There are a huge variety of commercial rail systems available. These
- Hardened steel shafts with recirculating ball bearings. These are
very low friction and have little backlash. The shafts must be protected
from sparks. Any sparks that weld themselves to the shaft will rapidly
damage the bearings and seals.
- Hardened steel shafts with polymer plain bearings. These are
cheaper but dust ingress is a problem.
- Linear slides with grooved bearings that run on a hardened,
ground rail. Low friction, cheaper than recirculating ball bearings but
again, dust and sparks can be a problem.
Unfortunately they have one thing in common. They are very expensive.
If you are building a machine that runs 24 hours a day then you can
justify the cost. However, I am building a budget home machine here so
another solution has to be found.
My solution was to use rubber sealed skateboard wheel bearings running
directly on the box-section framework. It is crude and will make any
professional CNC machine designer cringe but it does work!
Steel box-section is not accurate to a fraction of a thou like
the purpose-made linear systems so the bearings have to be spring loaded
to keep backlash to a minimum. Spring loading potentially reduces the
stiffness of the system but the loads are very light so this is not a
The X axis
The X axis carriage uses a total of seven bearings. Two run on the top
of the rail and take the weight of the Y axis motor assembly. Two run on
the rear of the rail and keep the Y axis square to the X axis. The last
two bearings run on the front of the rail and are spring loaded to
prevent backlash. The seventh bearing is on the far end of the carriage
to take the weight.
X axis carriage, showing the motor mount and adjustable brace to ensure
that the Y axis is at exactly 90 degrees to the X axis.
rear of the X axis carriage, showing the fixed bearings and the Y
axis motor mount.
compensate for any variation in the width of the rail, the front
bearings are preloaded with a spring. You can also see the adjuster to
set the Y axis to exactly 90 degrees.
The Y axis
The Y axis carriage is even simpler. It uses three bearings and two
steel rubbing pads (er.. actually screw heads). Two bearings run
on the front of the rail and one bearing runs on the back. The pads take
the weight of the assembly. The weight of the cutter head tries to
twist the carriage round on the rail, providing the preload for the
bearings. The unsightly chunk of angle sticking out is to increase
the preload because I had problems with the carriage lifting.
This is the rear of the Y axis carriage, showing the single bearing and
This is the Y axis carriage from the side. The front bearings sit lower
than the rear bearings so that the carriage doesn't twist itself off the
rail. The horrible bit of angle iron acts as a weigt to provide enough
load on the bearings to stop the carriage from lifting.
is the Y axis from the front. The cylindrical object just behind the
torch is the Z axis solenoid.
The Z axis
The Z axis is very simple. When the arc is established a solenoid pulls
the torch down onto the work. When the arc stops the solenoid is
de-energised and a spring lifts the torch. The torch is a drag type so
it is designed to work with the tip in contact with the work.
torch fits in an aluminium cup which is attached to the solenoid. An
aluminium bar positioned between two bolts stops the torch from twisting.