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  • different bits, different speeds

    Total noob question here, and for good reason. I have heard, and read from various sources that hss and carbide bits require different cutting speeds. It has only been casually mentioned, and no details are given. All research in the matter result in the same old, "4 X sfm/diameter", and cutting bit material is never specified unlike the material being cut. Am I missing something important?

  • #2
    It isn't so much a matter of "requiring" different cutting speeds, as it is a matter of "permitting" higher speeds for carbide which can tolerate much more heat. ...Making carbide more productive and longer lasting for commercial/production milling operations.

    Also, carbide, being much harder than HSS, will cut much harder materials that would quickly resist and destroy HSS.
    That's generalizing of course, since both HSS and carbide come in various flavors.
    Lynn (Huntsville, AL)

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    • #3
      Welcome to our forum. HSS and carbide tools do not require different cutting speeds, but the maximum allowable cutting speed depends heavily on the tool material. For example for turning a 1018 steel on the lathe Machinery's Handbook recommends about 110 fpm for HSS tools, but 800 fpm for coated carbide tools. The numbers are not absolute, but depend on a particular application. Heavy roughing is usually done at lower speeds than finishing.
      Again, you can go lower on speeds, but you should not go much higher. Otherwise the tool damage would result.

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      • #4
        You're just not finding the right charts. Sadly it doesn't seem to be common and I can't put my finger on an example just now but when I went searching for Surface Feet per Minute for various materials I found a few charts that had typical values for both HSS and carbide.

        Mikey makes a good point too. The numbers on those charts are suggestions for the maximum speeds that provide an OK but not great lifespan for the cutter while turning out a profitable volume of work within a reasonable time. Most of us home shop types run at well below those numbers to extend the life spans of our end mills, drills and even lathe tools.

        I know that SFM charts that show both HSS and carbide are out there but after a good 5 to 7 minutes of searching they are simply not coming up. But I saw them about a year back.

        In use I also find that the machines will "speak" to us and mostly they counsel me to keep well away from those maximum SFM numbers by whining and moaning if I push too hard. That and the cost of decent end mills is enough to convince me that I should pay attention to what they are telling me....
        Chilliwack BC, Canada

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        • #5
          Start slow and work the speed up from one part to the next, by the time that you reach the 50th part you will have it figured out (-:

          https://www.suncoasttools.com/PDFFIL...Catalog/35.pdf

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          • #6
            One thing that is not mentioned is the coating, if any, usually allows a greater SFM. The material being cut changes the max SFM also. For example, uncoated carbide can run at 1200 SFM in 6061, but add a ZrN coating and that increases to 1560, go to PCD coating and that jumps to 2200. HSS endmill is good for 600 SFM. (numbers taken from GWizard for reference).

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            • #7
              Much appreciation to all of you. Not required, but permitted. That is how the articles and videos should put it. Special thanks to Bented, I will print that chart first chance I get.Looks like that one sheet will replace a small pile I have now. I never really worry about feed speed as, number one, I have no reliable way to measure that, and two, I don't care. I take it slow and easy listening to the cutter do its thing. My main concern is rpm, and if it mattered between bit type.

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              • #8
                Certainly in the home shop, "listening to the cutter" would have been the best advice that anyone could have given. Many of us have older / less rigid machines with smaller motors when compared to modern industrial machines (which carbide bits are designed for). There's nothing at all wrong with hand ground HSS bits in the home workshop - they'll do 95% of what you're likely to need, and you'll learn a lot about rake, clearance, feed & speed in using it.

                Ian
                All of the gear, no idea...

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                • #9
                  I don't like to waste my time grinding. Buying carbide inserts allows me to actually build things in my shop utilizing my limited time. Do you want to stand at a grinder or do you want to remove metal? Get a speeds and feed app. such as HSHadvisor. It will give you exactly what you are looking for right at your finger tips at your machine.

                  https://hsmadvisor.com/
                  Location: The Black Forest in Germany

                  How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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                  • #10
                    One other thing that differentiates HSS from carbide is the brittleness of the carbide. Since carbide is more brittle, often the carbide is not as sharp as that fine edge is easy to break off. Industrial machines with plenty of rigidity and power can push this "duller" carbide into the material to be cut and remove a lot of material in a hurry. Lighter machines with less power need the sharper edge but are not often used where a few seconds per cut difference will matter so HSS is a good choice for many materials. There are carbides that are sharper, usually made for working in softer material where the edge won't break off so easily. They will often be specified as for use in aluminum. They (mostly) will work well in steel if one isn't pushing the cutter to the max.

