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?
Suggestion on reaming 7/32" hole?
- Thread starter mfarris
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Here's what I would do...
1) Lock down x/y in the desired location.
2) Use a center drill to start the hole true.
3) Drill a hole 1/64 less than the desired size. ie- 13/64. Check that the drill you are using is in good shape. I've known drills to go oversize to the extent that reamer has nothing left to ream - that's not good. Pick an RPM that sounds/feel good and has nice chips. Maybe 1000 rpm in Al. Since this hole is around 1/4" there's no need to step drill.
4) Ream the hole to 7/32" with cutting fluid. Do it relatively slow. Maybe 300 rpm.
Also - 2.8" - is that the depth at the center of the hole, or at the hole side? If it's a blind hole there'll be depth variance at the bottom because of the shape of the drill bit. Make sure that's not going to be an issue.
Regards,
Jason H.
1) Lock down x/y in the desired location.
2) Use a center drill to start the hole true.
3) Drill a hole 1/64 less than the desired size. ie- 13/64. Check that the drill you are using is in good shape. I've known drills to go oversize to the extent that reamer has nothing left to ream - that's not good. Pick an RPM that sounds/feel good and has nice chips. Maybe 1000 rpm in Al. Since this hole is around 1/4" there's no need to step drill.
4) Ream the hole to 7/32" with cutting fluid. Do it relatively slow. Maybe 300 rpm.
Also - 2.8" - is that the depth at the center of the hole, or at the hole side? If it's a blind hole there'll be depth variance at the bottom because of the shape of the drill bit. Make sure that's not going to be an issue.
Regards,
Jason H.
Watch the end of the drill when you turn it on. If it doesn't run really true, but instead orbits a bit at the tip, then go .030" under the 7/32" reamer. I have a cheap Chinese drill set, and there is enough wobble in them that I have fooled myself on these reamed holes. If the drill doesn't run perfectly true, you won't have any material left to ream. A reamer cam easily work in aluminum on a 0.030 undersize hole.
You can prick punch the intersection of the scribed lines and then use the wiggler in the spindle to line up with the desired location. With good technique you can probably hit the location within +- 5 thousands of an inch in the x/y directions. If you want to do better than that then you could edge find the material and then crank in to the desired location.
It depends on how critical the hole position is. If it isn't critical, sharpen a steel dowel to a point in your lathe, put it in the chuck on your mill, and eyeball the location in respect to the centerlines. Other than that, as JasonH says, use the edgefinder and dials.
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
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