Question about drilling holes

I drill as large as my drill press motor will cope without stalling.
I dont agree with being able to bring your hole back on centre with consecutively larger bits. There is just too much flex in drill bits to pull a hole back on centre once you have drilled it wrong. Yes you can get back to centre by doing various things, (such as George suggests with the cold chisel) but if you have put your pilot drill through and stepping up to 3/4 (final size) for example, if your not centre with your 1/2" or 5/8 hole then you are stuck with where you are. Drill bits also do not drill perfectly round holes, they are often eliptical or clover leafed so any of the previous induced errors will be amplified as you go up.
Drill your pilot hole and then drill the biggest that your press will cope with, but avoid doing it in uniform incremental steps.

Cheers Phil
 
I generally don't like to step up in drilling unless absolutely necessary. I find that the drill cuts pretty close if not right on size, and that causes binding on the sides of the flutes, which do not cut. This robs horsepower needed to cut. Also, especially on the lathe, if I drill through a part, and then step it up, coolant is free to exit the opposite end and makes a mess. Of course, this isn't much of a problem on the mill. As most of you know, the surface speed slows to zero at the drill's center, where the web is already pretty blunt and the angles aren't really prime for cutting. I think this is behind the thinking that a smaller drill making a pilot hole about the size of the next drill's web width give the advantage of pressure needed to push the larger drill through the work. That much is true. I would argue that as far as accuracy is concerned, if the hole is very deep, a small drill is more flexible and is prone to wander off position more readily. If accurate placement is important, then after the pilot drill is used, an end mill could be plunged right on location to correct some of that drift.

Most of the larger drills I use I will narrow or thin the web, and that gives me some relief on the pressure required, but I won't drill a pilot hole. I don't typically drill holes very large on the mill, but on the lathe, I will drill as large as I have the hp to handle. But I sometimes cheat a little if I am going to bore to finished diameter by deliberately grinding the drill off center just a bit. This of course does make the drill cut oversized, but that is by design, so that I don't get the drag from the normal wear seen on the sides of a drill. As drills get used, they are measurably smaller near the end, and this can cause problems with binding as you drill deep enough to get into the not-so-worn section.

If on the lathe I need to drill a close running hole, as in for a reamed hole, I will often drill undersize for a depth of about half the drill diameter, or enough to engage the full body diameter of the drill, and bore it just to fit the drill I am going to use. I think Tom may use a similar method when he drills the barrel of his Stevens Favorite to prep it for the gundrill. It's a common practice for that.
 
you guys have been talking about drilling a hole without stepping up in size so i tried it today in my lathe.
i used a 1/2" center drill then i followed with a 1" drill in aluminum to a depth of 2". it came out perfect with almost no play around the bit in the hole.
in shop class in school (when dirt was young) i was told to step up gradually
I got a much better hole no chatter or wander.
thanks steve
 
Moparfever said:
As far as I know there is a "right" way. You drill a pilot hole for accuracy. The pilot hole should be just big enough for the web of the final drill to fit in. It's this web area that causes wander. By not going any larger you allow the full width of the cutting edge to get a bite making an even cut. If you step up incrementaly you cause edge chipping on the bits, therefore increased wear.
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I agree. Drilling in very small steps only causes more wear on your drills.
 
And yet,the book put out by the Moore jig bore company says to drill in tiny increments to final size when putting in holes accurate to tenths. They start out with #80 drills and work their way up drill by drill. This may be harder on drills,but if you are making a die worth $100,000.00,drills are cheaper than a slightly off center hole that messes up the whole job. It depends upon the context.

The Moore book is "Precision Hole Location." That and the Connely book(sp?) which I also have an original copy of,but which is now in reprint(about machine rebuilding),make an excellent pair of books to have and to study as a basis for finding your own way along in shop problems.(runaway sentence!!)
 
Moparfever said:
Thanks for sharing that info George. I always wanted to read some of the Moore's books, but couldn't get myself to spend the money. Those books are like gold! Does it mention anything related to the notion that a small diameter drill has a greater tendency to wander? The truth may be counter-intuitive.
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Larger drills have greater tendency to wander because the web of the drill is so large. In the next issue of Home Shop Machinist, I discuss this very topic in my column. The gist is to start with a small drill and work your way up.

Sandro Di Filippo
 
The Moore book states that the punch mark should be larger than the drill to start with,hence the #80 drill.
 
In tool steel I step drill but not in small increments. in brass or plastic I never step drill, Found out the hard way,:nono: It will suck the chuck right out of the taper.:rofl:
 
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