It's bent!
Above: before any straightening work. The platter is bent so far out of shape it rubs against the cast chassis instead of spinning freely and true as it should.
Straightening
Above: After initial efforts, the upper surface of the platter shows considerably less bow when compared against the straight edge of a machinists rule.
Above: Another view.
Above: and yet another view.
The outer platter shell is made from a soft aluminum alloy that is very easy to deform. My plan was to make inner and outer forms to press the platter against in order to restore it to its original shape. First, I made a flat disc out of mdf that fit within the inner dimensions of the platter. Next, I situated the platter upside down against a very flat and rigid surface, placing the mdf form within the inner dimensions of the platter. Following that I used a variety of clamps to press the outer platter between these two surfaces. To a large degree this helped to reduce the warping. But the part was still not straight enough. The shell did not spin true enough to function with the clutch of the TD124. Further straightening is carried out below.
The original damage to the shell caused the outside rim of it to wrinkle. I found that working the outside rim back into its original form was the key to restoring the piece back into useable condition.
1/30/07
Above: A thick heavy tempered glass plate is used in combination with a machinists square to evaluate the side surface of the outer shell for perpendicularity (square) to its upper surface. Notice how the shell's upper surface is laid face-down flat against the glass.
Above: In this view a small gap can be seen between the edge of the square and the side rim of the shell near the glass.
Above: In this next view the side of the shell shows truer form against the edge of the machinists square. When the work was done it was like this all around.
Above: Shown here is the method of straightening. Soft wood shapes are used to gently coerce the untrue areas back into form.
Above: Notice the space between the bottom of the outer shell rim and the clutch face.
The design intent is for the clutch to be used to raise the outer shell up off of or dropped down onto the still spinning inner iron platter each time a record is changed. This saves stop-and-go wear-and-tear on the rubber idler wheel that drives the inner rim of the iron platter. When the photos are viewed closely it should be seen that the height difference between "parked and spinning" is very slight. This requires that the outer shell run true enough to operate within some rather "tight" dimensional constraints. After some careful work the outer shell is now "functional" and I pronounce it ready to spin vinyl. But first I'll use some polishing compound to rub out some of the light markings I caused with my straightening work.
comments: This outer shell material is so soft that it will easily be bent with your hands. If, when handling this piece, you bump it into something hard it will likely become bent out of shape. Caution is in order. Fortunately, should you have the misfortune of damaging one of these, it can be straightened......but with care!
Straightening an outer-platter shell for another TD124 owner
notes about the manufacturing process:
The outer platter shell was manufactured by a process called "metal spinning". In which: A flat piece of ductile (soft) sheet metal is held by fixture onto the mandrel of a "spinning lathe" and "displaced", by the careful application of pressure, using a variety of purpose built tooling, while spun, to deform the once flat sheet into a variety of circular or cylindrical shapes.
link to more information on metal spinning:
http://www.franjometal.com/metal-spinning/hand-metal-spinning.html