Wednesday, July 11, 2018

Desktop thermometer

My office at work gets quite cold some days, and I wanted to know how cold. Also, I was at a loose end over the long July 4th break and looking for something to do. So I knocked together this project:

Inside the box there is an Arduino Nano, and a DHT11 temperature sensor, plus a 10k variable resistor to set the display brightness. The display is a 16x2 LCD. It displays the temperature and humidity and the range of temperatures it has seen. The readings can take a little while to stabilize, so until they do the C and F are displayed in lower case, and the % for the humidity is shown as H. The Arduino sketch is here, and the comments in this file also say how to connect it up. You might need to install the DHT library from Adafruit (info linked from this page). It uses a USB connector for power.

I designed the case with Fusion 360. It holds together with friction. I undersized the hole for the DHT11, meaning it's not quite large enough for the sensor to stick out. Rather that spend another 90 minutes printing a replacements, I just attached the DHT11 inside the case with hot glue, which works fine except that it came out a bit skew-whiff. But that's how you know it's home made.

Sunday, April 29, 2018

High altitude printing

Recently, I was printing a frame with a tall arch shaped part to it. The designer of the object  provides a version with built in support and one without. If I allow Cura to generate the support for the one without, the printing time is a lot less, so that's what I went with.

It was all going well until the support detached from the build plate. The printer then merrily continued, dumping the support material into thin air, where it gradually turned into a big pile of print snot. It is complete coincidence that I was using a green filament.

I trimmed away some of the slot and let it keep going. Amazingly, after a while, the printer managed to re-establish a bridge across the arch with a few layers of support to go, and get to the end of the print.
Here's the print with the snot that I trimmed away and the support that came loose.
After removing the support and the rest of the snot, the results are a bit ugly, but not too bad.

Sunday, April 22, 2018

Forcing variable layer heights in Cura

Recent versions of Cura have an experimental feature where it selects different layer heights according the slope of the model. As the slope changes from vertical to horizontal, it uses a lower layer height to get better shaped prints. Imagine a dome, where the later height gets smaller as it reaches the top, and you'll get the idea.

I have a piece that I wanted to print where I wanted most of it to print at 0.2mm or 0.3mm to keep the print time down, but wanted a small vertical extrusion to print at 0.1mm for strength and precision. As the extrusion is vertical, this won't trigger the adaptive layer heights.

A simple solution is to add a second object which has a dome at the same height a the extrusion. As the same layer height is used across all the objects, the low layer height needed for the dome also applies to the extrusion, while the remainder of the model prints quickly. Here is a screenshot from Cura to illustrate what I mean. I know it's a bit tiny; for a bigger version, look here. In short, the dark blue is 0.1mm layers and the yellow is 0.3mm.

Saturday, March 31, 2018

CoreXY frame

Continuing my noodling around from last week, I started to put together a frame for a CoreXY system. I could have done the right thing and designed this in a CAD package, got all the measurements right, blah-di-blah, but instead I'm just winging it and knocking together what I need in Blender. Here's the frame with motors and runners but no belts yet:
The motors are mounted on plates bolted to the bottom of the frame with spacers to raise them to the right height. These corners of the frame have internal L-shaped brackets.
The gantry is mounted on brackets with runner wheels:
They move quite smoothly, though I think the brackets will bend over time.

For the pulleys, I'm going to use flanged bearings. I really wanted ones with a 5mm inner diameter, but I would have had to wait a couple of weeks to get them, so I bought some with an 8mm inner diameter and added a insert.
I ran out of 5mm washers, but there is an easy solution to that:
The next stage will be to get all the pulleys set up. The motor height is designed to work with pulleys mounted on top of the gantry with a 30mm bolt as you can just see in the video clip. For the pulley fixed to the frame, I'll need a mount to raise them high enough. More to follow.

Sunday, March 25, 2018

Motion systems

I'm starting to play around with motion systems. I have no specific goal in mind: it's just noodling.

Here is a small testbed using a MGN12 linear guide that I bought for a printer upgrade and then never used, together with some parts salvaged from my now-defunct Folgertech printer and a few printed parts. There are printed T-nuts under there, though you can't see them.

