Center punch point angle for drilling: 120° or 140° instead of 90°?

Mitch Alsup said:
A center drill bit is better still.
A spotting drill bit is even better.
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i have found it better to start with a center drill then switch to a spotting drill. the spotting drill tends to walk
 
Karl A said:
Bob Korves,

Your message contains an idea that I have not encountered before: When step drilling, step down in included angle as one steps up in diameter. Doing so centers the larger drill in the smaller hole, and keeps the larger drill from catching an edge. Catching an edge risks damaging the drill, and/or drilling off-center, rough, or oversize hole.

To practice this idea, one can have 135° drills up to the size that one is able to drill in one step, and 118° drills beyond that for hole sizes that that one will drill in two steps.

What do you think of the idea of stepping up to a 140° center punch before drilling with a 135° drill?

Karl A
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That would work fine. Just let the centering of each change of tool center automatically in the middle of the previous taper by using the previous taper as a guide. Anything of a larger included angle, even a couple degrees, will help to center the next cut. The drills and the machine do not have the rigidity to disallow tool movements when catching the work externally.
 
deakin said:
i have found it better to start with a center drill then switch to a spotting drill. the spotting drill tends to walk
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sounds like your spotting drill is not sharp, or you have run out, or a loose quill.

I don't suffer that. I use a spotting drill in the mill, and a center in the lathe. There are times I will use a center in the mill, but it's rare.
I don't find that my spotting drill walks off center unless its on a cylinder side. A punch will set it. I usually prefer my punch and hammer to an automated punch. I find that I miss more with the automated punch... it skates off, or requires enough pressure (even if backing off the cap) that I don't hit my mark. I find a light tap on a punch is all I need to ensure I get it started. I will then hit it harder if need be, or recenter by pushing the point off center.

I generally use 118 drills.. so no need to worry about points.. I have a preference for screw drills, vs jobber, wish the screw drills weren't so damn expensive, and more readily available.
 
benmychree said:
I am impressed with a really sharp prick punch, when you hit with the hammer and let go of it, it will stand on its own in the impression.
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I am both impressed and amused by the thought of a sharp and hard prick punch standing in its own impression in a workpiece.

The thought inspired an idea for the opposite approach -- to not punch the workpiece at all. Here are the steps of the idea:
1. Apply smooth-plastic masking tape to the workpiece.
2. Transfer punch, or scribe and prick punch, only the masking tape.
3. Center the spindle over the punch mark in the masking tape using a wiggler.
4. Remove the masking tape from the workpiece.
5. Make the hole using a screw-machine-length drill that has a split point to prevent the drill from walking.
 
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Karl A said:
RJSakowski,

I wonder whether your prick punch is sharper (has a smaller angle) than your scribe. I am thinking that a prick punch that has an angle that is smaller than the scribe is better for accurately locating scribe marks.

I checked the angle of tip of my scribe. It's 90°. The scribe is Moody Tools 51-1762 retractable diamond point ball-point type. When I selected it, I gave no thought to the angle of the tip.

Karl A
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My scribes and prick punches have pretty much the same angle. I make my scribes from 1/8" carbide drill shanks and grind them to an ogive or bullet shaped tip. This gives me better strength at the tip while not reducing visibility.. I grind my prick punches and center punches the same way with a larger tip angle for the center punches.
 
My understanding has been that prick punches are tapered/shaped the way they are to enable them to easily follow a scribed line to the intersection of another scribe line, where they are strong enough to then be hit to make a light indentation at that intersection. This is far more accurate than trying to visually put the tip of the prick punch at the intersection. It is also more accurate than using a wiggler or an optical punch, at least in my experience. Punches are for rough work in any case.
 
OCJohn said:
This is my kinda thread. Just the sort of informative minutia I love.

Wish I’d found this forum sooner!
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OCJohn,

After I have reached the limit of a how much informative minutia I can inflict on my coworkers, I come and post something here.

People at work tend to see me as being detail oriented, but I feel I am the opposite. I want robust processes that are easy to perform and give good results reliably. I quickly become frustrated when I give something detailed attention and it doesn't produce what I intended.

I am at the beginning of my third year in the machining hobhy. I am nearly finished moditying a belt sander, for grinding lathe cutters. Through the fabrication work, I am learning how to drill a hole close to the intended spot.

Karl A
 
Watch this... NYCCNC tests different points on drill bits, you may want to stick with 118 degree...

Just saw this the other day, it's from Jun 2019.
 
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