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Vernier for a Rotary Table

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  • #31
    It would be instructive if all the people who built something IN THEIR HOME SHOPS that required sub arcminute machining accuracy would chime in and briefly describe their projects.
    Regards, Marv

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

    Location: LA, CA, USA

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    • #32
      Originally posted by mklotz View Post
      It would be instructive if all the people who built something IN THEIR HOME SHOPS that required sub arcminute machining accuracy would chime in and briefly describe their projects.
      The times I used my rotary table I'm not sure I even used the coarser minutes part of the scales. I've certainly never used more than the center "0" on the vernier.

      To be fair though when making an actual tool I feel it's OK to go a bit OCD on ensuring accuracy. It's nice to start out with something more trustworthy.

      And despite what I just wrote I don't feel that indexing with saw blade wrapped around a plywood disc is really the best option if we're going to get OCD over accuracy on the tools we make.

      Chilliwack BC, Canada

      Comment


      • #33
        Originally posted by BCRider View Post

        The times I used my rotary table I'm not sure I even used the coarser minutes part of the scales. I've certainly never used more than the center "0" on the vernier.

        To be fair though when making an actual tool I feel it's OK to go a bit OCD on ensuring accuracy. It's nice to start out with something more trustworthy.
        ...
        So far in this thread I haven't seen anyone describe any home shop method for measuring the alignment accuracy of a rotary table. Slapping a vernier on an RT crank doesn't magically give the table sub arcminute accuracy unless you can prove it with a measurement.

        Regards, Marv

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

        Location: LA, CA, USA

        Comment


        • #34
          Originally posted by mklotz View Post

          There are many ways to build it so I'll just try to outline the principle which can then be mechanically implemented in a number of ways.

          Imagine that you had a circular disk (the 'disk' could be a circular sleeve) that was marked with 4 * 20 = 80 divisions. You can attach this disk to the shaft that drives the worm that turns the table. By providing a pointer fixed to the table, you would have your four degrees per worm revolution as well as the three arcminute subdivisions of each of the four degrees.

          So the problem becomes one of coming up with a scheme to subdivide a circle into 80 parts. If you could find an 80 tooth gear you could use it directly or attach a disk to it and use it as a reference to index the disk and mark the 80 divisions.

          Another approach is to find a friend or machinist club member who can be bribed with beer (that ought to be easy) and have him use his rotary table to divide the disk for you.

          Another approach is to make yourself a cardstock master and use that to mark the disk. If you draw a large circle of diameter D, and then set your dividers to a length given by:

          L = D * sin(180/N)

          you can use them to strike off N divisions around the circle.

          As an example, take D to be 255 mm, then with N = 80 we have L = 10.01 mm. (Yes, I adjusted the numbers to make it come out to about a centimeter).

          Yes, using cardboard will incur some error but remember that any error occurred here will be reduced by a factor of 90 when translated into table movement. We can use that reduction iteratively to further reduce the error. Divide one disk using the cardstock model, mount it and use it to divide another disk which will be 90 times better. Rinse and repeat until you perish from boredom or accept the fact that you will never make anything that requires sub-arcminute accuracy.

          Once you decide how you will approach the task, keep me informed. I'll be glad to provide whatever help I can.
          I have not problem with the first step. I take the outside diameter of the chuck on my Clausing 7" x 36" that is 8" diameter, I can easily with a strip of paper of approx length= : 638.37 mm
          8" x 25.4 x 3.1416=638.37 mm I can divide the strip of paper fairly accurately to 180 divisions so: 638.37/180= 3.5465 mm which will be 2 degree arc/division.

          Then I a take a sector (angle at the chuck) of 19 degrees (9.5 divisions) and make a circular vernier dividing these 9.5 div by 10; I would be able to get close to one tenth of a degree. This will be equivalent to and arc/cord at the 8" diameter of the chuck to approx error of 0.1 degree which is approx cord of 638.37 / 360 / 10= 0.18mm (0.177325 mm arc).

          Did I get it right?
          It would be able to for example make a gear 127 teeth, 16 DP using 360 degree /127 teeth = 2.834645 degree/tooth that for 14 degree pressure angle DP 16 is 7.938" = 633.43 mm at the circumference of the gear; which gives approx 633.43 / 127 = 4.98 pitch at the OD of the gear.

          Now I just read that maybe this ratio is practically better == 53 x 56 / 41 x 57 = 1.270004, err = 0.000003 but requires 4 gears. I am not sure my Clausing with take the 127 teeth as the banjo is right now.

          my table for cutting would be:



          You may only view thumbnails in this gallery. This gallery has 1 photos.

