Rotary table vs dro bolt pattern accuracy
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  1. #1
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    Default Rotary table vs dro bolt pattern accuracy

    Need to make 2 plates both 10"x10" 2" thick mild steel one on top of each other with bolts holding them together. I will need to be able to lift the top plate off and rotate 360 degrees in 90 degree increments.

    I plan to use 4 dowel or gage pins pressed in on the bottom plate and 4 drill bushings pressed into the top plate for location.

    Have a knee mill with DRO for making the bolt pattern or should I use a rotary table? Each plate will have a center hole. Aiming for pins that are .0005 undersize of drill bushings. Not too sure if one method or the other is the clear winner in accuracy. My gut is use the DRO but wondered if Im missing something.

    The plates are not square with each other so I can't use the outside edge and just forget about the pins all together.

    Thanks in advance.

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    From what I can make of your specs it sounds to me more like proper jig borer territory.

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    I was afraid I might be asking too much out of my equipment.

    Thanks for the reply.

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    Ouch,
    How far out from center are the holes?
    How square are your x&y axis to each other?
    How good is your rotary and know how appoach.
    You have for all practical purposes .00025 max to play with here.
    My gut would be the rotary used from the same starting zero index position but this is asking a lot from a Phase II or such.
    Add in drill bushing tolerance plus install and you run into no fit fast.
    Two, there or four locator holes, one is easy if you have a hub. Two will not locate you with no hub, Three and four are just plain hard to do on your machine.
    There has to be an easier way to skin this cat when making a high precision 90 indexer.
    Bob

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    turnworks --

    How precise do the 90-degree increments need to be?

    I recommend hesitating to use more than two pins, but instead using a pair of pins located "on the diagonal" of the square bolt-hole pattern. Three or four pins might possibly be justified if you are after gage-class squareness, but put you into the realm of "overconstraint with elastic averaging" . . . which is very difficult territory for pin-in-hole geometry.

    For standard-class work, I'd go with one cylindrical pin and one diamond-shaped pin -- commercially available from the makers of jig-and-fixture components -- and put the time and effort into the geometry of the hole pattern.

    John

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    I need to keep the outside corners positions to be within .003 every 90 degrees. There is more to it but that will work for what Im aiming for. Pin pattern size is up for grabs right now. I know the bigger the pattern then less tolerance pin to bushing I have to hold to maintain the .003 position.

    Rotary is an older bridgeport one and not parallel with the bottom anymore(.002 over the plate) I normally tram the head to the top of the rotary table first.

    Two pins would get the job done not sure why I just assumed I needed 4 pins.

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    Can you have a dowel in the center that you rotate the plates about? Then with 1 additional dowel hole in the upper plate you match drill and ream the 4 holes in the lower. If you start with the plates doweled together on the center hole, and then square the edges using the center as the datum, you should be able to meet your edge tolerances.

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    There will be a shaft attached to the top plate in the center that will run through the bottom plate and fixed to a pneumatic cylinder that will pop up the top plate to allow rotating. I plan to use a UHMW bearing in the bottom plate. The plates will always be in horizontal position. Im wanting the pins only to locate the top plate in relationship to the bottom plate.

    Making a sliding shaft that can rotate that accurate and stay that accurate over time is outside my skill set.

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    First, four pins is an over constrained solution to this problem. Only two are needed to locate the top plate and the most sensible arrangement would be a central hole that the top plate pivots on and a single pin located as far from that central hole as possible in the bottom plate with a second pin there. The top plate would have five holes, one in the center and four for the 90 degree rotations.

    Now, which method would be the most accurate? The answer to that would lie in a combination of the accuracy of your tooling and in your ability to use it. The sources for error that I see are:

    X-Y Coordinates:
    How accurate are the X and Y DRO scales on your mill? Any differences will put the holes on one axis at different distances than those on the other.
    How square are the X and Y axis of your mill to each other. Any error will produce an angular error in the four right angle rotations: two will be greater than 90 degrees and the other two will be less.
    Will you lock the X and Y axes down for each hole location? Will your moves from one hole to the next properly account for any backlash in the screws. I know you have DRO scales, but any looseness in the gibs can bring the table to different locations if a hole is approached from different locations. Don't believe me, just try it.
    How accurately can you drill the holes using those scales? What technique will you use to locate them? For best accuracy with a mill I would mark, but not punch the locations. Then use a short, spotting drill to establish the locations. Then a short, stout drill at a smaller diameter to drill a starter hole. Then a drill that is a few thousandths small for the final drilling. And finally a reamer for finishing the hole. Yes, that is a lot of drilling. All these drills should be very sharp and as accurately ground as possible to avoid any deflections or other errors.

    Rotary Table:
    You don't say what size your table is. A 10 or 12 inch one would provide a lot more accuracy than a 4 or 6 inch one. Most rotary tables have Vernier scales that allow setting it down to tens of seconds with a basic accuracy also in that range. That should provide enough accuracy for your purposes. But you say yours is old so there could be wear.
    Any slop in the center pivot of the RT would provide inaccuracy in both the X and Y location of the holes.
    How accurately can you locate the table under your quill's axis?
    How accurately can you locate the two plates on the RT?
    To the above sequence of drilling and reaming, will your lock the RT down after each 90 degree move? If it is not locked, drilling forces can move the table within the backlash available in the worm gear and that can be a significant amount.
    Will you lock the X - Y mill movements for each hole? Again, they should be approached from the same direction in spite of the presence of the DRO.

    Personally, I would use my 10" RT as I do not trust the angle between the X and Y axis of my mill to be that accurate.

    Note: I do not trust DRO scales to necessarily locate holes to their stated accuracy. This is not a mistrust of that stated accuracy but rather due to how they are used/mounted. Of necessity, DRO scales are mounted at a point that is offset from the quill's axis. So, any looseness in the gibs can allow the table to sit at an angle to the actual ways and this angle can be different with different sequences of movements prior to reaching a reading on the DRO scale. Errors of one or more thousandths can easily be produced this way. In a way, the scales on the handwheels which read the position from the rotation of the X and Y screws which intersect at or near the quill axis, are less susceptible to this kind of error. But I said "less susceptible", not immune.

    For the best location accuracy using either handwheel scales or DRO scales, the gibs should be as tight as possible and that location should be approached with a sequence of moves that ensures that any cocking of the table inside the looseness of those gibs is always the same. That movement should be at least one full rotation of the handwheel, preferably more and always with the rotation in the same direction. Even a small backwards motion can destroy this accuracy.

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    from reading between the lines of your post it seems you want to build a pneumatic indexer and are trying to locate the holes such that you don't bind the cylinder bearings. I have great doubts that the cylinders rod thread is actually that concentric to it's mounting.

    Asking if "a dro" is more or less accurate than "a rotary table" is insane without any background information. It's insane anyway as the tolerance your trying to hit is not measurable by you anyway. -If it was, and you had any experience doing it, you would know which to use already.

    You might instead post a picture and ask for ideas on improving your design to something you can actually hope to build.

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    Quote Originally Posted by turnworks View Post
    There will be a shaft attached to the top plate in the center that will run through the bottom plate and fixed to a pneumatic cylinder that will pop up the top plate to allow rotating. I plan to use a UHMW bearing in the bottom plate. The plates will always be in horizontal position. Im wanting the pins only to locate the top plate in relationship to the bottom plate.

    Making a sliding shaft that can rotate that accurate and stay that accurate over time is outside my skill set.
    I can't picture what you're scheming, but UHMW (in fact most ) plastics and fine close tolerances do not mix.

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    I agree with the air cylinder and the UHMW comments hence the reasons I picked them to be forgiving and only the pins to be the method to hold location.

    I do have ways to measure the holes after I make them.

    But I do agree this is the first indexer type piece I have made. Trail and error time.

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    I think I've gotcha

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    We make parts with decent accuracy, but after years of tears we've learned never to use two dowel pins in holes. Always one pin in a hole and one pin in a slot.

    As far as accuracy, I've always taken the orthogonality of X and Y as a given, but it's something I really need to check!

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    Take a look at this round-and-diamond pin combination: Bullet-Nose Pins | Carr Lane

    My link is to Carr-Lane's website, but other jig-and-fixture-component makers offer similar products.

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    Even if you make the parts perfectly, with two round pins it can jam or not assemble if one plate gets warmer than the other.

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    Quote Originally Posted by turnworks View Post
    Have a knee mill with DRO for making the bolt pattern or should I use a rotary table? Each plate will have a center hole. Aiming for pins that are .0005 undersize of drill bushings. Not too sure if one method or the other is the clear winner in accuracy.
    DRO will measure linear distance with an error of +/- 1 resolution count + glass error + temperature error + way straightness + play in the gibs + drilling/reaming accuracy. Doing square pattern will also add an orthogonality error.

    Rotary table measures angle, your error will be radius times the angular error which is resolution + worm drive error + plus bearing run-out+ eccentricity. Add temperature effects & drilling/reaming accuracy.

    Just to give you an idea, assuming both parts are done while temperature in the shop is the same and ignoring common factors such as drilling/reaming accuracy:

    5um scales with 10um accuracy grade will introduce +/-0.0003" error over 10" if approaching from the same direction (yes, scales have backlash too, not noticeable though as it is typically less than 1 count of resolution). Add way straightness & orthogonality, table deflection and gibs error to that.

    Bridgeport rotary table claims 5 arc sec resolution with 30 arc sec accuracy (probably includes runout, drive errors, etc). +/-30 arc seconds translate to +/-0.0007" at 5" radius.

    If you have some similar size scrap, try practicing on those. If you can measure the holes precisely, orthogonality, way straightness and gibs error can be compensated for on the next run. You will need to lock the gibs using the same torque every time to get the best repeatability. To achieve 0.003" final accuracy, I would suggest dowels which are at least 0.002" undersized, as it translates to a maximum play of +/-0.001". That will reduce your positioning requirements significantly. Using just 2 pins pressed into the first part and 4 holes on the other will help too.

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    Quote Originally Posted by John Garner View Post
    Take a look at this round-and-diamond pin combination: Bullet-Nose Pins | Carr Lane

    My link is to Carr-Lane's website, but other jig-and-fixture-component makers offer similar products.
    I used these all the time in making quick and dirty molds for the foundry. We would just drill and ream thru both plates at the same time for a perfect alignment. In the old days we would use these and square the mold after pining them together in a drill press. If your looking to index, then use one diamond pin and one round pin but you'll need to use a mill for location preferably with a R.T. If you limited to a BP then any time you can do both plates at the same time do it. If you want to use just a pin and bushing you can always go back and open up any of the holes later for the bushing using a boring head. Like others have said, there are a number of way to do this, even in a BP.

    BTW a standard dowel is .0002 oversize. And are pressed into hole that is .0005 undersize.

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    Lots of a great info thanks.

    If my skill and equipment are going to limit my chances of hitting my tolerance maybe Im going in the wrong direction. Instead of me trying to make tight tolerance with limited room for error could I go the offset down pin route then after assembly cam the pins in to get my positional tolerance(for each 90 position)then secure the pins once Im happy? The bolts will hold the position once the pins put it into position.

    Again thanks for the replies.

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    With all the computing power do any DROs comp for axis squareness errors?
    So that if calibrated and you move only the X the Y number changes or vice versa?
    Out side of square the problem with any scale is that is does not see pitch, yaw, roll.
    Ideally the reader head would be right under the cutting action. This is not practical so you get abbe error ( google it).
    This sensor to working tip "twist" is a problem measuring machine people loose sleep over.
    I have not seen backlash in a scale, seen a lot of motion on a axis slide that reader head does not.

    Sometimes with money no object you put two scales per axis.
    Think one in front of your B-port table and one on the back so that the reversal twist is measured but where you are in Y matters.
    I know of no standard DRO that can do this, It's the world of specials

    With all this new computer stuff and the price of scales you'd think somebody....
    Bob


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