Announcement

Collapse
No announcement yet.

Work Hardening?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Work Hardening?

    I've got a pair of Irwin Adj. Tap Holders that gr8tlife (Ed) put me onto for a tapping attachment I'm workin' on. These particular ones have 3/8" socket drives on the back so I set about turning that square hole into a round one when things began to go astray. Nothing new there!

    I'm having to hold it on a threaded section so I padded my 4 jaw with some beer can shims and because I didn't want to crank down real hard on the threads, I went light on the jaw pressure and kept myself to no more than .010 cuts with a spring pass everytime.

    Life seemed wonderful. The 4 corners were beautifully symetrical after every cut. As soon as those corners were gone I was gonna call it done.

    But right when I got close I started getting an occassional chirping noise that became a constant noise in very short order. It got to where the bit was entering with a curling cut but after maybe an 1/8" here comes the chirp and Jeez, was it ever getting shiny in there!

    I bellmouthed it about 20 thou before I realized this wasn't working, lol, but I do believe it musta work hardened on me and all that carbide bit was doing was polishing it. To the best of my knowledge this has never happened to me before, so, my question is, what should I have done to prevent it?

    The particulars. 600rpm on the spindle. .5" boring range. 3/8" solid carbide boring bar. Light cuts. Unfortunately, the key ingredient, which is what metal Irwin uses in their tap holders, is in the "unknown to me" category.

    SP

  • #2
    Originally posted by pntrbl
    I've got a pair of Irwin Adj. Tap Holders that gr8tlife (Ed) put me onto for a tapping attachment I'm workin' on. These particular ones have 3/8" socket drives on the back so I set about turning that square hole into a round one when things began to go astray. Nothing new there!

    I'm having to hold it on a threaded section so I padded my 4 jaw with some beer can shims and because I didn't want to crank down real hard on the threads, I went light on the jaw pressure and kept myself to no more than .010 cuts with a spring pass everytime.

    Life seemed wonderful. The 4 corners were beautifully symetrical after every cut. As soon as those corners were gone I was gonna call it done.

    But right when I got close I started getting an occassional chirping noise that became a constant noise in very short order. It got to where the bit was entering with a curling cut but after maybe an 1/8" here comes the chirp and Jeez, was it ever getting shiny in there!

    I bellmouthed it about 20 thou before I realized this wasn't working, lol, but I do believe it musta work hardened on me and all that carbide bit was doing was polishing it. To the best of my knowledge this has never happened to me before, so, my question is, what should I have done to prevent it?

    The particulars. 600rpm on the spindle. .5" boring range. 3/8" solid carbide boring bar. Light cuts. Unfortunately, the key ingredient, which is what metal Irwin uses in their tap holders, is in the "unknown to me" category.

    SP
    I remember drilling some stainless one time on a drill press. It was for a project in a blacksmith class I was taking. Things were going great until I got distracted and let up a tad on the feed. Boom! Instant work hardening and I was done with that hole until I found a carbide bit.

    Light cuts are bad news with work hardening materials. Also, don't spare the feed and don't stop or dwell in the cut (my mistake on the drill press). You've got to dig in under the work hardened surface and keep going. Coolant helps too, especially in operations like boring that are internal and prone to recutting the chips.

    That 600 rpm may have been light for carbide too.

    Cheers,

    BW
    ---------------------------------------------------

    http://www.cnccookbook.com/index.htm
    Try G-Wizard Machinist's Calculator for free:
    http://www.cnccookbook.com/CCGWizard.html

    Comment


    • #3
      The answer is simple. Don't take light cuts on material that works hardens easily. To implement that you needed to improve the hold on the work of course. The easy way to do that was to bore and thread a piece of round stock to fit the threads in question, then saw it in half and use it to clamp the part in the 4 jaw.
      Free software for calculating bolt circles and similar: Click Here

      Comment


      • #4
        It is also quite possible that the work hardening occurred when the square hole was initially formed or as a result of subsequent heat treating, not as a function of your machining.
        Jim H.

