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:

size-based distortion 2

size-based distortion 1

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.

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:

large thing 01

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.

large thing 02

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:

disappointment 01

and this:

disappointment 02

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.

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.

Hot Pi

In my last post, I explained how to set up the Pi with the drivers needed to allow it to read temperatures from DS120B sensors. In this post, I’ll show you the code and setup required to make it start taking readings automatically at bootup, and store the information in a Mysql database for future use. I’ll also add a couple of LEDs to the circuit, so that the system can provide some feedback and also to pave the way for controlling a relay or two.

Here’s the updated circuit schematic (click on it for a bigger version).


There are a few things to note about this slightly more complex schematic:

  1. I’ve added another temperature sensor. Its pins are simply connected directly to the pins of the first one (and any subsequent ones). The one-wire protocol and the fact that each sensor has a unique ID means that they can share a common bus.
  2. I’ve added two LEDs, just so that the system can indicate some state to me. One will be replaced with a relay in future.
  3. I’ve added an Adafruit level converter (the blue pcb). This is there to protect the Pi: its GPIOs run at 3.3V, and are not designed to carry much current. overloading them will cause damage to the Pi. The cheap level shifter allows the 3.3V Pi pins to be connected to 5V input or output from other devices, and protects the Pi from accidental over-voltage. It’s not strictly necessary just to drive LEDs, but the relays I want to use later need 5V.

I’m going to use Python as the main language for running the system. The reason for this is that I don’t know it very well, and there’s no better way to learn than to dive in and try and do something with it. Of course, this also means that my Python code may well be what is technically-termed ‘Not Very Good’. Use it at your peril. To make it work, you’ll have o install a couple of other things. One is MySql, the database into which the program will write the recorded temperature values (you’ll also need the associated Python library). The other is pigpiod which is, of course, a Pi GPIO library, though the name always looks like pig-pio to me. Pigpio (also available in its useful daemon form pigpiod) is a C library which exposes all the GPIO to C programs. Not useful for Python, I hear you say. True enough, but it also exposes the same functionality through a TCP socket interface, and it’s dead easy to use this from Python without installing any special python libraries at all. We’ll deal with that in my next post.

Installing MySql

In Linux, it down’t get much easier than this:

sudo apt-get update
sudo apt-get install mysql-server

At some point, the installer will ask you for a password for the root MySql user. In any serious situation, this should be a good, secure, password. I just used the same one as I’m using for everything else on this device. It’s only a toy, after all. Once MySql is installed, you can use the command line client to create a new user and to make the ;temperatures’ database and the ‘data’ table in which the readings will be stored. NB: when you are using the MySql command line client, don’t forget the semicolon at the end of a SQL command line. Multi-line commands (such as the CREATE TABLE command below) are fine in SQL, and they don’t get executed until the parser sees a semicolon. It catches me out all the time.

mysql --user=root --password=My5ecurePa$5word

mysql> create user 'pi'@'localhost' identified by 'raspberry';
mysql> grant all privileges on *.* to 'pi'@'localhost';
mysql> create database temperatures;
mysql> use temperatures;
mysql> CREATE TABLE 'data' ('id' int(11) NOT NULL AUTO_INCREMENT, 
'timestamp' datetime NOT NULL, 'sensor_id' int(11) NOT NULL, 
'temperature' float NOT NULL,  PRIMARY KEY ('id'));
mysql> quit;

When that’s done, you will have a database with an empty table in it, and a user ‘pi’ which you can use to connect to it from python. For that’ we’ll need the python-mysqldb module. This can be installed thus:

sudo apt-get install python-mysqldb

We can check that this works with a small python script:

import MySQLdb as mdb
import sys

    con = mdb.connect('localhost', 'pi', 'raspberry', 'temperatures');
    cur = con.cursor()
    cur.execute("SELECT VERSION()")
    ver = cur.fetchone()
    print ("Database version : %s " % ver)
except mdb.Error, e:
    print ("Error %d: %s" % (e.args[0],e.args[1]))
    if con:    

Run this, and if all is well you will see something like

Database version 5.5.37-0+wheezy1

Or possibly an error. With unbridled optimism, I’m going to assume that you are not seeing an error. It’s now time to create the code which will actually read the temperature sensors and write the data to the database. Here it is:

#!/usr/bin/env python
import datetime, time, sys
import MySQLdb as mdb

def getTemp(chipid):
   tempdata=secondline.split(" ")[9]
   return temperature

   con = mdb.connect('localhost','pi','raspberry','temperatures')
   with con:
      cur = con.cursor()
          timestamp = datetime.datetime.fromtimestamp(time.time()).strftime('%Y-%m-%d %H:%M:%S')
          t1 = getTemp("28-000001a9b68a")
          t2 = getTemp("28-0000009bba2b")
          query = "INSERT INTO data(timestamp,sensor_id,temperature) VALUES ('%s',%s,%s)"%(timestamp,'1',str(t1))
          result = cur.execute(query)
          query = "INSERT INTO data(timestamp,sensor_id,temperature) VALUES ('%s',%s,%s)"%(timestamp,'2',str(t2))
          result = cur.execute(query)
          print ("%s %s %s"%(timestamp, str(t1), str(t2))
   print ("Goodbye")


