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  1. #16
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    930

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    The 2 x 7204 angular contact bearings can be purchased as a matched pair that provide virtually zero axial float.
    But, only available with open construction necessitating seals with an oiling facility to be added

    I was told the 3204 C2 is virtually unavailable in Australia (at unspecified high cost).
    The normal range is specified as 7 to 25 microns, and available at a reasonable cost.

    Fitted a FAG 3204 with appropriate spacers to allow for the reduced width.

    The shaft float when jogging X +/- was about 40 microns, much improved.

    Operating the X-axis with rapid moves of +/- 100 mm from the zero position shows the actual on-table backlash as 60 microns.
    This is an acceptable value and I don't think I will pursue further improvements.

    I have another FAG bearing and will soon look at improving the Y-axis.
    Keep well,
    John.

  2. #17
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    930

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    A) I thought I should check the backlash using a Mitutoyo 1 micron/division Dial Indicator, 200 per rotation, 1 mm total.

    This Di is a difficult beast to hold steady, but I made a special rigid mount hanging off the mill head
    The photo shows the arrangement.
    There is a box angle plate on the table, the DI positioned to allow pass-by of the box, and a parallel that is slid back for the pass-by, and carefully angled in to the tip for the measurement.
    A setup as stiff and repeatable as I could quickly arrange.

    Test started with the DI upscale, G0 move (600 mm/mim) to X+100 mm, G0 to X0, note the DI reading, G0 X-100, G0 X0, note the reading.
    I take the backlash to be the difference in the two readings.

    Consistently got about 70 microns for the backlash.

    B) Then, something interesting.
    Instead of G0 commands, I tried G1 F100 to +/- 50 mm.
    Now consistently getting 90 microns.

    I have no idea why this would be.
    Any suggestions ?
    I did go back and check the value at G0 and 600 mm/min, still 70 microns.

    C) I then repeated the exercise with the original Dial Test Indicator, typical common unit labelled Insys.
    10 micron /div, 800 per rotation, 1 mm total.
    This is my goto DTI on the mill.

    Got a virtual repeat of the values from the DI.

    D) I then tried jogging using the CNC manual commands with 0.1, 0.05 and 0.01 mm steps.
    Now getting about 100 microns change on the CNC before the DTI detects the movement.

    E) In the real world of cutting metal, feeds tend to be around 100 mm/min, so perhaps I must rate the backlash at 90 microns, not the 60 previously advised.

    Keep well,
    John.
    Attached Images Attached Images

  3. #18
    Join Date
    Sep 2009
    Location
    Newcastle
    Posts
    341

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    I don't understand why are moving to x100 and how you are measuring backlash that way.

    The way I do it:
    Firmly mounted indicator.
    Move towards (into) the indicator until indicator is well in contact.
    Now you tell controller to move away from the indicator until the indicator reading actually changes. (Usually easiest to do this in steps).
    The difference between the CNC DRO and the actual movement is the backlash.


    Stick slip can be a factor in different readings at different speeds.
    Depending on steppr / servo resolution you may also have errors / differences also. With steppers, microsteps are not as precise as a full step and if you are trying to read a difference within the microstep realm you will not be accurate.

  4. #19
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    930

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    The +/-100 mm at 600 mm/min. and +/- 50mm at 100 mm/min were aimed at duplicating real-world situations that affect jobs.
    The backlash is the difference between approaching the Touch-Off zero position from the two directions.

    You mentioned stick/slip, and I think you have to add inertia, and I have a belt drive ( stiff and short, but still a belt ).
    I have always wondered about the track in the ball nut that the balls take, bottom of the valley or up one of the sides.

    Our methods for short-range jogs whilst in contact with the DI are similar, but could be slightly different.
    It appears you move slowly in, whereas I was jogging with high acceleration, followed by a high deceleration, but the maximum speed reached is still very low because of such short steps.

    I will investigate further the question of the microsteps.
    Cannot do much about it as the machine needs microsteps to get acceptable parts.

    Keep well,
    John.

  5. #20
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    930

    Default

    As the X-axis bearing change went Ok, I went ahead to check the Y-axis.
    Discovered the same 2 x 6204 plus 100 micron spacer between the outer rings.
    Changed to the 3204 and tested the backlash as previously described for the X-axis, and got similar results.

    So, to complete the trifector, I tested the Z-axis.
    Got somewhat similar results.

    Summary of backlash test results:
    X-axis:
    28 microns - rapid moves at 600 mm/min,
    66 microns - moves at 100 mm/min,
    120 microns - jog in steps of 10 microns at 6 mm/min.

    Y-axis:
    38 microns - rapid moves at 600 mm/min,
    49 microns - moves at 100 mm/min,
    90 microns - jog in steps of 10 microns at 6 mm/min.

    Z-axis:
    45 microns - rapid moves at 180 mm/min,
    53 microns - moves at 30 mm/min,
    70 microns - jog in steps of 10 microns at 6 mm/min.

    I observed an interesting difference in the mill movement responses comparing the X, Y axes to the Z-axis when jogging in 10 micron steps.
    For the X and Y axes, the mill would not move until a number of steps were entered, then linear per step.
    For the Z axis, observed a small movement for each of a few entered steps, then linear per step.

    Then I tried an actual cut test.
    Programed a circle Dia 74 mm and 5 mm deep on CI stock Dia 75 mm.
    No backlash compensation in the computer.
    The photo shows the flats observed at the quadrant points as alternately the X and Y-axis hit their reversal limits as the tool tracked around the circle.
    The flat is about 5 mm wide.
    If you do the trigonometry, this width flat infers a radial step in of about 80 microns.

    Repeated the test with backlash compensation entered into the computer and a cut of 0.2 mm.
    X-axis 66 microns, Y-axis 49 microns, Z-axis 53 microns.
    The photo shows the result, a flat that is extremely small, and cannot be felt by touch.
    Tried running a DTI around the test piece, but the background fluctuations masked any flat detection.
    These fluctuations reflect the limitations of doing circles with stepper motors on an old mill.

    This, I think, completes my investigations under this subject.
    I am happy that the mill is performing as good as I can expect.

    Keep well guys,
    John.CI Backlash Test with Compensation 5Sep20 compr.JPGCI Backlash Test No Compensation 5Sep20 compr.JPG

    Edit: The Z-axis is still the original factory-fitted trapezoidal (metric equivalent to Acme) screw.

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