I am about to turn a recess in an aluminum plate which is mounted on a face plate. The plate is 1/4" thick. The proposed recess is 5.400" x
0.125". The diameter is critical, the depth is not as long as the floor of the recess is flat. I am not clear what is the best way to approach this.1) Drill a center hole (the middle 2" or so of the recess do not matter). Cut the recess using an AR tool from the middle stopping at the circumference of the recess. The tool path will be 5.400" minus whatever the hole diameter is. This way I can get a nice flat bottom of the recess. The problem I see is getting the diameter accurate. I usually like to "sneak" upon the final dimension which in this case is not possible (at least I do not see how).
Theoretically also the AR cutter might rub at the start of the cut on the small diameter of the starter hole. I wonder if one should do this with a boring bar.
2) Start with a boring bar, enlarging the hole to the required depth of 1/8" in small passes. I have a depth stop for the carriage so consistency should not be a problem. This will allow for frequent checks as one approaches the diameter, reducing the depth of cut etc. OTOH one might die of old age doing this.3) Combination of the two: Cut a smaller recess to the required depth as in (1) and then carefully enlarge it with a boring bar as in (2).
What would a real machinist do?
Michael Koblic, Campbell River, BC