DIY industrial CNC router

CNC heavy chassis

CNC heavy chassis

Just think; if you had a CNC router you could start on the Original Twist 3-wheeler, make some furniture, maybe even earn a good living with contract work.
Cheap computers and software have brought CNC machining within range of the DIY brigade and Youtube reveals some totally heroic machines made in plastic and MDF with frames in off-the-shelf extruded aluminium beams. Time to join a mass of DIY enthusiasts and knock one up yourself?
I’d like to continue in this spirit but head towards something much bigger and heavier without increasing the cost to anywhere as much as that of a professionally made router.
Size: In order to machine round the outside of an 8’x4’ sheet, or bigger, we’ll need a long and strong X-axis. A heavy steel RSJ should 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. We can get to two router motors and two drills pretty cheaply with the tipping tool holder concept. More details here https://originaltwist.com/2015/01/31/cnc-router-slide-box-and-tool-changer/

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:  https://originaltwist.com/2015/01/31/cnc-router-caterpillar-drives/
Torsional rigidity is not a strong point but is easily enhanced by welding a heavy tube or two inside one of the webs. Steel can pick up vibration so filling a 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 Y table so that the two major movements – X and Y – are as light as possible. The Z frame 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 rough sketch doesn’t show the tractor drives or the box slides; one for the tool head and 2 to hold up the X-axis beam; they are described in detail later.
The rear mounted drive shaft drives the two differential caterpillar drives that power the Y-axis.

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 minor 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.

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

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