Freesteel Blog » 2014 » December

Tuesday, December 30th, 2014 at 2:27 pm - - Cave

This trip round the Lleyn followed on from last year’s Avoiding Nadilog by Walking in Wales. In retrospect nothing much happened. But it was exciting at the time due to the lack of planning and the risk of things going wrong.

Our white Xmas on Whistling Sands

Departure was delayed till the morning of the 24th because someone couldn’t possibly miss their 14th digging trip of the year in ODB. Anyway it was raining and we weren’t packed yet.


Tuesday, December 23rd, 2014 at 7:39 pm - - Machining

I printed this beautiful box for my speedy accurate barometer at the DoESLiverpool Xmas party designed on Openscad.


Friday, December 19th, 2014 at 9:46 pm - - Machining

I’ve been having “fun” trying to deal with the noise on the MS5611 altitude sensor. It’s an extremely accurate piece of kit (detecting approx 10cm of altitude). Last month I discovered that it is very sensitive to its own working temperature, which depends on how frequently it is read. You miss one interrupt and it throws everything out of whack.

So I got one of these Adafruit Trinkets, which is a tiny microcontroller board just to poll the barometer on a timed loop, and attempted to bit-bang the values down one of its output pins to a digital read pin on the main Arduino. Here’s what the the code looks like:

void sendbitbangbyte(uint8_t v)
    digitalWrite(PinOut, LOW); 
    digitalWrite(PinOut, (v & 0x01 ? HIGH : LOW)); 
    digitalWrite(PinOut, (v & 0x02 ? HIGH : LOW)); 
    digitalWrite(PinOut, (v & 0x04 ? HIGH : LOW)); 
    digitalWrite(PinOut, (v & 0x08 ? HIGH : LOW)); 
    digitalWrite(PinOut, (v & 0x10 ? HIGH : LOW)); 
    digitalWrite(PinOut, (v & 0x20 ? HIGH : LOW)); 
    digitalWrite(PinOut, (v & 0x40 ? HIGH : LOW)); 
    digitalWrite(PinOut, (v & 0x80 ? HIGH : LOW)); 
    if (v & 0x80) {
        digitalWrite(PinOut, LOW);
    digitalWrite(PinOut, HIGH);

Sensible programmers wouldn’t do this because they’d use one of the many standard libraries available that operats on a standard protocol. But I wanted to tactically slot this into the main loop that polls and waits (9 milliseconds) for the barometer/thermometer to make its reading with the minimum disturbance possible. It’s important to keep everything regular and synchronized.

Friday, December 12th, 2014 at 9:02 pm - - Machining 1 Comment »

The fact is that this is probably what most visitors to this blog are going to be most interested in.

I’ve another session hacking the barmesh slicing code, which is now creating these interesting subdivisions:

It can even generate 17 slices of a part without crashing, though it takes a few minutes:

It’s still only testing against the points, and not the edges and faces (hence the arcs), but that will only make it crash less as the shapes will be smoother.

It’s a little unclear what to do next. Maybe I should tidy the code further and clear up all these special cases I’ve been hitting and had to hack in to make it work. When a subdividing line crosses the r=0.5 threshold and I calculate it’s location, I’m setting it back to exactly 0.5. I don’t think this is the most reliable way to make it work.

The crucial functions are:

Sunday, December 7th, 2014 at 4:14 pm - - Cave, Kayak Dive, Machining

I’m going to do some other coding, now that I got this result. The code would fall apart if I touched it again.

Next on the list of things to do is clear out the vast quantity of rubbish left in the code, completely redo the subdivision loops and make the logic robust, apply it to multiple z-levels and plot slices, then make it test against edges and faces (not just points), and package it into a self-contained (but very slow) version of the slicer.

I don’t know how long this will take, as there are many other distractions available.

Wednesday, December 3rd, 2014 at 7:43 pm - - Machining 1 Comment »

By popular demand, I am working on a new Z-slicing algorithm, which is open source and in Python and can be found here. (My latest parts order is taking too long to be picked and come in the post.)

The code is not in any state to be used by anyone as I conduct some very meandering software development. I am unexpectedly basing everything on these BarMesh structures. This is a neat way to represent a triangulated manifold, such as the STL triangle file that contains the 3D input geometry. But also, instead of basing the slice on a strict XY grid (or weave) as I’ve done before, I’m using a second BarMesh to handle the 2D partitioning of the plane.


I don’t really know what I am doing, but if it gets messy and results in malformed folded cells, at least I can choose to constrain the BarMesh to conform to an XY grid weave structure, which I know works.

I’m just slicing (with a radius) against the points in the input geometry as this creates a more difficult slice geometry to begin with (here it’s an impeller shape with 38474 triangles, so I’m not starting with a toy example).

When the slicer is working, I can extend the code to test against the edges and faces of the input geometry.

I’m going for simplicity, and not being too constrained by speed or memory useage. There’s a lot more memory available than we need, and I’m counting on investigations into some weird Python compiler systems to provide the performance of C++, without the disadvantages of using C++.

I maintain the fact that if you take away the speed advantage of C++ on a particular platform, it loses its point of existence. Therefore the question of whether you should be using C++ is not to be found by looking at C++ itself, but by trying to beat what it supposedly does best using another language.

I can’t predict what will win. But the experience I am about to have ought to be extremely relevant to a programming team that is starting a new product and is having to choose what language they commit to.

This particular slicer will be for 3D printing and it will not notice the problem of mismatching triangle edges or self intersecting input geometry. (Self-intersecting inputs can come when you throw in some support structures.) It will be optimized for taking hundreds of slices and different Z-levels. It will work by finding the offset surface at a particular radius, and then offsetting back in by that radius to get the “true” surface, after the interior contours have been identified by tracking them up and down in 3D to prove full enclosure.