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  • Carbide rake

    It seams that most carbide inserts are negative rake, is it better than a positive rake or is it that you can get more cutting edges that way.

    My understanding of it is that negative requires more HP and positive less while freer cutting. I use TMNG inserts on a 13" lathe with a 1 1/2 HP (???Chinese 1 1/2 hp) motor and they work fine but would positive inserts work better on the same types of steel. They have a molded chip breaker so they not have to much of a negative rake.

    What is the reason for using a neg. rake and would a positive one be any better.
    The shortest distance between two points is a circle of infinite diameter.

    Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

    Southwestern Ontario. Canada

  • #2
    Has to do with the cutting dynamics of turning. When you're turning, you're not "slicing" the material at the cutting edge, you're peeling it off just prior to the edge of the tool.

    There's a really slick video we saw in my machining program that was excellent in demonstrating what's actually happening between the tool and the work. I wish I could share it here.

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    • #3
      Okay, I'm far from an expert on this one. To date, I don't even own a single neg. rake toolholder...

      Everytime someone seems to ask about rake (at least on PM), the mantra ends up: positive, positive, positive. Want a face mill? Get the high-positive one. Want a lathe toolholder, get the positive one, etc. etc.

      Yet, when you look in namebrand catalogs, most lathe tools (to take an example) are negative. Wha?

      My understanding is the negative tools are best for hogging material. The insert is supported better because of the pressure angle. Positive would seem to me to put more uneven stress on the carbide itself rather than compressive force supported by the thickness of your toolholder backing it up. So 1) you can take larger DOC, and 2) your inserts last longer. Both of which only matter in high production environments.

      As I said, though, my qualifications on this subject are slim to none. We'll see if others prove my thoughts wrong or right I will be curious to hear an informed perspective on when negative rake actually excels in any area other than efficiency.
      Last edited by Arthur.Marks; 06-11-2010, 08:53 PM.

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      • #4
        Would a positive rake insert be better for lighter finishing cuts compared to a negative rake insert, would it give a better finish.
        The shortest distance between two points is a circle of infinite diameter.

        Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

        Southwestern Ontario. Canada

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        • #5
          I put positive rake rake WNMG inserts (kennametal WNMG431-FW) into my negative rake insert holder. I still get 6 cuttting tips... and a better surface finish on lighter cuts. On heavy cuts, negative rake gives me better results, and longer life.


          Surface finish is complex... and minimum DOC complicates things on lighter machines. Look at Kennametal catolog -you can get inserts in both positive and negative rake for negative rake holders that have a minimum DOC of 75 thou (or more), and down to 2 thou.
          Last edited by lakeside53; 06-12-2010, 12:38 AM.

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          • #6
            This is the last test cut I did on the "Wreck" after installing new rear spindle bearings. The first 2" or so were done at 1500 RPM and then the speed was increased to 3000. DOC was .0075" feed .006". Insert is TPG 321 with a molded chip breaker. Material is 1018.

            Harry

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            • #7
              Originally posted by Arthur.Marks
              Everytime someone seems to ask about rake (at least on PM), the mantra ends up: positive, positive, positive. Want a face mill? Get the high-positive one. Want a lathe toolholder, get the positive one, etc. etc.
              -It's my impression that the positives are recommended for home-shop and relatively small machine use. They take less HP and work better with less-than-rigid machines.

              The negatives are better for high-production-rate, high-metal-removal applications with heavy machines and rigid automated machines.

              That's not a hard-and-fast rule, of course, but that's generally it, and why people here tend to recommend the positives.

              Doc.
              Doc's Machine. (Probably not what you expect.)

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              • #8
                A carbide inserts cutting edge is pretty fragile. By having it at 90° this strengthens the edge as much as possible. The holder’s pocket is then angled down to give the insert clearance.
                An insert that has a 10° chip breaker and is raked forward 5° in the holder gives you 5° double positive cutting configuration.
                The edge prep on the insert determines the minimum depth of cut (along with feed rate). On a insert with edge prep, if you feed slow with a light cut you are actually cutting below center on most of the inserts cutting edge. This will destroy the insert and give you a real nice finish (just kidding on the nice finish)
                You do get more edges out of a negative insert also, and they do require more horsepower over a positive. Another advantage is they are usually less expensive.
                If they work for you, use them for roughing down and then finish with a positive.


                Mike

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                • #9
                  Negative or neutral rake inserts are also best on graby materials like brass where a posative insert will just give you a bad day.
                  Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                  • #10
                    The inserts are negative to give the edge more strength - carbide is brittle and the acute angle formed by positive rake doesn't last well. Negative does require more cutting force meaning more hp and rigidity (hence the common recommendation its not the best choice for light home shop machines), but creates longer lasting tooling when using carbide
                    .

