Twist Drill Angle with Center Drills vs Spotting Drills

wildo

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I read this message from Bob and it got me thinking. I didn't even recognize the difference in twist drill split point angle (note the beginner's forum!) but when checking my drill bits, I find them to be 135 degree tips. Sigh, and I just ordered a set of 60 degree center drills. Looking at 140 degree spot drills- they seem to be quite a bit more expensive.

  1. Can I use these 60 degree center drills with my 135 degree drill bits? I mean- better than nothing, right?
  2. Why does it seem that 135 degree twist drills are far more common than 118 degree?
  3. Besides accounting for the drill bit tip angle, when does one use a spotting drill vs a center drill? What is the intended purpose of those two things? I had never heard of a spotting drill before.
  4. Does there exist a 140 degree (or maybe it's 70 degree?) center drill?
  5. Is it worth investing in a couple $30 spotting drills, or buying a whole new set of 118 degree twist bits?

Seems like some good advice below, Bob. Just looking to fill in some holes in my understanding.
I am assuming the main drill has a 118 degree point angle here. The starting tip of the center drill is ground at 120 degrees. The spotting/center drill should have a LARGER angle than the drill that follows for it to follow with proper guidance, not walking around as the cutting lips make contact with a 90 degree hole part way out on the cutting lips, which leads to chatter and poor following. The chisel point of the larger drill should be first to contact the work in the starter hole and start cutting, with the cutting lips taking over the job from the center outward, not starting by taking notches from some random points on the cutting lips. In this way the starter hole is truly guiding the drill.

Using the 60 degree main cutting portion of a center drill to make a starting hole for a drill is a really poor idea, even worse than the 90 degree spotting drill, but it is very commonly done. Using the parallel bore caused by the entire tip of the center drill penetrating the work is not quite as bad, but still wrong, and is also sometimes done in error.

Again, the only way a starting drill can facilitate centering of the drill and locating where the drill should follow is for the included angle of the starting/spotting drill to be slightly larger than the drill which will follow. That would mean a 120 degree spotting drill for a 118 degree drill to follow, and a 140 degree spotting drill for a 135 degree drill to follow.

I understand quite well that what I am posting here is not common practice. If you disagree with it, please explain to me why it is wrong.

Specifically, in the case of starting a hole in a curved surface, which is what the OP was trying to do, the center drill is a pretty good choice because it is extremely rigid, has a 120 degree starting angle, and can leave a small, well centered starting hole if pecked lightly and carefully, avoiding the need for milling the curved surface flat first, which would be probably be best for higher precision work. A spotting drill has a larger chisel point and is also less rigid than the center drill is, and is therefore more likely to walk off the curved surface.

Does that make sense? It has worked well for me...
 
60 degree center drills are designed for drilling centers on a lathe for use with 60 degree center in the tailstock. They also are used for other things, including spotting holes for drilling. Millions, no, billions of holes have been started with 60 degree center drills, and they do work for that job. The most common point angle for drills is 118 degrees, a general purpose grind. Flatter point angles are for harder materials, though I include all steels in that group. Softer materials can be cut with 135 degree drills, though somewhat slower. Split points are better at drilling holes in flat and perpendicular surfaces without any spotting, and they also require less pressure to feed them into the work. To me it makes much more sense to use a larger angle spotting drill than the main drill, but I am quite sure that many others disagree, if only because they have been doing so as long as they have been drilling holes. Try drilling a bunch of holes both ways and see what you prefer...
 
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?
 
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.
 
I totally forgot to add that when choosing a spotting drill, it only has to be big enough to span the web of the main drill. A 1/4" spotting drill will work for most drills in your drawer up to and including 1/2". If your drills are 118 degree points, use a 120 degree spotting drill. If using 135 degree split points, a 140 degree spotting drill works.
 
I totally forgot to add that when choosing a spotting drill, it only has to be big enough to span the web of the main drill. A 1/4" spotting drill will work for most drills in your drawer up to and including 1/2". If your drills are 118 degree points, use a 120 degree spotting drill. If using 135 degree split points, a 140 degree spotting drill works.
Exactly how I see things, too, Mike. As long as the chisel point gets into the spotting cone, it is big enough. Some drills have thinned flutes, so it is the chisel point size that needs to fit in the spotting hole, though a little oversize spotting hole does not hurt anything. The main drill cutting edges should not contact anything until the chisel point has established the hole location. The sides of the spotting drill should never enter the hole, it should be a pure cone, no cylinder portion at all.
 
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Lots of good information here, thank you.
Now I am looking for affordable spotting drills and find that 90 degrees seems to be very popular.

When would one use a 90 degree spotting drill?

David
 
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