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Grinding Length of HSS

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  • Grinding Length of HSS

    If I'm only taking a .003" cut, how far back do I need to grind my tool bit? After all, it's just that leading tip that does the cutting, right?

    I played with it a bit, and found a .040" or so grind lets chips bind really easily. I've also found the 1/2" or so of the preground bit is longer than I actually need--and encourages excessive stick out from the tool post.

    I'm using 1/4" HSS with a Taig mini lathe. I usually play with aluminum and plastics, but today it's A36 steel. I had to switch to carbide to knock off the mill scale. It was really cool watching a sharp HSS bit make 6's but the edge didn't last.

  • #2
    If the edge is not lasting on mild steel, reduce the rpms. turning 1 inch diameter steel should use 400 rpm. Everything scales from there. 1/2 inch 800 rpm

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    • #3
      How far back to grind it? The minimum length of grind is equal to your greatest depth of cut. There is a real advantage in grinding much farther back. Each time the tool dulls you will be grinding a little off the end. With a longer ground tip you get a lot of re-sharpenings without having to reshape the entire tool.

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      • #4
        Grind as much as required to achieve the desired geometry for the material.

        Originally posted by Illinoyance View Post
        How far back to grind it? The minimum length of grind is equal to your greatest depth of cut. There is a real advantage in grinding much farther back. Each time the tool dulls you will be grinding a little off the end. With a longer ground tip you get a lot of re-sharpenings without having to reshape the entire tool.
        I find that my eye sight is really the limiting detail.

        If one is only taking a .004" DOC, then a cutter edge .005" would be fine. But the shape would need confirmation under high magnification.

        I find it much to my advantage to shape the entire tool tip to the geometry required, and then know that the cutting end is correct.

        The gentleman from Ill- annoyance, makes a good point! ;-)

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        • #5
          Thanks for the suggestions and help!

          I played a little more today and just couldn't get the edge to last. I'll reduce the RPMs and give it another try.

          It looks like about 1/4" will work out to be a pretty decent length. Quick to make, plenty of square tool bit to clamp to, and I can get the stone in to clean up the edge.

          I'm open for suggestions on the tip geometry, I couldn't quite get things working nicely again--but see the RPM issue.

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          • #6
            Originally posted by Puckdropper View Post
            Thanks for the suggestions and help!

            I'm open for suggestions on the tip geometry, I couldn't quite get things working nicely again--but see the RPM issue.
            If speed, tip geometry and cutting oil are not helping there is also possibility of crappy HSS blank.
            Lots of substandard HSS on the "hobby" market. Check with a file, file shouldn't bite the HSS at all. (instead the HSS should leave gouge on a file.)

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            • #7
              The edge will also wear faster if you let it rub too much. Try to enter the cuts with a feed rate that generates a proper chip right away. And try to work the handle so there is no pause that leaves the edge rubbing.

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              • #8
                Hi,

                If you are taking light cuts to remove the bark off the A36, then HSS will dull very quickly. You need to cut deep enough to get under that bark in the first cut. That often means a .025" to .05" DOC. A bit difficult to accomplish with a light machine sometimes. Switching to carbide helped because that will tolerate the hard and abrasive mill scale far better than HSS.

                Dalee
                If you think you understand what is going on, you haven't been paying attention.

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                • #9
                  You are taking out the grinder marks with a fine hand stone until it looks good with a 5-10x loupe aren't you?

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                  • #10
                    Mr Pete's

                    https://youtu.be/hrDr4rYLiAk

                    Tony's

                    https://youtu.be/__A2xtLF0AU

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                    • #11
                      I admit this is kind of extreme. I made a toolholder for my BXA toolpost to use HSS more effectively. The slot that holds the cutter is inclined 12* to give me positive back rake. On the surface grinder I ground an 8* clearance angle the full length of the tool bit. On the top of the tool I ground a narrow groove inclined 12* to give me both 12* side rake plus a chipbreaker. As the tool wears resharpening merely involves taking a little off the end, maintaining about 8* clearance angles.

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                      • #12
                        Originally posted by Puckdropper View Post
                        Thanks for the suggestions and help!

                        I played a little more today and just couldn't get the edge to last. I'll reduce the RPMs and give it another try.

                        It looks like about 1/4" will work out to be a pretty decent length. Quick to make, plenty of square tool bit to clamp to, and I can get the stone in to clean up the edge.

                        I'm open for suggestions on the tip geometry, I couldn't quite get things working nicely again--but see the RPM issue.
                        Did you lower the rpm before you played a little more ?

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                        • #13
                          you're not finished with the bit after grinding....you need to stone it to the sharpness desired. Assuming correct speeds, that I suspect is why you are having difficulty with make a long .003 DOC and quickly get degradation. Whatever stalligmites protrude from the bit into the work break off and you've a crappy finish, seems like tool wear etc.

                          The edge has to be in keeping with the DOC. Its easy to take a tenth DOC for example if the tool is sharpened for it.

                          Put a proper edge on, use some cutting oil, the right speed (or under) and they last a long time. Literally, sometimes I go months months without having to regrind a hss with home shop use.
                          .

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                          • #14
                            For the shallow depth of cut I prefer a tangential tool. The holder has about 12* angle in two directions. The bit is square. It has a face ground 30* measured non the diagonal. A properly honed tool of this style is capable of taking a .0002 cut. At least that is how much my DRO said I did. There is a lot of info on the net regarding making these tools and at least one vendor. Another useful tool for very fine cuts is the vertical shear tool. Google both terms.

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                            • #15
                              I lowered the RPM and got much better results. I'm still playing with tool geometry, but the results have been encouraging. The last cut left a finish that felt smooth (not polished smooth, but all the tops were the same height smooth, kinda like a 2-56 screw.)

                              I have been stoning the edges. Focusing on it tonight didn't seem to help any more than just doing it, but I'm sure there'd be a big difference doing it vs not doing it.

                              Those shear tools are awesome. I made one some time ago as a result of discussion here, and wouldn't feel bad about not doing anything else to refine the finish the tool left in most cases.

                              The tangential tool looks interesting. I found an Instructable that showed how to make a tangential tool holder for the Taig lathe. (They usually annoy me, but this one is really quite good!) http://www.instructables.com/id/Tang...or-Taig-Lathe/

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