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Is a grade 8 bolt a good material to make an insert toolholder out of?

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  • Is a grade 8 bolt a good material to make an insert toolholder out of?

    I want to make an insert boring bar, but I don't want to deal with that heat treating stuff. How would a grade 8 bolt work? I would need to do a little off-center turning to make the head vs shank, and then CNC a little insert pocket in it. My other option is that I have a bunch of 1/2" square 4140 annealed.

  • #2
    Does not even need to be heat treated. Any steel will work.

    But have you looked how cheap a basic boring bar is? Even the American made ones are stupid dirt cheap. I can't justify making one. The borite ones are pretty good. Pretty cheap inserts too.

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    • #3
      Grade of steel will not change how rigid the bar is. It will somewhat change how easily the pocket wears, mainly due to damage/failure events. And mainly determin if your boring bar snaps, Or bends, when a serious failure happens. (Come to think of it, Snaping would likey be the better failure mode then bending for a boring bar)

      That said just about any steel is fine.
      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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      • #4
        more specifically, one that takes a cpgt insert. Not ccgt.

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        • #5
          Originally posted by Black_Moons
          Grade of steel will not change how rigid the bar is. It will somewhat change how easily the pocket wears, mainly due to damage/failure events. And mainly determin if your boring bar snaps, Or bends, when a serious failure happens. (Come to think of it, Snaping would likey be the better failure mode then bending for a boring bar)

          That said just about any steel is fine.
          But the grade 8 bolt is STRONGER than my stick of 4140. That means it will take much more force before it bends/breaks, and the pocket should be harder to deform as well.

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          • #6
            I've got a couple dozen home brew boring bars in my collection. Most are intended for HSS or brazed-in HSS or carbide. Most were crude works of the moment but a few are carefully made with built in dampers. Except for the fancy ones most all were made from un-heat treated drill rod or plain vanilla mild steel.

            A boring bar sees no great stress in service. It's made stout to be stiff; to resisst deflection in the cut. The factory made boring bars are made hardened to make them more durable to abuse, chip wash, and mild collisions.

            When pressed, back in the day, I could make a quick and dirty boring bar in about a quarter hour - unless it was made for boring Acme threads in a lead screw nut. Sometimes those had to be a works of art.
            Last edited by Forrest Addy; 11-01-2011, 07:04 AM.

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            • #7
              Originally posted by beanbag
              But the grade 8 bolt is STRONGER than my stick of 4140. That means it will take much more force before it bends/breaks, and the pocket should be harder to deform as well.
              Stronger is necessarily the right word. It has a higher tensile strength, but along with that higher tensile strength comes a higher likelihood that when if it fails, it will snap as opposed to bend.

              The grade 8 bolt will have less flex and be more ridged, that is for sure.

              Beanbag, I have a crap load of B7 threaded bar you can have.. Its 4140 heat treated to below gr 8, but higher than a 5. B7 is used for holding all your local refineries/chemical processors flanges together. You need carbide to turn it and its cheaper to use coolant than burn up inserts.

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              • #8
                They are both made of steel therefore they have the same modulus of elasticity, each has the same flexibility/rigidity for the same geometry.

                Has anybody ever actually "broken" a steel boring bar?

                Phil

                Originally posted by cuemaker
                The grade 8 bolt will have less flex and be more ridged, that is for sure.

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                • #9
                  Originally posted by philbur
                  They are both made of steel therefore they have the same modulus of elasticity, each has the same flexibility/rigidity for the same geometry.

                  Has anybody ever actually "broken" a steel boring bar?

                  Phil
                  Isn't it amazing how various people interpret some words? :-)
                  I think the TV ad people have about ruined the English language.
                  (American version at least, sorry Brits, Ausies,etc.)
                  ...lew...

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                  • #10
                    philbur is correct. The slope of the stress/strain curve for steel (i.e. its "stiffness") is virtually independent of hardness in the elastic region--the range of stress in which the steel returns to its original form when the stress is relieved.

                    Hardening increases the elastic region --- the steel can withstand a greater stress before it take a permanent set , i.e. bends, but that's all.
                    ----------
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                    • #11
                      But the grade 8 bolt is STRONGER than my stick of 4140. That means it will take much more force before it bends/breaks, and the pocket should be harder to deform as well.
                      To reiterate, all steels and steel alloys have about the same resistance to bending (given the same size of material) up to the point of permanent deformation.

                      The "strength" of the alloy determines how easily it deforms permanently. It has no significant effect on how easy it is to deflect before deformation.

                      Different materials have a different modulus of elasticity. That is what determines how easy deflection is. Tungsten carbide is about three times more resistant to deflection than steel alloys. That means it is three times stiffer which is why the best boring bars are made from tungsten carbide.


                      HOWEVER, as usual, there are some exceptions. If steel is prestressed far into the work hardening portion of the stress-strain curve it changes the modulus of elasticity. The material will not return back down the curve to where it began during deflection and following deformation. This effect occurs just before failure.

                      If you look at the stress strain curve for steels it rises linearly until the point of deformation. When deformation occurs the curve climbs for a bit and then falls as the applied stress required to produce further deformation decreases. As work hardening begins the curve then rises again. If the stress is removed just before failure the steel comes back down the curve but not back up over the hump. This moves the bottom of the curve to the right.

                      That will make it harder to deflect than usual. The effect is quite pronounced in some alloys, especially ones that are not considered hardenable such as mild steel.
                      Last edited by Evan; 11-01-2011, 02:39 PM.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        Original Question: Is a grade 8 bolt a good material to make an insert tool holder out of? Answer: Yes

                        OP: My other option is that I have a bunch of 1/2" square 4140 annealed. Answer: That would work also.

                        Other things being equal, I tend to use what ever I have the most of or what is more available.

                        I also stock some 12L14 which machines so nice.
                        Byron Boucher
                        Burnet, TX

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                        • #13
                          Originally posted by cuemaker
                          The grade 8 bolt will have less flex and be more ridged, that is for sure.
                          .
                          As others have pointed out - the grade 8 will flex just the same as 4140 and be no more rigid.
                          "Patriotism is the last refuge of a scoundrel"

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                          • #14
                            Yes, a grade 8 bolt will work well. Here is how to make an insert tool holder from one:



                            And this is how it works:



                            And yes, it will also work as a boring bar.
                            Last edited by Evan; 11-01-2011, 03:54 PM.
                            Free software for calculating bolt circles and similar: Click Here

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                            • #15
                              "Has anybody ever actually "broken" a steel boring bar?"

                              I crashed one into the chuck and broke the end off .

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