        Comment


        • #5
          Taking the spring pass each cut is the real cause of the problem. Carbide is stiffer than steel but it still will flex, especially in a hard material. That's why it work hardened at the bottom of the hole first. During the spring pass as the amount of material that needed to be removed grew greater the bar began to flex and rub instead of cut.

          Most likely the job can still be completed satisfactorily by improving the hold on the part and taking a deeper cut to cut under the hardened surface.
          Free software for calculating bolt circles and similar: Click Here

          Comment


          • #6
            Sharpen your cutting edge or rotate the insert,drop the speed down a notch and increase the feed.Coolant wouldn't be a bad idea either.
            I just need one more tool,just one!

            Comment


            • #7
              I would use thicker shims and grab it somewhere other than the threads. Surely you can somehow hold it at the square drive area if you think about it. For instance, how about a split collar to slide over the square drive area and chuck on that. Keeping the work concentric will have to be part of the solution.
              It's only ink and paper

              Comment


              • #8
                Originally posted by wierdscience
                Sharpen your cutting edge or rotate the insert,drop the speed down a notch and increase the feed.Coolant wouldn't be a bad idea either.
                Yep. although if your work size was 0.5", the speed wasn't necessarily really high, it still may have been almost double the usual SFM for a tool steel material.

                But less SFM allows more D.O.C.

                of course, if you actually have 10 Hp behind that cutter, pour it on and don't slow down. it's primarily Hp that limits the D.O.C. with higher SFM, so long as the cutter will take the force.

                Never tease nasty materials with a light cut.
                1601

                Keep eye on ball.
                Hashim Khan

                Comment


                • #9
                  The hard part about work hardening is you have to take deep cuts, your finishing cuts have to be like 0.01" deep or so and accurate the first time, theres no going back to shave 0.001" off.
                  Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                  Comment


                  • #10
                    If your taking light cuts then slow the speed way down and use a very small radius on the tip. The high speed light cut will generate heat.

                    Blowing air on the work or using a mist coolant system works.

                    Think no heat.
                    It's only ink and paper

                    Comment


                    • #11
                      Thanx once again guys. We'll be having another shot with some man sized cuts this time. I normally run carbide dry but we'll get the mister out ......

                      SP

                      Comment


                      • #12
                        Carld: I may be mistaken but I don't think its heat that work hardens material, I think its 'rubbing', as in, the actual 'working' of the surface without cutting it properly hardens it.
                        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                        Comment


                        • #13
                          That's two different processes. Work hardening is caused by plastic deformation of the material. Excess heat can cause the material to harden also but it must get hot enough to transform the crystal structure.
                          Free software for calculating bolt circles and similar: Click Here

                          Comment


                          • #14
                            Hmm, Any recommendation/insight on front/back rake vs work hardening?
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                            Comment


                            • #15
                              Ideally the material that is subject to plastic deformation is the chip and is removed, same thing as far as heat goes although to harden from heat it must reach red temperatures. Not at all impossible right at the point of contact.

                              The shape of the cutting edge will be dictated by the strength required for the cutting edge to remain intact under the sort of loads that hard materials demand. That usually rules out high rake and poorly supported edges. Part of the reason for taking a deep cut is that it insures material removal with a neutral or negative rake tool.

                              I have been doing quite a bit of hard turning in the last couple of years including materials in the RC60 ranges. Cutting edges must be very strong because the working loads are much higher than with softer materials. I have tried using high rake edges and it greatly reduces the cutting forces but the edge breaks down quickly.

                              There is always a tradeoff between hardness and toughness. The two are normally mutually exclusive in nearly all materials. One very important exception is a new material that is so far in development as transparent armor for glass. It's aluminum oxynitride and not only is it as hard as sapphire but it has tremendous impact resistance. As far as I know it isn't yet available in cutting tools.
                              Free software for calculating bolt circles and similar: Click Here

                              Comment

                              Working...
                              X