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  • Symmetrical parts from a mill

    What is the accepted practice when creating symmetrical parts in a mill? Say, something like a t-nut.

    Is it best to:

    1. Locate one edge, offset, machine, then repeat with the other side/edge.
    2. Use a work stop, locate edge, machine, and rotate the part against the work stop and machine.

    I have done it both ways with acceptable results, just curious what the pros might do.

    Keith
    Last edited by RKW; 12-17-2009, 12:05 AM.
    "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

    -- Harold "Doc" Edgerton

  • #2
    On a tee nut type part I find the face of the fixed vise jaw,offset to the opposite side of the workpiece and climb mill that feature,then flip the part and climb mill the other feature.
    I just need one more tool,just one!

    Comment


    • #3
      Maybe a bit unusual, but it works for me-----

      My designer, in the last job I had, introduced me to a way of working which I had not previously considered. Since beginning to use it I have seldom worked any other way. The scheme is best done on machines with digital readouts. The first thing done is to find the centre of the blanked out squared up piece you will be machining and then zero out the readouts at that position. Assuming you can use your edgefinder reasonably well then that position will be within about 2 thous or less of the true centre. Next a quick look at the drawing will show if you can easily work from that position, or whether you had better work from a " false centre position" which relates to matching the part to some other part. If the part is symmetrical, like a Tee nut, you can begin by drilling and tapping the hole on the centre point, then you can work out the offsets on the cutter to give the size needed, for example if the part to fit in the narrow bit of the tee slot is to be5/8 wide and 1" long and you use a 1/2" cutter you can confidently machine down to . 562 for each side and .750 for the length. Using this method, works well for me, it is tolerant of slight mistakes and It does not suffer fromtolerance stack up problems which sometimes occur when working from one corner. Hope this is of interest David Powell.

      Comment


      • #4
        I like to use a jig whenever I can. Often times it's so quick and easy to make that I just do it. Mdf and pvc are my workhorse materials. Cut an L shape, clamp it to the mill table, then adjust positions for the workpiece. One after the other, tuck the piece into the L, clamp, drill or mill, etc.

        Many times the jig consists of a piece of mdf pilot drilled for some screws, then aligned on the mill table with a workpiece tucked into the corner formed by the screws. The mdf is consumable, so you can drill or mill into it without fear of marking up your mill table.

        Another way to quickly align several pieces one by one for machining that I use is a home made mountable square. I took a piece of channel, cleaned up three sides, then drilled five holes in a pattern at one end. A piece of angle cleaned up on two sides and squared is drilled with one hole on one face, and two holes to match the groove on the side of the mill table on the other face. With one bolt holding the two pieces together, it is set on the mill table and the angle part secured with hardware to the side of the mill table. From there, the channel piece is carefully aligned to be square across the table and secured using two bolts and t-slot nuts. Then the rest of the holes are drilled through, all of them getting spring pins pressed in before the jig is removed from the table. Now I can mount that anytime, knowing it's square, then clamp a block to it at an appropriate point and I have my corner to locate a workpiece into. When there's several to drill or mill, it makes shorter work of aligning each piece.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

        Comment


        • #5
          Workstops of all kinds seem like they save a lot of time. I got to the point where if I didn't start with a work stop, I often slap one on the vise before removing the part just in case I want to put it back.

          BTW, a small Kant-Twist is a useful stop for many circumstances.

          Cheers,

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

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

          Comment


          • #6
            A T nut or several of them? For several a bar the width for the base and a little longer than the vise jaws. Mount in vise on parallels so you can mill the sides and locate center. Working from center cut each side the required depth Z and Y on both sides. Drill and tap bolt holes. Saw them the length you want and deburr them.
            It's only ink and paper

            Comment


            • #7
              Carl,

              The t-nut was only a hypothetical, so it wasn't really a question of how to make anything specifically so much as it was keeping anything (t-nut or not) symmetrical and the preferred method.

              But I do agree with your method in this case ...

              Keith

              Originally posted by Carld
              A T nut or several of them? For several a bar the width for the base and a little longer than the vise jaws. Mount in vise on parallels so you can mill the sides and locate center. Working from center cut each side the required depth Z and Y on both sides. Drill and tap bolt holes. Saw them the length you want and deburr them.
              "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

              -- Harold "Doc" Edgerton

              Comment


              • #8
                I make t-nuts with a shaper and a hacksaw.

                Comment


                • #9
                  (do stuff as usual)
                  g52 x[...] y[...] (center of symmetry x2)
                  g51 x-1 y-1
                  (repeat: do stuff as usual)

                  Comment


                  • #10
                    Originally posted by RKW
                    What is the accepted practice when creating symmetrical parts in a mill? Say, something like a t-nut.
                    I' ve done it both ways with acceptable results, just curious what the pros might do.

                    Keith
                    You ask a good question !

                    There is no correct answer.
                    The first question a pro asks, is "what are the tolerances ?"
                    The second is "how many"
                    The third is "what material will I be using"
                    The fourth is 'What machines do I have available"

                    This is not a smart aleck response, but shows the diversity of the answer.
                    Each Machinist has a certain skill level that he feels comfortable with, and that is the one where he makes the fewest mistakes.
                    The real question I think you are asking is :
                    "What method will produce the best results for me ?"

                    The fact that you ask that important question, tells me and others here, that you want to improve your skills.....hooray, that is great !
                    Keep all of this in mind, both when you read the answers of others , and try the various approaches yourself.
                    I confound some friends when I cut parts with a bandsaw, instead of milling,
                    or drill in the mill, instead of the drill press.
                    I look at the ultimate use/need and then ask the above questions.

