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  • machining an arc.

    I need to turn/mill an arc, 2.66" radius and the chord length is about 2" long. I plan on mounting it on a temporary plate on the lathe to do the turning. Since it is only part of the circle how can I measure the radius. I don't have the material to turn a full disk (and none available locally) and then cut down, besides that would be to easy (where's the fun if you don't have to beat your head against the wall). Is there an accepted method of making the measurements or machining something like this. The only idea I can think of is to mount a sacrificial piece on the opposite side of the plate and then I could measure the diam. but that seems like a cumbersome way to do it.
    The shortest distance between two points is a circle of infinite diameter.

    Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

  • #2
    I have a cheat for that kind of thing - a piece of 1" bar that's machined to a "half-circle" on one end (easy to measure, 0.500" from diameter to flat!), centre it in the chuck, bring the tool up to kiss it (pinching a thinnest-weight rolling paper works well) and it's on centre, take out the slack / zero the dial then wind back out to the radius I need and set a DTI against the toolpost (if turning with the topslide) or a Strategic Point on the cross-slide - then I can wind it out to clear the workpiece, take cuts until it reads zero and I should be on the desired radius, backlash permitting...
    Rules are for the obedience of fools, and the guidance of wise men.

    Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

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    • #3
      chuck up a stub and turn to whatever. Measure it, note the crossfeed reading, say its 1.000" so long as you don't disturb the tool, just remove the stub, mount the work and turn, and advance the crossfeed in, until it is out 2.16" from where it created the 1" dia. clear as mud?
      .

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      • #4
        Several ways you can do it.

        First, for all methods, lock down the compound slide.

        Most Accurate: Use a sacrificial scrap of any material. Mount it and turn to the desired radius/diameter. Zero the cross feed dial. Put a mark on the carriage/ways (use masking tape) so you can return to the same rotation of the cross feed. Mount your work and go down to that mark and the dial zero. Should be good to a thousandth.

        Slightly Less Accurate: Chuck up any round stock with a nice OD or a drill. It does not need to be perfectly centered. Touch off with the tool and zero the dial. Rotate the chuck 180 degrees and touch off again. Note the difference. Zero the dial at 1/2 of that difference from the first zero. Measure that diameter across those same two spots with your mike and divide by two. That is the radius at the zero you just marked. Calculate from there to the radius you need and back out to there. REMEMBER TO APPROACH YOUR FINAL POINT IN THE INWARD DIRECTION TO AVOID BACKLASH ERRORS. Now zero there for the final time and add the marks on the cross slide like above. That's your final radius. Due to more steps, this should be good to 1 or 2 thousandths.

        Quickie: If you can stand more error, you could just zero the cutter against the tip of a dead center in the head stock spindle. With a magnifier you can easily hit +/- 0.003" this way. Then back out, again keeping backlash in mind, to your desired radius and mark it off as described above.
        Paul A.

        Make it fit.
        You can't win and there is a penalty for trying!

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        • #5
          Is there a center hole, or can you bore one into your fixture plate? If yes, fit a plug into the hole, measure the plug, then measure from the OD to the plug.

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          • #6
            Or, depending on the accuracy needed and the size of the stock; bore a hole in the mounting plate to a nice round number and use calipers to measure from it to the radius, adding the radius of the bored hole.
            George

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            • #7
              Definitely remember the added radius. Also remember the lathe removes material by the diameter. I just ruined 6 hours of work because I got my measurements "Bass-Ackwards.

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              • #8
                Having no idea what your part looks like I have just guessed when making this drawing. If you do not have an NC mill then.

                Clamp part to mill table or rotary table, find the position on it where you would like the radius to be, move the table to where the center of said circle is, clamp a piece of sacrificial material 180° opposite at a distance that the tool will cut both parts. Use a boring head to bore both parts then measure across them with a tool that meets the accuracy that you require. Adjust the boring head until the diameter is reached, if using a rotary table just mill the circle until the diameter is reached.

                In this drawing the part is at the top and the sacrificial material is at the bottom.
                Good Luck

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                • #9
                  Originally posted by Illinoyance View Post
                  Is there a center hole, or can you bore one into your fixture plate? If yes, fit a plug into the hole, measure the plug, then measure from the OD to the plug.
                  I guess great minds think alike. Just after I posted this thread I got the idea to drill and ream a 1/4" hole in center of the mounting plate and fit a .250" gauge pin into it. It is a simple matter to measure across the OD of the part and the pin and subtract .125".

                  Thanks guys.
                  The shortest distance between two points is a circle of infinite diameter.

                  Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

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                  • #10
                    Try this link. It provides a formula to determine the radius of an arc.

                    http://www.mathopenref.com/arcradius.html

                    Harold
                    For those having fought for it, Freedom has a flavor the protected will never know.
                    Freedom is only one generation away from extinction.

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                    • #11
                      Originally posted by hwingo View Post
                      Try this link. It provides a formula to determine the radius of an arc.

                      http://www.mathopenref.com/arcradius.html

                      Harold
                      The OP desires to measure the part with simple tools such as a vernier caliper or inside micrometer. This requires 2 points 180° opposed.

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                      • #12
                        Originally posted by hwingo View Post
                        Try this link. It provides a formula to determine the radius of an arc.

                        http://www.mathopenref.com/arcradius.html

                        Harold
                        That link worked fine but when clicking on any of the "live links" in that view it only goes to some google thing and NOT the supposed operation.
                        Any Idea how to view the others?
                        In any event that is a very slick thing.
                        ...lew... OK I finally figgured out how to get to the home and from there to all sorts of fun and games things. Thanks!
                        Last edited by Lew Hartswick; 11-08-2015, 09:41 AM.

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                        • #13
                          Originally posted by Lew Hartswick View Post
                          That link worked fine but when clicking on any of the "live links" in that view it only goes to some google thing and NOT the supposed operation.
                          Any Idea how to view the others?
                          In any event that is a very slick thing.
                          ...lew... OK I finally figgured out how to get to the home and from there to all sorts of fun and games things. Thanks!
                          Glad it helped. I was doing a project similar to yours and it worked fine for me when having to literally measure existing dimensions.

                          Harold
                          For those having fought for it, Freedom has a flavor the protected will never know.
                          Freedom is only one generation away from extinction.

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