Rotary table vs dro bolt pattern accuracy - Page 2
Close
Login to Your Account
Page 2 of 2 FirstFirst 12
Results 21 to 38 of 38
  1. #21
    Join Date
    May 2005
    Location
    AZ
    Posts
    2,353
    Post Thanks / Like
    Likes (Given)
    215
    Likes (Received)
    802

    Default

    I'm in the round pin-diamond pin camp with others. When somebody showed me diamond pins about 30 years ago, it was a revelation. They help a LOT. for this sort of issue.

    Another perhaps minor point: If you are using mild steel, and clamping the plates together, if there is any out-of-flat condition, that could bite hard on 2" thk plates if they have been clamped hard and then released after machining.

  2. #22
    Join Date
    May 2012
    Location
    Pittsford, NY
    Posts
    1,036
    Post Thanks / Like
    Likes (Given)
    714
    Likes (Received)
    667

    Default

    I don't know what budget or space constraints you have, but there are a couple of curvic coupling halves on ebay for $75 each. The pins could be sloppy and the coupling would then locate the plates accurately when the coupling halves mate as the plate drops. The ones on ebay have an 20 teeth if I counted correctly, so they would index 90 degrees with a 5 tooth rotation.

    P.S. Looked up the new price for a 868182-1 coupling---$4390

  3. Likes tdmidget liked this post
  4. #23
    Join Date
    Dec 2008
    Location
    Vt USA
    Posts
    6,796
    Post Thanks / Like
    Likes (Given)
    745
    Likes (Received)
    2356

    Default

    It would be good to know the expected accuracy of both procedures. Advantage to one or the other? Sort of like "Probable hole size" when drilling holes that will be tapped.

    Me? I use the dividing head or rotary table for round or radial work, and trig out the hole locations for flat work. It seems to make sense to me, so that's why I do it.

  5. #24
    Join Date
    Mar 2005
    Location
    Maine
    Posts
    1,323
    Post Thanks / Like
    Likes (Given)
    9
    Likes (Received)
    532

    Question

    This was posted by John Oder over in the Antique section.
    Pretty interesting.
    It's a little hard to read.
    Keep clicking on the picture to make it larger.
    Attached Thumbnails Attached Thumbnails p-w-jig-borer4.jpg  

  6. Likes neanderthal mach, Winterfalke liked this post
  7. #25
    Join Date
    Jan 2006
    Location
    Angier, North Carolina
    Posts
    2,609
    Post Thanks / Like
    Likes (Given)
    1100
    Likes (Received)
    973

    Default

    Quote Originally Posted by EPAIII View Post
    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.
    EPA, this was a great explanation of the multiple factors adding up to affect the accuracy of a mill.

    Reading this, I was reminded of something I read somewhere - maybe even in a thread on this forum? - that suggested that +/-.001" was about the limit of accuracy for a standard BP mill -- or was it +/-.002"? Something like that; I remember being aware that it was certainly far less accuracy than one could achieve on a lathe.

    Since I don't recall the source, and my memory is fuzzy, I have no idea whether the above is correct - maybe someone can refute or support. But I have to say, with EPA's discussion in mind, plus my own relatively limited experience, it *sounds* about right to me. Of course, that may just mean that I'm better on a lathe than I am on a mill, which *definitely* sounds right to me!

  8. #26
    Join Date
    Dec 2008
    Country
    UNITED STATES
    State/Province
    New York
    Posts
    10,215
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    2653

    Default

    tram error or the head leaning effects XY as tool tip get longer or extended out. between raising Z by raising table and using a spindle with tram lean you can use a short center or spot drill but when you switch to longer drills the tool tip will be off in XY
    .
    just saying i have seen .002 to .005" errors cause machinist didnt fully appreciate the tram error effects
    .
    literally you can bore a hole with a short length boring bar and then put indicator in spindle and if longer than boring bar it will say the bore you just did is off center. i probably see some error even if just .0003" (often more) maybe 98% of the time. depends on machine and how used of course
    .
    rotary tables have their own errors and slides have a tilting action at times effecting position
    .
    usually use a granite square and indicator to check machine travel. not unusual to have table lean change as it travels down a slide. just saying not everything is easily adjustable.

  9. #27
    Join Date
    Feb 2011
    Country
    UNITED STATES
    State/Province
    Indiana
    Posts
    34
    Post Thanks / Like
    Likes (Given)
    22
    Likes (Received)
    15

    Default

    Quote Originally Posted by CarbideBob View Post
    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?
    Very good point. We will consider adding this feature to the next software release for DR400 series of Digital Readouts.

