Cnc project

Aim of this project

To design and build a table-top CNC machine on a budget of £150.

Background

Waking up the first day of 2009 with a mild hangover and randomly looking around the internet for non-porn I came across a few pages for CNC mills and figured I really needed one to build a spaceship, but the price tag on most CNC's are prohibitive for a hobbyist. Luckily there are many design plans out there for home-build machines. Eager to jump on the bandwagon, I have set myself the goal of spending a little time and money this year designing and building my own CNC machine, and posting the development here.

Specifications

(Subject to change)

Interface: USB (probably)
3-axis design :

• bed size (x,y) : 600x600mm
• bed depth (z) : 150mm

Tool type: Interchangeable, but will probably start with a router
resolution: better than 0.1mm

Design considerations and challenges

• Resolution
  presents two issues
  1. firstly how to measure such high accuracy, several solutions come to mind, if using stepper motors for example, the software can take care of positioning, but would require a "home location" to recalibrate every now and then. Optical disks could be used on the drive motors (this also would allow the use of normal DC motors instead of stepper motors), or (my favorite idea so far) would be to use a pulsed infra-red LED, with the light bounced off a mirror attached to the moving axis and detected by a photo-diode. the phase difference between the LED and the photo diode output would directly correlate to the distance, which can, given fast enough electronics, give fairly accurate distance measurements.
  2. Second is how to achieve such high accuracy with the drive shafts. The motors will need to be geared (or very high resolution, and expensive stepper motors will be needed), and/or the thread on the screw drives will need to be very fine. High resolution could also mean a very slow machine, which may not be desirable
• Workpiece clamping
  How to keep the piece we're working on in place
  

Components

Drive
Each axis will run along two parallel cylindrical metal rods of appropriate length, and driven by a long threaded rod powered by a motor. the moving components will be attached to the threaded rod by 2 long (1") bolts - on fixed bolt, and one 'floating' bolt attached with a stiff spring - this will compensate for any play in the bolts.

Progress report

05 jan 2009

I have had a look through my spare parts bin and found 3 stepper motors that should be suitable for the drive system of the CNC, they all appear to be 1.8 degree/step which gives the desired precision. They have been sitting in my parts bin for some time, and if memory serves they have been salvaged from old printers (probably dot-matrix) recovered from skips or throw-aways from friends. Salvage seems to be the best source of stepper motors since these seem to carry a price tag of around £30 and up (each) from most reputable dealers.
The main question is weather they can provide sufficient torque, to that uncertainty the system will need to be designed to house larger motors if necessary.

Additionally I have been working on some design schematics and will post them when they are finished.

More to come as soon as I can be arsed, with pictures