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