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Back Rake on HSS parting tool?

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  • Back Rake on HSS parting tool?

    After asking the question about rake angles on threading tools, Similar questions arise with parting tools. These parting tools illustrate the differences.


    The larger cutoff blank has a rounded like groove down the middle. Gadgetbuilder posted some instructions for grinding a similar groove to control the chip. There has been some discussion of a cross groove behind the tip that sort of establishes some back rake and rolls the chip into a small clock spring slug. Your thoughts and experience with HSS parting tools are solicited.
    Byron Boucher
    Burnet, TX

  • #2
    The groove in the top of the blade causes the chip to fold in the center. That helps prevents the chip from becoming wedged between the blade and the work piece. Gary P. Hansen
    In memory of Marine Engineer Paul Miller who gave his life for his country 7-19-2010 Helmand Province, Afghanistan. Freedom is not free, it is paid for with blood.

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    • #3
      well, these tools dont have a side clearance, thats why they dont work especially well. im too lazy to go downstairs and take a picture of a proper parting tool (i made before a got married). now its carbide only (brazed!).

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      • #4
        Your tool holder on the right has back rake built in, can't tell about the one on the left. It looks like you've grooved the right holder so the T blade sits perpendicular, as it should.

        Some books advocate grinding back rake into the parting tool, possibly for holders that don't have the back rake built in. The reason back rake is built in is to allow sharpening by grinding only the front edge. When back rake is ground into the top of the parting tool, after sharpening the front a couple times you need to grind a deeper groove in the top, then after doing that a few times you're below the top of the T and you must grind that section off and start over. Takes time and uses the blade up much faster.

        The T shape blades I have include side clearance so I sharpen the blade without removing it from the holder. Just set the table on the grinder to 7 degrees, put the holder on the table and touch it to the wheel. It is very quick to do so it gets sharpened often but little is ground away on each sharpening. Sharpening leaves a little burr on the sides so a touch is needed with a slip stone to remove that. I grind the little V you mentioned in the top of the blade (with a Dremel) for 1/8" or so back from the tip so this needs to be refreshed occasionally when it gets ground away.

        The V causes the chip to have a bump in the center plus the edges are wrinkled looking and are fragile. Chips generally exit the slot as an irregular curl or as short sections. In some materials the chip curls up straight and breaks off as little round coils. Since the V chip is narrower than the slot the swarf exits easily.

        Parting on my 7x12 can be an adventure because it is relatively limber. Keeping the parting tool's tip about 1% of the work diameter above center minimizes problems although it means pausing to adjust tool height occasionally when parting work over 1".

        John
        Location: Newtown, CT USA

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        • #5
          T shaped parting blades

          Where do you guys get your T shaped parting blades and how much do they cost? I placed an order from Shars for 3 different sizes of parting blades, all of which showed a T shape in cross section on their web site and were listed as Type P blades. Only one of the three came through as a T shaped blade (0.040 width). The 1/16 and 3/32 blades were the usual tapered sides. I called them to inquire and they said they aren't stocking the T shaped blades any more. I kept what they sent because the price (roughly $5 apiece) was reasonable and I needed them, but I'd like to try the T shaped blade, particularly in the two larger sizes.

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          • #6
            The problem with the T blades is that your holder has to be cut to fit them, or else they try to cock to the side by resting on the bottom and one arm of the T.

            I do just fine with the cheap straight one I've had for years, really. Grind the top of the cutting edge flat (It has a vertical point, wtf?) or even groove it a hair if I have the dremel handy, grind a little clearance at the sides.

            T's would be nice, but this thing came with a $8 holder and has outlasted it and the lathe it was originall on. I'm too lazy to buy more.

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            • #7
              I use the T blades in a regular holder with no back rake and they work find.

              I have quite a stash of them so I am unlikely to buy any for the foreseeable future. I have bought some from these guys in the past:

              http://www.tools-n-gizmos.com/store/..._sale&ppinc=MT

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              • #8
                Originally posted by Deus Machina
                The problem with the T blades is that your holder has to be cut to fit them, or else they try to cock to the side by resting on the bottom and one arm of the T.
                Several solutions for that, like inserting a feeler gauge (or any other shim) on the side of the blade before tightening the holder, and tapered blades have basicly the exact same problem of being able to install crooked without a shim or toolholder properly cut.

                Btw technicaly, its only a problem once the bottom of the blade gets further out then the 'top' since then you lose side relief, Also why you don't need to align a tapered blade exactly, but just ensure theres some taper on both sides to vertical to allow some side relief.
                (Athough I guess being crooked would make the cutting forces much more likey to bend the blade off to one side)
                Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                • #9
                  There is side clearance/relief on those blades.

                  The width of evey part of the parting tool that goes into the groove cut by the parting tool should be not wider than the cutting edge else it will "grab" and "dip/dive" and quite often jamb or break tthe tool - and/or stuff the job up.

                  The rigidity of the top/compound slide is everything!!!

                  I always set the height of the cutting edge above centre of the anticipated "dip" so that when cutting it is at or just above centre height. That way I minise the tool "dipping" and being pulled forwad and in and under the small remaining "tit" and perhaps breaking the tool.

                  A parting tool should have as much back rake as is practical so as to ease both the down load on the tool and the up-lift of the job.

                  I'd be more inclined to limit front clearance to strengthen the tool edge.

                  Here is my "shi**y old Chinese" lathe parting off a bit of 2" diameter cold rolled steel with minimum "stick-out" or "grip".



                  with this parting (lower left) parting tool (which has limited cutting depth):



                  I am successful with HSS and TC inserts too - but I prefer HSS.

                  But I have my share of "misses" too if I am silly or too ambitious.

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                  • #10
                    Originally posted by Boucher
                    ....There has been some discussion of a cross groove behind the tip that sort of establishes some back rake and rolls the chip into a small clock spring slug. Your thoughts and experience with HSS parting tools are solicited.
                    Putting a cross groove behind the cutting edge will lower the cutting forces, but tremendously shortens the tools life.
                    Once your cutting edge drops lower than the top of the rest of the blade, going deeper into a cutoff job cannot be done, without binding occuring.
                    To prevent binding, you need to grind back the entire cross groove to return to the original top ( a loss in length of maybe .3"), or relieve the sides of the parting tool, which basically turns it into junk for future work. Don't do it !

                    Rich
                    Green Bay, WI

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                    • #11
                      Parting-off tools are "consumables" here - the small cost being part of running a small shop.

                      All HSS tools and TC inserts are the same.

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                      • #12
                        One fellow at a shop I worked at would always grind a V groove or a small radius cross ways on the parting tool. I didn't like the way it cut and kept a blade in my box to use.

                        The best thing I did was to buy an insert parting tool holder.
                        It's only ink and paper

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                        • #13
                          + 1 on good tools

                          Someone recommended the A3 Kennametal tool to me. I kind of flinched at the $135 price tag then again at the $18 on the inserts. That was 3 years ago and I am still using the first insert and I parted some 2” steel with it this morning. I also bought one of the Newcomer tools from Enco. It is better than the HSS blades but far below the Kennametal in performance. I do ok with the HSS on some materials but am still trying to learn to use it for parting steel, hence the original question.
                          Byron Boucher
                          Burnet, TX

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