[kentavv]

Perhaps someone will be able to offer an opinion on the following. I hope the picture is clear enough... This is a corner of a 3.04x3.04x.25" Al plate. An 8x8 grid of 1/8" holes have been drilled (first with a 1/8" spotter and with 1/8" jobber on a .1" peck cycle). After the holes are drilled an 1/8" 2 flute end mill is used to clean out material .125 deep with .0625 z steps. The walls of the tubes should be .04" thick. As you can hopefully see the holes are eccentric to the tubes. I have repeated this process get a consistent error. The outer diameter of the tube is pretty good at .205 +/- .001 so the arcs seem to be cut fine. Measuring the tube wall is between .030 and .44 (should be .040) The inner diameter of the tube is roughly .006 over sized. This actually accounts for quite a bit of the error with a .004 lateral shift remaining. So the bit has wondered to the left on each hole. This is also easy to see by restarting the program and re-drill the holes with the end mill. The end mill shaves .006 off the part of the and the wall thickness where once was too large is now fine (the other side is still too thin though).

Should I be drilling these holes somewhat differently to avoid the deflection of the bit and subsequent hole enlarging? I don't know if I could reasonably use a centering drilling 1/8 end mill to drill out all these .25 deep holes without breaking it. What do you think? I wonder if a drill mill such the following might help?

http://www.use-enco.com/CGI/INSRIT?P...PARTPG=INLMK32

Thanks