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Vernier for a Rotary Table

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  • ElectronMini
    replied
    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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  • dian
    replied
    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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  • darryl
    replied
    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-

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  • Paul Alciatore
    replied
    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.

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  • Paul Alciatore
    replied
    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

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  • J Tiers
    replied
    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...................

    Leave a comment:


  • bollie7
    replied
    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

    Leave a comment:


  • ElectronMini
    replied
    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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  • mklotz
    replied
    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.

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  • ElectronMini
    replied
    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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  • ElectronMini
    replied
    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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  • mklotz
    replied
    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.

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  • BCRider
    replied
    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.

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  • mklotz
    replied
    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.

    Leave a comment:


  • BCRider
    replied
    Originally posted by ElectronMini View Post

    Bented: I love the picture with the circle divided so evenly. Any pointers on how could I implement that and build myself for example a collar of 8-12" diameter and scribe the divisions?
    I was thinking of using an old saw blade of let's say 1/2" pitch, but I only have 3/4" PITCH from my LumberMate 2000 Sawmill.
    Thanks
    Odd as it might sound doing a layout like Bented's circle in CAD is only the work of a few minutes. The trick once you have the drawing is to print it accurately. Many printers compress or stretch the image slightly. I've run into that on occasion where it was important to get a really good proper size. Like even the roughly 1% distortion in the one direction was too much for what I wanted to do. So I copied my original CAD drawing to a new document and stretched or compressed (many years ago but I think it needed stretching by some decimal amount like 1.3 or 0.85 or some such nonsense) the drawing and saved it as a new file which I then printed.

    I found the error by drawing a simple box on a sheet of paper that was near the maximum printable size and measured it. Then I scaled the one direction.

    Bottom line is that if you would like to work with a printed page drawing it would not be hard to send you a file with a test box which you print and measure. Then I'd scale the box as needed and send it back to you. When we determine the scaling factor needed I could send you a PDF file that would print out in Acrobat on two legal size pages which you would trim in one case and tape it over the other to get a nice big indexing circle with 360 marks. And a big one for every 10th mark and half big at each 5th mark between.

    You didn't mention yet if you're making a simple rotary table or if you're doing the whole worm gear setup. It sort of sounds like you're thinking of the whole full meal deal. that's a LOT of pretty darn precise work. And for doing that sort of work you would generally need an indexing head or at least a place to borrow one for such a job. Not to mention some pretty fancy welded assemblies or castings to make the base and the wheel and such.

    Bet that as it may I found this picture online which I think illustrates how most 90:1 tables with 4° handwheels are set up. It's certainly how my 6" table is marked. The file is pretty big so click on the pic below to see the full size version.

    To set the stage what you're seeing is the 0 index on the 60-0-60 vernier scale is sitting on the 40 minute marker that is resting between the 2° and 3° marks which can be seen just outside the shadow bar on the lower side for the "2" and just before the shadow line near the top for the "3".

    Each mark from the 30m mark to the next bigger mark which is the 40M mark is two minutes of arc. The vernier is also two minutes of range being bascially -60sec to 0 to +60sec in 20 sec increments. So technically this table would be good for + or - 20 seconds. But I'm sure that with a magnifier you could fudge it to closer to + or - 10sec of arc. Whether or not the table will respond to such subtle shifts is another thing. But all else being equal it should.

    To use this particular style of vernier when it is between two marks on the minutes index you would find which minute marker the 0 line is closer to. Then find the vernier mark to that side which is closest to one of the other marks. You then either add to the lower minutes mark or subtract from the higher minutes mark as needed to correctly split the 2min division. Clear as mud? I know it took me a fair few minutes of head scratching when I got the table and it's still a little bit of stop and ponder moment when I use it even now.



    Click image for larger version  Name:	rotarytablevernier.jpg Views:	0 Size:	155.5 KB ID:	1928032
    If you want to make a bandsaw blade indexing wheel perhaps invest in a shorter and cheaper 4 or 6 TPI blade from a 14" saw?

    For example, if you were to gash out your own 90T worm driven wheel. I think I'd want to start with something like bandsaw stock which is 4TPI. Then I'd want to make the wheel pretty large to minimize any creeping errors. If I used 4TPI stock and went with 180 teeth and used every second one to produce the driven side of the worm set the band would be 45 inches long. And that would fit around a disc that was roughly 45/3.14= 14.3 inch diameter. I say "roughly" because you'd want to join the band with some care to ensure that the teeth at the joint are spaced accurately as you can manage and that the disk would be a hair smaller to account for half the thickness of the blade. At that point using every other tooth would also reduce any build up of errors. Or another option might be some 6TPI stock and go for 360 teeth. That would give you an indexing disc to work with that was 19.1" in diameter. Some extra care would want to be taken if you went with this idea so that the center hole is more accurately located within the outside diameter than is seen in most wood working. Perhaps a flanged bushing or shaft for the center? Then skim the OD for a snug fit of the blade?

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