    Quote Originally Posted by CarbideBob View Post
    I have not seen backlash in a scale, seen a lot of motion on a axis slide that reader head does not.
    It is small, typically less than 1 resolution count, but every sealed encoder has backlash or reversal error. We investigated several designs comparing readings with laser interferometer. Reversal error can be as high as several microns. Older scales which used friction pads instead of ball bearings, showed some of the highest reversal errors. Higher end sealed encoders intended for OEM application have some of the smallest errors while exposed non contact encoders have virtually no backlash.

  10. #28
    Join Date
    Dec 2008
    Country
    UNITED STATES
    State/Province
    New York
    Posts
    10,215
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    2653

    Default

    DRO the better ones need calibration. that is using a 6.0000 gage block and indicator you zero indicator to gage block and zero DRO. remove gage block travel to stop til indicator reads zero and see what DRO says
    .
    quite common to be .0005 to .0020" in error and its often different in X than Y. usually special mode if DRO says X moved 6.0010 you tell it it moved 6.0000 and retest to confirm
    .
    used a DRO with error compensation mode a light would blink confirming its on. if somebody pressing buttons trying to zero DRO turns it off the DRO would often be off .002" per 6", it also needed to be turned on error compensation if DRO was turned off
    .
    old shop we did DRO calibration check yearly or before every tight tolerance job
    .
    and of course Bridgeport type mill the whole table bends like a banana as you put more weight on it. if part all the way over to one side as you travel in X if a precision level was on part you would see the table bending the more it overhangs to the right side of the saddle. bending and slide tile can be considerable on many mills

  11. #29
    Join Date
    Dec 2008
    Country
    UNITED STATES
    State/Province
    New York
    Posts
    10,215
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    2653

    Default

    obviously if part is getting hot a 10" part 100F hotter will grow .006", lack of temperature control has also cause part size changes later as part cooled to room temperature.
    even a 10F change will effect part .0006" can cause problems
    .
    many a hot part on a lathe will measure considerably smaller later when it cools off

  12. Likes zsinstruments liked this post
  13. #30
    Join Date
    Feb 2011
    Country
    UNITED STATES
    State/Province
    Indiana
    Posts
    34
    Post Thanks / Like
    Likes (Given)
    22
    Likes (Received)
    15

    Default

    Quote Originally Posted by DMF_TomB View Post
    DRO the better ones need calibration. that is using a 6.0000 gage block and indicator you zero indicator to gage block and zero DRO. remove gage block travel to stop til indicator reads zero and see what DRO says
    .
    quite common to be .0005 to .0020" in error and its often different in X than Y. usually special mode if DRO says X moved 6.0010 you tell it it moved 6.0000 and retest to confirm
    A word of caution here. Many things can go wrong with this procedure if the operator is not experienced. Just one of the examples: if stop is not indicated in, each degree of tilt will cause 0.001" of cosine error over 6". Quality scales have accuracy of at least 10um/meter which is better than 0.0005 over 40" (as measured at 68F). And over 6" it's typically better than 0.0001".

    So the measured error is very likely originates not in the scales, but instead caused by table bending, loose gibs, temperature etc. Those errors will change in both amplitude and direction depending on load, time of the day, approach direction, locking screw torque, location along the axis, etc. Only consistent and repeatable errors can be compensated for. Assuming all errors are systematic and using a total error to calculate a compensation factor might do more harm than good.

  14. Likes memphisjed, EPAIII liked this post
  15. #31
    Join Date
    May 2003
    Location
    Walla Walla Wine and Wild Turkey
    Posts
    4,625
    Post Thanks / Like
    Likes (Given)
    1
    Likes (Received)
    323

    Default

    I do alot of dowel pin work using a jig borer, something like this appears very doable. P&W demonstrated in the 1920s multiple pins in plates done in different shops, and having them fit together when finished using their jigborers
    Way back in time, work like this could be done on a good lathe and a face plate, using toolmakers buttons, another old one is, setting a rotary table using a sine bar.
    Perhaps asking a bit much in these more or less helpless times.

  16. Likes zsinstruments liked this post
  17. #32
    Join Date
    Mar 2005
    Location
    Maine
    Posts
    1,323
    Post Thanks / Like
    Likes (Given)
    9
    Likes (Received)
    532

    Default

    Quote Originally Posted by donie View Post
    P&W demonstrated in the 1920s multiple pins in plates done in different shops, and having them fit together when finished using their jigborers.

    See my attachment above.

  18. #33
    Join Date
    Mar 2007
    Country
    UNITED STATES
    State/Province
    Ohio
    Posts
    8
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    1

    Default

    Quote Originally Posted by wheels17 View Post
    I don't know what budget or space constraints you have, but there are a couple of curvic coupling halves on ebay for $75 each. The pins could be sloppy and the coupling would then locate the plates accurately when the coupling halves mate as the plate drops. The ones on ebay have an 20 teeth if I counted correctly, so they would index 90 degrees with a 5 tooth rotation.

