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There are a couple of articles on the Trott double arms showing 4 different designs. The Trott Type 4 has much smaller errors 2" over 2 hrs. The errors in Trott's article are based on starting with the arms parallel and pushing apart. If you space the boards and start with a negative angle, the accurate tracking time is doubled.
The tolerances for certain components are very important. Most important, the pivoting nut and drive thread must both pass at right angles through the pivoting axis. This could be given to a commercial machining company to make for you. Other dimensions are less critical eg hinge spacings, if a little less than perfect, can be compensated by a variable motor speed controller which as already mentioned needs to be accurate.
Polar alignment is especially important because you can't make dec corrections for this type of drive without significant modifications. A good quality polar finder scope would be essential.
Joe
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