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                    • #11
                      This has become an interesting subject to me. Beckley23's photo shows what I have found about inserts. He didn't say whether the insert was negative, neutral or positive to the work. Speed always produces a better finish with carbide.

                      I have mostly used 0 rake HSS and brazed carbide cutters and when roughing I didn't worry much about the sharpness. When taking finish cuts I was careful to get a sharp edge and radius on the nose that related to the feed I would use. The best finish on finish cuts with carbide was at .005" more or less depending on the spindle speed and I found high spindle speed was important with carbide.

                      Now that I am starting to use inserts more I have some issues with them. For one thing inserts are NOT sharp and looking at them with a magnifying glass will show you that. I guess you can buy inserts that are sharp but you would have to order them that way and probably a higher price.

                      Using an insert held at a neg. rake works ok for heavy roughing cuts but is worthless for finish cuts. I also don't buy that an insert held at a neg rake has better strength of the cutting edge. Draw it on paper and then tell me the cutting force on the downward angled insert doesn't tend to try to break the edge off. The strongest edge is at 0 rake with minimal front clearance where the cutting force is straight down on the insert.

                      Inserts held at 0 rake cut well under most conditions I have used them in, but still require high speed and sometimes slightly fast feeds.

                      I have not had good luck with HSS or carbide at positive rakes except for aluminum and sometimes brass/bronze.

                      It's for the above reasons I stick to inserts at 0 rake as they perform better for what I want. For manual lathe operations a cutter at center line 0 rake does a better job for me than anything I have tried. That holds true for brazed or insert carbide and HSS.

                      As to a chip breaker, on the inserts I have with a chip breaker there is a flat area after the cutting edge and then a dropping radius to make the chip break. That is, the chip breaker radius is NOT right behind the cutting edge on the inserts I have and have seen. On the other hand, I have not seen all the inserts that are made and really don't care to. If I need something special I inquire about it.

                      Since I am not into high speed production or CNC I can't say what would help or hurt there.

                      The problem is that there are so many different inserts it is hard to decide which one will do what you want without calling the makers. Even then you may get false info. One maker will say this type and grade is best and another will say something different. I even had two different salesmen at the same company give different recommendations, go figure .

                      It seems experimenting was the best test for me but that can get expensive with inserts for special alloy metals. Some companies will send samples and that helps.

                      All I can say is EXPERIMENT, your results may differ from others and nothing is written in stone.
                      Last edited by Carld; 06-12-2010, 10:47 AM.
                      It's only ink and paper

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                      • #12
                        Originally posted by Carld
                        For one thing inserts are NOT sharp and looking at them with a magnifying glass will show you that.
                        Very true. This is part of the reason for the heavy DOC needed, to my understanding. I have never seen a specifically noted "sharp" insert for steel, but there are ones for aluminum. I can attest that they are indeed sharp! The copy sounds a little too good to be true, but they do work very very well. This is one insert geometry example: http://latheinserts.com/product.sc?p...&categoryId=88

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                        • #13
                          Very interesting thread.

                          I have to wonder about the sharpness of inserts. Is the lack of sharpness on most inserts deliberate or is it just a consequence of how they are made? I suspect that they are made from powder that is first formed in some kind of mold and then baked to fuse it into a solid. I would appreciate corrections or further insite into this from anyone knowledgable.

                          But that kind of process would make forming a really sharp edge difficult. A further sharpening operation would be needed to produce a truely sharp edge and that would add to the expense. So are most inserts made to a price point and the slightly dull edges just a result of this? Or is there a real advantage to a slightly drll edge?

                          I may have to sharpen the edges of some truely dull inserts I have to see how they would perform in steel.
                          Paul A.
                          SE Texas

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

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                          • #14
                            Sharp edges are much more prone to chiping. more unsupported edge.

                            Try an interrupted cut in mild steel with a 'supersharp' aluminum insert.. then check how 'dull' it is after the edge chips off.
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                            • #15
                              In "Design and Use of Cutting Tools" by St. Clair, he mentions that carbide fails more quickly when ground to a sharp edge. Carbide is hard but brittle so it tends to develop a micro-chip at the cutting point; this then expands,leading to failure. He found that dulling the edge very slightly on carbide led to much longer tool life.

                              St. Clair also found that negative rake tooling can be advantageous with carbide because there is more carbide material adjacent to the edge. That is, the edge is 90 degrees and relief is provided by angling the whole bit down. On positive rake tooling the included angle is less than 90 so there is less carbide adjacent to the cutting edge for support and heat conduction.

                              John
                              Location: Newtown, CT USA

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