Twist Drill Angle with Center Drills vs Spotting Drills

[Bob Korves]

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.

 

[mikey]

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]

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]

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]

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]

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]

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]

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]

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!

 

[higgite]

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