Printer: ABL Assisted Bed Leveling

ABL still needs a fairly level bed for it to do its thing. I've found if your bed is within 0.2mm of level, then ABL will work great. Fortunately, once you have ABL installed, you can use it to help level your bed. Using the ABL is fast, easy, accurate, and you let the printer do most of the work.


After you open a terminal to the printer using Octoprint, Pronterface, etc., send a G28 to auto home the printer, then add a G29 to do an ABL pass. Once the pass is complete, the ABL's matrix of measurements in millimeters will be displayed in the terminal window. The top left number is the front left corner, and the top right number is the front right corner.

On the Ender 3 the bed leveling M5 screws have a 0.8mm pitch, so each full revolution of a leveling wheel will move the corner 0.8mm. Once you've adjusted the wheels, send another G29 to the printer to do another ABL pass, and keep adjusting as necessary. It's usually taken me three passes to get all my corners within 0.2mm of level.

Printer: Belt Tension

There is very little good information on what correct belt tension is, and virtually nothing on empirically setting belt tension.

Gates, the creator of the GT2 belt, recommends a tension of 6 to 8 pounds of tension when using a 6mm GT2 belt for registration like in a printer. A NEMA 17 stepper motor can handle 6.7 pounds (30N) of radial force centered at 17mm from the motor's face (something to bear in mind if you're using dampers). 

Because the belt wraps around the motor's shaft, the radial force on the stepper is double the belt tension, at the minimum of 6 pounds tension recommended by Gates, there's 12 pounds of radial force on the shaft. However, the allowed radial force increases linearly inversely proportional to the pulley center to motor's face distance. For example, if the pulley is centered at 8.5mm from the motor's face (half of 17mm), then the maximum allowed tension doubles to 13.4 pounds.

NOTE: Moon's bearing life charts indicate the radial limit can go as high as 8 pounds, though with reduced bearing life.

Given this, we want a belt tension of 6 pounds with the pulley centered 9.5mm or less from the motor's face. If the pulley center is further out, then the belt must either be tensioned lower or reduced bearing life must be accepted.

NOTE: The Ender 3 pulleys are centered 10mm from the motor's face. In practice I've found tensioning the belts at 6 pounds doesn't appreciably decrease the stepper bearing life

From the Moons' catalog (see link below).

From the Gates catalog (see link below).

To actually measure the tension the easiest and most precise way is via its vibration frequency. To calculate our target frequency we use Mersenne's equation 22:

Where f is the frequency, L is the length of the belt between contact points in meters, μ is the belt density in kilograms per meter, and F is the tension in Newtons (1 pound equals 4.44822 N). The density of a 6mm GT2 belt is 0.0083 kg/m (both stock and genuine Gates).

Simplified, for 6 pounds force with a GT2 belt, the formula is:

Hertz = 28531 / mm

On the Ender 3 specifically, with the hotend carriage against the limit switch, the belt between the carriage and idler is 0.302 meters (302mm) long, so the target frequency is 94 Hz.

NOTE: Previously this number was mistakenly changed to 86 Hz. The correct figure is 94 Hz. 

Again on the Ender 3, with the bed pushed all the way back, the exposed belt is 0.253 meters (253mm) long, so the target frequency is 113 Hz.

I found the best way to measure the frequency is the Spectroid app for Android. It produces a waterfall display of the frequency, so it's easy to see what the belt frequency actually is. Pluck the belt like a guitar string and look for the biggest spike. Just tap the waterfall display and it'll give you the frequency selected. The belt will naturally have multiple harmonics, but those will be significantly smaller on the waterfall display and can be ignored. From there just adjust the belt tension until you hit the target frequency.

Spectroid showing the primary frequency at 50 Hz.

GT2 design manual

NEMA stepper specs

Printer: Creality Eccentric

The stock Creality extruder side v-slot wheel eccentric is an eccentric only on one side and just a regular spacer on the other. This not only means the wheel move in a cone shape instead of just in/out, it also means the two outside wheels ALSO slightly move in a cone shape. So you end up with the wheels riding slightly cocked to one side in the extrusion.

 

Stock setup.

So I drilled out plate with a 9/32" drill bit and added a second eccentric in place of the spacer. If you don't use a drill press or mill, a step drill bit should do a good job of keeping it centered. I marked the spacers with one divot on the short side and two divots on the long side, so I can easily see which direction they're facing. I now need to manually keep the two eccentrics synced up, but that's not really a problem, and it's much better than stock setup. The adjustment is much more predictable now, and I can get all the play out with the wheels still relatively loose. 

 

Dual marked eccentrics.

 

Printer: Applying PEI Sheet

Almost all PEI sheets use 3M 486MP adhesive (transfer tape) to glue them to the bed. Most methods involve a dry application where you roll the adhesive on while slowly squeezing bubbles out. Regardless of how well you do it, you'll still end up with tiny bubbles. If you get a bigger bubble it will bulge up when the bed is heated. Plus, 486MP is incredibly tenacious stuff, and if you apply it even a little wrong there's no way to recover from it.

The solution is doing a wet install. 3M doesn't officially recommend it, but it works well. You want to mix 1 litter of water with 3 drops of liquid dish soap and put it in a sprayer. Clean the bed well, then liberally spray it down with the water. Peel the back off the 486MP and liberally spray it down with water as well. Then roll it onto the bed, and squeegeeing the water out from under as you go; I usually use a credit card. Then put it between two flat surfaces, put some weight on top, and let it sit for two days.

If the 486MP didn't come pre-applied on the PEI sheet,  repeat the previous steps to glue the PEI to the adhesive installed on the bed, and sandwich it again for two days.