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          • #35
            Thanks a lot for the suggestions and the picture. I am bound inside now for this Unusual Cold weather in Austin-Texas, but as soon as it clear up I will do some preliminary work. Somehow I think that the wood piece (bottom of a wire reel) could allow fairly precise divisions. Not Sure if I want to go with the 127 or the 53 x 56 / 41 x 57 = 1.270004, err = 0.000003 that I just saw on the web.
            Need also to get some more of my behind and finish the propane burner to melt the gear blanks. I have been building a little cutter for the gears but needs more work yet. Too many projects and little time.
            Here is a pic of my work-in-progress cutter hear. Willl use the Clausing spindle to hold the gears, Need to rig some power belting for the cutter.

            You may only view thumbnails in this gallery. This gallery has 1 photos.

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            • #36
              Probably the best practical two gear pair for a ratio of 1.27 is 80/63 with an error of 0.0125% as this table shows...

              Desired ratio = 1.27

              19:15 => 1.266667 (-0.262467%) **
              28:22 => 1.272727 (0.214746%) **
              33:26 => 1.269231 (-0.060569%) **
              37:29 => 1.275862 (0.461580%)
              38:30 => 1.266667 (-0.262467%)
              42:33 => 1.272727 (0.214746%)
              43:34 => 1.264706 (-0.416860%)
              47:37 => 1.270270 (0.021281%) **
              51:40 => 1.275000 (0.393701%)
              52:41 => 1.268293 (-0.134434%)
              56:44 => 1.272727 (0.214746%)
              57:45 => 1.266667 (-0.262467%)
              61:48 => 1.270833 (0.065617%)
              62:49 => 1.265306 (-0.369597%)
              65:51 => 1.274510 (0.355103%)
              66:52 => 1.269231 (-0.060569%)
              67:53 => 1.264151 (-0.460556%)
              70:55 => 1.272727 (0.214746%)
              71:56 => 1.267857 (-0.168729%)
              74:58 => 1.275862 (0.461580%)
              75:59 => 1.271186 (0.093421%)
              76:60 => 1.266667 (-0.262467%)
              79:62 => 1.274194 (0.330201%)
              80:63 => 1.269841 (-0.012498%) **
              81:64 => 1.265625 (-0.344488%)
              84:66 => 1.272727 (0.214746%)
              85:67 => 1.268657 (-0.105770%)
              86:68 => 1.264706 (-0.416860%)
              88:69 => 1.275362 (0.422230%)
              89:70 => 1.271429 (0.112486%)
              90:71 => 1.267606 (-0.188533%)
              91:72 => 1.263889 (-0.481190%)
              93:73 => 1.273973 (0.312803%)
              94:74 => 1.270270 (0.021281%)
              95:75 => 1.266667 (-0.262467%)
              97:76 => 1.276316 (0.497306%)
              98:77 => 1.272727 (0.214746%)
              99:78 => 1.269231 (-0.060569%)
              100:79 => 1.265823 (-0.328915%)
              Last edited by mklotz; 02-14-2021, 10:31 AM.
              Regards, Marv

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

              Location: LA, CA, USA

              Comment


              • #37
                Originally posted by garyhlucas View Post
                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.
                Gary: really appreciate the offer. I think that after reading and digesting all suggestions and knowing that most of my projects are done by God and By eye, I think the accuracy of the wooden top of a cable reel that I have ( about 24" diameter) with a carefully strip of paper and a sharp pointer mounted on the back of my lathe should be good enough for the two gears than I decided to cut to cut metric threads. If I remember they are 37 and 47 so it will be a fairly accurate gear or at least I will not be able to see the difference.
                Thanks

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                • #38
                  Just as a matter of interest for potential uses where the level of accuracy required is no where near what has been discussed here already.
                  Old manual drafting machines have protractors built into the head. Some of them its just printed onto an aluminium disc and others (better quality) have it cut into a disc. Usually around 120mm dia. Some also have a Vernier scale fitted as well. Sometimes these old machines can be bought real cheap at a garage/yard sale.
                  25 years ago as part of my job I used to service machines in 5 or 6 rooms with 18 machines each in them. Now that CAD has taken over there would be lucky to be 2 or 3 machines on site. When they were dumped I managed to salvage a couple of heads for myself.
                  10 - 15 years later I still haven't needed any of them but I'm sure something will come up one day.
                  Peter

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                  • #39
                    Originally posted by mklotz View Post
                    It would be instructive if all the people who built something IN THEIR HOME SHOPS that required sub arcminute machining accuracy would chime in and briefly describe their projects.
                    Evan has not posted in years.... He got tenths accuracy from his dial calipers with a magnifying glass, so I am sure that the simple division you described would have been no problem for him...................
                    2801 3147 6749 8779 4900 4900 4900

                    Keep eye on ball.
                    Hashim Khan


                    It's just a box of rain, I don't know who put it there.

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                    • #40
                      Sorry, I just saw this. I will attempt to take some photos tomorrow.



