Thursday, 20 November 2008

Hot Stuff

When I was making the chuck grip I noticed that the raft changed colour part way across the top layer of the raft.

The heater seemed to be on 100%, so it looked like the plastic was way too hot. By the time I noticed it seemed to have reached thermal equilibrium and apart from some snap crackle and pop sounds, and a bit of smoke the extruder seemed happy. I was reluctant to abort the build because it had taken about an hour to get this far.

When it finished the raft it cooled down to the right temperature and built the object. The surface of the raft has a completely different texture and it seemed easier than usual to peel off the object. Despite that, it managed to hold down what was a very big object. The shape of the object was less prone to curling than most, being a large circle (no corners to curl up) split into three segments and with a corrugated outside perimeter, which could absorb shrinkage. I need to do more experiments to know if it is beneficial to deliberately make a raft like this.

This is what the normal and hot rafts look like close up :-

And here they are under a microscope :-

To investigate further I ran a test with the heater target temperature set to 300°C and monitored the thermistor reading. It maxed out at 290°C. That is fortunate as it is just below the point where PTFE is supposed to start decomposing into poisonous substances. For some reason the PTFE holds up mechanically, I would have expected the barrel to pop out. Perhaps the ABS becomes so fluid that there is very little pressure required for extrusion. Anyway, the extruder seems happy operating at 280°C, where it just about manages to control the temperature with 96% PWM.

The filament changes from green and smooth to almost cyan and a rough texture: -

Again under the microscope the surface looks very different :-

My theory as to what is happening is that the green dye is composed of yellow and cyan dyes, and the yellow component is boiling off, disrupting the surface.

I had a go at making some objects at 240°C, 260°C and 280°C :-

It seems that 240°C is about the limit for green ABS before it starts to change colour and texture. The bottom of each object has to be at the correct temperature so it can be separated from the raft but other layers could be chosen to be different temperatures to give a stripy effect. The hot objects seem very strong and feel like they wont de-laminate in a hurry.

I don't think you can keep the plastic long at those temperatures, I found this mess under a raft. I think the temperature had gone wrong during warm up.

Initially I had no idea why my temperature control was occasionally going wrong. The thermistor is still well attached. I caught the effect with some logging and discovered that the temperature was reading about 40°C low some of the time. Touching a connector seemed to fix it. I could not find a loose connection so I just unplugged it and plugged it in again. I has been OK since. With a 10K thermistor you only need a few ohms to make a big difference at the high end.

So an interesting effect that might be exploitable for support material or aesthetic effects.

Monday, 17 November 2008

Key Things

Zach Smith and Bre Pettis have created a web site called Thingiverse, which is designed to make it easy for people to share digital designs of real objects. I have put most of the things I have designed for printing on a RepRap on there. I even created one especially for it, a key for reading utility meters :-

You can get these free from the utility companies, but not a quickly as you can RepRap one for a few pennies worth of plastic, and they are easily lost. The files can be found here :-

I also keep a gallery of all the things I have made with HydraRaptor here: They all now have links back to Thingiverse for the files.

A lot of the objects on Thingiverse are for a laser cutter, but I downloaded this twisted star box designed by Marius Kintel and printed it.

One trick I have learnt is that you can shrink objects a little by heating them with a hot air gun. When I first made this the top was too tight, so I shrank the bottom a little with the heat gun. It is now a perfect fit. You have to be careful not to get it too hot, or it will sag.

Tuesday, 11 November 2008

New Toy

The company that I have worked at for 25 years gave me a long service award recently. I could choose anything worth £500 so I chose this small lathe / drill / milling machine combo. The tiny watchmaker's lathe I have been using up to now is not really big enough for RepRap parts. This should be just about right.

A combo like this is a bit of a compromise and only recommended if, like me, you have limited space. It gives me a lathe, pillar drill and milling machine in a small footprint.

I also bought accessories with my own money, which came to about another £300 :-
  • Cutting tools - essential.
  • Compound cross slide - for cutting tapers.
  • Drill vice - to hold things on the cross slide when milling or drilling.
  • Tailstock chuck - to drill down the axis of round things.
  • 4 Jaw chuck - for turning square and irregular shapes.
  • Milling chuck and collet set - to hold milling bits.
  • Wiggler - for finding centres and edges when milling and drilling.
  • Die holder - for tapping threads.
  • Headstock centre - for turning between centres.
I have also made a couple of accessories with HydraRaptor. The first is very simple: a t-slot cleaner for removing swarf from the cross slide's t-slots.

That saved me about £2.80.

The second object is the biggest thing I have RepRapped so far. It is a cover to go over the chuck to make it easier to turn by hand when tapping. It took over 7 hours to build and weighs 77g.

Here it is installed :-

I haven't used it yet but it feels like it should work well.