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Vernier for a Rotary Table

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  • Vernier for a Rotary Table

    Looking for some information on what could be available to use to build a vernier for a DIY rotary Table.
    Basically looking for some ideas on something available already with some kind of fairly precise graduation. I was thinking on a diameter of 6 - 12 inches. Then I would fabricate a vernier and with a 80 or 90 to 1 DIY rotary table I could basically have any number of division of reasonable accuracy.
    Thanks

  • #2
    What kind of accuracy are you looking for? a 90:1 gives 1 degree every 1/4 turn. Attach a simple spindex to divide that by 360 and it should satisfy most needs.

    Mike

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    • #3
      With a 90:1 worm gear, one rotation of the crank will move the table 360/90 = 4 degrees.

      Provide 4 divisions on the crank disk for 4 degrees. Subdivide each of those one deg segments into N divisions. Then each division will correspond to 60/N arcminutes.

      With N = 20 a division will correspond to 3 arcminutes.

      3 arcminutes = 0.00087 radians (call it 1 milliradian)

      A milliradian is very small; if you misalign your laser by 1 milliradian at a target 100 yards (3600 in) away, you'll miss the bullseye by only 3.6 in.

      This level of precision is better than almost any DIY project could require.

      Regards, Marv

      Home Shop Freeware - Tools for People Who Build Things
      http://www.myvirtualnetwork.com/mklotz

      Location: LA, CA, USA

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      • #4
        If you can get a resolution of 1 minute of angle, it will be more than adequate for home shop use. Any more is overkill.

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        • #5
          i dont know if this is what you were asking: find a program (=app) that lets you print out a scale. maybe somebody else will know what these are called. (there is one living somewhere in the guts of one of my computers, but i dont remember which one.) glue it on and seal it with nail polish. good for many years.

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          • #6
            That would certainly be a job for either a rotary table with indexing wheels or a proper dividing head.

            I'm also guessing from your post that you're not really looking to make a geared table but instead one which is positionable. You'll swing it to some value then lock it down for the actual machining? Is that the intention or is there something more grand in mind?

            As for making yourself a vernier you got me going on this. I checked the couple I've got in my own shop and came up with this illustration below.

            The top and bottom verniers in this case do the same thing. The supposition here is that we're indexing the worm wheel for a 90:1 table with 4 degrees per turn and each degree (quarter turn) split into 10'ths. And the vernier set up to split each 1/10 unit into 5 parts so we're resolving down to a 1/50th of a degree over a quarter turn of the handwheel.

            Both of these below do the same job. If not sure print this picture out and cut along the division lines and try them. But obviously the lower one has more resolving power by making the vernier divisions larger. The downside is that the lower option represents a little less than half the movement around the wheel. Or a quarter turn per side depending on which direction you're moving. But splitting the divisions for aiming at 1/100th of a degree becomes viable. The upper more compressed version would easily manage with splitting the 10'ths into 5 steps but trying to split the settings for 100ths would be a guess. The upside of the upper option is that you can see all the marks without needing to shift your head.


            But if you were looking to index around the edge of a 10 or 12" diameter lathe faceplate sized platter on a suitable base with 1° marks around the periphery then each degree would be just over 0.087" for a 10" plate or 0.105" for a 12" plate. So if our vernier scale as per the lower option spans 9 degree divisions each side of the 0 index we're looking at 1.566" (total for each side from "0")) for the 10" table and 1.89" for the 12" table. Into that you need 10 divisions made using 11 marks. The center "0" and 5 each side. So 0.1566'ish for the one and 0.189" for the other. This is realistically something you could do on a mill where you scratch the marks into a thin plate with a pointed "D" shaped scriber. Then fix it to the base of your table set into a slight recess so the surface of the plate is flush with the OD of the rotating table plate.

            And if you're actually going to make the table geared like it would be on a "proper" rotary table then just apply the upper option to the worm handwheel.

