This page explains how I changed my Atom 2 printer’s hotend from the one supplied by the manufacturer to a 3rd party one I found online.

Why I decided to change hotends

My printer came with a proprietary hotend that included a 0.400 mm nozzle made from titanium. This nozzle is part of a hotend configuration that included a proprietary heater (actually a ceramic disk), a standard thermistor, a heatsink, and 3 fans – 2 “side blowing” fans intended to cool the just-extruded filament, and a third fan intended to cool the nozzle and it’s attached heatsink.

Over time I discovered that this configuration was prone to nozzle jams resulting from a buildup of filament inside the nozzle. I found the best way to clear these jams was to completely disassemble the hotend and heat the nozzle with a propane torch. This melted and/or evaporated the filament that was stuck inside and made the nozzle like new.

Because of how the Atom 2 hotend is designed each time I did this I had to cut one of the thermistor’s leads and re-solder it after reassembling everything. Doing this was tedious and time consuming, plus I also sometimes broke one of the other wires connected to one of the fans, and this required even more time to correct.

I finally grew weary of all this and decided to replace the hotend with one that was easier to deal with and (hopefully) not prone to internal jams.

The hotend I chose

After looking online at numerous hotend options I settled on this one: https://www.deltaprintr.com/product/mini-hotend/. I picked this one because it was specifically designed to be used in a delta style printer. It was small, looked like I could install it on my printer without too much trouble, and was reasonably inexpensive.

The following paragraphs explain how I went about making the hotend change.

The Atom 2 Hotend Assembly

The Atom2 has a 3-part hotend assembly. The largest part is called the “central hub”; this is what is connected to the 6 arms of the printer and its purpose is to hold the nozzle firmly in place. The “hub cover” attaches to the central hub and the nozzle, with heater and thermistor, is attached to it. Here is a photo of the Atom 2 central hub, hub cover, and nozzle:

My Plan

These are the 4 parts to change the hotend:

  1. Install the hotend in the hub cover and attach the hub cover to the central hub
  2. Connect the wires for the heater, thermistor, and 3 fans
  3. Verify the printer can read the new thermistor properly
  4. Re-do the Auto Home function and recheck the Z-offset value since the Mini’s nozzle tip will be at a location that is different from the Atom 2’s nozzle tip.

The only one of these I can do before installing the Mini hotend is figure out how to do the third one. To do this I’ll have to make sure the temperature the printer sees is close to or equal to the real temperature of the hotend.

To accomplish this I decided to use an instant read cooking thermometer I’ve had for quite a while. This is what it looks like:

This thermometer works very well. It is fast and accurate, and of course it can be set to either C or F. In the above photo it shows 20.3 degrees C; this was the room temperature at the time I took the photo. At the same time the Atom 2 printer showed its hotend temperature as 20 C, so it was nice to verify the 2 devices were in close agreement.

But of course I don’t care about room temperature. So I measured the temperature of the Atom 2 hotend at different temperatures to see what sort of agreement I could get. Here are the results:

PrinterThermometer
2020.3
150145
200180
220193

I wasn’t surprised by the difference between the 2 readings because only a tiny part of the thermometer’s probe could contact the tip of the nozzle. But I was happy to see a direct correlation between the printer’s temperature and the thermometer. I think this will be good enough to tell if the printer is getting the correct information from the new thermistor.

Dealing with the Thermistor

A thermistor is a special type of resistor that emits different voltages when it is at different temperatures. To display it’s current temperature the printer’s motherboard contains a circuit that reads the thermistor’s current voltage and converts that to the corresponding temperature for display on the LCD.

There is a tricky parts to producing an accurate temperature from a thermistor: different thermistors produce different voltages for the same temperature ranges, and no thermistor has a direct or linear relationship between temperature and voltage.

To resolve this the Marlin firmware that drives 3D printers contains a set of tables that relates the temperatures and voltages for different types of thermistors. Each printer’s firmware specifies the table for the type of thermistor it uses. So this gives me 2 problems to solve:

  1. How to tell what thermistor table my Atom 2 firmware uses
  2. How to tell what table values, if any, match the Mini’s thermistor which the manufacturer specifies is a Semitec 104NT-4-R025H42G

Google was my friend here. By doing a search for “Marlin Semitec 104NT-4-R025H42G” I found two web pages that said this thermistor uses table 5. So I realized that I’d be in luck if the Atom 2’s firmware was set to use Table 5. To determine this I looked at the Configuration.h file in the Marlin directory and found this line of code in the Thermal Settings section:

define TEMP_SENSOR_0 5

This says to set the thermistor table for sensor 0 – the hotend sensor is always number 0 – to 5. So I was i luck – my printer’s firmware was already set to the correct thermistor table.

That means all i should have to do is verify the hotend temperatures across the same range as the table above. When I do this they will either match ok, or not. If not, I’ll have a new problem to solve.

Received the Mini – and my first mistake

The hotend arrived with no instructions and without the Bowden tube attachment part shown on the Mini’s web page. I did notice a plastic insert on the top of the heatsink and I thought this might be there to hold the PTFE tube my printer requires. So I pushed in a test piece of PTFE and indeed it did hold tight.

But then I could not get the test PTFE out of the Mini. So I had to pry it out, thus destroying the plastic insert. At that point the only way I had to hold the PTFE in the hotend was to replace the plastic insert with a metal one I took off an extra extruder motor I have that was a left-over from when I did beta testing for the dual extruder Atom 2-EX.

Fortunately the metal insert fit ok, so after installing that the Mmini looked like this:

From left to right: 2 screws to attach the fan, the Mini with the metal Bowden tube connector, pieces of the plastic insert, and the cooling fan.

