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

[Karl A]

Here is a paragraph from the book The Home Machinist's Handbook (1983), by Doug Briney:

"Prick Punches and Center Punches. Prick punches and center punches (Fig. 3-6) are quite similar. Both are used to locate the center before drilling a hole. The center punch has its point ground to approximately 90°. The prick punch is ground with a sharper point, usually around 60°. Prick punches are used when doing extra fine layout work. The sharper point makes it easier to position accurately. Because the depression left by a center punch is better suited for starting a drill, the prick punch is usually used first, then a center drill is used to shape the depression. The use of a prick punch is not necessary when the location of a hole is not critical."

If a 90° depression is better suited for starting a drill than a 60° depression, then it stands to reason that a 120° depression is better suited than a 90° depression for starting a 118° drill, and a 140° depression is better suited than a 90° depression for starting a 135° drill.

In an Internet search, I found a optical punch set with a 120° center punch.
https://www.hhip.com/optical-punch-set-8600-4120.html
I did not find a 140° center punch.

 

[Mitch Alsup]

A center drill bit is better still.
A spotting drill bit is even better.

 

[benmychree]

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.

 

[Karl A]

A center drill bit is better still.
A spotting drill bit is even better.

I made a series of depressions in a low-carbon steel bar using a transfer punch. The point of the transfer punch is about 60°. I will drill and ream the depressions to 0.25 inch diameter. I generally locate the position of a hole without a punch, and use a single screw-machine-length 135° split-point drill to make the hole before reaming. In this case, I plan to locate the position of the depressions using a wiggler, and drill directly.

 

[Karl A]

I found this summary in an Internet search:

"Center punches are used in two different point angles. A sharp 50 to 60 degree layout and marking punch often called a prick punch, and a 90-120 degree drill starting punch that is used after the marking punch. I bought a pair like this a decade ago but the same source now only carries one unspecified point angle but in many sizes. . . More industrial decline. . . But you can grind your own. Prick punches are usually more slender than center punches." https://www.anvilfire.com/21st-century-blacksmithing/basic-tools/center-punch/

 

[Bob Korves]

I have found that in general when making holes, it makes sense to start with wider included angle cutters and follow it with smaller and smaller included angles as the hole progresses. If not, the wider angle of the following drill will catch on the edge of the cut, making the cutter dig into the diameter, which pulls the following drill off center, often leaving an ugly and oversize hole. The smaller included angles center themselves in the previous larger angles. It sure works for me...

 

[OCJohn]

This is my kinda thread. Just the sort of informative minutia I love.

Wish I’d found this forum sooner!

 

[RJSakowski]

I prefer a needle sharp point for accurately locating a scribe mark. I make my own punches from old chain saw files and broken tap shanks. I sharpen the files to approximately a 30º point and anneal the struck end and use these as a prick punch.

I sharpen tap shanks from pulley taps to an 80 - 90º point. Heating the struck end to a red heat will soften the end enough to prevent shattering.

 

[Karl A]

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

 

[Karl A]

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