Using Custom Hotends on a mk3.5  

Using Custom Hotends on a mk3.5

This is a guide to configuring a custom hotend on the mk3.5. I spent a while trying to make my Revo Hemera work 100% on mine, and I think I finally have it down enough to make a guide.

Disclaimer: my only experience with custom hotends on a mk3.5 is with this Hemera kit so things may be missing. If there are other things I'm missing please let me know in the comments. My goal is for this to be a somewhat central place for the collective knowledge, and to make anyone else's lives significantly easier.

Installing the hotend:

Installing the hotend is the same as the mk3/s/+. Follow whatever tutorial there is for your design. If there isn't a tutorial, good luck.

Changing the esteps:

If you change the stepper motor or drive gearing on your hotend, you'll need to change your esteps (how many steps your extruder needs to take for 1mm of filament) you can measure this, although most manufacturers have a general guideline that should work well. For me it was 786.5.

To change the value on the printer

1. Turn on the printer
2. On the home screen select settings
3. In the settings menu, hold down the knob for 3 or 4 seconds.
4. When you let go of the knob you will be in the experimental settings window
5. Go down to "Extruder Steps Per Unit" and set it to your value
Note: The esteps value on the printer only does whole numbers, I just rounded my value down
6. Select "Save and return" at the top
7. Select "YES" to reboot your printer

Changing the Z height:

Some hotend changes affect your max height. If that is the case for you, you can change the z height in that same experimental menu.

To change that value

1. Turn on the printer
2. On the home screen select settings
3. In the settings menu, hold down the knob for 3 or 4 seconds.
4. When you let go of the knob you will be in the experimental settings window
5. Go down to "Z-axis length" and set it to your value
6. Select "Save and return" at the top
7. Select "YES" to reboot your printer

Pinda positioning:

The pinda probe that handles bed leveling may be in a different position with hotend upgrades. This value is actually changed in the slicer.

To change it

1. Go into prusaslicer and make sure your mk3.5/s preset, or physical printer is selected
2. Click the printers tab on the top
3. Click "Custom G-Code"
4. Find the G28 line

G28 ; home all

5. Above the G28 line paste in 2 M851s. These are the Marlin firmware codes to change the probe offset. In my case the X was 30.60 and the Y was 11.60. These values will be different if you have a different hotend design. Make sure to set them to the values given by your mount. You can measure the values on your printer if values are not given. If only one offset needs to be changed, you can delete one of the M851 lines

M851 X30.60
M851 Y11.60

6. Click the save button next to the printer name in the top left. I would recommend putting "- {hotend name}". My profile is "... mk3.5s 0.4 nozzle - Hemera"
7. Make sure you use this profile anytime you slice for this printer.

Bonus: You can also change the Z height under general in the printers tab.

Input shaping:

When you change the hotend the resonant frequencies of the built-in input shaping will be totally off. So you have to use the accelerometer to calibrate it. Getting the accelerometer to calibrate fully was somewhat finicky, but here's what worked for me.

To calibrate your accelerometer

1. Follow the prusa guide up to step 10 
2. Since there is no convenient fan slot, you'll have to get a bit creative. I believe the accelerometer has to be parallel to the X axis, although that isn't 100% confirmed. Since my kit is angled, I couldn't just stick it on the front, so I ended up trying different methods to attach it to the vertical backplate of the hotend. Initially I tried VHB tape, but it kept giving errors about not getting enough data. The solution that worked for me was to put a single piece of duct tape on top of the hotend to hold the accelerometer in place. This seems to have transferred enough energy to the accelerometer to make it happy

3. You can then follow step 11 and beyond in the instructions
4. At the end, you will have resonance values that are saved to the control board

You'll have to repeat this process if you make any significant changes to the hotend

Other things:
- The hemera comes with a 2 pin fan, which means that it won't report RPM back to the control board. This means I can't properly run the heater test so the printer gives me a warning to finish calibration every time I turn it on. I can just ignore the warning, so that's what I've been doing. Although it's not idea

If you did all of that, you may now eat a small bag of Harbios 🙂

That should be it! If there's anything I messed let me know in the comments and I'll be happy to update this guide. Hopefully this gets indexed by Google and is able to help some people