>Subject: Re: gnomon shape and EoT >Sent: 17/9/98 7:09 am >Received: 17/9/98 9:06 am >From: John Pickard, [EMAIL PROTECTED] >To: David Higgon, [EMAIL PROTECTED] > all, [email protected] > Dear John, David and Alberto,
There's a very well illustrated and explained section in Frank Cousins' book "Sundials" on correction for the equation of time showing the Oliver dial (similar to Silas Higgon's) the Negretti & Zambra version with a pierced analemma aperture and the very sophistocated Sunquest Local Meantime Dial by Schmoyer. > And in any event, the gears on a rack >include BOTH the teeth AND the spaces. Anyone got any suggestions on how to >make such a rack? > >In case you can't visualise it, imagine a U-shape with a semi-circle forming >the base of the U, and two parallel arms coming out. What you need is a >properly formed set of teeth on the inside. I guess you use a shaper, but I >only have access to a mill. A similar process to cutting an internal gear if I visualise the problem correctly. As you suggest a basic milling machine is useless in this situation although the Bridgeport has an attachment to deal with it. A shaper is the obvious choice for a 'one-off' using three stages. A parallel 'gashing' cut to remove most of the tooth space followed by independent profile cuts. The interesting bit would be where the teeth on the semi-circular section transformed into the straight arms as there would be two distinct tooth-space profiles - an internal involute on the curve becoming a straight-sided rack tooth on the arms which would require some careful pre-planning. Some years ago I made a model of Matthew Murray's Hypocycloidal Engine with the internal gear ring shaped as above. Not a job to be undertaken lightly but worth it just to see the amazement on the faces of anyone watching it directly convert straight line motion into rotary motion. Tony Moss.
