seXY – computer-controlled mechanism for a new clock

s e X Y

Computer-controlled mechanism for a new clock

seXY is a motorized mechanism that under computer control can move a carriage slide in two separate axes: up to 380mm (15″) in the X-axis (side to side) and 350mm (13.5″) in the Y-axis (front to back). A huge “design space” to play with! My intention is to mount a head to the slider equipped with a servo that engages either a pen or an eraser… More to come on that front!

In the following video, the left and right steppers are being controlled by the micro-controller which is just moving the slide to and fro the X and Y axis, and the four diagonals.

seXY: Stepper-driven X-Y mechanism

So, this weekend CNC project was largely inspired by an interesting product called AxiDraw,  that is described as “the personal writing and drawing machine that mixes the precision of robotics with the warmth of a hand-drawn note.”  I was primarily fascinated by the control belt arrangement that allowed the two steppers to be stationary. This avoids all the complications of creating wiring harnesses that have to flex with the machine movements.

It appeared from the video and other similar designs that the two steppers are fixed at either end of the x-axis and that axis movement is achieved by moving BOTH. This belt arrangement has an interesting effect. When both steppers turn in the same direction, the carriage slide moves along the x-axis. When the steppers move in opposite directions, the carriage slide moves along the y-axis. If either stepper is stopped, the carriage slide moves along a diagonal.

Neat, huh.

So I decided to design and build one.

Design Details

The design revolves around two pairs of 400mm long steel rails – 10mm diameter for the X-axis (as it is carrying the weight of the Y-axis mechanism) and 8mm for the Y-axis. A central carriage block consists of a pair of plates that mount the rail bearings and a set of 4 GT2 idler pulleys.

The x-axis end cheeks hold the X-axis rails and provides mounts for the Nema17 stepper motors. Small set screws in the end cheeks bear down on the rods to hold them fast. Similarly, a pair of end cheeks hold the y-axis rods; one also holds a free-running 20-tooth drive pulley while the other allows the GT2 timing belt to be terminated. Simply put, the belt starts at one of the Y-axis cheeks and runs around the periphery of a “+” sign (think of the carriage assembly at the centre of the +) and terminates at the same y-axis cheek.

I think that the following pictures will show and explain the salient design ideas…

seXY: showing details of the central carriage that contains the rail bearings

seXY: driven to the upper limit of the slide Y direction

seXY: closeup of the stepper motor mount and drive pulley at each end of the x-axis

seXY: underside view of the central carriage and the x-axis rail bearing blocks

seXY: underside view of the stepper motor mount

seXY: belt termination at one of the y-axis end cheeks

seXY: closeup of the y-axis end cheek and free running drive pulley

seXY:closeup of the central carriage showing the idler belt pulleys.

I designed all parts using SketchUp with CAM functions using SketchUCam and cut them on my own CNC machine. All parts were machined from 12mm HDPE with the exception of the upper carriage bearing block which was machined using 6mm HDPE. All hardware was M3, M4 and M5 Allen-head machine bolts.

Additions to come soon will include magnetic homing for both axis and a pen/eraser servo plate to bolt to the slider.

Standby while I power it up and run the steppers!

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