There were no instructions with the Mini and no indication on the fan showing airflow direction, so I assumed it should be mounted with the label facing out. There are threaded grooves at the top of the heatsink, so mounting the fan was easy.

Now I had to find a way to mount the Mini on the Atom2’s hub cover piece. To do this I’ll have to increase the size of the central opening in the hub conver, and also drill 2 holes for the 2 M3 screws that will attach the Mini. Here’s how everything looks before making those changes to the hub cover:

You can see the 2 threaded holes in the bottom layer of the Mini’s heatsink. I’ll have to drill 2 matching holes in the hub cover and make the center hole large enough to fit the red insulator on the Mini’s hotend.

Assembling the hub cover

I tried using a power drill to enlarge the hub cover’s central hole, but this did not work. So I used a Demel toolwith a couple of different grinding heads to simply enlarge it. Then I switch to a Dremel cutoff wheel and made a slot on each side of the hole wide enough to accommodate an M3 screw. The slot approach eliminated the problem of drilling 2 holes at precisely the right location.

After doing this it was fairly easy to mount the Mini onto the hub cover. Here are some photos of the finished assembly.

Top view showing Mini mounting screws & PTFE adapter in center; side blowing fans, and screws holding Z-Min microswitch at bottom center

Front view showing side blowing fans, Mini nozzle, heatsink fan, and Z-Min microswitch above fan

This assembly uses several different types of wire connectors and I want to standardize on one type. So I’m going to see if I can purchase some that will allow me to easily switch between the Mini and my current hotend.

Completing the Central Hub

Installing the hub cover on the central hub was fairly easy – just 2 screws. It’s a bit awkward installing the screws due to some interference with the side blowing fan screws, but some tweezers enabled me to get the job done. Here are some photos of the completed assembly mounted on the printer:

Unfortunately the electronics store from which I wanted to purchase new wire connectors has closed, so I had to order them from Amazon. I’ll complete the wiring once I get them.

Redoing Auto Home and Z-Offset

I realized that even though I don’t have the hotend heater, thermistor, and fans connected I could still redo the Auto Home and Z-Offset functions. This is because, while I was beta-testing the dual extruder update, the hardware I used did not have the a way to attach or use the Z-Min microswitch. So I printed my own attachment piece for the microswitch and only connected it when I changed my printbed surface. But doing this meant I had to invert the signal sent by the microswitch.

So to get the Z-Min function to work the way the original Atom 2 worked I had to go back to the original firmware setting. To do this I updated Line #288 of file Configuration.h in the Atom-2.0.7 ver2 firmware to this:

const bool Z_MIN_ENDSTOP_INVERTING = false; // set to true to invert the logic of the endstop. Atom2 with Mini hotend

Previously the value had been “true” for my beta-testing work. (Note that in the actual file everything is on a single line of code.)

Happily, after making this change the Auto Level function worked perfectly and the default Z-Min value appears to be fine also. Naturally I’ll have to re-check this when I get the hotend 100% operational – sometimes a small change is needed to the Z-Offset value to get good first layer adhesion.

Completing the change

These are the connectors I used to connect the Mini’s wires to the wiring supplied with the Atom 2 printer: https://www.amazon.com/gp/product/B071XN7C43/ref=ppx_yo_dt_b_asin_title_o00_s01?ie=UTF8&psc=1. They worked very well. I should have shortened the leads to about 2 inches in length because I later found out the extra lengths of wire were difficult to fit inside the plastic Bowden tube cover that came with the printer.

Here’s a photo of the assembled hotend ready for the new connectors:

I simply cut off all the existing connectors and replaced them with males from the above set from Amazon. After this I tried testing the nozzle temperature. But I really couldn’t do that because there is only a tiny part of the nozzle protruding from it’s insulated cover:

This made it impossible to get a decent reading with the thermometer, so I just gave up on this test. To make sure I got the rest of the wires connected properly I used this image from the Atom 2 assembly manual:

I connected female connectors to the ends of these wires and then just plugged the correct pairs together. Once all the wires were connected I turned the printer on and everything seemed to work. So I proceeded to stuff all the wires and the PTFE tube into the plastic cover. This is when I realized I needed copious amounts of black electrician tape to close up gaps in the plastic cover due to the extra lengths of wire from the new connectors.

Here are 2 photos of the completed hotend assembly with all wires attached.

First test print

I made this test part to check for things like stringing, bed ahesion, side seams, etc. It prints in about 10 minutes. I was primarily interested in seeing how the temperature worked since I really had no idea what the real nozzle temperature was. Fortunately the test printed just fine and I also needed no adjustments to Z-offset.

First real print

I’ve had a couple of parts ready for printing for a while now. I picked this one to start with:

And here’s how it looks after 39 minutes of printing:

Assuming this print finishes OK I’ll be posting this part on Thingiverse tomorrow. It’s name is TwistedFlutes8.

It’s the end result that counts

My first attempt to print TwistedFlutes8 failed because my printer stopped feeding filament to the Mini hotend. After some investigation I traced this to two things: the printer’s PTFE Bowden tube had developed some kinks in it, and the teeth on the extruder’s gear had become slightly clogged with bits of filament. Considering all the printing I’ve done over the past 3 years and the number of times I’ve had to take the hotend apart to clear jams I wasn’t surprised by these results. So after replacing the PTFE tube and the extruder gear with one that had more teeth I made this print:

You can get this STL file here:
https://www.thingiverse.com/thing:3554522 .

Last update: 10 Apr 2019