Bottoming

As well as repairing my extruder I aimed to bring it closer to the latest RepRap design. The machined heater barrel has been replaced by a drilled out brass bolt. Partly out of laziness, and partly out of a lack of confidence in my skill with a lathe, I bought a ready made barrel from BitsFromBytes.



I was surprised to find it came with a modification that I had blogged when I made my first nozzle. That was to turn down the end that fits into the PTFE holder to get round the fact that I didn't have a bottoming tap to make the thread go right to the end of the hole in the PTFE. Ironically, I had bought a set of taps which included a bottoming one in the meantime. Well at least I thought I had :-



The tap on the left is the one from my cheap set of taps which only has one for each thread. The boxed set are the new ones that I bought with a taper, second and bottom tap. The single tap seems to correspond to the second tap, which makes sense for a compromise. The thing I don't understand is why the bottom tap still has a point and some taper. I was expecting it to be straight with a flat end, so I made it thus with a grinder :-



To try it out, I made a test thread in a scrap of PTFE and cross sectioned it :-



As you can see the flat end of the heater barrel butts up nicely to the end of the thread in the PTFE. I think it is important not to have a void here as it will fill with molten plastic which will freeze when the heater is switched off. It might then be difficult to melt it again as it is insulated from the heater.

The end of the barrel that is turned down is then a pretty good fit for the acorn nut, although a simpler solution is probably to bottom the thread in the nut and the PTFE and then go back to a plain heater barrel.

Nuts

Since my first attempt at making the RepRap extruder the design has moved on to use a brass acorn nut as the nozzle. It has the advantages of making the extruder easier to fabricate, allows the aperture size to be changed by swapping nuts and allows blockages to be cleared. I have to say that I never experienced a blockage with my single piece nozzle, but I can see how it could easily happen if a bit of dirt bigger than the aperture gets into the barrel.

Unfortunately brass acorn nuts, otherwise known as dome nuts and cap nuts, are expensive and hard to get hold of. I got a couple of un-drilled ones from BitsFromBytes.



My plan was to start with the smallest hole I could drill and expand upwards to see what effect it had and then drill the other to the size I found to be the best. Stupidly, I overestimated how thick the dome was and put a centre drill right through the first one. So now I have one with a 0.3mm hole and the other is about 1.1mm.

This is the 0.3mm bit I used :-



If you use a drill or a drill press it is easy to snap drill bits this small but it is actually very easy to drill 0.3mm holes with a lathe. I spin the nut in the chuck and hold the drill in my fingers. I drill from the inside of the dome. The drill finds its own centre and then I apply light pressure. I expect the same could be done by spinning the nut in a drill chuck.

The RepRap design for the heater barrel is just a flat ended threaded brass tube made from an M6 bolt. This is easy to make but not the ideal shape. Brass acorn nuts seem to be machined from a solid piece of brass. The internal thread is made by drilling and tapping. Because it is a blind hole that means that the thread does not go all the way to the end. If you screw a flat ended barrel into it then it stops short of the end, leaving a void that will fill with molten plastic as can be seen here. Molten thermoplastics compress under pressure, so ideally the amount of molten plastic in the extruder should be kept as small as possible to make the start stop response as fast as possible.

I decided to sacrifice my over drilled nut to find out the inside profile by cross sectioning it :-



Not surprisingly, the inside profile matches a 5mm drill as that is the correct size for tapping an M5x1 thread.



This is how far a flat ended barrel can enter :-



This is my attempt to match the profile :-



And this is the improved fit :-



The chamfer at the end is not quite right. My DeWalt bits have a 110° angle but the standard appears to be 118°.

I decided to take a look at steel acorn nuts :-



These are a completely different animal. Rather than being machined out of one piece they consist of a nut with a dome pressed into it.



They have some advantages and disadvantages : -

• They are cheaper and more commonly available.
• They are smaller so less thermal mass.
• The dome is much thinner, about 0.4mm rather than 1mm, so it is easier to get a short hole.
• Steel has a much lower thermal conductivity than brass so plastic may cool down in the nozzle.
• Steel has a different thermal expansion rate than brass. Fortunately it is less so it should get tighter as the extruder warms up.
• The steel dome might spring out under the pressure of extrusion.
• The flat ended heater barrel goes in further but leaves voids at the side.

