Slide rails

There are a huge variety of commercial rail systems available. These include:
  • 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 major problem.

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.

The 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.

The rear of the X axis carriage, showing the fixed  bearings and the Y axis motor mount.

To 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 cable guides.

Y axis side  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.

This 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.

The torch fits in an aluminium cup which is attached to the solenoid. An aluminium bar positioned between two bolts stops the torch from twisting.