CNC heavy chassis

Cheap computers and software have brought CNC machining within the realms of DIY.

Some smaller routers are not much more than toys but for a machine that can commercially earn its keep we need some size and the ability to hang more tools.
So here we have something much bigger and heavier without increasing the cost to anything like a professionally made router.
Size: In order to machine round the outside of a standard 8’x4’ sheet we’ll need a long and strong X-axis. A heavy steel RSJ will resist the tendency to sag under the weight of two 4.5kW router motors and two drills. Of course a machine that holds multiple tools is ultra productive but also very expensive; usually that is.

We can get to two router motors and two drills pretty cheaply with the tipping tool changer concept. More details here 

The neat thing about an RSJ is that it is not only cheap but the ‘I’ section enables us to put drive gears and the drive rack neatly inside the web.  more details on drives here:
Torsional rigidity is not a strong point but is easily enhanced by welding a heavy tube inside one of the webs. Steel can pick up vibration so filling the tube with heavy chain and oil will make a cheap but effective damper. You only get really smooth finishes on a machine that is well damped.
Z axis: You could buy an off the shelf Z slide and hang that off the X axis for a conventional and simple layout but the logic of that is debatable. When machining flat boards etc there is very little Z movement so it would be better to put the Z under the table so that the two major movements – X and Y – are as light as possible. The Z frame (the whole bed) is raised like a garage car lift with 4 chassis mounted screw posts rotating to drive ball nuts on the frame. Unlike the continuous chain of the garage hoist a short timing belt for each screw connects to one of a pair of drive spindles which are both driven by a central belt and the stepper motor. This way we get reduced belt stretch and a built in reduction drive. This layout enables a much bigger Z travel – 3D modelers note – and makes a fast and responsive machine with a tidy uncluttered look. Air balancing means there would be no heavy lifting for the Z stepper motor.

This unusual configuration has a lot of advantages. The sketch above doesn’t show the tractor drives or the box slides (one for the tool head and 2 to hold up the X-axis beam) which are described in detail here.
The rear mounted drive shaft drives the two differential caterpillar drives that power the Y-axis.

Panels: MDF – easy to cut, cheap, noise suppressing and remarkably robust when painted with Hammerite.

Snags?
Weight: Chassis needs heavy welding skills and reasonable accuracy.
Z-axis: Quite elaborate 4 pillar lift mechanism with guides.

The advantages.
Size: Scaleable to industrial size. Pictured as 2m x 4m here.
Visibility: Whole work table visible without moving obstructions like gantry legs.
Safety: Work table has no sideways movement. No pinning accidents.
Space saving: No sideways movement of table so roughly half the footprint.
Convenient: Router motors present themselves right to the edge of the table.
Accurate: Very solid construction gives accuracy and good machining finish.
Neat: Hollow beams accommodate wires, lights and control panels.
Responsive: Weight of Z-axis is removed from the X and Y axes.
Large Z-axis: Big range possible – good for carving.
Short Y-axis: Single stepper motor drives both ends of X-axis beam so no crabbing.

…. and isn’t it the neatest CNC router ever?

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