Tuesday, December 02, 2008

 

McWire experiences

This is my first post here, so it's liable to be longer than normal as I've an amount to catch up on.

To introduce myself, I'm Jon, I'm English, I live in Germany, I'm a Java programmer by day and after following RepRap for a while, I've now been building myself a McWire in my spare time. I now have things up to the point where the machine is moving in three axes and (somewhat fitfully) extruding material. The electronics are generation 2 from RRRF, the extruder is the laser-cut one from Bits from Bytes.

A summary of some things I've encountered and the solutions I had for them:

  1. I was unable to find the normal steel/iron pipes that the original McWire build uses in local DIY stores. I suspect I was just being blind, since I've since now found them in the nearest two stores. Either way, I ended up using pipes and fittings made by Tubeclamps, for example RS part number 286-3459. (I ordered pretty much everything from RS. If anyone wants a complete list of the RS part numbers I ordered, mail jon at siliconcircus dot com.) These are big, thick, heavy pipes and fittings. They also have a 64mm hole diameter on the flange (as opposed to 70mm). Zach at RRRF kindly helped out here by laser-cutting the acrylic with a 64mm diameter. The pipes are a real pain to cut (you'll want a good pipe cutter and a spare blade) but other than that, they work fine and you end up with a very stable (and very heavy...) machine.

  2. Possibly due to the saddle bands I was using (RS part 283-2360), I couldn't get the captive nut for the Z stage to stay in place properly. My solution to this: solder the nut into the band using a whole lot of solder. While I realise that this is hardly the proper engineering solution, it works nicely.

  3. I managed to lose the extruder tip (the aluminium part with the 0.5mm hole) from the Bits from Bytes extruder kit. BfB were very helpful and sent me a new one at a reasonable price. Nonetheless, it cost me about a week. Don't do this :-)

  4. Once I got the extruder built, I found that the two cogs (the one on the motor and the one driving the M8 bolt) were slipping past each other. It's something of a makeshift solution, but I wrapped clear tape (Tesa Film/Sellotape to name two brands) around the motor cog. This provides sufficient pressure that they're now working nicely enough.

  5. Having done this, I got the extruder driving plastic forward and extruding. Unfortunately, I also got a leak at the barrel/PTFE insulator join. I'm using ABS. My first attempt at a solution was to wrap PTFE tape around the join and tighten a hose clamp over that. This seemed to work well for a couple of days, but I've just turned the heater on this evening and noticed a small leak again.

  6. Overall, the extruded filament didn't seem to want to stick to anything and the extruder didn't seem able to drive it sufficiently well. My conclusion was that the extruder tip (which isn't directly heated in the BfB design) wasn't getting hot enough for my ABS. I've now bought myself a Dremel, a 0.5mm drill bit and a brass M6 acorn nut. I drilled the hole and mounted this yesterday evening and results now seem better, though still not perfect. Certainly, when heating up to 235C (as registered by the thermistor in the barrel), I now see a little bit of extrudate leaking out of the hole, which definitely wasn't happening before. I've now applied some glass wool around the extruder tip and will see if that helps.

  7. It seems like the actual drive mechanism is having trouble driving the ABS. I've seen mentions elsewhere about sharpening the thread. If this is still a problem with the glass wool (and the consequently hotter and therefore presumably less resistive tip), then I plan to try this.


Comments:
I am having the same problem here with ABS and the BfB extruder. Has anyone tried this yet? Is there anything that Jon and/or me could try to fix this?
 
Oh, and I fixed the problem with the gears not really catching using the method described in Erik's blog here:

http://blog.erikdebruijn.nl/archives/60-RepRap-Acrylic-extruder-mods.html

It basically moves the motor nearer to the other gear so that it catches
 
Have you tried sharpening your drive screw? Use the appropriate sized thread cutting Die and just give it a couple passes on the drive screw to sharpen the threads up. Some people said it helped with the BfB design and I know it helped with the Mk II design.

Demented
 
Hi Demented,

Yep - see point 7, that's next on my list. I've now got the extruder off to do that. I've also taken the opportunity to remove the half-nut at that end of the extruder and just screw the nut right the way down, which should further aid the heating behaviour.
 
Hi Daniel,
Nice to see my blog helped out! It encourages me to keep blogging my RepRap work :)
 
You must sharpen the thread to use with any of the harder plastics (ABS, HDPE) i would also recommend running a hack saw on the thread to give it a squarer bottom profile this also helps a lot, the heater barrel must also be insulated, plumbers solder mat works very well and in not very bulky.
 
Hello Iain,

Plumbers solder mat is a great idea. I'll look into that.

Did you have to seal up the connection between heater and PTFE insulator too? I will have to do this to prevent leakage.

Also, did you do anything else to hold the washer on the opposite side of the filament in place? Mine seems to slip out every now and then... I might have sharpened my drive screw too good ;)
 
Hi Jon,

I'm making the same setup and was wondering how you mounted your extruder. A picture would be wonderful.

And I might need your advice on the software end. Did you use the reprapdvd0.7?

http://detroitreprap.blogspot.com
 
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