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Measuring the small end of a taper

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  • Measuring the small end of a taper

    I'm turning a taper on my lathe, measuring the big end is easy enough but how do you measure the small end. Even using a caliper across the closest point that you can get isn't really getting at the very edge of the small end taper diameter. I don't wish to remove the part from the lathe to do this.
    The shortest distance between two points is a circle of infinite diameter.

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

    Southwestern Ontario. Canada

  • #2
    Correct me if I am wrong, but Joe Pie did a video on that?

    I went to youtube and found this, go to about 20:00, and he uses ring gage and trig to get a number


    ​​​​​​https://www.youtube.com/watch?v=OfrRFCzGNTw

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    • #3
      If you're working with dimensions rather than the big end and then checking the taper to a sample to check the fit then the length from the one spot to the other you where you make the measurements is also important. So you'd need something to set a distance between measurements as well as the diameters at each end. Do you have anything for that?

      When I did some MT3 arbors for my old mill/drill and made three of my own tapers I used a commercial MT3 socket as my sample for final fitting. Is something like that not a possibility? Outside of that when I've had to do other very specific angle tapers they were to match other tapers. Like when I did my 5C collet holder for the lathe. In that case I used a DTI to match the compound slide up to the angle as best I could and called it a day.

      If I were to do something of the sort for a taper I was making to a specific taper I think I'd rig up something like a way to index the side of the frame of a micrometer to a flat side of my tool post and then move the tool post along a measured length of the taper. I'd rig up some way of accurately measuring off some distance of longitudinal travel and do a diameter at each end of that set length. Then I'd have the change over the distance and could figure out the angle of the taper. How this set distance between measurements is done would rely on what sort of taper it is and just exactly how accurate it needs to be. And I guess to some extent on the actual angle since if it's a really steep and short angle then it would magnify any potential errors in some possible measurement options and possibly the method for measuring would alter.

      Come to think of it you didn't even say if the taper is an external taper or an internal one. I just assumed it was from your description of the use of a caliper for the small end.
      Chilliwack BC, Canada

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      • #4
        It's an outside taper (male).

        It just hit me like a hammer, I can place straight edges down either side of the taper and then measure the distance across the small end with the inside jaws of a caliper. That will get me a measurement I need. Dohhhh!
        The shortest distance between two points is a circle of infinite diameter.

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

        Southwestern Ontario. Canada

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        • #5
          You did not know the tool position before starting the taper?

          If the tool position is known the starting diameter will be what you set it at plus any nose radius effect.

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          • #6
            Can you use the lathe itself to measure the part? I would touch off on the big end of the taper, zero everything (dials or DRO, whatever you have), then travel the carriage to the small end. Note the distance on the carriage dial. Now crank the compound in to touch off an the small end, and note that distance too.

            The numbers will give the taper per inch or per foot or whatever, but a bit of trig will give the taper in degrees.
            25 miles north of Buffalo NY, USA

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            • #7
              You can also use the lathe with a dial indicator on the toolpost or cross slide. Set the indicator on the middle of the part diameter. Measure the big end, compress the indicator .2 or .300, set to zero. Move the carriage (and indicator) down to the small end, keeping track of how far the carriage moves. The amount the indicator moves from the original setting is half the difference in diameters. The lathe itself serves to keep the measurements in perpendicular cartesian coordinates.
              Kansas City area

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