ROCKY WOODLAND FORGE
  • Store
    • Tools
    • Blacksmithing tools
    • Camping & Cooking
    • T-Shirts
    • Gift Cards
  • Classes
  • Artwork Gallery
  • Gallery
  • Blog
  • About
  • Testimonials
  • Events

Round punches vs slot punches

4/21/2017

5 Comments

 
​Tonight let’s take a look at slot punches vs round punches and the benefits and drawbacks of each.
Round punch benefits
  1. Most often can produce the needed size hole in 1 heat.
  2. Are commonly available for purchase.
  3. Remove more material than slot punches.
Round punch draw backs
  1. Remove more material than slot punches.
  2. Displace/deform the stock more so than slot punches.
  3. They are limited on the size stock that can be effectively punched.
Slot punch/drift benefits
  1. Remove less stock than do round punches.
  2. Require less effort to punch a hole than do round punches.
  3. Displace/ deform the stock less than round punches.
  4. Present far more options as far as what hole can be placed in a what size stock.
  5. Are easier to align in a straight piece of stock (in my opinion).
Slot punch/drift draw backs
  1. Remove less stock than round punches.
  2. In small sizes (1/2”) and less they are more prone to deformation and breakage.
  3. Usually not commercially available.
  4. Overheat (lose hardness faster due to reduced mass).
Sooooo…. What does all this mean? Which is best? Why do the benefits and drawbacks contradict each other?  There are no easy answers, it really all depends on what you are trying to accomplish and what will the end product be.
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. 
Picture
​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. 
Picture
​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. 
Picture
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. 
Picture
​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. 
Picture
​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. 
Picture
​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. 
Picture
​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. 
5 Comments
Wayne Elliott
6/6/2017 11:47:01 am

Great article! Very well thought out and the pics for illustration purposes were very helpful. Thanks.

Reply
Ben
3/5/2019 07:22:32 am

As a newcomer to the craft, this is information that has been poorly explained until now. Thank you very much.

Reply
Brad Holman
3/5/2019 08:22:34 am

Thanks, I'm glad you found it useful.

Reply
Erick Forsyth link
4/2/2020 03:23:35 pm

Question: how do you punch on/at an angle? I see some very high-level artistic work from time to time with punches going at a pretty extreme angles. I have no idea how to pull this off. Is it in the punch or a trick with drifting?

Reply
Brad Holman link
4/2/2020 05:04:25 pm

Hi Eric, thats a difficult one to explain with an easy way to draw, but I'll give it a shot.
If I understand, you want a bar to pass through another bar at an angle rather than 90 degrees as is more common.
So, lets say you want to pass a .5" square bar through a .5" square bar at greater than 90 deg. and 10" from the end.
You would measure 10" from the end of the horizontal bar, and slot punch a .5" half way through the bar.
Then flip the bar over, measure 9.75" and punch half way through based on that mark.
That will leave a small slug in the middle to be knocked out by a smaller tool.
That will give you an offset hole through the bar to be drifted to final shape and size at around half way between 90 and 180 deg.
I hope that makes some kind of sense.
If not, shoot me an email and ill try to use my limit drawing skills to illustrate.

Reply



Leave a Reply.

    Author

    This is a blog to address some incorrect information that is commonly spread as well as some how-to and technically information for those interested in forging. 

    Archives

    August 2017
    April 2017

    Categories

    All

    RSS Feed

​We will not be accepting any orders at least through mid January.
Thank you for your understanding, we are sorry for any inconvenience.
Contact us
Picture
Copyright © 2019
  • Store
    • Tools
    • Blacksmithing tools
    • Camping & Cooking
    • T-Shirts
    • Gift Cards
  • Classes
  • Artwork Gallery
  • Gallery
  • Blog
  • About
  • Testimonials
  • Events