My summary was just what Gotteswinter said in the video, followed by where I am myself on all this.
I took no apprenticeship training. Everything I know was learned by watching/listening to others. Especially the skilled trades wizards we had in our engineering prototyping lab. Those guys were the best of the best hand picked from machine shops and dealerships all across Canada. I have also learned a shit ton from the wizards here on the forum - but all tempered by my own science knowledge base.
I'll comment more below.
But the center hole isn't for starting drill bits?
Because a good center drill has a 120 degree cutting point, it can be used as you described as long as the first straight section of the center drill does not enter the hole.
But I don't often use center drills that way any more. It's too easy to drill too much and get into the straight section.
The second part of the profile is to make an angled hole that has relief at the center so that a tool (like a center) can then be engaged.
I think you mean third. In my mind, the second section deepens the first and provides a clean hole for a lathe center. But yes.
To start a hole for a drill bit, only a small portion of the first part of the profile should be used. Just enough to stabilize the point and prevent wandering until the drill itself becomes stabilized by the hole that it is cutting. And needed because the 'point' of a twist drill isn't a point - which is bad at the start of a hole, but very necessary as the whole becomes deeper
Yup, that's the gist of it.
And I guess that a spot drill may be better, but as long as the 'dent' from the center is slightly larger than the drill 'point' I'm not sure that it will matter much. But maybe I'm all wrong
Fundamentally, I agree. But I have found 4 things that I think make a proper spotting drill better.
1. The flutes on some center drills and the double ended nature of many often makes holding them properly in the chuck difficult.
2. As noted earlier, it's often too easy to drill too deeply and engage the straight section above the small tip. Even a tiny slip is bad because then the angles are wrong for a proper drill start.
3. Some center drills have a flat point just like a drill. In that case, their only advantage over a plain drill is their additional stiffness.
4. The size of a proper spotting drill is more appropriate for starting a drill, the angles (assuming you buy the right ones) are perfect, and they are readily available in carbide. Center drills are available in carbide too, but much harder to find.
I have a nice selection of carbide spotting drills in both 90 and 120 degrees. I almost always use them and prefer them.
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Please don't share anymore. Ignorance is bliss. I want to just buy something, anything for once and just be happy that its as designed / expected, and not have to rework it.
