July 2014 – Tiny gadgets

July 15, 2014

It’s not my fault!

It’s probably not my fault, at least.

If you’ve been following my blog (and why wouldn’t you?), you’ll know that I have had some issues with my 3D printer printing things skewed. It got particularly annoying a day or two ago when a five-hour print ended up unusable. It’s about time I got this problem sorted out. Annoyingly, it seems that every time I do a test print, the problem goes away. Indeed, a test print after the last failure was absolutely fine. Perhaps it is time to apply logic, and isolate the causal factor. My test prints tend to be small (so that they are quick and cheap), but I notice (or perhaps only care about) the problem on large objects which take a long time to print. This morning, I printed a set of test objects of increasing size, to see if the problem was size related. The objects I used as test pieces were simple hollow square-section blocks, connected by a thin strip. The model looked like this:

I printed each of the pieces individually, smallest first. The results were very interesting:

As you can see, the first three parts printed fine (I’m not worried about the finish quality, just the geometrical straightness). Only the largest one had any skew at all. The really weird thing is that the skew was pretty constant over the entire width. To me, this strongly suggests a printer firmware issue. If the hardware were misaligned, or a drive were slipping, I would expect to see this on all the prints. A structural problem would surely show up gradually as the parts grew bigger, or possibly show up more the further from the centre the print head moved. In fact, the skew at the centre is the same as the skew at the ends of the part, and there is no skew at all on the second-largest component.

So if the problem is not the hardware (that’s why it’s not my fault 🙂 ), what is it? It could be the slicer, but this seems unlikely because I have used both Slic3r and Cura. Printing gcode files from either can have the problem. This seems to leave only one culprit: the printer firmware. I’m thinking that there is some rounding error, or some motor step-counter in the firmware which is overrunning or accumulating a consistent error on long print head movements. As the print head moves back and forth along the x-axis, this error fails to get reset between z-slices. Each layer is slightly offset from the one before. With smaller parts, this error simply does not occur. This is speculation at the moment.

I’m currently using Marlin firmware. My install is at least a year old, so my first step will probably be to upgrade to the latest version. This might be a bit of a pain, because I made a few customisations when I installed the last version, to suit my printer. If that doesn’t sort the issue out, I’ll have to delve into the source code. At least Marlin is open source, so I can fiddle with it.

July 12, 2014

Disappointment is…

… finding your five-hour print is faulty

Hmmm. Yesterday I replaced the heater in the printer’s hot end, vastly improving printing. Today I replaced a faulty power switch, reducing the chance of the printer cutting out mid-print. There seemed to be no reason not to make a start printing some of the bigger components I’ve been working on. Here’s the first one:

Don’t worry what it is. All will be revealed eventually. it’s a substantial bit of plastic, and demonstrates why I wanted to build a 3D printer in the first place. How else could I make components like that without a proper machine shop? I still want a proper machine shop, by the way, but I love the fact that I can make lightweight, stiff components straight from CAD models. I mean, just look at it.

So, you might think all is well. This component took five hours to print, and the printer didn’t miss a beat. What it did do, however, is print the whole thing skewed. It’s not immediately obvious, but look at this:

and this:

Makes you want to cry, doesn’t it? Both of those sides should be vertical, in case you didn’t realise. In fact the whole thing has been printed with a slant on it. I’ve had this problem before, but I thought I’d cured it by increasing the tension in the drive strings (the fault being that one or more was slipping). Clearly I hadn’t. Back to the drawing board I go.

Always remember: that which does not kill us makes us really annoyed.

July 11, 2014

Things are hotting up

I haven’t posted much about the 3D printer recently (or indeed about anything else). This is partly because I have been trying to work out what was the cause of poor print quality. I’ve finally worked out what the cause of the problem was. I’d been finding it increasingly difficult to print larger objects, particularly those which involved long continuous print movements. The symptom seemed to be a lack of material being extruded. I’d tried turning up the filament feed rate, but this only led to the extruder slipping as it tried to drive more filament through the print head. I turned up the temperature setting, to make it easier to extrude, and this worked over shorter movements, or at lower speeds. I could only get half-decent results by printing very slowly, with a feed rate of 120%.

The cause of the problem turned out to be very simple. The resistor used to heat the hot end simply could not provide enough heat to melt filament at the required rate. At low speeds, no problem. At higher speeds, insufficient power meant taht the hot end simply could not maintain the required temperature. Short distances were OK, because the hot end had time to heat up between extrudes.

The solution was to replace the resistor in the hot end with a cartridge heater. The J-head kit I bought came supplied with both options, and for some reason I can’t remember, I originally went for the resistor (there’s a picture in this previous post). Ten minutes of disassembly, rewiring and reassembly is all it took to fit the new heater. The hot end now reaches operating temperature much more quickly, and stays there very accurately, even if I turn the feed rate up and use a cooling fan. Goodness knows why I didn’t try it before.