Newbie to machining here. I need to drill a precise hole 7/32" in 6061 aluminum. Will be using my mini mill. The dept of the hole will be about 2.8". Any suggestions on what drill size I should start with before the reamer? And what mill RPM should I use for both the drill and the reamer?
Hi,
First make sure that the mill z axis is perpendicular to the table over at least 4" of travel other wise the hole will drift in xy direction.
Then make sure that the work piece is held true to the table.
May I ask what is the purpose of this hole as you are attempting to drill a hole 12.8 times the diameter of your drill, it is not impossible but is difficult to drill a precise hole on hobby machines. You have also specified the size as 7/32", this indicates that a tolerance of 0.005" is acceptable, is this so or you need to hold it tighter?
Ordinarily I'd suggest that you use a coated stubb drill for maximum rigidity but you will not get the depth with a short stubb drill, the choice is then a carbide drill as it is 3 times stiffer than an HSS one. As has been mentioned try not using a cheap import drill, buy your self a couple of decent ones such as Dormer or similar, these do make a difference.
I would not use a chuck, if you have collets then use the collets as the run out is significantly less than a chuck.
Do not plunge drill , the drill is sure to wonder and you will not get a straight and true hole, intead peck drill, touch and lift very gently and this will take time to do . If you have a vacuum cleaner use it to suck up the swarf all the way out of the hole as this is particularly a problem with Aluminium, the metal has a tendency to fuse itself to the cutting tip and you will for sure drill over size and oval. Use wd40 as a lubricant and squirt it in the hole. This will bring the swarf up for the vacuum all the time.
Once you have done all this then you can think about reaming.
Best of luck,
A.G