To move the axes some system is needed
to convert the rotary movement of the stepper to linear motion. There
are several methods of doing this:
- Ball screws. These are very accurate, have very low backlash and
are ideal for CNC positioning systems. Unfortunately they are also very
expensive and difficult to protect from dust.
- Rack and pinion. These are a lot cheaper than ball screws and if
positioned carefully the dust just falls out of them. They are not
as accurate as ball screws but easily good enough for plasma work. They
are often used in commercial plasma cutters.
- Chain drive. Wrap a length of chain around a couple of sprockets
and away you go... Chain drives do tend to wear and can need
regular readjustment. It is also suprising how expensive 30' of chain
- Belt drive. Like chain drive but using toothed belts.
In the end I went for belt drive as it is the cheapest option and in my
opinion it is less prone to wear than chain.
Modern toothed belts are extremely strong and resistant to stretching.
A good example is a car timing belt. In 30000 miles a car
timing belt is still nearly as tight as the day it was
fitted. Of course nothing is as easy as it initially looks. 16' long
belts are hard to find. In the end I settled for 30mm wide HTD belts,
approx 3m long for the X axis and 1.5m long for the Y axis. The belts
were cut and extended with wire rope. At the far end of each axis the
cable runs over a large aluminium pulley that can be adjusted to set the
One problem with the wire rope system is that it tends to twist under
load as the rope tries to unwind. This does not affect accuracy but it
does not look very good.
This is one of the joiners that connects the wire
rope to the toothed belt
Y axis motor mount showing the motor and bearing blocks to support the
toothed belt pulley.
Y axis stepper motor mount and reduction belt drive
is the Y axis idler pulley and tensioner. The X axis tensioner is very