Thanks to Robin S. for answering my question. I'll have to review the various drafting symbols.
The jig is designed in chapter 9. It serves to drill two little holes in the annular plate. After that, the plate is placed in a vise-clamp fixture (designed in the chapter 10) for straddle milling. It is recommended that the part be located in the same way as in chapter 9, in order to maintain control of tolerances. In the preceding chapter, the primary locator was a pin in the big central hole and the secondary locator was a pin in the first of the two small holes to be drilled. The second little hole to be drilled is located relative to the center of the first little hole.
Since the second little hole doesn't necessarily satisfy the tolerances relative to the center of the big hole that the first little hole is required to satisfy, I think that when it comes to the next stage of straddle milling the butt plate, it might be important to keep track of which hole is the first little hole and which is the second little hole. The book doesn't comment on that. One could argue that the three centers are on a line which is parallel to the path that the milling cutters will follow, but I think that the uncertainty in locating the second hole allows the possibility that the three centers are not necessarily collinear. So the choice of which hole one uses could affect the direction of the milling.
Does one simply use a magic marker or something to mark the first hole before putting the part in the bin to be sent to the worker who is going to do the straddle milling (possibly the same worker a week later, after he's had time to forget)? Or is there some other convention?
I suppose that if one wants to be really picky, it might also matter in some way which of the two circular faces of the part is going to be the one that will be subjected to straddle milling. For example, maybe the tolerances on the hole center or diameter or shape that hold on the side where the drill enters the part don't necessarily hold on the other side where the drill emerges.
Ignorantly, Allan Adler snipped-for-privacy@zurich.ai.mit.edu
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