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  • Exploding cutoff blades

    All too often I'll have a hss cutoff blade break mid-cut. I just had one explode and get flung 5 feet from the lathe. This usually happens in deep cuts in steel, or even worse, aluminum. What am I doing wrong, if anything?

    Situation: hss 3/32nd's cutoff blade (import). Blade is tapered (bevelled.) Lathe in backgear, mid-slow rpms. Manual feed, trying to feel the chip lift off, then advance on the "soft" pocket that results. The blade is positioned slightly above centerline. Using Tap-magic generously.
    Going good until about halfway there, then BOOM ! I think tonight's instance (12L14 steel), the chip lifted but didn't peel away, and the cut off blade got hung-up on it.

    I've had a cutoff blade jam in a deeeeep cut in AL, and stop the lathe dead in backgear with the V belts squealing. Something tells me this should be avoided.

    Is 3/32nd's too thin? Should I be making a relief cut? Certainly I'm not the only one to experience this? I'm beginning to get gun-shy of cutting parts off.

    Thank you,

    Gary
    Gary


    Appearance is Everything...

  • #2
    A relief cut is a very good idea. The other is that you want the blade ON center, not above it.

    If it's above and it digs in and is forced down, it will dig even deeper. That's probably why it's breaking.

    Dan
    At the end of the project, there is a profound difference between spare parts and extra parts.

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    • #3
      I might be doing it wrong, but I cut off at or near the same speed as when turning, tool centered and exactly perpendicular to workpiece, and side clearance for the full depth of cut, flood coolant on. Right or wrong, I don't know, but it works for me. A few degrees top rake can help. Wearing safety glasses and not standing directly in line with the cut off tool might help protect you from a catastrophic failure.

      Comment


      • #4
        Make sure the blade is not extended any further than it has to be and on center. As for speed, I have variable speed control and start off slower on the larger OD and than gradually speed up when reaching the smaller diameters.

        Try to maintain a continuous chip and occasionally clearing the ones that don't make their way out.

        Still have to watch for aluminum and brass since they are so grabby.
        "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

        -- Harold "Doc" Edgerton

        Comment


        • #5
          what size lathe? (And weight)

          I don't think you need a thicker blade, I use 3/32" in my 12x36 1000lb lathe just fine, 1/8" seems too thick for steel on a lathe that small (Yes small)

          What would also help is if you checked out how fast you are feeding

          Engage automatic cross feed (At the same spindle RPM's) and see what setting corasponds to how fast you normaly feed. Or just time your revolutions and do math, maybe with a indicator for travel on the cross feed.

          Getting *dead* on center is very important. Too high and yea it can dig in, too low and the work can bend and ride up ontop of the blade (scary!)

          Is the top of your blade got a chipbraker? My blades are just flat toped and it works fine, too much rake can cause it to dig in.

          Are you aligning your blade to be *PERFICTLY* aligned to 90 degrees to the work, a degree or two off and the side of the blade will try and cut as its fed in deeper.

          You could also grind down the sides of the blade a few mils up to just before the tip of the blade to provide side relief along the length of the blade
          Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

          Comment


          • #6
            Gary,
            Make sure the blade is straight in the holder. It should have clearance on both sides of the blade. If it is not straight in the holder all the relief will be on one side. Don't be afraid to slow rpm's down a little.

            Mike

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            • #7
              What is the condition of the lathe? You can't part off successfully on a worn out machine.

              Comment


              • #8
                Oh, some other questions that came to mind:
                how far is the blade extended out of the holder? It should be no more then is needed (ie just over an inch for parting up to 2" diamiters is what I usally leave it at.. im a little shy about parting over 2")

                How far are you away from the chuck while parting? parting is best done within 1" of the chuck

                Are you using a tailstock? large tailstock ram forces can cause the work to collapse around the tool causing it to bind. its generaly recommended NOT to use a tailstock while parting.
                Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                Comment


                • #9
                  As recommended, I put the tool on center and also made a relief cut. (Went in a bit, backed out and moved over about .010", repeat)

                  These seemed to help alot, thanks.


                  Originally posted by Black_Moons

                  Getting *dead* on center is very important. Too high and yea it can dig in, too low and the work can bend and ride up ontop of the blade (scary!)
                  Scary....Dam right ! (that's why I had the blade just a hair above centerline.)

                  Anyways, lathe is about 1000 lbs, 11" by 36" 1.5 HP I was considering getting insert tooling, but I certainly don't want to invest in a holder and inserts and have it damaged by my stupidity.

                  So far, so good now, thanks all.


                  Gary
                  Gary


                  Appearance is Everything...

