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Twist Drill Angle with Center Drills vs Spotting Drills

wildo

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#1
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...
 

Bob Korves

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#2
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...
 

wildo

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#3
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?
 

mikey

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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.
 

mikey

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#8
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.
 

Bob Korves

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#9
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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David S

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#10
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
 

Bob Korves

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#11
When would one use a 90 degree spotting drill?
I have no idea, but you are correct that it is popular. 120 degree spotters are less common, but out there, and 140 degree spotters are like hen's teeth. I think a lot of this is due to sources of information, some of them mainstream and from well respected sources, that say the spotting drill should have a smaller angle than the following drill. I have never been able to get my head around their reasoning...

A spotting drill can be carefully ground to whatever point angle is desired with the right fixture and skills.
 
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mikey

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#12
When would one use a 90 degree spotting drill?
David, supposedly, 90 degree spotting drills are for use with HSS or cobalt drills, while the 120-140 degree spotting drills are for carbide drills with more brittle flutes. Carbide drills do not tolerate impact at the corners of the flutes and can chip; supposedly, HSS can tolerate impact.

The problem I have with this concept is that impact is impact. Spotting with a 90 degree drill is okay for softer stuff but try it in harder stuff like 4140 or even 1144 and the drill can jump on contact; this is most noticeable with smaller drills. I've had this happen enough to just prefer a wider angle on my spotting drills. Besides, I'd rather just stock 120's and not have to wonder which drill I'm grabbing. I normally use cobalt drills in harder stuff and my drills stay sharper longer with a 120 - just my personal observation.
 

David S

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#13
Mike thank you very much.

Against all my better (read cheap) judgement I went to my local industrial supply guy today and ordered a 1/4" hss, 120° spotting drill. Bugs me that they are more expensive than I think they should be.

However that aside. I am glad the OP brought this thread up, and everything makes sense to me.

David
 

Bob Korves

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Mike thank you very much.

Against all my better (read cheap) judgement I went to my local industrial supply guy today and ordered a 1/4" hss, 120° spotting drill. Bugs me that they are more expensive than I think they should be.

However that aside. I am glad the OP brought this thread up, and everything makes sense to me.

David
They last just about forever... Get a 140 degree as well...
 

EmilioG

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#15
I've been testing both spot and center drills for starting holes and finding punch marks, but now
stick with one drill, spot and drill and/or use a 142° Guhring spot drill. I've made that mistake of using
a narrow spot with a wider drill point and got horrible chatter when the drills lips hit the side of an improper spot.
I found out the hard way. So I now I choose my drills and spot drills carefully. Great information here.
I also found that running carbide spot drills faster preserves the tool.
 

darkzero

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#16
When I was researching on which spotting drills to buy, I found conflicting answers. Some guys say to use a spotting drill with a lower angle than the drill so the flutes contact first & create a sort or counterbore for the drill to follow, then some say to use a spotting drill that has a wider angle than the drill so the tip of the drill contacts the workpiece first.

The guys stating you should use a smaller angle spotting drill say you don't want the tip of the drill bit to contact first as the chisel point will tend to wander. Then the guys who say to use a wider angle spotting drill say you don't want the flutes to contact first cause if they were ground uneven you will get wander as well as for carbide drills cause the flutes could chip.

There's also info saying you should not have to use a spotting drill with carbide drills cause they are stout, are designed to be used without a starter, and the flutes will chip in harder materials which they are generally used for. So as mentioned, the spotting drill angle also is dependent on the type & material of the drill bit as well as the hardness of the material being drilled.

Then there are guys saying you should use a spotting drill that in between, so neither the outter most edge of the flutes or the tip of the drill should contact first but rather in the middle of the flute.

I forget who but I only found recommendations by manufacturers & in documents saying you should use wider than the drill angle like most are saying here, never the opposite. They guys saying the opposite are machinists by trade, saying "I've never had a problem, or I've been doing it this way for x amount of years, yadda yadda".

