Monday, 30 March 2009

I love ABS

I took delivery of some ABS just before the weekend and I have been running the repstrap for about 6 hrs a day since- what super stuff!

Having worked with HDPE for a long time the major problem was warping of any print. ABS nearly cures this although it smells a lot worse. I am now extruding onto a glass chopping board (heat resistant to 240 degrees ) and evostick.

The first major break through I got was reducing the extrusion rate. This had been puzzling me for a long time as every extrusion seemed to have an excess of plastic. I kept recalculating the and measuring the rate it was laid down at and the size but nothing seemed to work. Then after watching for a long time I noticed that when making small movements the machine never got to full speed- should be 720mm/min but on the on screen display I only got 2-300 mm/min- hence too much plastic! I now need to maximise the acceleration on all my steppers using the EMC set up programme.

I have also programmed the arduino so that when it switches off it goes backwards fast briefly and when it switches on it goes forward 1.5 times the reverse quickly to stop ooze and this seems to wok really well (although it stretched my limited programming capability)

The first thing I printed was Adrian's pinch wheel extruder. (I would show pictures but there are no batteries in my camera I will add them tomorrow). This came out really well and appears to be quiet strong- not quiet at Nopheads quality but good enough for now.

I then decided to print the wine glass as we were going out to a dinner party. Unfortunately the machine does not have enough height so I managed the stem- it looked great and only took 40 minutes. When I showed it to friends they did not understand the rep rap excitement. In fact Alex told her boyfriend Steve "Its a good job he already has a wife or he would stand no chance!"

I am now in the middle of printing a large assembly- it has been printing for over 9 hours now and is looking good despite the various breakdowns.

Next steps
1. Tidy all electronics/wires up.
2. Make sure the feed to the pinch wheel is constant using smaller hooks this appears to cause the pinch wheel jams
3. Fasten the extruder more securely to the machine
4. Fix the acceleration issue
5.Move the whole thing into the garage to reduce noise and smell
6. Increase the z axis movement

Questions to anyone

1. What is the biggest thing that anyone has made?
2. Using a pinch wheel extruder and the resistor/brass bar/welding tip/stainless set up could we just wack in solder and extrude it- what is the melting point?
3.Is acceleration an issue on Darwin?


Sunday, 15 March 2009

Pinch whell stepper extruder

I have battled for a long time to get an extruder drive that works reliably, delivering a steady speed especially when extruding HDPE. The problems were that the DC motor would slow under load and was often variable in speed as the load varied. Also the screw drive did not positivley grip the plastic rod

I have come up with a solution based on all the other pinchwheel extruders but made from easily accesible parts (Thanks Adrian Zak and everyone else).
From the photographs you should be able to see a 90 degree angle bracket (from B and Q) which I have fastened a stepper motor to. I have machined a pulley and given it a rough surface using the technique described in another blog (sorry I can't rember who) where you put a tap into the lathe and the work piece into a bracket so it rotates effectively cutting a bevel gear. This is better than knurling as 1. I don't have a knurler and 2. it gives a groove for the plastic to sit in so keeps it in the correct place.
Initially the stepper did not have enough torque to drive the plasitc consitantly but I rewired the stepper so the coils were bipolar parallel and it worked fine.
I have also made it so that the stainless tube comes as close to the pinch wheels as possible to prevent any buckling.
I just now need to work out how programme the arduino so that when it gets a stop signal the stepper reverses for a few steps then stops to stop oozing.

90 minute nozzle

I have been experimenting with NOPHEAD's nozzle design for a couple of weeks. The second picture shows one made with brass and a Peek tube. I had problems with the thermistor coming out so I held it in with twisted wire and fire cement. This worked OK but I melted the Peek when the thermistor fell out also when the nozzle hit the work piece, it bent.

I then put in place mark 2 using a stainless steel tube. This works very well using virtually the same design as nophead but without the cooling fin. It does have a narrowed wall just above the heater block anmd it is tapered at the end. The cooling comes from the plate that the nozzle is mounted into. The tube is made from 8mm stainless studding and the brass is 3/4" bar. It uses the ceramic resistor as a heater and this is sealed at either end with fire cement.
The welding tip is drilled out but is not drilled in as far as it would go as I suggested earlier on and it seems to work.
The great thing about it all is that from deciding to make it to having it heating on my machine using a pillar drill, lathe and taps it only took 90 minutes. Much faster than the old nozzle. Well done Nophead, genius design.