Tonight let’s take a look at slot punches vs round punches and the benefits and drawbacks of each.
Round punch benefits
Let’s look at a ½” round punch vs a ½” slot punch.
A ½” round punch has a surface area at the working edge of .196 square inches. Remember Pi R squared? A 1/8x1/2” slot punch has a surface area of about .0625 square inches. Why is this important? It matters because PSI (Pounds per square inch) matters. Given a hammer strike on both tools of X pounds the slot punch will penetrate deeper. Why? If both tools where struck with the same force the slot punch would penetrate deeper with less deformity of the surface of the stock because of the reduced surface area of the working end of the tool. Think of it like this. Given the same force, which would penetrate deeper into mud, a knife point or a sledge hammer? The knife would, but the sledge would deform the surface more.
What’s up with all this math and talk about psi? Who cares? I thought all you need to know to be a blacksmith was how to hit stuff with a hammer. Wrong, wrong, wrong. There is so much more to this than people realize. We are simply comparing surface area and it’s affects at this point with no consideration of heat, tool edge geometry, and a host of other factors.
So now lets look at some of this in pictures.
Round punch benefits
- Most often can produce the needed size hole in 1 heat.
- Are commonly available for purchase.
- Remove more material than slot punches.
- Remove more material than slot punches.
- Displace/deform the stock more so than slot punches.
- They are limited on the size stock that can be effectively punched.
- Remove less stock than do round punches.
- Require less effort to punch a hole than do round punches.
- Displace/ deform the stock less than round punches.
- Present far more options as far as what hole can be placed in a what size stock.
- Are easier to align in a straight piece of stock (in my opinion).
- Remove less stock than round punches.
- In small sizes (1/2”) and less they are more prone to deformation and breakage.
- Usually not commercially available.
- Overheat (lose hardness faster due to reduced mass).
Let’s look at a ½” round punch vs a ½” slot punch.
A ½” round punch has a surface area at the working edge of .196 square inches. Remember Pi R squared? A 1/8x1/2” slot punch has a surface area of about .0625 square inches. Why is this important? It matters because PSI (Pounds per square inch) matters. Given a hammer strike on both tools of X pounds the slot punch will penetrate deeper. Why? If both tools where struck with the same force the slot punch would penetrate deeper with less deformity of the surface of the stock because of the reduced surface area of the working end of the tool. Think of it like this. Given the same force, which would penetrate deeper into mud, a knife point or a sledge hammer? The knife would, but the sledge would deform the surface more.
What’s up with all this math and talk about psi? Who cares? I thought all you need to know to be a blacksmith was how to hit stuff with a hammer. Wrong, wrong, wrong. There is so much more to this than people realize. We are simply comparing surface area and it’s affects at this point with no consideration of heat, tool edge geometry, and a host of other factors.
So now lets look at some of this in pictures.
This pic is ½” stock. The top hole is a ½” round punch, the bottom a slot punch and ½” drift. Notice how much thicker the sides of the bottom hole are. Now look at a side view.
Remember the knife vs sledge comparison? This is how it plays out in real life. Both produced a ½” hole, however the slot punch made the hole with thicker sides(due to removing less material) and had much less deformity of the bar. The slot was punched in 1 heat and drifted with a second heat. So here is the dilemma, which is better? Again it depends on what you want the end result to be. Look at the resulting slugs from these 2 holes.
Notice how much less material was removed with the slot punch allowing more material in the sides of the hole. Also think about which would be easier to drive through a bar.
Now lets get to the major benefit of slot punching. Is a ½ hole in ½ stock possible with a round punch? Yes, with the right edge geometry, but that is about the limit. A slot punch has no such boundaries. By removing less stock from the walls of the hole you allow yourself room to expand the hole.
Now lets get to the major benefit of slot punching. Is a ½ hole in ½ stock possible with a round punch? Yes, with the right edge geometry, but that is about the limit. A slot punch has no such boundaries. By removing less stock from the walls of the hole you allow yourself room to expand the hole.
The above pic is a 1” hole through the same 1/2” stock. That’s a feat pretty hard to pull off with round punches. I only stopped there because I ran out of time, it could go bigger for sure.
Now look at some flat stock punched with the same punches.
Now look at some flat stock punched with the same punches.
This is 3/8x1” flat stock with the same punches. Similar results from this angle. The round punch on top removed more material leaving thinner walls. This is not a bad thing neseccarily. If you want the stock to remain 1” wide this is the way to go for sure. It is slightly faster and remains closer to original size.
The bottom hole was slot punched and has the thicker walls. Also not bad, and possibly preferred. If you want more strength, more room to expand the hole, want to create square blocking around the hole, or to draw the stock down into ears this is exactly what you want to see.
And here is the side view.
The bottom hole was slot punched and has the thicker walls. Also not bad, and possibly preferred. If you want more strength, more room to expand the hole, want to create square blocking around the hole, or to draw the stock down into ears this is exactly what you want to see.
And here is the side view.
You can see that the round punch did deform the stock more than the slot punch, but only slightly. So why did it deform less than the ½” square? In this case there is more stock surrounding the hole to help support the stock itself while punching. This stock would be a great opportunity to use the round punch to take advantage of the speed of the round punch, so long as you don’t need the extra wall material.
Below are my used and abused tools for producing these holes.
Below are my used and abused tools for producing these holes.
With all that said which is best? Like many things in life there is no easy answer. With experience and knowledge of not only what things happen, but also the why it starts to become more obvious which tool is better for which application.
Thanks for reading. I appreciate your support and hope this helps folks in their journey to become a better smith.
Thanks for reading. I appreciate your support and hope this helps folks in their journey to become a better smith.
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