Oh good! I'm glad to know that I didn't get the "wrong" center drills since my primary application is indeed starting holes on the lathe (or maybe in the mill). I guess I'm still a little confused about the purpose or point of the spotting drill, but I think this might be in definition alone. If I had to take a guess at what "spotting" a hole means, I'd imagine it meant to start the hole in the correct location. IF that's true, one could claim (maybe?) that a center drill is a specialized type of spotting drill since it is also used for starting the hole in the correct location, namely- the center of spinning stock. Am I on track here?
If all that's true, then I'd infer that a center drill is for starting a hole specifically on a lathe, and a spotting drill is for starting a hole everywhere else. Yes?
Wildo, Bob has given you accurate and very good advice. I just wanted to try to clarify some points of confusion. A center drill is intended to do what the name says - drill an accurate hole with the proper taper to fully seat a center, a dead or live center. Back in the day, dead centers were common and the tip of the center drill created a reservoir to hold lubricant; as the work piece heated up, the oil moved from this reservoir towards the taper to provide lubrication. In the USA, tailstock centers typically have a 60 degree taper and our center drills have a matching taper so they are actually the appropriate tool for only this purpose.
However, in addition to drilling holes for centers on a lathe, many guys use center drills as a general purpose hole locator. The tiny tip of the drill fits nicely into a center-punched hole and it begins to cut immediately; the taper it cuts tends to center the main drill that follows ... logical, right? The problem with this is what Bob pointed out. The taper created by a center drill allows the cutting edges of the flutes to contact first, not the tip of the drill, and this can damage the cutting edges of the drill. In soft materials this is not a major deal but it can be in hard materials or if you're using a carbide drill. You will definitely see this effect when using tiny carbide drills; they can snap when a cutting edge catches.
The ideal situation when drilling a hole is for the center of the drill to contact first and then gradually and continually engage the adjacent cutting edges as you apply downward force to begin the actual drilling operation. This locates the drill accurately and avoids impact damage to the cutting edges. This what a spotting drill does. The angle of the spotting drill is ideally wider than the angle of the tip of the drill and the center of the divot it creates allows the tip of the main drill to engage first, just at the center to eliminate walking of the drill. As you push the drill into the cut the cutting edges gradually engage, maintaining accuracy and avoiding damage to those edges. If the angle of the spotting drill is narrower than the main drill's tip, the cutting edges hit first and this can lead to damage to those edges, chatter and an inaccurate hole. This is what Bob alluded to and why he recommended that the angle of the spotting drill be wider than the main drill - to ensure the center of your main drill contacts before the edges.
We have all step-drilled larger holes. We drill a pilot hole first and follow with larger and larger drills until we get the hole size we want. The problem with this is that in every case, the cutting edges are hitting first. This is why we often have a chewed up, inaccurate hole as the drill bounces around on those flutes.
The most accurate way to drill a hole is to spot it first, then use an on-size drill without using pilot drills or stepping up in size. As long as the angle of your spotting drill is larger than the tip of your main drill, the center will hit first and the drill will smoothly engage the hole and cut cleanly. You will find that your drills will cut rounder, more accurate holes. The drill will also not tend to grab and this is worth remembering; when drilling brass, go directly to an on-size drill and it won't grab on you. If you drill a pilot hole in brass first, I guarantee the drill will grab on you. Avoid the pilot drill and save yourself a lot of peck-drilling. It is also good to know that you do not need to drill deep with a spotting drill; just go deep enough to locate the main drill and it will work fine.
As Bob indicates, you can drill without a spotting drill provided the drill is sharp and properly ground and the surface has no irregularities. If these conditions are not present then a spotting drill is appropriate. For me, I try to always use a spotting drill but if I don't then I try to use my screw machine drills.
So, use center drills for drilling holes for your live or dead center on the lathe. Use spotting drills with a cutting angle wider than your drills for all other jobs. Hope this makes sense.
