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HSS vs Carbon steel for finish

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  • HSS vs Carbon steel for finish

    Guero's thread about "resting" HSS, reminded me of another curious thing I read recently about lathe tooling. Rather than hijack that thread, here it is...

    In "Precision Workshop Methods" by HJ Davies, 1935, he says that it seems like carbon steel tools can give a better finish than HSS.
    In the middle of a few pages discussing Tolerances in Machining, the Failure of Tools, the Built-up Edge, Quality of Machined Surfaces etc, he says...(the last sentence is the one that interests me)

    "The built-up edge is much more noticable with high-speed steel tools than with carbon steel tools. Whether this is due to some difference in the surface quality of the steel or to the very much greater temperature reached when cutting with high-speed tools is not known. It is of interest that some, at least, of the new cutting alloys of the tungsten carbide class do not appear to form the built-up edge. This is in spite of the fact that they may be run through a range of speeds much greater than those possible with high-speed steels and including even higher temperatures. There fore temperature alone cannot be the factor which controls the formation of the built-up edge. It is likely that some peculiarity of the surface of the tool is more important.
    The question is of more than abstract interest, because there is no doubt that some tools give a better finish than others. Carbon tool steel has often been preferred for finishing cuts, because it appears to produce a smoother surface than high-speed steel."

  • #2
    Can you even get carbon tool steel blank cutters any more?. Most of the ones I see advertised are either M or T series lathe tools. But as to higher level of finishes from one or the other, from my own personal experience I would have to say that tool profiles in terms of nose radius, clearance angles plus speeds and feeds are the determining factors. It may be that at the time people were grinding carbon tool steel bits with larger nose radiuses that help promote better finishes. Actually if I need really high levels of finish on turned parts (say on bearing diameters on shafts to large to fit in our grinders I go with the scaper type tools described in HSM around ten years ago. By setting the compound at the proper angle it is possible to remove material in the .0001 ranges and produce a very good finsih
    Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.

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    • #3
      Time to conduct some experiments as 20 ft per minute?.

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      • #4
        The problem with trying to take one tenth cuts is even though we can set the compound to the proper degrees to make the cut most metals will 'grab' or 'push' the cutter depending on our lathes and the metals we're using.
        If you can't mount a tool post grinder then try holding a one or two inch piece of 400 or 500 grit paper around the shaft without touching the shaft with anything but the paper. Move it back and forth and measure often. this works well when bearings have to be mounted.
        Cast iron will not tolerate a one tenth cut with any accuracy.
        Rob

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        • #5
          Rob we would never make shafts out of cast iron and the only real use for it in lathe work for machine element besides gears and pulleys is for quill bearings. And those woud grind anyway. The scraper type tools I was refering to will remove very small amounts of material and the resulting chips look more like steel wool than anything else. I'll have to dig through the pile of old magazines in the basement and see if I can find the article.
          Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.

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          • #6
            I think I must have discovered that "scraper" method by accident. After making a cut I sometimes retract the tool without changing the in-feed or with less than 0.001" additional. This can produce a fine cut and a very good surface finish, even with tools that have a small or no nose radius. The cutter will cut along the edge behind the tip and tends to make a wide, ribbon like chip. It seems to dig in less and this may be the reason why this works.

            Paul A.
            Paul A.

            Make it fit.
            You can't win and there is a penalty for trying!

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            • #7
              The scaper type tools actually don't cut with the top of the tool as in a conventioal tool. The way they work is they have the leading side ground with a nagle from the bottom to the top just the reverse of a normal too with the work side being ground so that the angle goes from the bottom to the top. Just think of them as a facing tool put in the lathe upside down. the toll is then place so that the centerline of the tool matches the centerline of the lathe. What you are actually cutting with is the edge formed by the two clearance angles. This edge has to be very sharp and the polished. I know this is a pretty poor explanation and unfortunately I don't have access to the machines at work right now as I'm on vacation.
              Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.

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              • #8
                I sure would like to see a photo of one of these scraping tools. I've never seen one.
                Michael

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                • #9
                  Well, I don't know. I use drill rod quite often for cutting tools, and the finish I get with a sharp tool is pretty good. I usually use this for boring in small holes, but sometimes I get lazy and just rotate the tool to do a facing or turning. If anything the finish is a little better than with hss. If I don't overheat it, the edge lasts as long, but if I push it, it dulls much more quickly than hss. I'm not sure, but I think I'm getting a sharper and smoother edge on the drill rod (carbon steel) than on hss.
                  I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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