Dave Bell contributed" >I had been thinking about John Lynes' suggestion for using a laser pointer >to "copy" rays from a horizontal dial (or any other type, for that matter) >through a sill-mounted mirror to a ceiling dial. I'm attaching a (small!) >sketch of the sort of 3D pantograph support it would take. I'd appreciate >any suggestions from the machinists/mechanists out there, on making it >workable!
One practical caveat on the subject of laser pointers. The 'diode laser' these devices contain is unlikely to be mounted with any useful degree of precision as they are only intended as a hand-held pointer. Consequently the axis of the laser beam may well not align with the axis of the metal tube/case in which it is mounted. You can test this by placing the pointer in a vee-shaped trough (an engineers' vee block if you have one) and rotating it through 360° while holding it lightly in contact with the vee. If the spot projected on a distant wall remains in the same location you are lucky and alignment is spot on but if it describes a circle you can calculate its divergence from truly coaxial by noting the diameter of the circle and the distance of the wall. One simple corrective method used in the precision laser trigon I made for the 20 metre dia, Silverlink dial, where the dial 'plate' is a double cone, was to note the highest/lowest points of the inaccuracy circle and mount the device so that these lay in the plane of rotation of the trigon i.e. perpendicular to its pivot. In the final version of the laser trigon the pointer case was held in a vee groove milled/ground along a 60mm length of 16mm square steel block by a u-bolt which allowed rotary adjustment as above. The perpendicular bearing was again a vee groove milled/ground at right angles across the base of the steel block. A light leaf spring held this in contact with a ground cylindrical bar to provide a precision 'swinging laser' with absolutely no play in the joint. JPEG of the final device to anyone who contacts me directly. ************************ I love the lights of Paris I love the lights of Rome. But the finest of all are the tail lights Of the cars taking grandchildren back home. ************************ Just joking. Tony Moss