                      Originally posted by ElectronMini View Post

                      Paul: Any chance you could send me a close up picture of your table to see how the "vernier" is incorporated/built?
                      Thanks
                      Paul A.
                      SE Texas

                      And if you look REAL close at an analog signal,
                      You will find that it has discrete steps.

                      Comment


                      • #41
                        The problem with CAD programs is not in the program, but in the printer. Many printers do not have the same mechanical scale in the X and Y (page width and length) directions and will not print a circle as a circle. That, in turn means that your carefully divided drawing will be off when it comes off the printer. Some CAD programs can compensate for this. You can look into the menus and directions.



                        Originally posted by Bented View Post

                        Any simple free CAD program can do so with a few key strokes.

                        Moving from a simple drawing to machine instructions is a different matter that is often called CAM. A very accurate printer will do this.
                        Paul A.
                        SE Texas

                        And if you look REAL close at an analog signal,
                        You will find that it has discrete steps.

                        Comment


                        • #42
                          One of my rotary table projects used a toothed belt wrapped around the outside of an mdf disc. The belt was sliced at an angle so the junction wasn't a simple butt joint. The belt started with about 6 or 7 more teeth than required to make this happen, and the final number of teeth was some fixed number- it might have been 120, or 3 degrees per tooth spacing. I machined the od of the disc so the belt was snug to it when the teeth were aligned at the right point.

                          With the belt temporarily held to the disc, I mixed up some epoxy putty and pressed a bead of it into several teeth in a row, and also backed it up with a steel piece that was made to pivot. With the putty set up, I took that piece and pressed it over the overlapping area where the junction was to accurately align the teeth to each other at that point. I epoxied the belt to the disc.

                          In this case, the disc pivots on a point which I fasten into a T-slot on the mill table. The steel/epoxy putty 'indexer' pivots on another point which I fasten into a T-slot. I swing that away, rotate the table to where I want it, then swing it back in to engage the discs teeth. If I move it one tooth, that's 3 degrees- 10 teeth and it's 30 degrees, etc.

                          The mdf disc has a center hole which it pivots on, so the upper portion of that hole is concentric with the pivot point. That lets me use it to align a work piece on top of the mfd. This whole setup is crude and simple, but it works well. My first project was a sprocket for my experimental electric three wheeler, and it turned out to be very smooth with no eccentricity and no rhythmic variations.

                          Another rotary table project I did, a smaller one, uses the same basic system, but the pivot point for the indexer is adjustable. This let me notch into the teeth, but then break that spacing into smaller divisions. In this case the main rotary disc has 360 teeth on it. One day I wanted to be able to make a 100 division wheel with it, so that meant the indexer had to move .6 tooth spacing, .2 tooth spacing, etc. I would go 3 teeth, then add .6 of a tooth, then move to 7 teeth and add .2 of a tooth, move to 10 teeth and add .8 of a tooth distance, etc. Don't ask me how much of a pain that was-
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                          • #43
                            Originally posted by bollie7 View Post
                            Just as a matter of interest for potential uses where the level of accuracy required is no where near what has been discussed here already.
                            Old manual drafting machines have protractors built into the head. Some of them its just printed onto an aluminium disc and others (better quality) have it cut into a disc. Usually around 120mm dia. Some also have a Vernier scale fitted as well. Sometimes these old machines can be bought real cheap at a garage/yard sale.
                            25 years ago as part of my job I used to service machines in 5 or 6 rooms with 18 machines each in them. Now that CAD has taken over there would be lucky to be 2 or 3 machines on site. When they were dumped I managed to salvage a couple of heads for myself.
                            10 - 15 years later I still haven't needed any of them but I'm sure something will come up one day.
                            Peter
                            "10 - 15 years later I still haven't needed any of them but I'm sure something will come up one day."

                            me too:

                            Attached Files

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                            • #44
                              Yea.. I remember using one on my short drafting assignments at the power plan.
                              Good you mentioned, because I can look up on eBay.
                              The more I read the more I come up with ideas and I just waiting for better weather to go hands on. I will be making a jury rigged rotary table that will get me started with interchangeable gears & acme or big 3/4' NC taps to HOB aluminum gears. My Clausing lathe brought only the reg. gears as per QB. But I just got it 2006 so God knows if originally had another set.
                              I like also the idea of a BIG (24" wooden disk and a precisely divided paper strip with a pointy fix finger. Ineed to do all this with the lathe. I have a milling machine (Pederson) in Ontario-Canada but not easy to bring it here and never assembled after I bought it from High School. No room n the garage!
                              My scrap box is almost depleted since 8 or 10 years ago the local scrap yard in Round Rock-TX stopped the permission to select and buy scrap. None around here that I can get in. SAFETY is the claim reason.
                              Thanks for the picture, it brought memories.

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