What and Why?

If you remember from back in high school math, if you take any 3 points in (3d) space there is exactly 1 plane that goes through those 3 points. Adding a 4th point that doesn't happen to exactly land on the same plane will create a saddle shaped surface that is not flat.

What does that mean for 3d printers? Well, a plane is a flat surface and you really want to be printing on a flat surface so that means you want your bed to define a plane. If you have 3 points to level then you will always have a plane (aka flat surface, assuming that your print bed is relatively flat to start with). As you adjust the height of these three points the flat surface remains and you just need to get them all the same distance from the print head to get a level bed.

With a 4 point leveling system, you need to get 3 points level and then match the 4th point to exactly that plane. If your 4th point doesn't match then it distorts the entire surface and you have to figure out where the right adjustment is to get it flat.

Okay, so 3 is better, why would my printer come with 4 point leveling? For a couple of reasons. First, that's how things are always done. Most 3d printers being sold are far from revolutionary in their design opting instead of mimic what was done before them. Second, if you bed isn't rigid then the unsupported corners will start to sag due to gravity. On another printer that I own, I have a 6mm thick aluminum plate and that is a great candidate for 3 point leveling because it does not sag. The cheap ender beds, not the best choice without some reinforcement.

That means for a 3 point leveling system, your bed always stays flat (or as flat as it started out the process being) no matter how you adjust those 3 points. Adding a 4th point requires that you position it at exactly the right height to match the flat surface created by the other 3 points. If you don't get it in exactly the right position you force your print bed not to be flat but rather to be a saddle.

Options

You have 2 problems to solve to move to 3 point leveling. First, you need to attach the new leveling point to the print bed (the thin aluminum with a heating pad attached to it). Second you need to attach the new leveling point to the bed plate (the z axis that rides up and down and has the leveling wheels attached to it).

I, personally, can only think of 2 ways to attach a new leveling point to the print bed. Either you need to stick a hole in it to create this new point or you need to attach something to the existing points a put a hole in that thing.

If you put a hole in the existing print bed, you run the risk of hitting a trace in the heating pad and ruining it. If you do successfully drill a hole in it, I think that you run the risk of the unsupported corners sagging.

Instead, you can have an aluminum bar machined that attaches to the two existing corners and has a countersunk hole for a new 3rd leveling point in the middle. This was done for the Ender 3 by GulfCost Robotics: thing:3150257 and sold as a product. The idea is simple and elegant and if you want to try it out, you can print the STL from thingiverse and try it with an unheated bed before trying to source a bar of your own.

Which leads to the second problem, where do you stick that new leveling knob? With the Two Six Engineering Dual Z Upgrade you conveniently already have a hole in exactly the right place. If you don't have this upgrade, you'll need to use the bar to accurately locate and drill a new hole in your existing bed plate.

Installing the Bar

Installation is simple. Remove the front 2 leveling wheels and springs. Insert a new M4 screw into the middle countersunk hole and use 2 nuts to secure the bar to the print bed at the two corners. Once the bar is secured, place one spring on the middle screw and insert all 3 screws into their holes in the bed plate. Leave the original 2 screws loose in the holes and attach a leveling wheel to the middle screw.

That's it!

Leveling

Three point leveling is nice and easy because the points are independent. You can completely ignore the front point while getting the other 2 points level and then you can level the 3rd point without needing to worry about adjusting either of the first two!

If you have a bl touch and the Dual Z Upgrade this is super easy because all 3 points are rigidly supported and the bl touch will tell you exactly how far away you are from level! If you don't have this great combination, you'll have to go with a piece of paper and feel to get it right.

So, the process goes like this. Start with the back left point and adjust it the spring tension that you like (or if using paper, to the height that just touches the nozzle). Move over to the back right point and get it to match the height of the back left. Repeat a couple of times until it is stable and then adjust the front point. At this point there is no need to go back to the other 2 points, you're done!

Sourcing the Bar

This is where things get annoying if you don't have a CNC machine that can cut aluminum. One option is to buy an aluminum bar and hand drill the holes in it. This works, but the cheapest I could find cost about $16 to get a single bar shipped to me. If you have a good local source of aluminum bar stock your options may be better than what I had available.

You could buy the full kit from GulfCoast Robotics but that's pretty wasteful.

The laser cutting service sendcutsend.com offers really low prices (3mm 6061 aluminum is about $4 and 7075 is about $6) but their minimum order is $29 for 6061 and $39 for 7075 which means that you need to order 7 or 8 bars to cover the minimum price. If you go this route, hopefully you can arrange some kind of group purchase and many people can benefit from it!

Personally, I managed to grovel a returned piece from GulfCoast robotics and then replaced it with a 7075 laser cut bar thanks to the fact that I was placing an order that didn't meet the minimum price and therefore adding it was "free".

If you made the bar or had the bar cut for you, you'll still need to countersink the middle hole to hold the screw head. I just use a cheap set of countersink bits.

Last but not least, you should make sure that the bar you have is actually flat. The first one I had was slightly cupped which caused it to pull the two corners of my print bed a little lower than the middle front. It was easy enough to straighten it out so just check it before you install it and fix it if it's not perfectly flat.

Final Thoughts

If you are going to mod your printer (and particularly if you already have the Dual Z upgrade) this is a good contender for improving the quality of your life. I can get a very level bed in less than 5 minutes with little to no effort. There is so little effort involved that I don't even need to run a test print when I'm done, I just know it will work. At this point I've probably gone through the process 5 or 6 times as I've swapped out the PLA printed part I used to try out the idea and while figuring out that the bar was bent.