                    Jigs/Fixtures are a waste of time for one off jobs, UNLESS, the tolerence required makes some dimensions critical, then it can become imperative to make one.
                    Digitals on any machine tool can and will double the output along with improved tolerence control and elimination of backlash error calculations.
                    W/O digitals, the job becomes more difficult

                    Now to answer your question
                    If "symmetry" is the critical designation ( and its a good one), I would drill the tapped hole first to dowel pin size, and use the dowel as a depth stop on the vise jaws to do both sides, OR,
                    I would mount a scrap piece in the jaws and drill a dowel hole which establishes my center lines, and then clamp the predrilled part on the dowel
                    and machine both sides at the same setting
                    Neither of these methods match the others, but is equally of value
                    Note, in the first method, I used the part as a fixture, and in the second method, we made a fixture to reach an objective.

                    The first is easier, but requires more clamp/unclamp time ( and ERROR entry)
                    The second requires more time, but gives an operation that not only will provide symmetry, but allow machining steps without resetting the part, and the ability to measure the part while still machining.
                    Think about this if the part were made from Gold....and no errors were allowed.
                    You would want to be able to sneek up on every dimension .

                    Good question Keith..and good luck.
                    Hope this helps you when you see the myriad of answers
                    Rich
                    Green Bay, WI

                    Comment


                    • #11
                      Again, not really making t-nuts here, just asking about symmetry ...

                      Originally posted by dp
                      I make t-nuts with a shaper and a hacksaw.
                      "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

                      -- Harold "Doc" Edgerton

                      Comment


                      • #12
                        Thanks, but I never said anything about CNC ...

                        Originally posted by beanbag
                        (do stuff as usual)
                        g52 x[...] y[...] (center of symmetry x2)
                        g51 x-1 y-1
                        (repeat: do stuff as usual)
                        "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

                        -- Harold "Doc" Edgerton

                        Comment


                        • #13
                          Ah, finally a detailed answer to the actual question! I'm certainly greatfull to have this community to bounce things off of and really just love to read this forum like a newspaper (since I really don't care about the real "news"). But geesh, sometimes folks need to read to OP a little more carefully. In this case I was looking for an answer to a question not a solution to a problem ... there is a difference!

                          Rich,

                          You are right, I'm always looking to improve my skills not just turn the cranks and produce any old junk (we have China for that ;-) The fact that you answered my question with several others just gives me more room for thought. I have not had to use any fixtures to date as the non-moveable vise jaw and work stops have worked quite well. But it probably only a matter of time before I do. Tooling plates have also piqued my interest. I do like your suggestion of dowel pins for items with holes.

                          Thanks,
                          Keith

                          Originally posted by Rich Carlstedt
                          You ask a good question !

                          There is no correct answer.
                          The first question a pro asks, is "what are the tolerances ?"
                          The second is "how many"
                          The third is "what material will I be using"
                          The fourth is 'What machines do I have available"

                          This is not a smart aleck response, but shows the diversity of the answer.
                          Each Machinist has a certain skill level that he feels comfortable with, and that is the one where he makes the fewest mistakes.
                          The real question I think you are asking is :
                          "What method will produce the best results for me ?"

                          The fact that you ask that important question, tells me and others here, that you want to improve your skills.....hooray, that is great !
                          Keep all of this in mind, both when you read the answers of others , and try the various approaches yourself.
                          I confound some friends when I cut parts with a bandsaw, instead of milling,
                          or drill in the mill, instead of the drill press.
                          I look at the ultimate use/need and then ask the above questions.

                          Jigs/Fixtures are a waste of time for one off jobs, UNLESS, the tolerence required makes some dimensions critical, then it can become imperative to make one.
                          Digitals on any machine tool can and will double the output along with improved tolerence control and elimination of backlash error calculations.
                          W/O digitals, the job becomes more difficult

                          Now to answer your question
                          If "symmetry" is the critical designation ( and its a good one), I would drill the tapped hole first to dowel pin size, and use the dowel as a depth stop on the vise jaws to do both sides, OR,
                          I would mount a scrap piece in the jaws and drill a dowel hole which establishes my center lines, and then clamp the predrilled part on the dowel
                          and machine both sides at the same setting
                          Neither of these methods match the others, but is equally of value
                          Note, in the first method, I used the part as a fixture, and in the second method, we made a fixture to reach an objective.

                          The first is easier, but requires more clamp/unclamp time ( and ERROR entry)
                          The second requires more time, but gives an operation that not only will provide symmetry, but allow machining steps without resetting the part, and the ability to measure the part while still machining.
                          Think about this if the part were made from Gold....and no errors were allowed.
                          You would want to be able to sneek up on every dimension .

                          Good question Keith..and good luck.
                          Hope this helps you when you see the myriad of answers
                          Rich
                          Last edited by RKW; 12-17-2009, 09:09 PM.
                          "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

                          -- Harold "Doc" Edgerton

                          Comment


                          • #14
                            Short and sweet ... That's the solution I like too.

                            Originally posted by wierdscience
                            On a tee nut type part I find the face of the fixed vise jaw,offset to the opposite side of the workpiece and climb mill that feature,then flip the part and climb mill the other feature.
                            "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

                            -- Harold "Doc" Edgerton

                            Comment


                            • #15
                              Symetry

                              I like David Powell's idea about working from the center.
                              That's really the definition of symetry isn't it.
                              I know I do some parts that way sometimes because symetry
                              to the center facet is critical. But I never thought it
                              through to be a generalized system.

                              Comment

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