Retrofitting The Graves MK IV Faceter..
The Graves is unusual in that it appears to have been designed "with a clean sheet of paper" to be manufactured in large quantities, using mass production methods such as permanent-mold castings. Being a more "finished" unit, there is less flexibility for field modification. A few simple changes in design detail would have made this model simpler to retrofit, but it turned out there was a simple way to accomplish the digital retrofit after all!. To add to the challenge, this unit is left-handed.
It was only neccesary to make the usual mounting bracket for the potentiometer, and a simple bent wire, 0.092 thick, of stainless steel to mechanically couple the potentiometer to the angle axis. Because of the packaging design of the machine, there was not sufficient room to mount the readout box on the head itself, so I built a separate box which the user can mount on the base, the wall behind the machine, or to the mast, with some simple attachment method like sticky-backed Velcro.
The box is wired as with the other units. The box can be painted white to match the machine.
Here are some other views of the linkage details. The post which is drilled and tapped to the moving part of the angle assembly is placed so it does not interfere with motion at zero degrees through 180 degrees. There is only one place on the assembly where it is possible to drill and tap the hole for the post. As you move the machine through these angles, use a china marker to locate the clear area.
Rather than use a clamp on the potentiometer shaft, we have drilled a 0.092 hole through the shaft. The coupling rod tightly fits this bore with no play or lash. The post which is mounted to the moving section of the angle axis is likewise drilled with a 0.092" hole.
We have assembled the potentiometer so its electrical and mechanical zero at the the machine's Zero Position (Table polishing position)
, so now all we have to do is turn it on, and setting the angle exactly at 90 degrees with an accurate protractor or square (Never the one on the machine), turn the trimpot until it reads exactly 90 degrees.
Be sure to check several other positions such as 45 and 60o with an accurate triangle. Now, if you intend to keep the original protractor, set it to read the same as the accurate squares and triangles.
For a very accurate calibration method, Click Here.
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