Drilled A Tapered Hole. But Did Not Mean Too.

[Chipper5783]

Drilling is a roughing operation. It sounds like to took reasonable steps to drill a good quality hole. The suggestions given here are all good. In this activity (machining), you'll spend a lifetime chasing precision - trying to improve. I used to think drilling a hole was easy (just ask me and I'd tell you all about it). Now nearly 40 years later it sometimes feels like I know less than I did back then. The more I learn, the more I discover that there is a lot more that I don't know.

Drill a hole seems like such a simple task. Maybe not so - there is a lot that can give you a less than perfect result. That you recognize, even after making a good effort, the resulting hole is not that great - I say congratulations. You are on the right track of learning to do better.

 

[planeflyer21]

Something else that was shown to me, the lettering stamped into the shank of a drill bit can adversely affect your hole. At one shop we stoned off the stamps when we needed more accurate holes, as well as holding the bits in a collet rather than a chuck.

 

[brino]

the lettering stamped into the shank of a drill bit can adversely affect your hole. At one shop we stoned off the stamps when we needed more accurate holes

I always wondered about that whenever I could feel the stamping. I figured it must contribute to an error, however slight.
I never ran any tests though as I've never considered drilling a particularly accurate operation.
Also, I am not sure that I could tell with my (older, somewhat worn) equipment.

-brino

 

[Bob Korves]

I was drilling some aluminum today in the mill (g0704) and spotted all my holes with a #3 center drill. Drilled 10 at 3/8 and one at 1/2 inch. All through a piece of .600 inch stock. The half inch hole is .505 at the bottom and .520 at the top. All of the 3/8 holes were tapered as well. I did not run the bit much past the stock. Ran at 1500 rpm and had good ribbons coming off.

What went wrong? I don't recall ever having a hole come out tapered like this. Any advice to prevent this from happening. Yes I should ream to get an accurate hole but I was not expecting a taper. Also used a brand new set of hss bits. Any advice is welcome.

Using a center drill might be part of the problem. It is 60 degrees and the drill angle is larger. The means that the cutting edges intersect the 60 degree pilot hole at some distance out along the cutting edges. That can lead to the drill walking around the pilot hole until it gets established in the hole. That could cause your taper, as well as messing up the hole location. You used the center drill to center the hole, but the drill walking can make the drill not follow the pilot hole. A pilot/spotting drill with a larger angle than the drill cutting edges, and only slightly larger than the drill chisel point gives better results in my experience. If you use split point drills you can eliminate pilot holes altogether on nice flat 90 degree surfaces. Stiffer drills, like screw machine drills, are stiffer and help keep the holes to size and parallel walled, and often eliminate the need for starter drills. They also often have less runout because of their length and do not require as much moving the Z axis up and down for changing out tools for a lot of work.

 

[RJSakowski]

I just noticed a post from several years back in the "similar threads" footer.
http://www.hobby-machinist.com/threads/ring-at-the-bottom-of-drilled-hole.17803/

 

[modela]

Nice to see such a good topic on hole drilling.

My brother works a lot with aluminum. He uses WD-40 as a lubricant. It is probably not the problem, but worth a try.

 

[modela]

Using a center drill might be part of the problem. It is 60 degrees and the drill angle is larger. The means that the cutting edges intersect the 60 degree pilot hole at some distance out along the cutting edges. That can lead to the drill walking around the pilot hole until it gets established in the hole. That could cause your taper, as well as messing up the hole location. You used the center drill to center the hole, but the drill walking can make the drill not follow the pilot hole. A pilot/spotting drill with a larger angle than the drill cutting edges, and only slightly larger than the drill chisel point gives better results in my experience. If you use split point drills you can eliminate pilot holes altogether on nice flat 90 degree surfaces. Stiffer drills, like screw machine drills, are stiffer and help keep the holes to size and parallel walled, and often eliminate the need for starter drills. They also often have less runout because of their length and do not require as much moving the Z axis up and down for changing out tools for a lot of work.

I am impressed, but you realize that you just shot down one of my old "pilot hole" beliefs and gave me something else to worry about. I am definitely going to copy these and put it in my troubleshooting file.

Being an old mathematics guy made me appreciative of Charlie Brown's comment. "I prefer subjects that are a matter of opinion."

Jim

 

[modela]

Something else that was shown to me, the lettering stamped into the shank of a drill bit can adversely affect your hole. At one shop we stoned off the stamps when we needed more accurate holes, as well as holding the bits in a collet rather than a chuck.

I do this because it is easier than changing to a chuck. I will be aware of the stamps.

 

[joshua43214]

I am impressed, but you realize that you just shot down one of my old "pilot hole" beliefs and gave me something else to worry about. I am definitely going to copy these and put it in my troubleshooting file.

Being an old mathematics guy made me appreciative of Charlie Brown's comment. "I prefer subjects that are a matter of opinion."

Jim

I took so many math classes in college that I wound up with a second major. I like math because it gives me the illusion that some of my problems have solutions...

 

[Bob Korves]

I am impressed, but you realize that you just shot down one of my old "pilot hole" beliefs and gave me something else to worry about. I am definitely going to copy these and put it in my troubleshooting file.

Being an old mathematics guy made me appreciative of Charlie Brown's comment. "I prefer subjects that are a matter of opinion."

Jim

Jim, please note that the drilling ideas I posted are not universally accepted, by a long shot. If you go to "one of those other machinist forums" you will find a lot of different ideas that are all touted as the best thing since apple pie, and the arguments can get loud. Study the options, try some of them out, observe carefully what is happening when you are drilling, and see what seems to work best for you in different situations. I still drill holes that I swear at often enough to know I am no expert on the subject...