how hard it would be to “add” a probe to a printer.

Bed leveling is not necessary for great quality prints, but it is a very nice thing to have for repeatability. With this option, you basically never have to care about the first layer ever again. You just hit print, grab a beer and move on with your life until the print is finished.

Without it, provided you set up your bed well enough, you won’t have issues for most of the time, but if anything moves (which can happen easily especially if a print is hard to separate from the plate) then you might have unexpected issues on your next print and you’ll have to calibrate again.

It’s also useful if you have several glass plates. Once a print is finished you take off the plate, put another and hit print, so you don’t have to wait until it cools off. With the autolevel this task is a no brainer.

An other important feature is that it can take care of slight warping on your plate. The plate might look perfectly flat to the naked eye but it actually never is. If you have a low spot in the middle there’s no way for you to correct it with manual leveling. This aspect is crucial for very big printers.

In any case, it is easy enough and cheap to setup, I see no reason not to add it. I install it on all my machines, it saves a lot of hassle especially during the very first prints, when you just built your printer. It takes a lot of parameters out of the equation so you can focus on solving other issues instead of wasting your time tuning the first layer.

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Hi. The answers seem to have branched off into another direction compared with the original post. I get the debate about bed levelling on printers where the bed “floats”, but the OP was asking why not fix the bed permanently to TWO rails ensure it is level and forget about it. Once level, it will remain level despite knocks and bumps and troublesome piece removal.

It is actually simple to ensure perfect levelling with extruded aluminum rails.


Until your bed warps when heated to different temps. I 99% of the time use PLA at 65C bed temp, so my beds are set level at that temp, but off or at 120C I am sure they would require some work.

The first layer is 0.2mm or so. If you have an error of 0.1mm, it’s off by 50%. That has a dramatic affect on the way the layer cools, adheres and fills. You won’t get that kind of precision with a fixed set up (that I know of). My heated bed is 0.2mm higher in the middle than on the edges when heated.

Bolting it to the rails is what the Original Prusa printers have done. But like I mentioned they use a probe at the beginning of each print to test 9 points on the bed to measure for level and heat warps. Without a probe I don’t think it is very likely that you could get reliable first layers off of a bed that is fixed into position on the rails. Like others have mentioned before me in this thread. The bed could be unlevel for several reasons:

  • Bed material isn't level (happens a lot)
  • Bed warps with heat
  • Printer assembly is off level because of small burs in the metal screws, nuts or other debris.
  • Table top/feet can cause the rails to not be parallel
  • the rails could be out of true because of temperature differences.
The list goes on but the point is that you can't rely on the idea that just because the bed is fixed to the rails that it means it is close enough to level that you won't have problems. Just moving my Taz from one table to another was enough cause the bed to be unlevel. That was just from frame resting on a table that was more flat than the previous one. And that uses several extruded aluminum rails for the frame. You would think it would be rock solid.

Well, in theory yes.

In practice, clearly no. Profiles are mostly straight (I insist on the “mostly”), but you need some way to attach them together, which, from what I’ve seen on some of my builds using profiles, are not very precise.

What you ask for is theoretically possible. A fixed bed could prevent from having to care about bed leveling, in an ideal world.

Now in the real world, getting that kind of precision seems very unlikely. I can’t imagine how you could build a printer without having any way of adjusting the bed, unless maybe if the whole thing was milled from a giant block of metal somehow.

If you don’t have any adjustment on your bed then you will have adjustments to make somewhere else, probably in your sliding rods supports. So that just moves the issue elsewhere.

Hi guys. Thanks for your thoughtful replies. However, bolting the bed to two rails doesn’t preclude fine tuning. You can use your BLTouch or a piece of paper to fine tune the levels. But once dialled in, that’s it. Sure, check from time to time, but there are no moving parts…

As for the bed warping, that’s probably because the aluminum used for the heated bed is rolled stock. Simply replace it with 3mm 6061 sheet, for example which was never in a roll to begin with and there is no residual stress in the material and no chance of it warping. It would be a simple matter to buy some stock, cut it out on your cnc router and fit it to your printer.


It is thermal expansion. Tom currently has a great video demonstrating it with multiple heater configurations.