Twist Drill Angle with Center Drills vs Spotting Drills

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!
 
Randall Marx said:
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!
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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.
 
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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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.
 
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.
 
The 140° will work with all drills under 140, but I would also test the matched examples, 120 spot with 118, 140 spot with 135.
 
mikey said:
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.
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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
 
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.
 
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