There are two main parts to this code. The function getTemp(chipid) reads the virtual file created by the 1-wire driver discussed in my last post for a specific temperature sensor, and extracts the temperature data from it. There’s nothing clever about it, just python string processing. You’ll need to modify the code further down to use the chip ids of your own sensors, of course. The main body of the program establishes a connection to the database, then enters an unending loop inside which it gets the temperature from each sensor and writes it to the database with a timestamp using a simple SQL INSERT query. Adding a time.sleep(10) means that this process happens every 10 seconds. Assuming you run this without errors, you’ll find that your database will gradually be populated with temperature readings. You can check this using the mysql command line client again:

mysql --user=pi --password=raspberry

mysql> use temperatures;
mysql> select * from data limit 10;

| id | timestamp           | sensor_id | temperature |
|  9 | 2014-05-17 17:17:17 |         1 |        20.3 |
| 10 | 2014-05-17 16:45:10 |         1 |      22.187 |
| 11 | 2014-05-17 16:45:10 |         2 |      23.875 |
| 12 | 2014-05-17 16:45:21 |         1 |       23.75 |
| 13 | 2014-05-17 16:45:21 |         2 |      23.875 |
| 14 | 2014-05-17 16:45:33 |         1 |       23.75 |
| 15 | 2014-05-17 16:45:33 |         2 |      23.875 |
| 16 | 2014-05-17 16:45:45 |         1 |      23.812 |
| 17 | 2014-05-17 16:45:45 |         2 |      23.875 |
| 18 | 2014-05-17 16:45:56 |         1 |      23.812 |
10 rows in set (0.00 sec)

If you have got that far, and are seeing data in the database, that’s excellent. In my next post, I’ll show you how to install pigpio and use it to flash an LED to give confidence that the process is working without having to check the database all the time. The next step is then to create a webserver which will show the data without having to log on to the Pi. Keep watching this space.

Warming to the Pi and Python

I’ve had a Raspberry Pi since Christmas, but haven’t done much with it up till now.  That’s all changed since I moved house.  Getting to grips with a new central heating system, I find that (for reasons too dull to list here) I need to monitor and control it rather more flexibly than I can do with the standard timer.  I also don’t want to pay lots of money for a Nest controller, however beautiful it may expect itself to be considered.  My first thought was to use an Arduino – the problem I’m addressing does not need a lot of processing power, and I don’t want to spend a lot – but I came up against a problem which seems to me to be the Arduino’s biggest failing at the moment.  I want to be able to control the system over WiFi.  There are WiFi shields for Arduino, of course, but they start at £30, twice the price of the Arduino.  I could invest in a Yun which has WiFi on board, but that’s £70.  There simply seems to be no cheap way of getting WiFi onto an Arduino – a device which is crying out to be connected to the internet..  Rob suggested I could use a bluetooth adapter and control the system from a phone; that has attractions and is very cheap, but means I have to be quite close to use it (no controlling the heating system from work, for example).  This made me turn to the Pi.  At first sight it seems ludicrous to use a full Linux computer for this trivial task, but consider: the Pi costs £23 (or less, for a model A), and the tiny USB WiFi dongle was only £9.  That’s £32 for the whole thing.  Much cheaper than an Arduino and WiFi combination. 

The hardware requirements of the system I want to build are quite simple.  It must monitor two or more temperature sensors, and it must be able to control two or three relays capable of switching mains voltage.  Having a couple of controllable status LEDs would be good, too.  I don’t require any other user interface input or display in hardware, because I want to use a web interface.  The Pi is more than capable of handling all this, but it turns out that there is quite a bit of software installation and configuration to do.  All of it can be done over a ssh connection (I use PUTTY for this), which is fortunate because I don’t actually have a monitor with an HDMI input that I can use as a display for the Pi.

