Q: Drill diameter for reamed holes.

[jefilomeno]

I use a drill that is .0156"-.0312" smaller than the reamer.
The material being cut is also a large factor, as well a feeds and speeds.
Make sure that your center drill leaves a slightly larger chamfer than the reamer size.
In plain 1018 CRS steel I would use 300-400 rpm and a high feed .018" per. rev. for the larger drill size (.015") and a 4-6 flute carbide tip reamer, about 15-20 percent less feed for a HSS reamer. The higher feed insures a straighter, tighter hole size. Non-ferrous materials use abit faster RPM and about the same feed ratio.

[jefilomeno]

Also, I know this sounds crazy, but I have actually reamed a dead on hole size in non ferrous materials with a spiral flute bottom tap, in a rigid collet chuck feeding at a slow feed. The teeth on th tap act like a bunch of very precision cutting edges. Works great in aluminum. Nice finish to.

[HuFlungDung]

Originally Posted by jefilomeno Also, I know this sounds crazy, but I have actually reamed a dead on hole size in non ferrous materials with a spiral flute bottom tap, in a rigid collet chuck feeding at a slow feed. The teeth on th tap act like a bunch of very precision cutting edges. Works great in aluminum. Nice finish to.

And when you discovered this little tidbit, were you intending to ream the hole or thread the hole?

It does sound like a heck of an idea, better than using an endmill, because the upper body of a tap is not relieved, so it will burnish the hole, yet the contact is minimal, and coolant flow should be good.

Plus, the rake angle of the spiral flute will give a nice positive cutting action compared to the dumb scraping of a straight flute machine reamer.

[jefilomeno]

I was actually tapping a pretty deep hole, so I wasn't using the tapping cycle. The problem was getting coolant to the bottom and so I wrote a peck-tap cycle to reciprocate in and out but not comming out of the hole completely as to keep the threads timed correctly. Well as most of you know FEED HOLD dosen't work in a tapping cycle until the tap comes out. Not the case in just a standard G01. So, the tap stopped mid cut and just turned while inside the hole. The spiral is on a tight helix so it was like dozens of little boring bar edges polishing the hole to .250" As I investigated my screw-up, it came to end happily with a new found, as you said tidbit. And there you have it...

Originally Posted by HuFlungDung And when you discovered this little tidbit, were you intending to ream the hole or thread the hole? It does sound like a heck of an idea, better than using an endmill, because the upper body of a tap is not relieved, so it will burnish the hole, yet the contact is minimal, and coolant flow should be good. Plus, the rake angle of the spiral flute will give a nice positive cutting action compared to the dumb scraping of a straight flute machine reamer.