dtut wrote:Look at how the arm is built. Look at the geometry. There are four pivots, two unequal length arms, and a "head shell" that is part of the mechanics creating the geometry. If the stylus transcribes an arc across the record, it is very shallow or complex, or both and the "pivot point" will be a geometric product and well behind the support - in other words, a very long effective length, but if this is in fact a tangent arm, concepts like "under hang, over hang, offset," and even, to some extent, "pivot point" are moot.
The OP can establish this for himself by cutting a strip from an old calendar page so that there is a long line down the center of the strip with perpendicular lines at even spacings and putting a hole at one end centered on the long line which will fit over the spindle. Then he can put the stylus down on the long line at one of the cross lines at any point on the record and temporarily place the support so that the stylus/cart is tangent and perpendicular. Once that's done, he can move the stylus/cart to any other point - here's the tricky part - the strip may have be rotated to meet the stylus/cart, but the stylus/cart should still be tangent and perpendicular. I think I would start with the arms as parallel as possible, the cart carrier directly in front of the support, and the stylus in the middle of the record. Misalignment can be dealt with by careful repositioning or twisting of the support.
This is actually pretty easy and very understandable with pictures, but I've never figured out how to post pictures on VE.
I also think it might be worthwhile for the OP to ask his question on the "Angling for 90 degrees" thread on DIYaudio Analogue where the original pictures came from.
Well said! I, too, think an empirical approach is the most straightforward. I was able to get a copy of the original B-J patent and the patent drawing:
I then traced the B-J profile and scaled it best I could as a cad drawing on my computer. Having done that, I then manipulated it into 3 positions on the record surface:
The base of the overall arm does not rotate, but only pivots up and down. It's the 2 arms that pivot horizontally. The concepts of overhang and underhang really don't apply to an articulated arm like this. It DOES sort of have an offset angle, but it changes as it moves across the record so as to maintain tangency. It would be very interesting to see how accurately it can do that. The sight lines are perpendicular to the headshell at my best guess as to where the stylus would be in the headshell. The lines all pass pretty close through the spindle center, which is an indication of tangency. The patent defines 6 mathematical equations to describe the geometry of the arm movement, but I haven't tackled those yet.
Troglodyto: If you could precisely measure the distances between the 4 pivot points on the arm and the distance of the stylus from the front pivot points behind the headshell, I may be able to make a model accurate enough to plot on paper as a full size template.