These are just the nuts I have managed to buy. I have no idea how much they vary from one supplier to another.

The heat insulator in the picture above is an experimental one turned from a bar of soapstone.

Extruder postmortem

So my extruder died at the beginning of the month. I have been busy with other things so I have only just got round to thinking about rebuilding it. Here is a list of things that failed :-

I got my extruder working in mid August. At the end of September it ejected its PTFE barrel from the clamp, breaking the heater wires. This is a common problem and stems from the fact that PTFE has the lowest coefficient of friction of any known solid material. I tightened it further and it didn't slip out again. However, when I came to dismantling it I noticed that the end that was in the clamp has been compressed by about 0.3mm. The 3.2mm drill that I made the hole with is now quite a tight fit so the hole may have shrunk slightly. I will drill it out to 3.3mm because some of my 3mm HDPE filament is slightly more than 3.2mm where it is a bit oval.

One curious thing is that it now looks to have a thread in the entry hole.



I am struggling to explain this. The only way I can think it may have happened is as follows :-

The filament has a thread cut into one side by the drive screw. Before I added my feed spool the filament used to rotate as it went through the extruder and had a thread cut all the way round. Perhaps it transfered its thread to the PTFE, which is quite soft.

The final failure mode was the nozzle jumping a couple of threads and leaking molten HDPE. It rammed the nozzle through the object being made and damaged the bed underneath.



The bottom end of the PTFE barrel has swollen by about 0.3mm. PTFE is known to creep, i.e. when subjected to a prolonged force it very slowly flows. It has no memory so it does not spring back when the pressure is removed. I think this is why the top of the barrel shrank and the bottom expanded. There is a lot of pressure in the barrel and it is close to PTFE's maximum operating temperature. PTFE melts at 327°C but it starts to degrade at 260°C. I have been running at 240°C which is a bit too close for comfort.

My barrel, at 12mm, is smaller than the current recommendation which is 16mm. This may have contributed to the failure.

The recommended solution is to fit a pipe clip, but I didn't have one small enough, so I pressed a short section of 15mm OD pipe over the end, which is a tight fit. It had the desired effect. When I first made the barrel the thread was a snug fit. When I dismantled it I noticed it had become quite sloppy. With the ring of metal in place it is now a tight fit. I will use some plumber's PTFE tape to seal it as well.



The other thing that was on the point of failing was the flexible drive shaft. Strands started breaking and the more that broke the more it flexed, so it was a kind of avalanche effect. I estimate that the shaft had rotated about 100,000 times so the flexing backwards and forwards must have caused metal fatigue. I probably have the most mileage on this part of anybody so far so it could be a sign of a design flaw. There are a couple of problems with my implementation which certainly won't have helped.

The first is that I used some 2.5mm cable I had to hand rather than the 3mm recommended. It doesn't seem like a big difference but I think the rotary strength is at least a cube law which would make my cable roughly half as strong. It was left over from a garage door installation but I don't think I have any worries there as 100,000 flexes is about 75 years use!

The other contributory factor is that the bearing lands on my drive shaft are a little bit eccentric. This stems from the fact that my watchmaker's lathe is not really big enough for this work. Fundamentally the hole through the headstock is not big enough to take the 5mm threaded rod.

Each time the shaft rotates it opens and closes the pump halves a little. This makes a big torque variation over a revolution because of the strong springs holding it closed. That caused the cable to wind up and unwind a little each revolution. It actually modulated the filament width and gave the objects a basket weave appearance. Here is a good example :-



I bought some new parts from www.bitsfrombytes.com, which wasn't an option when I first made the extruder.



That will allow me to give the flexible drive a fair test. If it proves to be unreliable then I will switch to direct drive. I found out from the core team that it is not required for any of the polymers currently used, only things like Field's metal.

Another thing that was starting to fail was the J-B Weld holding the heater wire to the nozzle. It is supposed to be rated to 315°C but it had started to crumble with my extruder running at 240C. The other problem I had with my J-B Weld is that it does not cure in the specified time at room temperature. I have to bake it to make it strong. I emailed J-B Weld but got no response apart from an automated reply.

David commented on my last post suggesting Thermosteel ,which is good for 1300ºC, so I will try that next. A couple of the core team use BBQ paint which handles 600°C so I will try that as well.