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Twist Drill Angle with Center Drills vs Spotting Drills

Randall Marx

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#61
This long and interesting discussion has piqued a question: how well or how poorly would a 118 degree drill center in a hole spotted with a 140 degree spotting drill? I ask because I do not own any spotting drills. I've used just the tip on center drills to locate holes for 118 degree drills. That worked very well. I now have 135 degree cobalt split-point drills AND 118 degree drills. I'm considering getting some 140 degree spotting drills to use with the cobalt drills and wonder if I can safely and effectively use the same spotting drills for my 118 degree drills. If not, I might get some 120 degree spotting drills to use with the 118 degree drills and keep the spotting drills with their respective drills in the indexes (120 with 118's and 140 with 135's). If the 140's will work as well with the 118 degree drills, that is less to buy and track.
Thanks!
 

Bob Korves

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#62
This long and interesting discussion has piqued a question: how well or how poorly would a 118 degree drill center in a hole spotted with a 140 degree spotting drill? I ask because I do not own any spotting drills. I've used just the tip on center drills to locate holes for 118 degree drills. That worked very well. I now have 135 degree cobalt split-point drills AND 118 degree drills. I'm considering getting some 140 degree spotting drills to use with the cobalt drills and wonder if I can safely and effectively use the same spotting drills for my 118 degree drills. If not, I might get some 120 degree spotting drills to use with the 118 degree drills and keep the spotting drills with their respective drills in the indexes (120 with 118's and 140 with 135's). If the 140's will work as well with the 118 degree drills, that is less to buy and track.
Thanks!
I have not tried it, but I expect that the 140 degree starter drill followed by a 118 degree main drill would work just fine, probably no different than following a 120 degree starter drill. As long as the main drill has a smaller angle than the starter drill, the drill should center in the cone. I love the 135 degree split point cobalt drills, especially the machine screw (stub) length. If you are square to a smooth surface with them, you can pretty much forget about starter drills if you are using a drill press or a mill. I also have some of the jobber length ones as well, and they work almost, but not quite as well without a starter drill.
 

Scruffy

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#63
I quit using center drills for spotting when I broke the tip off one and spent an hour getting the broken piece out! I have bought sets of lettered, numbered and fractional in stub length and love them.
Posted from the osu heart center in Columbus Ohio. My 84 year old father had a bad episode Friday morning. This is night numer 2 for me , got one and a half hours sleep last night sure hope tonight is better.
Thanks scruffy
Update my father just asked me if I had a pocket knife? His iv line had got tangled and he wanted me to cut it. Gonna be a long night.
 
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EmilioG

Active User
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#64
The 140° to 150° spot drills will work for (me), on any drill below those included angles; 118, 135. With my 8mm Guhring 142 carbide spot drill, I can make the spot shallow or wider/deeper, depending on the drill size up to about 5/16". For small holes, I'll use a 1/4" spot drill. If the holes I need are not deep, I mostly use stub length drills.

Tom Griffin told me that he doesn't even bother with spots, he just pecks a spot with the final drill and goes.
I use spot drills most of the time, especially with smaller size drill bits or on round parts. I may start using a sharp conical point to find my punch marks.
I have used a Starrett wiggler and the conical point on a Starrett double end edge finder with some success. Hitting a target hole on center w/o a DRO is a challenge.
But I like a good challenge, so I keep drilling. I just have to take my time, I don't want to ruin the parts I'm working on.
 

Randall Marx

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#65
Thanks for the input everyone. Sounds like the consensus is that it should work for me to use one set of wide-angle spot drills for all of my drills. I plan to order some with my next tooling order. If anyone has a GOOD reason not to use the 140 degree spotters with 118 degree drills, please speak up. Otherwise, I'll get some.
Thanks again.
 

Mutt

Active Member
Active Member
#69
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.

the 140º spot drills are near impossible to find, but 135º drill bits are EVERYWHERE. Companies act like it is a major job to grind the spot drill to 140º instead of 120º ??????????????????????????? Seems to me, if I were in the drill bit making business, I would be offering the 140º spot drill before a 120º as both 118 and 135 drills would work using it, but not visa-versa. Besides, what happened to companies making what people want, instead of making what they think customers need?????????

After reading an article on this and buying a couple of spot drills, the concentricity of the holes being drilled on my lathe have improved immensely
 

mikey

Active User
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#70
Yup, 140 degree spotting drills are harder to find but they have a bunch on ebay and Amazon usually has some. They sort of last forever; I'm still using a 120 degree spotter that is old, not sure how old, but its still sharp. I have maybe 5 more as a backup but haven't had to pull one out. I buy cobalt spotters, not carbide, because if I drop it then it is more likely to survive the drop.
 

Wreck™Wreck

Active User
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#76
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?
Unless you are in a production environment any method of spotting a hole is valid, there is no right or wrong way there is only a faster or slower way.

Let us use a bit, no pun intended, of reason, lathe centers are typically 60 degrees so a Center drill is 60 degrees with a pilot diameter that ensures that the sharp point of the center does not reach the bottom of the hole. When spotting a drill location the pilot diameter is not in any way useful. A center drill is used to create work holding features.
Why do many taps have centers in each end? They are not there to make your life easier, they are there because this is how the manufacturer holds them, small taps are often sharply pointed ends where the point is held with an external center when being ground.

Spotting drills are often 90 degrees included angle which will create a 45 Degree chamfer on the hole entrance which will require no deburring afterwards, this in itself is an advantage.
They are less easily broken then center drills.
The angles are better when followed by a twist drill, this may help answer your original question. As a hobbyist are you drilling 10 holes per day or 1000 holes per day?
If only several holes per project use whatever method works for you and your equipment as this is what is the "right" method. Ignore most of the "How To Do This Correctly" web advice, for many people the exercise of thinking about a process is more important then actually doing it.