Drilling: very basic question

Here is one on accurately locating holes. It is a two part video. Maybe it is this one.

RandyM, thanks very much for that. I’m away from my computer just now, I’ll give it a look in a few hours.
 
I've been tinkering with a very small machine shop for several years, but I'm a perpetual newbie. My question is about what point angle I should use for my everyday drills.

OK, I've been in the habit of buying 135-degree split point for all my drills, mainly because they seem to be sturdy and resist walking during the first moment of entry. Most of the time they perform beautifully. Actually, all the time, really.

My question is about those times when I need to drill a small hole in a very controlled, precise location. I have a set of combined drill/countersink to locate a tiny hole into the material, and then follow with the actual drill bit. I should have thought of this a LONG time ago, but I just realized that the cutting angle is LESS than my 135-degree drill bits... I'm not sure of the angle, but they look like about 60 degrees. So when I press the drill bit into the hole made by my combined drill/countersink, my drill bit is not going to immediately hit bottom, right?

If I'm going to use 135-degree split point drills, what should I use to make my initial hole? Whatever I use needs to make a larger degree-angle than 135 degrees, right? I know there is such a thing as spotting drills, but they look just like normal drills... if I'm going to use those, what's the difference between (1) making a pilot hole with a spotting drill and following up with a normal drill, and (2) just making the entire drill hole with the spotting drill? I mean, if the spotting drill is so superior with making the original hole, then why not just finish the hole with it?

What am I missing? You can tell I'm confused....

I too use 135deg. split point cobalt drills for everyday work and feel good about it.

When precise location is required I use a carbide scribe to mark an X then centre punch it with a quality punch, sometimes using a magnifying glass to see well enough. This is usually followed with a short 135 deg. split point drill for less flex to start the hole then switch to a jobbers length drill if the short drill can't do the full depth. I reserve my centre drills for drilling centers. Unless there is a special job to do I like to stay with 135 deg. split points for everyday work. So far I have not felt the need to complicate things more than that.
 
I’m still not clear on the difference between spotting drills and center drills, though. Can you explain please?

Center drills are most commonly used for making holes for dead or live center work in the lathe. However a lot of people use them to make starter holes for drills when they first start out so they don't have to buy 2 sets.

here you can see the difference. Combined = center drill when the angle is 60°
Center_drill.png
 
To me the first question would be what do you consider a precise location +- 0.01",0.005",0.001"? The other factor is what sized drill are we talking about, as that also plays a part?
Dan-S, good question; my mistake for not being clear. Normally, my idea of a small hole is 0.125” or smaller. Sometimes as small as 0.060” or so. As far as placement accuracy, I guess I’ll say within 0.002” or 0.003”.
 
Center drills are most commonly used for making holes for dead or live center work in the lathe. However a lot of people use them to make starter holes for drills when they first start out so they don't have to buy 2 sets.

here you can see the difference. Combined = center drill when the angle is 60°
View attachment 247545
Dan-S, thank you! Nothing like pictures. The “combined” drills are the ones I’ve been using. They were the ones that gave me my original question since the included angle (60) is less than that of my drills (135).
 
Dan-S, good question; my mistake for not being clear. Normally, my idea of a small hole is 0.125” or smaller. Sometimes as small as 0.060” or so. As far as placement accuracy, I guess I’ll say within 0.002” or 0.003”.

.002" is getting into the iffy range. If you watch the Mr. Pete video posted above, he only got 2 holes out of 9 within 0.002". I bet if he examined both x and y position none of them would have been within 0.002"

I was taught if you need accurately positioned holes, you drill and then bore. If you need accurately positioned and sized, you drill bore and then ream (taking into account the size tolerance).
 
Mikey, thanks for the clear explanation of a spotting drill’s inability to clear chips. I’m not really willing to give up my 135-degree drills for 118’s, so it sounds like I’d better invest in some spotting drills.
I’m still not clear on the difference between spotting drills and center drills, though. Can you explain please?

A center drill has a small tip, followed by a 60 degree cutting shoulder. The hole it creates has a 60 degree included angle, right? A spotting drill has a 90/120/140/whatever degree tip only; it cuts the included angle at the tip from the get go. Both types of drills have their purpose.

A center drill is intended to create a 60 degree cone into which a dead or live center will fit so center drills are intended for use in the lathe. The fact that this drill is stiff and is easily used to precisely locate holes leads to it's use for most drilling - on the mill, drill press or in hand drills. The tip will slip into a center-punched hole readily. Quite often, we punch a location, let the tip of the center drill move the work piece around until it stops moving, then we clamp the work piece down and follow up with a drill. This works so well that I would guess most of us use a center drill this way.

A spotting drill is actually supposed to be used to start a hole. It cuts a divot that allows the very tip of a drill to center precisely. The very tip of a drill does not cut; it simply spins. The angle of the hole the spotter makes is wider than the angle at the tip of your drill so the tip locates in the center of the divot. As you apply pressure, the cutting edges adjacent to the tip begin to smoothly engage and the drill begins to cut gradually, without moving around. This allows a drill to drill more accurately and cleanly. In fact, if you use a spotter and follow with the main drill, it will drill more accurate, precise holes.

Of course, sometimes you must use a pilot drill and follow with a larger drill and when you do, you will see what happens when the flutes contact first instead of the very tip; the drill will tend to bounce around and the resulting hole will not be as accurate. This is what a spotter is intended to prevent.

If all you need is to locate a hole you can center punch it and use the tip of the spotter to locate you. With precision holes, though, I find the tip of a center drill is easier to use because I can see the tip. I prefer to use a center drill to come down into the intersection of a marked cross as opposed to using a punched hole because, for me, it is more accurate. I then follow this with a spotter so my drill cuts clean. When the hole position is not critical, I use a prick punch followed by a sharp center punch and allow the spotter or center drill to move the work piece around until it centers.

Hope that helps.
 
Like Mikey, I also like to put "crosshairs" when doing accurate hole positioning. If the drill starts to wander you can see it right away while
you can still do something about it like moving the punch mark before you get too deep...for those holes that "absolutely have to be right"
Mark S.
 
I've also been known to "correct" an off-center pilot hole with a rat-tail file before enlarging the hole with the final-size drill. Better to do a hail Mary save than ruin the part.. of course this was in the "before time" (before I had a milling machine, that is)
Mark
 
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