Finishing the Bottom plate

Now that all the round features are finished, I brought it back to the mill. Because we are drilling through the plate, mounting in the vice is not an option because we would drill into the vice. So, I mounted it directly to the table on top of parallels. I snugged the clamps down ran a dial indicator back and forth along the edge. Watching the indicator, I tapped the corners with a dead-blow hammer until the indicator did not move when swept across the edge.



To properly locate the holes, I used the dial indicator to find the center of the large hole. Placing the indicator against the inner surface of the hole, I rotated the quill and noted the how much the readings varried both in the X and Y axes. I then moved the table so that when the indicator was rotated within the hole, the reading was the same all around. The quill was now perfectly assigned with the center of the hole so I zeroed the X and Y axes on the digital read out (DRO).



The print gave polar (radius from center/angle) locations for the holes. I could have used a rotary table to place the holes but our rotary table is HEAVY so I avoid using it when I can. Instead I simply used the magic of trigonometry to find the X,Y coordinates of the holes from the center of the plate.



I realized that I had mounted the plate to close to the edge of the table so I wasn't able to drill the far corners. Also, one of the holes called for a tapered pipe thread and that had to be done from the other side of the plate. So I flipped the plate over, squared it and indicated the center.

I decided to tap the large center hole by hand but since it was already mounted, I used the machine to help me out. With the quill centered on the hole, I placed a spring center in the chuck and lowered it onto the tap before running my threads.



I started the pipe thread with the mill but I didn't tap deep enough. It would have been easy to destroy the threads, especially in the soft aluminum so I finished it off by hand. I used a brass plug to check my depth and stopped when there were about three threads still visible.





I finished my last two corner holes and cleaned it up. Our first part is done! Only 30 more to go!