The temperature sensors I am using are DS120B types.  Rather than being dumb devices like thermistors (whose resistance varies with temperature) they have logic inside which allows them to communicate with a host over a three wire digital serial connection.  The manufacturer calls this ‘1-wire’, because the actual communication only needs one, but you also need power and ground, so really there are three).  This has several benefits: it means that many devices can share the same wire (as each has its own address), it reduces the likelihood of inaccurate readings caused by long wires, and it means that the host does not need an analogue to digital converter to read them.  It also means that the host must implement the 3-wire protocol in order to read the sensors.  Fortunately, cleverer people than I have already done the hard work, and there are Linux drivers for these and other three-wire devices.  The relevant ones are already present in the raspbian distro, and simply need enabling.  Simply?  Did I say simply?  This is Linux – nothing is simple until you know the magic incantations.  I need Linux to load the appropriate 1-wire driver modules when it boots up, so it’s necessary to edit the /etc/modules file, which contains the list of drivers to be loaded.  It’s necessary to have root privileges to edit this file:

sudo nano /etc/modules


The lines w1-gpio and w1-therm tell the system to load the one-wire driver for the pi’s GPIO port (GPIO4 is used for this) and also the drier for the DS18B20 chip.  You can also load them manually from the command line if you wish:

modprobe wire
modprobe w1_gpio
modprobe w1_therm

Once these modules are loaded, the system will probe in the background to see if there are any sensors it recognises on the wire.  It will then create a new folder for each one under /sys/bus/w1/devices, and pop the data it retrieves from each in a file in there.  Reading the temperatures is a simple as reading the values in these files.  For example, as I write I have two sensors.on my Pi.  If I look in the devices folder, this is what I get:


The long number ‘28-0000009bba2b’ is the unique id of the sensor (hard-coded into the sensor itself).  The 1-wire driver identifies these and creates links to each.  By browsing to the folder and looking at the ‘w1_slave’ file within, the temperature is revealed.  it’s the bit saying ‘t=21250’, which is 1000 times the temperature in degrees Celsius (21.25 degrees).  If you want the temperature in Fahrenheit, I suggest you go back to the eighteenth century.  If you are actually executing the commands, you may notice a short delay between the ‘cat’ and the output.  That’s because when you read the file, the system actually contacts the sensor and takes a reading.  This is perhaps slower than you might expect, but it’s fast enough for my purposes.  For completion, here’s a quick wiring diagram of how to connect up the senor to the pi. the only other component needed is a 4k7 resistor, to tie the communication wire to the 5V line.


The next step is to use python to read these temperatures and store them in a database, then get a web-based interface up to control the relays.  I’ll also need to make sure that everything runs automatically, with no user intervention.  Watch this space.  Unless you have better things to do, which I fervently hope is the case.

Richmond revealed

I was at one of Rob’s excellent rather useful seminars yesterday.  As it was about 3D printing, Rob asked me to say a little bit about my printer.  Having not prepared to do anything, I relied on some bits of video I found on my tablet.  Thinking about it, I realised that although this blog has quite a few posts about specific parts of the building process, there isn’t really a post just simply describing the printer.  So this is it.  I call it Richmond.


It’s inspired by Johannn Rocholl’s Rostock printer, but the design is my own.  I wish I could blame someone else for the design flaws, but I can’t.  The basic design features three columns made of extruded aluminium section (bought).  Up and down each column runs a carriage (the pink bits), with spring-mounted ball bearings to make it run smoothly and without wobbling.   Rob printed these for me.  Here’s a close-up of one.


The blue parts are modifications (printed by me) to add extra ball-bearings.  The original design simply had screw in the plastic which gradually wore loose.  At the right of the picture, you can see the fishing line which is attached to the carriage to drive it.  This loops around a motor-driven pulley at the bottom of the pillar and and idler pulley at the top, to make it move up and down.  The grey rods are carbon fibre, and join the carriage to the tool head.  The rods are in pairs, to keep them in a parallelogram shape – this means that the print head can move in three axes, but will never rotate.  The tool head looks like this from above: The translucent tube in the centre guides the plastic filament into the heated part.


The print head looks like this if you happen to be underneath it (which I don’t recommend, because it heats up to 220 Celsius):


The underneath view shows you the hot end – the part that melts the plastic and squirts it out through a tiny (0.3mm) hole.  The plastic filament is driven through the flexible tube to the hot end by an extruder, in which it is pressed against a rotating gear.  It takes a surprising amount of force to push the filament though the tube and the hot end.  Here’s the machinery that does it (with its operator):


As usual, these components were printed for me by Rob, but they are not my design.  Rather, I downloaded the designs from thingiverse.  There is no sense in reinventing the wheel, especially when so much effort has gone into making it work well.  The plastic filament is taken from a reel, which sits close by on a home-printed stand:


The filament drive is powered by a NEMA17 stepper motor.  Each carriage is also driven by one of these, mounted at the bottom of each column:


Each motor has a printed pulley on it (printed and then machined with my printed lathe, in this case) which drives the Spectra non-stretch fishing line to move the carriage.  The filament is kept under tension by an adjusting screw on the carriage.  How much tension?  Until it goes ‘ping’ rather than ‘boing’ when you pluck it, that’s how much.  As an aside, it also makes an interesting Aeolian harp if you take it outside on a windy day.  Each stepper motor moves as finely as 3200 steps per revolution, so with the pulleys I have this means about 55 steps per millimetre of vertical movement.

