Granite Flat Calibration and Lapping - Page 4
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  1. #61
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    Lazlo,

    Re your questions on the indicator cariage:

    1. I described the button-head socket screws as having ". . . a small flat ground on each of them." These flats, measuring about .225" diameter, look as though they have also been lapped as they have a mirror finish. I believe setting up the carriage base this way would give a more precise reading of the area under the base than if the entire base were ground and lapped.

    2. What appears to be a "hand weld" is simply a bit of dust and lint that had collected in the junction between the base and the vertical support.

    3. With the exception of the "too short" spring in the adjuster, the indicator carriage appears to be as originally manufactured in every respect.

    Hope this helps.

    Frank

  2. #62
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    Frank-
    Thanks a million for the information.
    Bruce

  3. #63
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    Well - I built a crude setup and tried it this weekend. I only get a few hours a weekend in the shop, so my plans for this initial test were not very aggresive. Mostly I wanted to learn how to better design the parts. So I machined two 1x3x5 inch steel blocks to use as supports and rigged a carrier for an indicator using a 1x2x3 block. I did not scrape the parts after seeing Frank's supports - I will embelish them with three point feet later. Here is what I learned with my crude setup.

    Error caused by side to side tilt is important. This is seen when the indicator carriage moves perpendicularly off the intended track. I concluded that it is important to be able to adjust the side to side tilt of one of the supports - that being the one that supports the straight edge at two points. It would also be desirable to have something to guide the indicator holder along a specific track. For this first crude run, this type of error was enough to kill the effort.

    Though I tried hard to match the two supports, they differed in height by 2 thou. I could have made them closer with more effort, but I wanted to plow ahead and learn what I could without changing anything. It became clear that having one support adjustable in height would also be very desirable.

    In short, everything we learned from Frank's posts turns out to be important. In addition to what I learned from Frank's photos, I learned that it is important to control side to side tilt and to guide the indicator carriage along its intended track.

    In my free time, I will design supports along the lines of Frank's. Adjustable tilt will also be part of the design. Hopefull I can build them and test them in the next few weekends.

  4. #64
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    Frank_Dorion:

    First off, how are you progressing with your
    Hendey 12 inch high speed shaper(a shameless plug
    for the product line)? I hope that the items I
    sent you were of some help.

    Regarding your planekator indicator carriage,
    the spring will fill the entire space and the ends
    are flat, not rounded. Also, the carriage handle
    appears to be missing. It attaches to the bolt
    in the middle of the carriage base. One thing
    that has not been mentioned about the indicator,
    the contact point is jewelled. The screw heads,
    on my indicator carriage base, have also been
    lapped to provide a three point contact.

    The lack of appreciation for the amount of
    effort required to generate and maintain an ac-
    curate plane surface is truly amazing. The
    disposal of a number of surface plates and
    straight edges at the former Aable Machine Tool
    was a real problem for me. The tool dealers did
    not want them, complaining about freight cost and
    lack of customer interest. Many of the machinist
    who came in to buy machines or tools were not in-
    terested. If they did not have a surface plate,
    they did not want one. It seems that all lay out
    work can be done on a welding table and for the
    really accurate work the milling machine table.

    After several months of this foolishness, I hit
    upon a new sales trick---"buy a machine tool, get
    a free surface plate". This worked well enough,
    but entailed a screening process to insure that
    the plates would go to a good home. Education
    was the key. Besides explaining the usual uses
    and care of a surface plate, I made it a point to
    finish the discussion with an admonition about
    using the plate as a surface for punching gaskets.
    A recent survey showed that the plates are all in
    fine shape and considered a valuable asset to
    their new owners. The plates sizes were from 8"
    x 12", 12" x 18" and 18" x 24", all cast iron.
    The 24" x 36" cast iron, on a stand, went home
    with me, no one was interested in that size. For
    the work I do, this is a very convenient size.

    The last surface to be offered for sale, was
    the largest. It is a 54" x 108" x 12" black gran-
    ite two ledge plate, grade A (inspection). Same
    story, no takers. It was located at the back of
    the shop surrounded by cabinets and machine tools.
    It was one of the last items I moved out of that
    shop and the one that caused me the most concern
    over its disposal. When it became apparent that
    it was not going to sell as is, the owners asked
    me to contact a monumental mason or a building con
    tractor. The idea was to cut it up for headstones
    or have it slabbed up for kitchen counters in a
    custom home. I strongly resisted both suggestions
    which lead to a good bit of acrimony between the
    owners and myself. This went on for months and
    since I really liked that plate, I was hoping that
    someone would buy it. Fortunately, the property
    was sold before I had removed all the equipment.
    After a certain amount of time, any equipment re-
    maining on the property belonged to the new owner.
    In exchange for cleaning up the property, I was
    given any remaining equipment, including the plate
    It is now at my shop and I hope to have it set up
    in a couple of weeks. The surface plate and its
    custom built stand have a combined weight of about
    9000lbs.