            Or did I mis-read your intent on any of this? At any rate you should be able to see how to generate your own vernier now.

            EDIT- I messed up the units numbering on the lower scales in both of the sketches when I mirrored the numbers. But hopefully you get the idea. Just imagine the 1-8 on the right side of the big 1 as being the other way around.

            Click image for larger version  Name:	Vernier.JPG Views:	0 Size:	20.8 KB ID:	1927610
            Chilliwack BC, Canada

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            • #7
              It is kind of pointless going too far as the wormwheel won't be all that accurate. Simplest available already thing to start with is to make an adaptor that holds your changewheels, and a marker or detent.

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              • #8
                Originally posted by Baz View Post
                It is kind of pointless going too far as the wormwheel won't be all that accurate. Simplest available already thing to start with is to make an adaptor that holds your changewheels, and a marker or detent.
                Too right. There are 21600 arcminutes in a circle. If you think you can obtain meaningful, repeatable subdivisions of an arcminute with a DIY mechanical device, you're deluding yourself.

                Not to mention the fact that it's highly unlikely that anything built in a home machine shop is going to need that sort of accuracy.

                Regards, Marv

                Home Shop Freeware - Tools for People Who Build Things
                http://www.myvirtualnetwork.com/mklotz

                Location: LA, CA, USA

                Comment


                • #9
                  Take a look at George Thomas' "Versatile Dividing Head" , it's a marvel of elegant design IMHO. Only requires 2 gears at 60T each, and the ability to cut mating worms. http://www.hemingwaykits.com/acatalo...ding_Head.html

                  In this case, a 60:1 worm and wheel setup is further divided by a 60-hole plate. The 60-hole plate itself can be rotated with another 60:1 worm and wheel, if I understand the design correctly. I highly recommend his book on the tool, called "workshop techniques" --

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                  • #10
                    Originally posted by mklotz View Post

                    Too right. There are 21600 arcminutes in a circle. If you think you can obtain meaningful, repeatable subdivisions of an arcminute with a DIY mechanical device, you're deluding yourself.

                    Not to mention the fact that it's highly unlikely that anything built in a home machine shop is going to need that sort of accuracy.
                    I work in a machine shop, several years ago a customer ordered a mandrel, 7" dia. steel tube 86" long. Centerless ground then faced to length with a 7 minutes of arc taper on each end 6" long.

                    When first looking at the drawing I asked my employer if the customer could actually measure such a feature and he said, not in our lifetimes.

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                    • #11
                      If you can provide a Cad drawing I could try engraving it for you on my CNC mill. It is not heavy duty but tight and quite accurate as it has THK high precision linear ways and THK balls screws. I have a stepper driven 4th axis too so I could do a dial as well. I have a nice sharp 60 degree carbide engraving tool too.

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                      • #12
                        Originally posted by Bented View Post

                        I work in a machine shop, several years ago a customer ordered a mandrel, 7" dia. steel tube 86" long. Centerless ground then faced to length with a 7 minutes of arc taper on each end 6" long.

                        When first looking at the drawing I asked my employer if the customer could actually measure such a feature and he said, not in our lifetimes.
                        Wouldn’t that be somewhere around .025” diameter difference from big diameter to small diameter over 6”?

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                        • #13
                          Originally posted by oxford View Post

                          Wouldn’t that be somewhere around .025” diameter difference from big diameter to small diameter over 6”?
                          I calculate it to be around .0003 over 6".

                          -js
                          There are no stupid questions. But there are lots of stupid answers. This is the internet.

                          Location: SF Bay Area

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                          • #14
                            Originally posted by Jim Stewart View Post

                            I calculate it to around .0003 over 6".

                            -js
                            I could be way off, my numbers were off some online calculators.

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                            • #15
                              7 minutes of arc is .1166 Deg.
                              Tan = .002
                              .0122 X 6 = .0122
                              .025 included, this is close enough
                              .

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