Friday, November 14, 2008

More Geek Stuff: Simple Motion Control

After my crude success with CAD drawing for the snake grabber, I have now tried something a little more ambitious. This is a simple way to make something move in a more or less linear controlled fashion using leaf springs. If you just use a simple leaf spring and attach something to the top of it, the spring bends over. The classic solution to this is to attach something to counter bend the top of the spring so that the top of the spring stays more or less parallel to its original position, but displaced as desired. The sketch below (excuse my beginner CAD abilities) shows a two leaf spring device with the leaf springs in RED and the top controlled portion in black with something coming half way down to counter bend the top so that it remains parallel.

The thing labeled wobble pin is a method for accomodating the fact that the pusher (usually a screw with a dimple in the end) will never be exactly perpendicular to the motion control device. By inserting the wobble pin between a dimple on the motion control bar and the dimple on the screw, the wobble pin accommodates in accuracies of construction that are always present. The location of the dimple on the bar should be half way down but it is best to empirically locate the best point since sometimes materials are inhomogeneous. By attaching a mirror temporarily to the top the image can be viewed while pushing. Find a point where the image doesn't deviate. When the CAD drawing was made the RED leaf springs should have been shown as extending over the top and bottom pieces for attachment.

A more sophisticated version of this can use three balls in a cylinder at each end of the wobble pin. The cones of the wobble pin rest against the three balls. It is easy to drill a hole in the control bar for the bearing balls. In a pinch disassemble a ball bearing, but it is easier to just order a box of loose precision bearing balls. A box of 100 1/8" bearing balls will cost around $7.00.

A still better version of this device uses four leaf springs with the second set displaced from the first set so as to form four upright springs in a rectangular bedpost configuration. The counter-bending bar then sits between the two sets.

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