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  • Negative rake inserts

    Hi, recently been playing with some 1" diamiter negative rake 2 insert endmills. Kinda neat how I can use all 6 or 8 cutting edges (triangle/square inserts respectively) of the insert, but im wondering about SFM and cutting depth

    At first I tryed a light cut, 0.01" 6IPM, mild steel, 3/4" wide workpeice, rapidtap cutting fluid covering work, and got a nasty surface finish at 500rpm (120SFM)
    Apon incressing SFM to 200, I got a much better surface finish and less vibration.. but cuts much past 0.02" and the poor finish comes back. Seemed to even overshoot the dialed cutting depth, ie the poor finish is due to material building up on the tip, not the tip skiping over material or deflecting away.

    My mill is an IH mill. Quill, Y and Z axis locked.

    Any tips on getting good surface finishs with this style endmill? And just how hard I can drive it?
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  • #2
    - rake

    I think you will find you will get excellent results with this tool if you ever need to machine bronze or other hard brittle type material.
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    • #3
      Heh, Also how is it for removing mill scale? id assume not technicaly having a cutting edge to dull it should cut through abrasive material with longer tooling life?
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      • #4
        mill scale

        no sure about the mill scale but they are a life saver on hard bronze. we machine alot of ampco bronze at work and -rake tools do a very good job
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        • #5
          What's the actual insert? There are "negative" inserts that have a little chip-breaker groove that gives an effective positive rake cutting edge.

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          • #6
            Nowhere near enough spindle/surface speed for negative carbide. Try at least three times what you are using.

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            • #7
              600SFM on mild steel??? that seems way excessive for any of the charts iv seen..
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              • #8
                Originally posted by Black_Moons
                600SFM on mild steel??? that seems way excessive for any of the charts iv seen..
                Thats on the low end, just popped up a couple of programs and my 3" 45 degree lead facemill, generally run at 900sfm in mild steel(1140 rpms). Ran that one set of inserts for over a year (4 cutting edges per, literally tons of material on one set of inserts). .050 - .100 doc, feeding about .009-.011 per tooth, 50-70 ipm. On a light finish pass, bump it up to 1500 sfm and slow down the feed a bit.

                Sandvik R390's mild steel, I used to run 1100 sfm on the conservative side and up to 1400sfm, not taking light cuts, tool life was fantastic, 1-3 hours of cut time. It took a while to get up the balls to realize that I could run that fast.

                Your built up edge, indicates too low of a speed and too low of a feed.

                600sfm is a good starting point, and make sure you are feeding it hard enough, on the very low end, depending on the insert geometry I wouldn't want to feed less than .0025 per tooth.

                Ditch the oil too, unless you like sticky chips. Run it dry. You will get a good finish.

                I'm not familiar with your mill, but you may run into rigidity and HP issues.

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                • #9
                  Hmmm, I still can't find much refrence online to going faster then 400SFM with uncoated inserts in mild steel. 50% faster with coated inserts...
                  Also I might note these are random crappy uncoated inserts included with my cheap chinese insert tooling, so I suspect they are carbide made from the left over floor sweeping from around the carbide press :P
                  I'll look on ebay for some new inserts..
                  Now I just need to figure out what kinda insert I want...
                  TNMA TNMG TNGA TNGG..
                  M or G accuracy...
                  G chipbraker or A none.... Q and U are also options that might work judgeing from my endmill holding screws but those seem rare?

                  those seem to be the 4 common types that will fit this style of mill.. Unless anyone else knows a common TNxx insert type.
                  And then I just need to decide on thickness, coating, carbide grade, type of chipbraker.. *sigh*
                  So many decisions...
                  Last edited by Black_Moons; 12-23-2009, 08:04 AM.
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                  • #10
                    I have been running the positive rake Tungalloy carbide inserts as high as 1000 sfm and my negative cermets as high as 2000 sfm. I really like the light show.


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                    • #11
                      Originally posted by Black_Moons
                      Also I might note these are random crappy uncoated inserts included with my cheap chinese insert tooling, so I suspect they are carbide made from the left over floor sweeping from around the carbide press :P
                      Now THAT changes everything. Give 400sfm a shot. Some of the new wizz bang coatings and geometries really let you get after it.

                      Also, be aware that a lot of the TNxx inserts are for turning, and you could have less than desirable results. Look for the softer carbides, generally a roughing insert, one that will wear rather than shatter.

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                      • #12
                        Originally posted by Black_Moons
                        600SFM on mild steel??? that seems way excessive for any of the charts iv seen..
                        I run 1200 fpm on pre hard 4140 on the lathe for finish cuts.

                        Throw away the charts, buy good inserts, and look at the finish to determine the speed.

                        You already found out that it improved a little when you sped it up a little so....

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                        • #13
                          bobw53: Ah. I'll try the 1000 SFM when I get some new inserts then.
                          As for 400SFM with these crummie inserts, its also an interrupted cut, does that factor into going slower with negative rake inserts? Or should I just feed less and cut shallower?

                          Wow evan, nice show. Did'nt know posative inserts could run so fast! Maybe i'll try cranking the speed up on my lathe next time im playing with carbide. Sure would solve workpeice lighting issues while working! hah.

                          How is negative rake on softer materials like aluminum?

                          I wonder why all those charts list lame SFM's like 100~400 for mild steels.
                          Charts for inserts 10~20 years ago?
                          Really conservative ratings?

                          Iv seen those 'negative into posative' rake inserts, TNxP oddly enough.. even though my carbide insert chart doesnt list P as a hole/chipbraker type, I guess it stands for 'posative'
                          Not really sure what a negative posative insert qualifys as.. Id guess its just as weak yet free cutting as a typical posative insert but just fits in negative holders and has 2x the cutting tips as posative inserts.
                          Last edited by Black_Moons; 12-23-2009, 08:35 AM.
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                          • #14
                            tryed the inserts at 400sfm, Much nicer surface finish, my first pass through the scale only took about half the scale off but it gave a near mirror finish.. I was impressed, to bad it did'nt last for the next pass.. that gave the typical 'smearing' pattren I often get with mild steel, where it looks like someone took your mirror finish and galled it with aluminum or something.

                            One thing to note was SERIOUS pain from those BLACK chips, And lots of black spots from the oil + mill scale dust on my shirt now.. -_-; maybe I should start using them dry after all..
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                            • #15
                              Wait til I send you some of those solid carbide sticks I promised. Use those to remove the scale and save your inserts for clean cuts.

                              Negative rake works fine on hard aluminum, especially if you can go fast enough. There is no top limit for machining aluminum.
                              Last edited by Evan; 12-23-2009, 11:02 AM.
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