Re: Isn't this Bass-Ackwards ? - NCRS Discussion Boards

Re: Isn't this Bass-Ackwards ?

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  • John M.
    Expired
    • January 1, 1999
    • 1553

    Re: Isn't this Bass-Ackwards ?

    Wayne,

    I do not think that the roughness of the finish was necessarily a desired thing, but merely a byproduct of the blades in the broaching machine wearing and getting chipped. I am not sure how you could achieve an uniform agressive finish in a linear machine like a broach. It is easy to do in the case of a circualr mill, by adjusting the feed rate to leave ridges, but I do not believe that you could do so with a linear cutter.

    I would hazzard a guess that the blades were only changed when the machine could no longer produce a deck that was within tolerances. It would only make sense that all blades were all changed at the same time and properly set to their proper incremental cut. A fresh set of blades, set correctly, should leave almost no marks behind until the edges of the blades began to nick and wear as time on the machine progressed. We use proggessive broaches in our shop to cut internal keyways, and they leave almost polished surfaces when done, as long as they are not worn out. It is really only the last blade that passes over the material which leaves the mark for the most part, since each progressive set of blades cuts slightly more metal. As long as the last blade in the cutter is neat and clean, the cut will probably be so as well.
    My opinion is that not all pads had anywhere near the same ammount and depth of broach marks, and to judge that as if they did is silly. Hopefully, most judges, realize this and judge the pad finish accordingly. In the case of my car, I had several mechanical judges look judge my pad, and several made comments about the total lack of broach marks, but none ever failed the pad finish. I think that most just recognized the pad for what it really was, an untouched, original pad, and judged it accordingly.

    Regards, John Mcgraw
  • Tim G.
    Extremely Frequent Poster
    • March 1, 1990
    • 1358

    #2
    Re: Isn't this Bass-Ackwards ?

    Guys, I agree with Wayne. I've always understood that the machining process left definite broach marks when the blades were fresh. I've also understood that that is why big blocks had more evident broach marks, because they were more quality conscious and kept the blades in better shape. I've even heard that they would deck the four bolt main blocks when the blades were fresh due to the high performance nature of those engines and that the four bolt blocks should have more evident broach marks. It the world as I know it collapsing.........

    Comment

    • Tracy C.
      Expired
      • July 31, 2003
      • 2739

      #3
      You are correct John. My machine shop

      experience directly lines up with your comments. Cutting tools are expensive and as long as they produce a part that will meet the engineering specs, they are used to make chips. In some case pulled off early for resharping before they self destruct. There is a point whereby the tool becomes so worn that it begins to degrade rapidly with continued use. The resultant machine finish will be far from pretty.

      tc

      Comment

      • Chuck S.
        Expired
        • April 1, 1992
        • 4668

        #4
        Making Chips

        Good to see a couple of members know something about "making chips"...the steel kind; not cow chips, wood chips, buffalo chips, poker chips, potato chips, or bone chips.

        Comment

        • Tracy C.
          Expired
          • July 31, 2003
          • 2739

          #5
          Re: Isn't this Bass-Ackwards ?

          Tim,

          I've never heard the "more pronounced broach marks on big blocks" theory before. I would seriously doubt there is any logical basis associating higher horsepower to a requirement for a rougher machine surface for the head / block interface.

          Throw out the gasket and think of the extreme in each senario. Two mating surfaces machined glass smooth would provide a perfect seal. Surfaces with linear grooves deep enough to hang a fingernail on would provide a perfect escape routes to allow compressed air/gas mixture to leak.

          Me thinks somebody came up with that one because of the propensity of big block cars to be "created" by those with crude broach marks replication "talents".

          tc

          Comment

          • Dick W.
            Former NCRS Director Region IV
            • June 30, 1985
            • 10483

            #6
            Re: Isn't this Bass-Ackwards ?

            TC, I do not know if the finish, which is expressed in RMS, has anything to do with the HP on the big blocks. I do know, from my experience with the early second design Mopar hemi race engines (1964 up)that you can get too fine a finish. Teams were having problems with head gaskets leaking. Turns out that it was that the pad surface needed a rougher finish to hold the gasket.
            Dick Whittington

            Comment

            • Wayne M.
              Expired
              • March 1, 1980
              • 6414

              #7
              Petroleum Refinery piping + Felpro says

              the bolted refinery flanges have a specification for surface serrations (125-250 microinch) to grab the stainess spiral-would gasket.

              My 842 aluminum BB heads definitely have serrations.

              From Felpro Head gasket tech tips brochure: Too smooth a surface allows excessive lateral motion of the head gasket which can deform the combustion seal. Too rough a surface creates small leakage paths. Prepare the head and block surface in the 90-110 RMS (80-100 RA) range. That is just enough to feel with your fingernail.

              Comment

              • Dick W.
                Former NCRS Director Region IV
                • June 30, 1985
                • 10483

                #8
                Re: Petroleum Refinery piping + Felpro says

                TC, if my feeeeeble memory serves me correctly, Chrysler was finishing the blocks to about 60 RMS. Almost no roughness
                Dick Whittington

                Comment

                • Chuck S.
                  Expired
                  • April 1, 1992
                  • 4668

                  #9
                  Re: Petroleum Refinery piping + Felpro says

                  RMS = Root Mean Square. Its a way of calculating an average. My memory is a little hazy here, but I think its the average of the sum of the squares of measured surface irregularities.

                  We are still left not knowing what the units for the 60 are, e.g. inches, micro-meters, micro-inches (0.000001 inch). If it's microinches, a common unit for machining irregularities, then 60 RMS is about the smoothness of PVC pipe. Now, that's an eye-opener.

                  Comment

                  • Tracy C.
                    Expired
                    • July 31, 2003
                    • 2739

                    #10
                    Re: Petroleum Refinery piping + Felpro says

                    thanks Dick,

                    I was just trying to illustrate a point. It isn't possible to finish a block and heads flat and smooth enough to go without a gasket.

                    tc

                    Comment

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