Also mounted at the base of each pillar is an adjustable mount for the print platform (which is just a circle of glass from a local glass shop).  These are ugly and badly designed, but they do the job for the time being.  When I redesign the base, they will go.  Using three supports means that I can ensure the print surface is as close as possible to being perpendicular to the pillars.  This is important for ensuring good prints, particularly so for getting the first layer of plastic to stick to the base.  If the base is not level relative to the  and y movement of the print head, then it’s likely that the first layer will vary between being too thin (resulting in nothing but an impression in the masking tape) and too thick (resulting in a strand of plastic not actually stuck to the base).  Bear in mind that this means the print plate has to be level to within plus or minus a tenth of a millimetre over its diameter, and you’ll see why this calibration has been such an issue for me.


The hardware is mounted on a box made out of plywood I had lying around, painted purple because I had a can of purple spray paint to hand.  On the front of the box is a basic controller interface – an LCD and a simple rotary/click controller. 


The LCD provides status information about the printer, and a menu system to allow various parameters to be adjusted without using an attached PC.  It is possible to fit it with an SD card holder, which will allow printing completely independently, but I have no need for that.  The display and controller are driven by and provide input to an an Arduino Mega controller inside the main box.  The arduino is fitted with a Ramps 1.4 shield, which provides the stepper motor driver electronics and the higher power switching for the hot end.  The arduino runs Marlin firmware, which generates the stepper motor motions required to translate standard cartesian G-codes to carriage movements for the delta configuration.  The firmware also controls the extruder, monitors the temperature of the hot end, and drives the LCD.  The power supply driving all this is also in the purple box.  It’s a scrap supply from a PC, which provides handy 5V and 12V outputs, at high enough currents to drive the motors and the hot end.

Looking back, it doesn’t seem that complicated.  Makes me wonder why it took me so long to build.

Turn again, Whittington!

As a devoted reader of my blog, you will know that I’m engaged in a continuing quest to improve the precision of my 3D printer.  The most recent improvement was to replace all the pivots in my original design, which were simply composed of screws passing through holes in plastic, with proper miniature ball bearings.  This has had a huge benefit – there is now very little play in the movement of the print head.  Printed shapes are much more precise than they were.

The next issue to tackle is one I’ve been aware of for a while, but have not had the means to fix.  First, a recap: the whole print mechanism is driven by three stepper motors.  Each stepper motor has a pulley mounted on its output shaft, which drives a belt made of stretch-free fishing line.  Each belt goes the length of a vertical column, passes around an idler pulley at the top, and is attached to a carriage, so as the motor turns, the carriage moves up and down the column (a picture may not be worth a thousand words in this case, but if I had had one to hand it would certainly have saved me fifty or so).  The amount by which the carriage moves depends on the number of steps the output shaft of the motor rotates and the diameter of the output pulley.  Knowledge of these two things allows the printer firmware to be calibrated for each motor with the number of steps required per millimetre of carriage movement.  Because this figure is set in firmware, it doesn’t matter what each pulley diameter actually is – I can simply measure it and do the calculation.  However…

The problem I found on measuring the pulleys (kindly printed for me by Rob) is that they are not perfectly circular – the diameter varies by about half a millimetre around the circumference.  Add to that the fact that when a pulley is mounted on the motor shaft there is a little added eccentricity, and you end up with a drive pulley whose radius varies by about 5% as it rotates.  Over long movements, this is not a problem – the eccentricity averages out – but for small movements this results in unevenness.  It’s particularly obvious on the first print layer, which is often thin.  It manifests itself as patches of thin or thick deposition, and it just won’t do!

The cure is obvious: I need pulleys which, when mounted on the motor shaft, are round.  The standard method of making round things is with a lathe.  I would dearly love to have a proper modelmaker’s lathe, but they are expensive.  I can’t really justify spending £400 to make three plastic pulleys. I tried printing some more pulleys myself, but the accuracy of my prints was no better than Rob’s (not surprising, given that the pulleys weren’t round). 

I need a lathe.  I have a 3D printer.  You are ahead of me.  I had a search online to see if anyone had published designs for a 3D printed lathe, and the only one I could find was this one,  by a guy calling himself Sublime.  It’s a great piece of work, but it doesn’t look precise enough for my purpose, and I don’t need the three-jaw chuck.  For the time being, I only need to turn pulleys.  So I stole some ideas from Sublime, and designed my own lathe.  Here’s a video of it in operation:

I’m happy to say that it works.  It’s far from perfect, but the pulleys I have turned with it now have a variation in radius of 0.1mm or less, which makes for much better prints.  Now it’s on to the next improvement: automatic calibration of the print surface.  Watch this space.