The tools and stock for this post. 1/4x1.5 flat mild, 1" slot punch, and 1/2" round and 90 deg. punches
The holes laid out and punched with the 1" slot punch.
Using the hardy tool and 1/2" rounded punch to set down one side of the stock and then the other. The stock is forged down on one side, resulting in the end of the bar being distorted. But no worries, it will correct it's self when the stock is flipped over and the other side forged down
Stock forged down on both sides of the slot to allow 1/2" stock to pass through the flat stock. Note that forging the second side down corrected the crookedness of the end of the stock.
Using the same tools to set down the center first and then both sides (in the opposite directions of the first set down) to allow 1/2" round to pass through
The same procedure can be used with a 90 deg. punch to create a pass through for 1/2" square on the diamond.
The finished project with 1/2" round and 1/2" square passed through flat stock.
The test subjects for this post. 1” mild steel bar and a ¾” slot punch. Now we are looking at the difference between punching from mainly one side as opposed to punching half way through and then flipping over and punching from the other side.
Here are the resulting holes and slugs. The hole nearest the end is punched from one side and the slug knocked out from the other. The other is punched half way from one side and half way from the other side.
So this tells us the resulting slug, at least in this application, are close enough to not significantly effect the outcome. For all intents and purposes the same amount of material was removed from both holes.
You can see an obvious difference here, but nothing drastic
Here are the holes after drifting to ¾” Now the difference appears more obvious.
And now it is obvious. So what is the difference and why is happening? When you punch from mainly one side you are driving that material down and when the material meets the anvil it spreads out. Resulting in the stock spreading out more at the bottom side than the top where you started punching.
When punched half way through and then half way through from the other side you limit the “bottom spread”. Equalizing the spread, so to speak.
So how does this affect anything? Well, if you need more even and visually appealing holes then punch half way, flip and punch again to make the hole more even.
However, if you need more material in a certain spot, then punch from the appropriate side and flip when the punch meets the anvil . Larger cheeks on the bottom of an eye hole is an example of this. The extra material at the bottom would allow longer checks on the bottom of the eye and shorter checks on the top, for example.
I hope this gives you something to think about next time you need to punch a hole. There is so much more to this craft than what meets the eye.
Nothing is as simple as it seems.
I like to compare forging to running a bulldozer. With a few exceptions of sawing, chisels, etc. dozers do not remove dirt they only move it from one place to another, and yet accomplish amazing feats by simple relocating material. Such is forging, the hard part is to determine when, where, and how to move the material from one place to where it is needed in an efficient manner that results in the desired outcome.
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.
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.
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.
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.
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.
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.