Theory & the basics of the design
Mechanical Design
The “chassis” of the machine has been built from a single
full-size sheet of 18mm MDF and divided in such a way as to reduce the cutting
and to use the already, reasonably square edges left by the board makers. The linear bearings used on the axis’s have
been sourced from kitchen draws, granted the accuracy of these runners isn’t the
best, but by installing them in such a way that they are either under
compression or being opposed, deflection can be minimised. These draw slides have been chosen as they are
cheap and easy and not because they are accurate or suited to the task in hand!
The threaded rod is M8 x 1.25mm all-thread
bar and again this is chosen as it is cheap and easy to acquire, not because it
is accurate or ideally suited to my application.
Spindle
To keep costs down, for the spindle (cutting motor) I’ve
used a cheap dremel syle rotary tool.
This has a 3.2mm collet fitted as standard to allow the use of small
tooling and spins up to 35’000RPM with 130 Watts of power. Obviously this isn’t as powerful as the
motors found in commercial machines but it’s cheap and allows scope for
improvement at a later date. The
multi-tools high speed is ideally suited to the small cutting tools used, which
must be spun at a much higher speed than larger diameter tools. The tool also has an in-built speed control,
allowing the spindle speed to be adjusted manually.
An important point to consider when choosing one of these
tools to use as a cutting spindle is the amount of run-out at the tool tip,
this can be seen as the “wobble” of the tool as it turns. This run-out will cause inaccuracies in the
machining of the part but will also induce a radius of gyration and cause a
vibration in the machine. This vibration
will show as “tool-chatter” and leave marks on the parts surface.
Once the machine is up and running I’m going to be looking at
making a new spindle from a Radio Control Model Plane DC Motor and a ER11
spindle extension, but that can wait!
Tooling
Due to the 3.2mm collet, there is an awful lot of cutting
bits and burrs available from the internet for next to nothing! I bought a 3mm
carbide end mill from eBay for £2 delivered and this has been used for most of
the cutting so far, as I’m only cutting MDF and soft materials, this end mill
should last a pretty long time. I’ve
also got some “V-Carve” tools to try and some 3mm drill bits as well as some
larger 6mm ball end mills for when I get chance to develop the new spindle.
Control System
The control system to be used is the tried and tested
stepper motors via serial com port and a driver board. The system works in the following way;
firstly a model is generated in CAD of the finished part. This is then exported into a CAM program
which generates “G-Code”, this G-Code data is transferred to the machine
controller software which translates this G-Code into step and direction pulses
out of the serial coms port of the PC (at 5V and very low current). These signals are sent down a serial cable to
the stepper motor driver board which uses them to drive the stepper motors by
energizing the stepper coils in such a manner that they move the required
amount. The power supply to the stepper
motor driver board comes from an old PC ATX power supply and these supplies the
required current to the board to enable it to drive the stepper motors.
As you can see above, the opportunity for errors in the
finished part can creep in to the system in many areas, the main one being the
computer running the control software is required to generate “pulses” for the
step and direction signal, if these pulses vary in there speed then the machine
will vary the distance it travels for each one, that is to say that if the
frequency of these pulses alters from the expected frequency then the machine
could travel at a distance of less or more than expected and the finished
part’s size will vary away from that expected.
The amount of this frequency deviation will vary on the “load” on the
computers CPU whilst running the “heartbeat” algorithm that controls this
output frequency. For instance starting
a program whilst the PC controller is running could cause a load on the CPU and
an error in the machined parts finished size!
There could also be an error induced if the parameters of
the machine are not correctly entered into the machine controller software, if
for instance the leadscrew pitch was entered as 1.2mm instead of 1.25mm the
machine would assume that to travel 50mm it would need to rotate the axis
41.666 times as opposed to 40 and this would give and error of 2.08mm on a 50mm
cut!
