Twist Drill Angle with Center Drills vs Spotting Drills

[David S]

First let me say that I think Bob's post above is an excellent summary and guideline.

For years I always used centre drills for spotting and was often not too happy with the results but just figured "that's the way it goes some times".

When this thread started I did what lots would do and googled it. My second suggestion is to forget doing a google search. I found much conflicting information.

Instead I suggest that you get a couple of spotting drills and do experiments. This is what I did and was very impressed with the improvement. I use 120° spotters for 118° drills and a large 135° stub drill for spotting all my smaller 135° stub drills.

I normally use small bits under .250" and under magnification it was easy to see the following bit wobbling around when spotting with a centre drill and how steady the bit was when using a 120°spotter.

David

 

[petertha]

Actually, I think the spotting drill should have a slightly larger included angle than the following drill for most scenarios..

You are exactly right. I had a brain fart. A 118-deg spot cone would see the entire cutting edge of a 118-deg drill in simultaneously contact when they meet. Maybe the only reason it is even appearing to work without problems is one or both are not ground to 1-deg precision.
Now I will write on the afterschool blackboard: 120 for 118... 120 for 118.... LOL

 

[eeler1]

I drilled a hole today

 

[Jimsehr]

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...

Bob and I talked about spotting drills one day and I said I grind my own out of regular drills that I shorten. I grind spotting drills to the angle of the drill I will be using. All a spotting drill is is a short stout drill. Be it hss or carbide. I have ground them out of broken center drills . That is what protractors are for.