Why is it important to know?
Knowing the hardness of the material you are machining is important. It helps determine the appropriate speeds and feeds for your application and can affect the design of the tool being used in the machining process. Despite this importance, it is surprising, although very common, that many machinists do not know the hardness of the material they are working with. This can lead to any number of adverse machining results such as poor finish, reduced tool life, tool breakage, and even scrapping out the part you are working on.
In our industry, hardness is typically measured in Rockwell (HRc) or Brinell and can be obtained by using a hardness tester; a fairly common device found in many machine shops. The tester consists of a diamond tipped indenter that is pressed into the material with a given amount of force. The deeper it goes, the softer the material. The hardness could also be obtained from the vendor that provided the material provided no other processes, such as heat treating, were performed on the material.
Every week I quote a lot of specials tools. Sometimes I know the material that is going to be machined and sometimes I do not. Of the times I do know the material seldom does the customer provide the hardness. This is an important variable to know and can drastically effect the design of the tool and the way that we would recommend the tool be ran. If I have the opportunity to get more information from the customer, one of my questions is always “What is the hardness?” All too often, the customer cannot provide an answer, so I’m forced to guess. If you want your tool to perform optimally it is worth the extra time to figure it out.
As an example of how widely the speeds and feeds can vary based on the hardness within the same type of material, take a look at our Milling Cutter speeds and feeds chart (http://www.supertoolinc.com/uploads/MillingFeedsandSpeeds.pdf). Scroll down to the “Medium Strength Steels” Class of Material, specifically the Stainless Steel 400 series. The Brinell hardness can range from 135-325. At 135 Brinell the recommended starting Surface Feet Per Minute is 400 which for a 3” diameter milling cutter works out to 509.3 RPM’s. At 325 Brinell the recommended starting Surface Feet Per Minute is 200 which for a 3” diameter milling cutter works out to 254.7 RPM’s. . 509.3 RPM’s versus 254.7 RPM’s. That is twice as many RPM’s cutting the same material with different hardnesses! You can see why knowing the hardness is important. It is not just important in milling; it is important in all machining applications including reaming, drilling, counterboring, turning, etc.