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                    • #11
                      Originally posted by unreal203 View Post
                      Total noob question here, and for good reason. I have heard, and read from various sources that hss and carbide bits require different cutting speeds. It has only been casually mentioned, and no details are given. All research in the matter result in the same old, "4 X sfm/diameter", and cutting bit material is never specified unlike the material being cut. Am I missing something important?
                      Your understanding is correct. What you're missing is that the SFM will vary by material - both the cutting tool and the work. Each combination of cutting tool and work material has a different theoretical best SFM speed.....that's how the universal formula accommodates both hss and carbide. Obviously the common ones are carbide and hss, a list of SFM's is often just for one or other - the list provide should state somewhere what tool material they are referring to.

                      If someone is stating the cutting speed and giving the material and not the cutting tools material, its because its obvious, implied or they're being a bit sloppy. "The piece of steel cut well and I kept the speed under 100rpm" implies the cutting tool is HSS for example because its such a common scenario and we all know 100 sfm is the cutting speed for hss & steel. Doesn't help the beginner, but the point is, just because the cutting tool material isn't stated, it is nevertheless a critical part of determining what is the best SFM figure to use
                      Mcgyver
                      Senior Member
                      Last edited by Mcgyver; 01-12-2022, 07:34 AM.
                      in Toronto Ontario - where are you?

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                      • #12
                        If you are new to metalworking
                        just use these rules.
                        If you are machining steel, go as
                        fast as you dare until the steel just
                        starts to turn brown or blue.
                        Brown is safe. Blue is getting a
                        bit fast. For aluminum, I don't
                        think you have to worry about
                        cutting too fast, as generally
                        you can cut aluminum at wood
                        working speeds. Keep it simple.
                        When you are cutting and turning
                        steel blue, then pay more attention
                        to speeds and feeds. Else just use
                        your machines and have fun.

                        -Doozer
                        DZER

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                        • #13
                          Originally posted by Black Forest View Post
                          I don't like to waste my time grinding. Buying carbide inserts allows me to actually build things in my shop utilizing my limited time. Do you want to stand at a grinder or do you want to remove metal? Get a speeds and feed app. such as HSHadvisor. It will give you exactly what you are looking for right at your finger tips at your machine.

                          https://hsmadvisor.com/
                          I sympathize. But the cost of replacing carbide bits time and time again gets a bit much for me. HSS blanks take some time to shape, but after that its a very quick matter to keep them sharp, and they have a lot of life to give. I only use carbide when absolutely necessary. I used to buy HSS inserts, but small items like that too difficult to sharpen, so replacement is needed, and thats when I went to blanks.

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                          • #14
                            Originally posted by unreal203 View Post
                            Total noob question here, and for good reason. I have heard, and read from various sources that hss and carbide bits require different cutting speeds. It has only been casually mentioned, and no details are given. All research in the matter result in the same old, "4 X sfm/diameter", and cutting bit material is never specified unlike the material being cut. Am I missing something important?
                            First, welcome to the forum. This is the place to ask questions, noob or not.

                            Your formula (4xsfm/dia) to get a spindle RPM is correct. All that's needed is to fill in the blanks for sfm. Also, yes, HSS and carbide run at different speeds. I'm going to offer some approximate starting points for a few common materials. Keep in mind these are not carved in stone, merely starting points. Expressed in Surface Feet per Minute, or sfm, for mild steel; 100sfm, for aluminum; 200sfm, for brass/bronze; 150sfm, for difficult to cut tool and high alloy steels; 50~75sfm. Note that these numbers are for HSS cutters. When using carbide multiply the numbers by a factor of 3.

                            The above numbers are for open cutting tools like you find on a lathe, shaper, or mill. For drilling operations cut the numbers by 1/2, for reaming cut speeds by 1/4.

                            Again, these numbers are a starting point. In some applications where the machine and part setup can handle it the speeds can be increased. On smaller lower powered machines the speed might need to be reduced. Also, the availability and application of cutting fluids will influence speeds, too.

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                            • #15
                              Do you want to stand at a grinder or do you want to remove metal?
                              Hey, when I'm standing at my grinder with a HSS bit, 'removing metal' is exactly what I AM doing! But I get your point.

                              About that "Surface Feet per Minute". With a drill bit, for example, you are concerned with the speed at the outer edge of the drill - which with a 1 in. drill will be 3.141592(blah blah) inches per revolution * minute. The edge closer to the center of the bit it will be moving at a lower sfpm - it's the outer edge that you are concerned about overheating and burning up.

                              Speed is also related to feed, i.e., how deep a chunk of metal you are attempting to remove with each revolution of the bit. You might normally run a 1/8th in drill bit at, say, 800 rpm in your drill press. There is no reason you can't run that same bit at 100 rpm, it will still drill just as good a hole. BUT! If you crank down just as quickly as you would at 800 rpm you are then attempting to remove 8 times as much metal per revolution, and that poor little drill bit is likely gonna snap right off. However, depending on the job, and the machine, there may be times when changing speeds is more trouble than drilling that one hole a little more slowly - with a lower feed rate.
                              "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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