Re: ATM 19th Nervous Breakdown

["James Lerch" <jlerch1@tampabay.rr.com>]

Hi Bob

Good timing, I'll use you and your mirror as an example of how I'd use my 3D mirror App to aid in advising you on figuring your mirror. On a side note, you helped me find a bug in my code, but I'll address that in another post.

(** To the List,  The Purpose of this post is to gather comments and advice, on the comments and advice I give Bob below.   Try to be nice :)

Bob, Lets start with some images generated by my code, based on the Sixtests results from you post.

#1 http://lerch.no-ip.com/atm/Bobw2.jpg (42KB, Shades of Red)
#2 http://lerch.no-ip.com/atm/BobW.jpg (51KB, Green to Red)

In Image #1, Bright Red is the High spots, dark Red the low spots on you mirror.  In image #2 , Bright Red/Orange are the high spots, Green is the Low spots on your mirror.  Both images agree with the mental picture I got while reading you original post.  Keep in mind, we can't do much about the low spots, and can only rub off the high spots!

Since your high spots are the middle and 80% radius of your mirror and your edge is low I offer the Following Two ideas:

Using your Existing 12" Lap, or Making an 8" lap, using a W stroke where: **1

#1) The Left/Right of the W puts the center of the lap on the Center of your outer hill.
#2) The Up/Down of the W puts the Center of the Lap Inside the Outer hill by 50% of the hill slope
#3) During the W stroke, Concentrate most of the work on the Center of the W and the Edge of the W, Moving quickly between the two so as not to rub too much on the central valley.
#4) Do 6 Laps around the Table and Re-test
#5) If Center Hill and Outer hill are reducing height evenly then repeat above
    A) Else, if Center hill Gets higher than outer hill:
        I) Decrease Width/Height of your W stroke, OR
        II) Keep stroke size, just do more strokes on the Center of the W
    B) Else, if Center hill Gets lower than outer hill
        I) Increase Width/Height of your W stroke, OR
        II) Keep stroke size, just do more strokes on the Outer edge of the W stroke
#6 Repeat above steps, testing Frequently, until the mirror
    A) No longer behaves as expected (re-think plan of attack)
    B) You find yourself with a nice Parabola :)

<End>

Using a Full Sized Lap:

#1 Stop doing tool on top, and start doing Mirror on Top with a good press.
#2 Do a W stroke where:
    A) The Up/Down length puts the Peak of the outer hill on the edge of your lap
    B) The Left/Right width puts the Peak of the outer hill outside the edge of your lap
#3 Do this for 6 laps around the table
#4 Re-Test
#5 If The center hill and outer hill are reducing in height evenly then repeat above
    A) Else, If the Center hill gets higher then Outer hill, Increase Up/Down and Left/Right stroke length
    B) Else, If the Center hill gets lower then the Outer hill, Decrease Up/Down and Left/Right stroke length
#6 Keep doing this until the mirror
    A) No longer behaves as expected (re-think plan of attack)
    B) You find yourself with a nice Parabola :)

<End>

Note **1
    In the lab, it has been my experience that when using a Sub-Diameter lap, the user often:
        #1 Uses to short of a Stroke length, which results in the Center 70% of the mirror gets more glass removed, shortening its ROC, while the Edge Roc isn't as effected as much.

        #2 The User Concentrates the work on the Mid point of the W stroke. (IE Does Less strokes Center over Center, and at the outer edge of the W, and more strokes near the 50% width of the W which creates a Hill in the center of the mirror

        The two errors above, create what appears to be a Rolled Edge, Followed by a hill at the 70 - 80% range,  A Valley, then a Hill at the center of the mirror.  The Fix is Counter Intuitive, The user needs to do more work Near the 70- 80% radius, and Center over Center. To do this requires what feels as an uncomfortable, but necessary, amount of lap over-hang.  

Take Care,
James Lerch (Wearing an Industrial Strength Flame Retardant Fire Suit!)
http://Lerch.yi.org/atm (My Telescope  Construction Webpage)