    After all the problems with the surface plates
    I decided to aquire all the straight edges for my
    shop. These included a pair of 10 footers, which
    definitely require two men to handle them properly

    Hendeyman

  5. #65
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    I also have a Rahn Planekator. The indicator adjustment mechanism and height adjustment mechanism is what is called a bowed flexure. Alexander Slocum describes the bowed flexure is his book “Precision Machine Design” in chapter 8.6 Flexural Bearings and on line here in section 17-14:

    http://pergatory.mit.edu/2.75/2_75%2...d%20flexure%22

    Here are some pictures of my Planekator adjustment mechanism disassembled:

    http://i72.photobucket.com/albums/i1...exureParts.jpg
    http://i72.photobucket.com/albums/i1...wedFlexure.jpg

    The bowed flexure is capable of amazing resolution.

    Don Clement
    Running Springs, California

  6. #66
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    Though I tried hard to match the two supports, they differed in height by 2 thou. I could have made them closer with more effort
    Bruce,

    Did you read my comment above about nullifying the slope error by lifting the straight edge without disturbing the feet and swapping ends?

    This should work the same way you nullify the straight edge error itself by flipping it over.

    Robert

  7. #67
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    Robert-
    At first I was worried about the slope due to differences in the two supports. I also concluded that it could be taken out by an end for end flip. I do intend to look at that sort of correction. But I thought further about the problem and realized that the only data that is needed for this correction (in principle) is the actual relative heights of the supports. This is measured directly in the first pass. (Some interpolation will be required because you cannot measure directly under the supports) This difference in height can be used to adjust the data from the second pass I described above. The overall error can then be calculated. This is all theoretical at this point and practical considerations may get in the way. So the end for end flip may turn out to be necessary. Until I get a decent setup constructed, I will not know. And my progress is limited by very limited shop time in the next few months. If others are working on this idea, they will probably get ahead of my progress and I will then learn from them.
    As I said before, I intend to do the end for end flip anyway, as it will provide a check of the results.

  8. #68
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    But I thought further about the problem and realized that the only data that is needed for this correction (in principle) is the actual relative heights of the supports.
    Doh! That's so obvious, and much simpler - thanks Bruce!!

  9. #69
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    There is a good article by R.J Rahn called “A simplified method for Surface Plate Calibration” that appeared in the January 1970 issue of "Manufacturing engineering & Management”
    “This straightforward approach to surface plate calibration requires only a desk-top calculator or slide rule, and an autocollimator or electronic level accurate to 0.1 seconds of arc”

    Also there is an article from the Collins microflat technical manual by J.C. Moody " how to calibrate surface plates in the plant" using similar methods and autocollimator

    I believe the same methods described by R.J. Rahn, using the autocollimator, could be used with the Planekator.

    BTW, my Rahn indicator carriage has the same three button head screws on the bottom.

    Don Clement
    Running Springs, California

    [ 09-26-2006, 09:24 AM: Message edited by: Winfield ]

  10. #70
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    Have you wondered why Rahn used the bowed flexure for the indicator carriage to smoothly move a “millionths” indicator? I use 18 pairs of SS flexures in making my patented telescope focuser for the same reason... ultra-smooth movement. http://www.clementfocuser.com

    Alexander Slocum “Precision Machine Design” http://www.amazon.com/Precision-Mach.../dp/0872634922 p521:
    “Sliding, rolling, and fluid film bearings all rely on some form of mechanical or fluid contact to maintain the distance between two objects while allowing for relative motion between them. Flexural bearings (also called flexural pivots), on the other hand, rely on the stretching of atomic bonds during elastic motion to attain smooth motion. Since there are millions of planes of atoms in a typical flexural bearing, an averaging effect is produced that allows flexural bearings to achieve atomically smooth motion. For example, flexural bearings allow the tip of a scanning tunneling microscope to scan the surface of a sample with subatomic resolution*
    *See, for example, G. Binning and H. Rohrer, “Scanning Electron Microscopy”, Helv. Phys. Acta, Vol. 55, 1982.pp 726-735

  11. #71
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    Slocum does some consulting for my employer. Next time he is here I will discuss this with him.

  12. #72
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    Thank you for the photos, I have a "Repeat o meter" that I've nearly finished building. If I can find a long enough granite straight edge, I'll make a Planekator! As I live in France dial indicators that read in microns come up on eBay cheaply, I bought two recently at €20 and €24 (about $24 and $28)

    Matthew


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