Rockwell Hardness Test

Introduction

You’ve probably come across the letter and numbers Rc60 or Rc58 when it comes to the quality of tool steel. If you are the sort of person that operates on a need to know basis, it might be enough for you to know that a higher number equals a harder steel. And that hand tools normally range between Rc58 to 62. For every one else, keep reading!

If you are more of a visual person, enjoy this very informative but not at all entertaining video.

The rest of you, keep reading this informative but not at all entertaining article.

Rockwell hardness test

This is a method of testing that determines how hard a material is, it is mostly used to test the hardness of metal. The method of testing is very easy to perform and quite fast, so it’s one of the most popular ones. You can put a Rockwell testing machine right on a production line to verify the quality of a metal right after it’s made.

How is it performed?

Well first of all you load the machine with a small diamond that’s cut to a specific shape. Then you put the material that you want test in the machine and press play. And voila! you get your answer.

What the machine does

While you wait this is what happens in the machine. First it applies a minor load on the material then proceeds to put on a major load for a certain amount of time. Then backs of to the initial minor load and you measure the difference in depth between the first and second minor loads. If all of this is confusing, have a look at my awesome drawing skills below. It might help.

When you have your measurement you just put it in a specific mathematical formula (provided below) and you will get your hardness rating.

Math formula

Still, what does the letters and numbers mean?

Well lets begin with the R because it’s the first and the easiest. It stands for Rockwell. So how about the next one, C. That just means that we are measuring on the Rockwell C Scale. Rockwell represent most of the alphabet, but they skipped a few and I don’t know why they picked the once they did. Ask Stanley P. Rockwell.

Did you know

Cabinet scrapers have been bumped up a few numbers on the scale in recent years to around Rc50. Before you used to make them from old handsaws but they are a bit too soft to make a really good scraper.

Each letter represents a different cut on the diamond or diameter of the ball that makes the indentation on the material. You can have a look below to see all the letters and what they represent as well as which type of material that is suited for which letter.

Are you hard or are you soft?

Now that you know what it all means there’s only one question remaining? Do you want a tool with a hard or soft steel? Well of course there’s not an easy answer, it depends on how you use it and what you prefer. A hard steel is more brittle and takes longer to sharpen but can take a sharper edge that holds for a long time. While a softer steel is more forgiving but doesn’t hold an edge quite as long.

Take for instance a mortise chisel, you are going to pry and bend a fair bit with a tool like that, so you might be better off with a little softer steel so you don’t chip your edge so easily and have to spend all day sharpening.

But for the absolute best result and finish, you need a chisel with harder steel so you can hone a really sharp edge and be able to take of ridiculously thin shavings.