## Thursday, 29 December 2011

### Mendel90 extruder

The Mendel90 parametric design starts from the extruder dimensions and works outwards. I used a Wade's extruder for the Mendel sized version of the machine (I will need to sort out a smaller extruder for the Huxley sized version). My starting point was the Prusa version of Wade's. I tidied it up a bit aesthetically and made a few tweaks to the design and that had the side effect of making it easier for Skeinforge to slice correctly. The old version caused it to think layers were bridges erroneously. It now looks like this: -

The functional things I tweaked were: -
• I added nut traps for captive hex head bolts. That allows me to fasten it under the carriage with a couple of wing nuts, so I can swap extruders very easily.
• I brought the front of the bearing holder forwards 2mm. That stops the idler closing fully, which  makes it easier to feed in new filament and allows the hobbed bolt to be removed without having to remove the idler. The downside is it would be less tolerant of smaller diameter hobbed bolts.
• I made the idler bolt holes slightly further apart so that I could make them larger without intruding into the bearing holders.
• I added a slot around the top of the hole for the insulator. When it was simply a blind hole it had radiused corners at the end due to the fact that the filament has a minimum bend radius. That meant that, unless the insulator was chamfered, it did not go all the way to the end of the hole.

I use hobbed bolts and 10mm hot ends from  reprap-fab.org. Wolfgang makes the bolts so that the big gear can be spaced off from the bearing with 5 washers. That allows the small gear to be placed the right way round, allowing the big gear to be removed easily. M8 washers can vary in thickness so I made a printed spacer 7.5mm long to replace them.

I don't use Greg's accessible version of the extruder because I never remove the idler. Once I have got the spring tension correct I don't like to change it. If I need to clean the hobbed bolt I simply reverse out the filament, remove the nut and then remove the big gear and hobbed bolt. It only needs cleaning if there has been a malfunction due to a filament tangle or a nozzle blockage.

To make the nut easy to remove, rather than use lock nuts or a Nyloc, I use a single nut and a weak spring. The spring stops the nut vibrating loose and gives enough pressure to keep the bolt in the correct position but it can be removed without using a spanner.

The extruder is the only part of the machine that wears out, so I have made it easy to swap out by adding a 9 way D type connector. D connectors screw together and have good strain relief for the cable, so they are reliable when subjected to constant movement. They are also rated for 5A per pin and 125°C, which is a good margin for this application.

I attach the connector with a bracket that is screwed to the motor by removing two of the motor's screws and replacing them with screws that are 5mm longer.

I have several extruders with difference nozzle sizes that I can change very quickly.

#### 22 comments:

1. I'm curious about the shaft that actually drives the plastic into the hot end. What material is that? Metal? And what do you do to it so that it bites into the plastic and doesn't slip?

2. Hello James,

The drive is via an M8 steel bolt that has been "hobbed" by running it against a tap spinning in a lathe or a drill. That makes a groove in the bolt with teeth to grip the filament. See http://hydraraptor.blogspot.com/2009/04/all-torque-and-no-traction.html for the technique.

3. Excellent! I'm interesting in purchasing something like this if you decide to make them available.

4. There is a Mendel90 on thingiverse now I think someone got tired of waiting for the original source to produce something. You can get the sizes of the MDF from this blog and if you already have a 3D printer you can print off the parts on thingiverse and make yourself a Mendel90!

1. Yes no need to publish mine now, saved me a load of work.

2. Hey, we are still waiting for urs )

3. Im not a big profi in reprap building, but I wouldnt recommend anyone to build something from this files. (
Really have author try to make something real?

5. Nophead, what parts wear out fastest in your experience? (what part of the extruder assembly?) I just got up and running, and now I am curious what I should have on hand.

Ben--

6. The extruder little gear wears out first. It tends to crack with all the backwards and forwards motion. If not the teeth wear down and fall off eventually. I print it with 100% fill these days. The big gear lasts a lot longer. The other extruder parts eventually develop stress fractures and fall apart. This is after months of 24 / 7 printing in a chamber at 45C.

7. Hello Chris
what material do you advise for printed parts?
I'm not very keen on ABS... I print only with PLA

Is PLA suitable for part close to temperature, like extruder body, stepper motor mount, or timing belt pulley ?

regards

8. I wouldn't use PLA for any of those uses as it can easily melt. It also creeps under pressure so screws don't stay tight.

If you are only going to use the machine to print PLA then you can get away with making it from PLA but you have to keep the motor current low, bed temperature low and have a hot end that is well insulated.

9. Man I love what you have done and are doing.

Do you know about this grooved bearing?

Maybe it would reduce feed friction by not compressing the filiment so much?

http://www.ebay.com/itm/Grooved-Bearing-use-any-printer-using-Wade-hobbed-Extruder-drive-/130648464693?pt=LH_DefaultDomain_0&hash=item1e6b415d35

:)

10. Yes I have seen that. They are rather expensive though. I did a similar thing by pressing the bearing into a brass ring that I had hobbed the same way as the bolt.

11. Oh very good then.

About the brass ring hobbing though, if it's not pulling and just pressing it should remain smooth and energy is not wasted deforming that side of the filament for no reason. Just my thought there.

I'm really looking forward to your release version(s). Your Mendel90 is just fantastic so far by the looks of it.

I love it.

12. Yes the pulley doesn't need hobbing. It was just an easy way to make a semi-circular channel without having a semi-circular tool.

13. can the J-head with groovemount be fitted to the extruder body?

14. I have fitted one. The slot doesn't line up with the fixing screws but it is close. I drilled it and put the screws through. It would be better to get a blank one as the off centre slot makes the drill wander.

I have never understood how groove mount works. The hot end mount has to be absolutely solid as minute movement at the top would make the nozzle wander.

1. Oke, thanks.

I will use a gregs hinged then, with groovemount :)
like this one: http://www.thingiverse.com/thing:12661

2. Check that it doesn't limit the X travel as the motor is at an angle on Greg's so I assume it sticks out further.

15. Hi You say here that you use 10mm hot ends but the extruder body I printed seems to have a 16mm hole for mounting. Am I missing something?

16. It's a parameter in wade.scad. I set it to 16mm as the default as that is the most common size for J-Head, etc. At the time I designed the machine I was using 10mm ones but they all failed so I am trying J-heads at the moment.

17. Brilliant guide, thanks for sharing this.