barrel_forging_2009_by_irishmage-d3fo0udKnowing 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.

Below is a Rockwell/Brinell hardenss conversion chart that I pulled from Wikipedia for your reference:

Brinell HB

Rockwell C HRC

(10 mm Ball, 3000 kg load)

(120 degree cone 150 kg)

800

72

780

71

760

70

745

68

725

67

712

66

682

65

668

64

652

63

626

62

614

61

601

60

590

59

576

57

552

56

545

55

529

54

514

53

502

52

495

51

477

49

461

48

451

47

444

46

427

45

415

44

401

43

388

42

375

41

370

40

362

39

351

38

346

37

341

37

331

36

323

35

311

34

301

33

293

32

285

31

276

30

269

29

261

28

258

27

249

25

245

24

240

23

237

23

229

22

224

21

217

20

211

19

206

18

203

17

200

16

196

15

191

14

187

13

185

12

183

11

180

10

175

9

170

7

167

6

165

5

163

4

160

3

156

2

154

1

(http://en.wikipedia.org/wiki/Hardness_comparison)

If you have any questions or would like more information on hardness please leave us a comment here on our blog or fill out the form on Super Tool’s Contact Us Page.

Bryan Enander
Super Tool, Inc.
941-751-9677

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  1. Bryan,
    You a very technical and scientific approach and we do the same at Apache Aerospace but most of the US machine shop are dominated by big catalogue houses and tools are bought based on price and not performance. The cost per part is the approach across the Atlantic and in Asia but not in the US unfortunately. Let us talk how we can work together in the Aerospace industry.
    Regards,
    S.Lamba

    • brednane1981 says:

      Dear S. Lamba,
      Thanks for your comment. I agree with you that most tools in the US are bought on price and not performance. Purchasing agents are not usually machinists and don’t always understand that the more expensive tools will actually save them money on a cost per part basis. That being said we have worked with customers and showed them the savings on a cost per part basis and they continue to use that approach. Because cutting tools are somewhat of a commodity it is sometimes discouraging to work closely with a customer, design and engineer a tool that achieves optimum performance only to have the customer take the print of the tool that you designed and send it out for bid the next time the job comes around. I guess that’s the free market and loyalty doesn’t always factor into the cost analysis.

      I would be happy to work with you. Let me know how we can help.

      Thanks,
      Bryan Enander

  2. Dean Bartles says:

    How did you determine your “Milling Cutter speeds and feeds” published in your chart?

    • brednane1981 says:

      Hi Dean,
      Thanks for your comment. The speeds and feeds in our charts are general starting parameters for each specific material listed. The numbers are considered conservative meaning they are at the low end. I should point out that because these are “starting parameters” they are not “optimum parameters.” To achieve optimum performance slowly increase the speeds and feeds from the starting parameters until unacceptable finish or wear of the cutter occurs and then back off a bit to the point of acceptable finish and wear. The speeds and feeds published in our charts come from generally published materials such as the Machinist Handbook and the USCTI (United States Cutting Tool Institute).
      Thanks,
      Bryan Enander

  3. al says:

    Bryan, a lot of speed charts are structured with ranges as the one you provide does. However, I always thought that the lower range of the speeds was to be paired with the harder material, not the way you describe in your article. Do you have it reversed? For instance soft steels can be run to 500 sfm (by your chart) and high strength steels only to 150 sfm, so why does the harder range of the hard material category call for a higher speed? Seems backwards?
    Thanks,
    Al

    • brednane1981 says:

      You are correct! In my haste to show the difference in speeds and feeds from the softer material to the harder I reversed the numbers. I’ve corrected it so it should read correct now.

  4. James says:

    I have a Question. I’ve looked up how Hardness is Proven on YouTube Videos(Brinell or Rockwell). Very Interesting! My Question is….(and I am having a hard time finding info on this anywhere) How do you take the Results for a Hardness Test and Convert them to SFM (Surface Feet Per Min)? I know you have a chart set up here and that is nice. But if I Had some material that has a hardness test done on it. How do I determine the SFM for it? Is there a Formula? That is what i am asking really. Any Info is helpful thanks.

    • brednane1981 says:

      Hi James,
      The SFM is determined from both the material that is being machined and the hardness of that material. Different metals have different metallurgical structures. As a result two different materials with the same hardness (say steel and aluminum) may have different optimum surface feet per minute numbers. I don’t know of a formula that will allow you to determine SFM from the hardness alone because I don’t think there is one. It is a combination of the material and the hardness that determines the optimum SFM and these SFM recommendations have been refined over the years through trial and error by machinists. Also, keep in mind that the numbers we recommend in our charts are starting recommendations only and not necessarily the optimum. Every machine and every material is slightly different (one cast of aluminum may have slightly different metallurgical structure from the next cast just as one machine may have a more rigid setup then the next machine). To determine the optimum speeds and feeds you must slowly increase the speeds and feeds until you achieve a result that is unacceptable and then reduce the speeds and feeds back to the last point with an acceptable result. To answer your question about “How do you take the hardness (of a particular material) and convert it to SFM?” take the first material listed in the Milling Cutter Speeds and Feeds Chart (Aluminum Alloy – Wrought). The Brinell hardness can range from 30 – 150. The recommended starting SFM will range from 1000 – 2000. If the material is on the softer side at 30 Brinell the starting SFM should be 2000. If the material is on the hard side at 150 Brinell the starting SFM should be 1000. If the hardness is somewhere in the middle at say 90 Brinell the starting SFM should be 1500.
      I hope this helps answer your questions. Let me know if you need further clarification.
      Thanks,
      Bryan