                  Comment


                  • #10
                    RPMs too low. You don't need to cut off in back gears. Even if you only use a brush to drip oil/coolant in the groove, do it. back the tool out to clear chips every once in awhile. Two reasons most cutoff blades break. Dull tool and cut clogged with chips.

                    Comment


                    • #11
                      Originally posted by goose
                      All too often I'll have a hss cutoff blade break mid-cut. I just had one explode and get flung 5 feet from the lathe. This usually happens in deep cuts in steel, or even worse, aluminum. What am I doing wrong, if anything?

                      Situation: hss 3/32nd's cutoff blade (import). Blade is tapered (bevelled.) Lathe in backgear, mid-slow rpms. Manual feed, trying to feel the chip lift off, then advance on the "soft" pocket that results. The blade is positioned slightly above centerline. Using Tap-magic generously.
                      Going good until about halfway there, then BOOM ! I think tonight's instance (12L14 steel), the chip lifted but didn't peel away, and the cut off blade got hung-up on it.

                      I've had a cutoff blade jam in a deeeeep cut in AL, and stop the lathe dead in backgear with the V belts squealing. Something tells me this should be avoided.

                      Is 3/32nd's too thin? Should I be making a relief cut? Certainly I'm not the only one to experience this? I'm beginning to get gun-shy of cutting parts off.

                      Thank you,

                      Gary

                      first off you need to set the blade a slightest hair below center,
                      2 make sure its sharp
                      3 use cutting fluiid and ots of it even on aluim
                      4 take extreamly slow feed and i do mean slow and the slowest speed you can get out of the lathe
                      5 not nessasarry for a relief cut but its not a bad idea either,
                      6 do not oever extend the blade length start at 1 inch if you need more you can adjust later if need be..
                      7 follow these steps and you will do just great, ignore them and you will be asking this same question again ,,

                      8 i have parted off 2 inch aluim and steel and have had no problems doing it this way, as soon as ai try and push it i get trouble so i leanred not to push it,
                      i can now and most often do part at 230 for lauim and steel and brass and bronze i have done at upto 975 rpm cause brass try;s to catch and bit all the time ,, dont push the feeds and speeds untill you can do it slow take your time there is no rush ..
                      9 does not matter how small or how big of a machine you got ..if you push it you will have problems

                      Comment


                      • #12
                        Oh, some other good advice: Lock your carriage, lock your compound, And as an intresting option you can run the blade upside down (and in reverse, or on the back of your cross slide) to provide much better chip clearance (as chips fall outta the cut insted of built up ontop of the blade), continious air blast can work to clear chips just as well though im sure, But this just another 'advanced' option, it should work fine normaly.

                        Getting a *taller* blade can help.
                        If you have a BXA toolpost, the standard cutoff holder for it can take 11/16" blades, or 3/4" if you file the top of the blade holder groove like I did (Doesnt even contact the blade in normal operation, so the modification doesnt affect its ability to hold the smaller blade or have any chance of screwing up the holder if you do it wrong)
                        Note that it does add a little bit of rake that can help, Don't add any (well, as much) rake on your tool if your using these
                        (For the record, I use the affordmentioned 3/4" tall blades at 3/32" thickness on my 12x36 without problems in both wedge and T style)

                        Theres also 'cutoff blade' to square shank holder dealies you can buy. these generaly do not add rake.

                        (mmm at air/mist system for cutoff)
                        Last edited by Black_Moons; 01-30-2010, 10:43 PM.
                        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                        Comment


                        • #13
                          I can see that play in the crosslide and compound can easily allow the workpiece to drag the cutter inwards by friction from the sides of the cutoff blade. All of the area on the workpiece in the groove that's below the center axis can impart both a downward and an inward force to the cutter. Normally we only support the cutter against downward forces.

                          My thinking in theory says that the cutting edge should be just a tad above center to impart some small force tending to keep the crosslide positioned against the crosslide lead screw. That may tend to counteract the tendency of the cutter to dig in because of the top rake angle. Just what seems right to me, in theory at least. I have not done a lot of cutoff work on the lathe, preferring to start the mark on the lathe, then cut it off on the bandsaw. I would normally be facing the cutoff ends anyway, so there's always a remount of the workpiece required.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                          • #14
                            One more thing - don't forget to lock the carriage.

                            Comment


                            • #15
                              http://www.gadgetbuilder.com/Vertica...t.html#Parting

                              Adding the "V" notch to the top edge of your parting tool will work wonders. I part lots of 4140 and the notch makes the chips flow so much more freely out of the cut. Give it a try...you'll like it!

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