    P.S. Looked up the new price for a 868182-1 coupling---$4390
    Yes! Don't fight it with dowels, make something that self-aligns.

    Actually I was thinking he should make a crude 90 degree version of a curvic coupling.

    Imagine each plate as a clock face. Use a chamfer mill or similar tooling to make two v-slots across the face of the part. One from 3 o-clock to 9 o-clock, the other from 12 to 6. Repeat on the second part. Cut four fairly short lengths of square key stock and install them in the v-slots at 3, 6, 9, and 12 o-clock in one part. Now you have four raised V-ways. Invert the other part and mate its v-slots with the keys. You can still use the center shaft to lift and rotate.

    Same principle as the v-ways of a lathe - self centering.

    You do need to get the two slots in each plate as close to 90 degrees to each other as possible. I'd cut one slot, then clamp an accurate round bar into the slot, and clamp my best square along the bar. Then indicate along the blade of the square before cutting the second slot.

    For better straightness and surface finish on the slots, use a v-cutter on a horizontal mill.

    For really good accuracy, grind the slots on a surface grinder. Since you rotate the part 90 degrees between cutting the two slots, and dial it in for the seoond slot, the X-Y angle of your machine axes doesn't matter at all (nor does your DRO accuracy).

    You oould still use dowels. Make the slots with a ball end mill, depth just a little less than the ball radius. Then lay the dowels flat in the slots. Round keys self center too!

  19. #34
    Join Date
    Nov 2003
    Country
    UNITED STATES
    State/Province
    Texas
    Posts
    5,042
    Post Thanks / Like
    Likes (Given)
    176
    Likes (Received)
    1553

    Default

    Yes, I would faster blame looseness in the machine and the operator's technique for errors than almost any commercial DRO scales.

    Any CALIBRATION of a DRO scale should only be done by an experienced operator who completely understands all the sources of error and who takes the time and effort to eliminate or minimize all of them. Otherwise you are just fooling yourself. If you have two inexperienced operators do a calibration, you will likely get two completely different results. I would not be at all surprised at 0.002" differences or even more. Three operators will get you a third result that will not agree with either of the other two. Etc.

    Frankly, unless you have a very good person doing the calibration, you are probably better off trusting the DRO scales and working on the other sources of error.

    If you don't believe me, try running a calibration with two or more operators, each of which has no knowledge of the fact that the others will be doing it.



    Quote Originally Posted by zsinstruments View Post
    A word of caution here. Many things can go wrong with this procedure if the operator is not experienced. Just one of the examples: if stop is not indicated in, each degree of tilt will cause 0.001" of cosine error over 6". Quality scales have accuracy of at least 10um/meter which is better than 0.0005 over 40" (as measured at 68F). And over 6" it's typically better than 0.0001".

    So the measured error is very likely originates not in the scales, but instead caused by table bending, loose gibs, temperature etc. Those errors will change in both amplitude and direction depending on load, time of the day, approach direction, locking screw torque, location along the axis, etc. Only consistent and repeatable errors can be compensated for. Assuming all errors are systematic and using a total error to calculate a compensation factor might do more harm than good.

  20. #35
    Join Date
    Mar 2011
    Location
    Huron
    Posts
    1,355
    Post Thanks / Like
    Likes (Given)
    1909
    Likes (Received)
    846

    Default

    Quote Originally Posted by maynah View Post
    This was posted by John Oder over in the Antique section.
    Pretty interesting.
    It's a little hard to read.
    Keep clicking on the picture to make it larger.
    Was literally thinking of this article while reading the thread. And I agree with the round/diamond pin configuration, I found fixtures so much easier to make when I finally could order these.

  21. #36
    Join Date
    Sep 2018
    Country
    RUSSIAN FEDERATION
    Posts
    28
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    2

    Default

    Manual punch over line made with heght gauge is enough precise for most dowel pins usages...Just choose one (best) edge as reference.


  22. #37
    Join Date
    Sep 2009
    Location
    barcelona, spain
    Posts
    2,394
    Post Thanks / Like
    Likes (Given)
    535
    Likes (Received)
    1438

    Default

    1.
    The std approach for 40 years in jig borers and similar high accuracy stuff was always location and never a rotary table.

    2.
    Forget drilling anything.
    Drill undersize. By about 0.2 mm-0.5 mm depending on thickness, and how good your drills are.
    Hopefully using short stub drills, screw machine drills, or similar.

    Bore the holes to (under)size.
    About 0.02 mm - 0.1 mm undersize.

    Boring will make the holes cylindrical, and straight, to tolerances of your spindle, quill travel error, and Tir error in the spindle.

