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

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  • #16
    Here is a good example of a boring bar.

    Free software for calculating bolt circles and similar: Click Here

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    • #17
      Originally posted by philbur

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

      Phil
      I had one break off at the thread for the insert screw.

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      • #18
        IMO - The degree of how well a boring bar cuts is primarily the insert geometry and secondarily the density (or stiffness) of the bar itself. That density helps damp vibrations, and is why solid carbide and heavy metal (high tungsten content steel) bars seem to cut smoother.

        What diameters and length-to-diameter bores are you looking to finish with this homemade boring bar? And is there some reason you've chosen CPGT over the more-common CCGT or the more-edges WCGT? I don't know what equipment you intend to use for milling the insert pocket sides, but a 7؛ (14؛ included angle) cutter for a CCGT side clearance angle is probably easier to find than an 11؛ (22؛ included) cutter.

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        • #19
          I got a ccgt bar and returned it immediately because the degree that it needs to tilt the insert downwards in order to provide enough relief for small bores (<.5"). A CPGT needs to be tilted less.

          The aim is for bores from slightly under 1/2" and larger. The length will be adjustable because I will also make a holder for it that it can slide in and out of.
          I'm going to CNC out the pocket, so its particular shape isn't an issue.

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          • #20
            secondarily the density (or stiffness) of the bar itself.
            Density and stiffness are not related. Think lead.
            Free software for calculating bolt circles and similar: Click Here

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            • #21
              True, that. Carbide is pretty dense though, and remarkably stiff too. A lead boring bar, not so much.

              Originally posted by beanbag
              I got a ccgt bar and returned it immediately because the degree that it needs to tilt the insert downwards in order to provide enough relief for small bores (<.5"). A CPGT needs to be tilted less.

              The aim is for bores from slightly under 1/2" and larger. The length will be adjustable because I will also make a holder for it that it can slide in and out of.
              I'm going to CNC out the pocket, so its particular shape isn't an issue.
              The inclination angle of the insert, especially with high-positive screw-down inserts intended for boring operations, is easily offset by the chipbreaker/top form geometry design. I've never seen any great advantage to CPGT over CCGT in performance, and there sure are a LOT more CCxxT inserts out there to choose from.
              Last edited by PixMan; 11-01-2011, 11:42 PM.

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              • #22
                Originally posted by PixMan
                The inclination angle of the insert, especially with high-positive screw-down inserts intended for boring operations, is easily offset by the chipbreaker/top form geometry design. I've never seen any great advantage to CPGT over CCGT in performance, and there sure are a LOT more CCxxT inserts out there to choose from.
                For a bore of just under .5", a ccgt insert needs an inclination of -9.x degrees whereas a cpgt only needs -5. Besides, I only need one cpgt insert, namely a Kennametal cpgt-HP 21.51 which can be had on ebay. The KC730 coating seems to work on all materials. There is also a TiB2 coated version that is supposed to work for aluminum, but I have that coating on another insert, and it doesn't seem all that great.

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                • #23
                  Even though all steels are supposed to have the same stiffness, I can just pick up a stick of stainless 304 and it somehow "feels" easier to flex (even before taking a permanent set) than a hardened steel, like an aircraft drill.

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                  • #24
                    The other day I needed a small boring bar and couldn't find what I needed in the tool room so I took a piece of water hardening drill rod and made my own. Rough machined the shape, then took it up to cherry red, swished in a bucket of water to cool then took it up to pale gold to draw and finally polished and sharpened. Made two of them and you can't tell them apart from a Bokum brand tool of the same size. For the cost of a set of quality purchased boring bars you could make half a bucket of your own.

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                    • #25
                      I make my boring bars out of what ever it don't matter for the stuff I do the small bar 1/4 X 1/4 is quite good I don't pack it up in the tool holder (non height adjustable) I just bend it up to the right height the sharpening of the tip is the critical thing The one on the far left is broken i think it had too much heat when brazing

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                      • #26
                        There is a small difference in Young's modulus between stainless and carbon steels but not one you would notice by "feel".

                        It's all in the mind. Within their respective elastic limits reinforced concrete and granite are 3 to 6 times more flexible than steel.

                        Phil


                        Originally posted by beanbag
                        Even though all steels are supposed to have the same stiffness, I can just pick up a stick of stainless 304 and it somehow "feels" easier to flex (even before taking a permanent set) than a hardened steel, like an aircraft drill.

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                        • #27
                          Regardless of the modulus, you can "push harder" with hardened tools, and that is why everyone thinks they are more rigid.....

                          Because they ARE more rigid......

                          At least in the definition most people use. I can guarantee you that an unhardened lockpick, prybar, or similar tool will not work as well as a similar hardened one, if it works at all.... with hardening, the ultimate force that can be applied is larger, because you go right past the stress that causes deformation in the unhardened tool.

                          Sure, it eventually breaks, but.............

                          As for grade 8 bolts, grade 8 or grade5 bolts are often a material that workhardens ferociously, and can be a bear to work with if you try to sneak up on dimensions.
                          1601

                          Keep eye on ball.
                          Hashim Khan

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                          • #28
                            I've made one out of a cummins valve.

                            Andy

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                            • #29
                              Trouble is a boring bar doesn't have the same functional requirements as a lockpick or a prybar so the analogy is a bad one.

                              You can push harder because they are stronger not because they are more rigid.

                              Phil

                              Originally posted by J Tiers
                              Regardless of the modulus, you can "push harder" with hardened tools, and that is why everyone thinks they are more rigid.....

                              Because they ARE more rigid......

                              At least in the definition most people use. I can guarantee you that an unhardened lockpick, prybar, or similar tool will not work as well as a similar hardened one, if it works at all.... with hardening, the ultimate force that can be applied is larger, because you go right past the stress that causes deformation in the unhardened tool.

                              Sure, it eventually breaks, but.............

                              As for grade 8 bolts, grade 8 or grade5 bolts are often a material that workhardens ferociously, and can be a bear to work with if you try to sneak up on dimensions.

                              Comment


                              • #30
                                Originally posted by Evan
                                Here is a good example of a boring bar.

                                Evan, this is one of the things I like about you. You have the ability to go so far in subject depth as to cause eye glazing if coffee isn't close by. Then there's the other Even... brief and funny!

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