Sunday, July 16, 2017

3D Printed Tourbillons, a coda

In my previous post, I wrote about making 3D printed tourbillon mechanisms. There are a few parts in these designs which are not printed, specifically two small bearings, and a few 2mm and 3mm shafts made of steel. There is not much I can do about the bearings, but I was curious to see if I could replace the shafts with plastic and used them in the version III design. I printed 1.8mm, 2mm and 2.8 mm diameter cylinders at 0.1mm layer resolution. They had to be printed with brim, so I then trimmed the brim off with a sharp knife and lightly sanded them. The 1.8mm shafts work quite smoothly for the spring/balance wheel, the escapement gear and the anchor. The 2mm shafts replace the two shafts which hold the frame together. The 2.8mm is a replacement for the the main 3mm shaft. It's a tight fit in the bearings, and is also too loose for the sprocket to engage with it well. I tried to shim it with a little blue tape the way I had done with the steel shaft, but this didn't work. However, putting a small blob of blu-tak (or similar) in the axis hole of the sprocket did. Here is the tourbillon working with plastic shafts (which I wave at the camera at the start).

One though which follows is that you could made the drive shaft better by printing it and the sprocket with a non-round cross section, like the flattened shafts you get on some stepper motors. They probably would not last as well as the steel ones, but it's an interesting idea, as you can cut the plastic to the exact length very easily.

Saturday, July 15, 2017

3D Printed Tourbillon

I've tried several times to print clocks and clock mechanisms, with not much success. Here are three models that I tried:

I printed the first with my original Folger 2020 and the other two with my Eclips3D. None of them ran very well: the clocks would tick for only a few seconds, and the tourbillon watch would not run at all, though I was able to get the tourbillon subassembly to work for a short while.

I don't really know how to debug mechanical systems. With code, I have intuitions about where to start looking and techniques that I've learned over the years for breaking down and isolating the problem. But with most mechanical systems of any complexity, I don't really know how to get started, apart from looking for obvious things such as malformed parts. With a tourbillon, the problem is particularly hard as it isn't a linear sequence of parts, unlike (say) the spring motor, where each gear just turns the next one. In a tourbillon you have a power source which turns a mechanism, and the mechanism gets blocked from turning beyond a point by the anchor, and this causes the balance wheel to turn and then bounce back, nudging the anchor and allowing the mechanism to turn further, consuming a bit more energy from the power source. If any stage goes wrong, the whole thing just deadlocks. In the context of the tourbillon watch, this is made much more complex by the gears to divide down from seconds to minutes and hours.

I like the tourbillon as a design. It just looks interesting when it's moving. So I was happy to find a couple of designs on thingiverse for just the core tourbillon on its own. Here is one. And here is the other. I spent last weekend working on the first one, and almost gave up when it didn't run at all. But unlike the toubillon watch, the mechanism is simple enough that I could look at it part by part and see which parts weren't moving freely or slipped slightly on their shafts under load. With some reprints and adjustments, it worked: not perfectly, but enough that I had some confidence that continuing to tweak it would fix it. When it does stick it is always at the same position, implying that either there are some gear teeth that need adjusting, or possibly that the weight of some of the parts throws the mechanism slightly out of alignment. Here are some pictures and video. It's a bit wobbly in the wideo, and I have since improved this.

I then went on to the other design (it's called design III - the person who made them recommends not trying version II). Again, it took some fiddling and reprinting parts which were a little off. I think this is a less good design than version I. In particular, the power transmission from the sprocket (the wheel with the dimples) is done purely through the shaft that it is attached to. This means it slips quite easily, and once it has slipped a few times, the wear on the inner hole of the sprocket means it will keep slipping. In version I, the sprocket was locked to the mechanism through a hexagonal hole which meshed with a hexagonal shaft on another part. I was able to overcome the slipping with a shim, in the form of a little blue tape wrapped round the shaft. I don't think this is a good long term solution. There are related problems with the other shafts. There are two in the outer cylindrical parts of the frame which hold the tourbillon together, but they are too loose to do this, again requiring a shim to fix them. The shafts for the balance wheel, escapement wheel and anchor are not enclosed at one end, so they can slip out. Here's the video and pictures. Again, it runs but does slow down or stop in some positions.

All in all, I am quite happy with these. Neither runs perfectly, but they've restored my faith that I might be able to get things like this to work. I would really like to tackle this amazing device. Perhaps I will one day.