    2.1
    Typically, a Bp type machine like yours would make holes of about 0.01 mm error in tir, or less, and about 0.02 mm or less in travel over 3-4" or 100 mm, give or take a bit.

    Drills would likely / typically have errors of 0.05 mm or much more, in straightness / cylindricity.
    Errors over 0.1 mm are typical, over any thickness like 2D(iameter) or more in steel, say 20 mm thick in 10 mm holes.

    3.
    Drilled holes are banana-shaped and not round.

    4.
    Finish with a reamer in a bored hole.

    A finish-bored hole will be straight and cylindrical to a very high degree.
    0.01 mm in accuracy, 0.02 mm in overall specs wrt. cylindricity, straightness, etc., something around there.

    Finishing it with a reamer tends to make the hole very smooth, uniform, and very good in size, about 0.005 mm, or 5 microns in size with great finish.
    The reamer will bend and follow the hole, so ..

    4.1
    a drilled hole, reamed, can have great finish but not be very straight.
    4.2
    A bored hole, finish reamed, will usually be very straight, and very uniform, with great finish.

    4.3
    Lacking a boring head or boring bars, you can bore with the edge of an old hss end mill.
    Aka single-lip cutter.
    Just grind off a bit from the edges of all but one lip.
    Mark the flutes of the lip used.
    IF HSS or non-carbide tool, you could progressively bend the cutter a bit, to get the desired hole size.
    Clamp endmill in v block with spacers, measure, cut, adjust.

    --
    All above is laborious, but should take less than 2 hours in work to make the first excellent hole.
    And maybe 30 mins each for next 3 holes, total 4.

    Accuracy should be about 0.01 - 0.005 degrees in rotational angle, possibly better, on a say 150 mm D circle.
    0.001 degrees, +/- 0.002 degrees, could well be achieved.

    Amateurs do this stuff all the time in hobby telescopes.
    Without real mills, or DROs.

    --
    A Very sligthly tapered reamer, and pins, very slightly tapered, could easily get you better repeatability, in each of the 4 angular positions.
    Hard to predict, but something on the order of 1 micron in position and 0.000x in degrees of repeatability.
    Repeatability, not accuracy.

    Abrasives like 800-2000 grit si-ca or 2 micron films (3M) or lapping powders or diamond grits, just slightly tapering/reducing the reamer end and pin ends.
    Hand work with a rotating spindle in mill or lathe, and a bit of practice, backing piece like a ground flat or v-block flat side.
    Measure with dti and micrometers.
    Takes less than a minute to minutely taper a drill rod or gage pin by 2 microns or so.
    Or to taper the reamer to finish the holes.
    Sure the tapers wont be the same, 1 to 4 pins, or uniform in profile.

    But the tapared pins will likely locate the piece extremely accurately and repeatably in each of the 4 positions.
    More or less 1-2 microns in positional repeatability, and thus sub 0.001 degrees in angular repeatability- not accuracy.
    --

    The holes might be out 0.005 mm plus/minus a few microns in location.
    Using a 0.01 mm accurate DRO to position them.

    Using 3 holes and the custom tapered reamer, and hand-tapered pins, should always locate and geometrically-average the error.
    I think std gage pins are about 0.01 mm, 0.001 mm +/- 5 microns, accurate, tending to less than 5 microns error, tending 2 microns.

    If you hand-taper the reamer tip and shank by about 0.008 mm, or 0.01 mm, the final holes at bottom will be a bit tight for std gage pins.
    This makes the assy better in repeatability.

    --
    Amateurs lap complex mirrors and glass flats to much better than 1 micron, overall, thousands per year.
    And fit stirling engines with pistons and rods, to 1-2 microns in size and 1-2 microns in tir.
    And make model engines to 2 micron fits between piston and bore.
    --

    A DRO allows You accuracy in hole location similar to a high end jig bore, 40 years ago.
    Todays *commodity* CNCs do about the same, not quite, by using very good ballscrews and some compensation.

    Todays high end CNCs do much better, via even better ballscrews, extreme mass and rigidity reducing thermal effects in the short term.
    And secondary feedback via 0.01 micron glass scales (Heller Germany, Mori Seiki Japan, Moore & Wright USA Nanotech lathe).

    --

  23. #38
    Join Date
    Dec 2018
    Country
    UNITED STATES
    State/Province
    Indiana
    Posts
    104
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    12

    Default

    Thanks all for the replies some very good info and I learned a lot.

    I did a trial piece with two dowels and failed at getting repeatable results indexing the plates. I ended up scoring a very cheap barrel gear 4 position indexer and it worked perfectly for the application.

    Hirth and curvic couplings Ive never heard of and enjoyed learning about them.


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •