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lathe DOC question

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  • lathe DOC question

    morning gang.
    earlier today I set out to make a 3" face mill.. 6 inserts with integral
    R8 shank. now usually I don't like to take stock down that far and would
    opt for a press fit or something similar.. but I've had this 3" mystery metal
    piece a long time.. was just the right length.. and figure if all turns out
    well I'll have the tool a long time and worth the investment.

    the material is mystery metal to me.. probably like a 1040 or 1050.

    To start I had to get my blank down from 3" to 1.25" over the first 4".

    I was getting very long stringy chips.. dangerous kind. So I started to slowly
    increase both my DOC and my feedrate.

    Now, as a part-time HSMers, I've never worried much about feeds / speed
    and basically go on feel / finish / cutter life. I probably err on the slow side
    but I'm in no rush.

    Lathe: colchester student
    tooling: old carbide insert I can no longer read the code on .. Mitsubishi
    ANMM220412P maybe? not sure.. but triangular and a big chip breaker.
    almost looks like top rake. The holder has negative rake.

    At a consistent 500 rpm I had to increase the DOC to 0.060" (so .120" off
    the diameter) and feed to about 0.010" according to the chart.

    at this point I'm getting nice blue 6's and 9's but the tip of my insert
    is discoloring (from heat).

    Just curious, do those numbers sound outlandish to anyone?
    have I been overly conservative to date? I'm really moving material
    compared to when I'd just put up with the long stringy chips and clean
    them after every pass.

    I've never done more than 0.020 or so DOC on this lathe.. haven't
    had it long, still getting to know eachother.. but its not even breaking
    a sweat at 060 DOC (though i wouldn't want to push it any further!)


  • #2
    It sounds to me like you've actually got a TNMM433 insert there. That would be a negative rake, single-sided insert. While they are primarily used for roughing, you may actually have one that has such a large up-sharp positive top form geometry (a.k.a. "chipbreaker") that it's more of a finishing insert. The "P" might also indicate the high-positive chipbreaker that gives nice shearing action and low cutting forces.

    I commend you for doing the right thing in increasing depth of cut and feed rate. Many people have a natural inclination to do the opposite when things aren't working quite right.

    You are getting the correct chip form with the 6-9 shape, and the blue means you have the cutting speed correct. That color is a sign that the heat of the turning process is being carried away with the chip rather than excessively heating the tool or workpiece. Well done!


    • #3
      Horsepower is your friend, here. Is the lathe straining to make the cut, if not increase the DOC to .100-.200, keeping the feed rate the same, and see what happens.


      • #4
        Sounds all fine, though you might wanna go with a stronger insert. The more negative rake and less chipbraker/relief, the more the tip can transfer heat into the bulk of the insert and hence the harder it can work.

        0.1" DOC in steel is no big deal for carbide. your lathe complain bitterly when you take it too far.

        When the chips start burning you and melting into plastic things they land on, then you have a problem... Because yaknow, your chips are burning you and melting into things. the carbide still does not care.
        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


        • #5
          holy smokes 0.2" DOC? wow I'll have to give that a try.
          The fact that my insert tip is changing colors isn't a bad sign?
          Its still cutting.

          Thing with taking that deep of a cut is that my radius is changing
          so far I'm no longer sure what parameters I'm changing.

          I'll post some pics of the insert when I have my camera handy.



          • #6
            Greater depth of cut can make the chip harder to break.

            Higher feed rate, (more ipr) will usually help break the chip.



            • #7
              Radius does not matter too much with roughing deep cuts, you are not going for a good finish, just try not to leave the metal riped up deeper then you plan to finish. Its better to have a larger radius when roughing for durability however (Fine points snap off)

              Note my lathe takes 0.1" DOC in steel, But its 1000lbs and 2HP and I can hear the motor starting to slow down at higher feed rates and work over 1". I have had 1018 steel chips come off my cheap chinese carbide that turn black, Burn anything they touch, Yet the carbide seems happy.

              Insert tip changing colors is more likey due to gunk stuck on it, or the coatings wearing off. Carbide does not go funny colors with tempature like HSS. If you snap the carbide, thats too deep a cut. If you wear it down, thats too shallow a cut, You want just below snaping the carbide and stalling the lathe. Meaning you will wear it out, but you will still likey end up doing so much work with a single insert you are more likey to do something stupid and snap/chip it due to work impact or something like that, then actualy wear it out from cutting action.

              Inspect your carbide inserts under a 10x+ magnafyer like a lupe or microscope, Its the only way to actualy tell how intact the cutting edge is. Don't go by color, go by its physical shape (And be prepaired to scrape off chunks of metal that have stuck to the carbide)

              Infact, chunks of metal getting stuck to the carbide is what will 'rip up' metal deeper then you are planing. Random chunks of metal make poor cutting tips and cut deeper then your insert. cutting oils really help, athough at higher feeds/speeds they boil instantly.
              Last edited by Black_Moons; 03-11-2012, 03:43 PM.
              Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


              • #8
                If you are getting "chunks of metal that have stuck to the carbide", that's a condition known as "Built Up Edge" and is easily corrected with more speed and more feed. In the home shop just starting out with carbide, it's the single most-common failure mode.

                The problem lies in most home shop guys not having enough speed on their machine to employ coated carbide tooling...or being too afraid to use it. The material builds up on the cutting edge, reaches critical mass, and then wipes out the insert in catastrophic failure.

                Then we get "That 'brand x' carbide sucks.


                • #9
                  Sorry for the typo when I said "radius" I meant the radius of the
                  work, not the radius of the insert. With the radius changing so fast
                  (with 0.1 DOC for example) my SFPM is changing with every pass.

                  But thats all well and good, I'll experiment more next time I have
                  lots of stock to remove. See if I can't get these belts to slip.

                  I snagged some pics of the insert and the flycutter for those interested..
                  just to flesh the thread out a bit:

                  note they're a bit over 3/4" on their long sides. I also didn't notice
                  (till now) that the top is dished. I got the tool and a pack of inserts
                  with the lathe purchase. This is probably the 3 or 4th time I've used
                  them. Yes they are single sided:

                  They mount in the holder with some negative rake:


                  • #10
                    Here's the cutter so far:

                    the chatter/steps you see in the radiused wall are due to the face mill
                    I was using.. don't have an endmill large enough. I suppose I could've
                    made it a square pocket.. but this just kinda looks better. 1.5" 3 insert
                    face mill taking 0.1" DOC and using a DRO to stop at a zero point.

                    Prime example of having stops on a mill table (mine doesn't.. yet!).

                    The finish there hasn't bothered me so far but I may just go in with
                    a small grinder and smooth it out a bit.



                    • #11
                      What inserts do you plan to use in that cutter?

                      Please don't take it personally, but I wouldn't ever bother making a milling cutter that can fit lathe inserts. In the past I've seen many people doing so find out why lathe inserts are made for smooth, uninterrupted cuts while milling inserts are made specifically for the hammering action of cutters like that.

                      From the looks of it you probably plan on using TPG43x style inserts. Look specifically for milling grades if that's the plan. Other than that, nice work so far.


                      • #12
                        I bought a lot of 60 Sandvik TPKN 22 04 PD R for $60 ($1/pc) milling inserts.
                        I haven't recieved them yet but made the seat to fit those.

                        Bad move? I still have some breathing room to modify the seat if I had to.



                        • #13
                          are you sure thats the correct holder for the inserts? (the turning tool)


                          • #14
                            No idea. Like I said it came with a box of tooling (most of them were whittled
                            away to get them to fit the original 4 position turret post) -- the holder
                            you see in the picture already had the insert in it.. and there was a box
                            with maybe 4-5 more.

                            Something makes you think thats not the right holder?


                            • #15
                              i would be worried about the overhang.