Well I went with spotting drills wider than the following drill bit. I mostly use 135° split points so I got a 140° spotter but I also picked up some 120° just to have for my 118s. But makes you wonder cause those 140 & 120 spotting drills are less common. 90° seemed to be the most common (at the time). I don't own a full set of screw/stub length drills yet but one day I will. I think I only own a few in one size only that I bought for 6/32 tap hole size.

One thing I found was 90° seemed to be popular with CNC guys cause it saves a countersinking operation. They would spott drill a bit deeper for a larger dia then the following drill hole size to provide a chamfer so no need to chamfer or deburr the hole after it is drilled.
 

Bob Korves

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#17
I have a hard time understanding how a small chisel point of slightly smaller angle than the spotting drill cone can wander compared to the cutting edges making first contact a large percentage of the radius out from the center, which I consider a recipe for grabbing, chattering, walking, and making lobed holes -- and I have seen and experienced those issues, and it is why I changed to a wider spotting drill angle, and those problems have gone away.
 

darkzero

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#18
Exactly why I chose to go with the wider angle spotter. I used to use center drills for spotting too, we all have, and I admit, I still tend to it do it ever so often with soft materials where it doesn't matter to me. But the chatter & sound it makes (in harder materials ) alone tells me it can't be right. Don't get that with the wider angle spotter drills. And my holes don't tend to wander as it did sometimes using center drills.
 

mikey

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I used to use center drills for spotting too, we all have, and I admit, I still tend to it do it ever so often with soft materials where it doesn't matter to me.
I never used a center drill to spot a hole, oh no, not me ... you see, like Tom (@higgite ), I'm a genius! :)
 
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higgite

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#20
I just call my center drills spotting drills. It’s cheaper, makes me sound smarter and I get fewer disparaging remarks that way. Us frugal geniuses don’t cotton too well to disparaging remarks.

But, after digesting this thread, I can see an authentic spotting drill or two in my future. New goodies! Yes!

Tom
 

Bob Korves

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Actually, using just the small tip of a center drill as a spotting drill is a fairly good idea. Those center drills are massively rigid, and the 118 degree point will spot a hole for a 118 or less angle drill. You just need to make sure not to let the center drill penetrate the work beyond the angled point. It usually takes a larger center drill to be useful for this strategy.
 

ericc

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Bob, that is a good strategy. I think I saw someone do it on Youtube. Big center drills can often be found used for very low prices.
 

Jimsehr

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#25
I grind all my drills by hand and split the point on allmost all of them and if I need a spotting drill to match the drill I 'm using I grind it out of an old broken center or any old stub drill I have laying around. I bought two drill grinding machines but I am to lazy to learn how to use them. I started to grind drills in 1954 so it is hard to break the habit.
Jimsehr
I even split the points on center drills.
 

mikey

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Jim, you da' Man! I can barely see the tip of a center drill anymore, much less attempt to split the point. I used to hand grind all my drills too but as I got older I also got lazier and my eyes got blurrier so I went to a Drill Doctor. Now I can grind drills and split points but I'm still working on the lazy part.
 

Mutt

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Here is a link on the subject. Do note it is on a CNC site, but that doesn't change much:
http://blog.cnccookbook.com/2017/02/27/when-to-use-a-spot-drill/

Ok, so say all my drills are 118º . If I use a 60º center drill to spot drill a piece in the lathe chuck, the drill would start cutting some where else on the drills face, rather than the drill's point? So if I used a spot drill, it would need to be 120º, to make the drill bit actually start drilling at the drill bit's tip?
 

Bob Korves

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Ok, so say all my drills are 118º . If I use a 60º center drill to spot drill a piece in the lathe chuck, the drill would start cutting some where else on the drills face, rather than the drill's point? So if I used a spot drill, it would need to be 120º, to make the drill bit actually start drilling at the drill bit's tip?
Correct.
 

David S

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#29
This is really embarrassing having been doing this wrong all these years. Even the "pros" use regular centre drills.

David
 

Bob Korves

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This is really embarrassing having been doing this wrong all these years. Even the "pros" use regular centre drills.

David
If you ask 10 machinists at random, most will say that the advice in this thread is all wrong. Try what we are saying here, and see how it works for you in your shop.