On 17 Nov 2019, at 04:49, Mark Cowan via Groups.Io <toolontop@...> wrote:

Hi,

Feel free to embed (paste) pictures in your post - this works for most people - or post them to the pictures section of the group and refer to them.  Pictures are very helpful.

I'm not quite sure what you're referring to with the "pivot position".

Mark

On Saturday, November 16, 2019, 11:11:36 AM PST, Rien <lugt@...> wrote:

[Edited Message Follows]

What kind of lap diameter do you use for polishing? Same as mirror?, or undersized but fixed, or variable sized depending on the correction you need to make?

 

Thanks for mentioning the pivot point position, it makes sense to choose that as low as possible. (This will also place a minimum on the lap size, i.e. of the kind: the minimum lap diameter should be N times the height of the pivot point above the mirror))

 

Edit: I see now in the excel sheet that the lap is '15.5', which I suppose is the size in inches... (that would represent an undersized but fixed sized lap which is kind-of confirmed by the picture)

On 16 Nov 2019, at 07:19, Bill Thomas <bthomas32000@...> wrote:

from what i have experienced, polishing requires the greatest amount force - my 24" machine http://www.yubagold.com/mom/index.html
uses 1/2 hp and 1/4 hp - two stock M-o-M systems - using ball bearings - more than enough power.  i should replace the M-o-M system with 2 monster variable speed control motors which a friend gave me - would eliminate the belt hassle changing.  the other change i need to make is moving the arm pivot back further from the table axis so as to increase the radius of  the "quill" arc -  also on the website pictures the "arm" is counter balanced like the old high end LP record player - so as to control the downward force on the lap - which i have found necessary to stop chattering of smaller laps (noticed this in a picture of a Zambuto machine).

(need to send along a picture - worth a thousand of the following words) also to that end found i needed to get the pivot (toggle pad) attachment as low as possible to the back of the tool/lap - use a 5/8" short threaded stud into the back of the aluminum plate of the tool/lap.  the toggle pad 5/8 NC female is then threaded on that stud.  the 1" top of the toggle pad then fits into a machined adapter which on the other end is then threaded on to the 5/8" NC arms quill.

attached is a "strokeSetup.pdf" which i use to setup my modified M-o-M and record the settings.  also attached is a spreadsheet i use with my homemade sphereometer - the radius of the 3 legs from the center of the dial indicator was done with a bar of approximate 3" radius - then the actual radius was determined by measuring a mirror of known focal length (using excel solver).  it is vary accurate.

but i don't think a M-o-M type machine will be what you need for a closed loop system - would be more like a Zeiss machine?  also think you will need to know the exact RPM of the table and arm to avoid repatriation of strokes  - one crude method is to mark on the edge of the mirror where the tool/lap crosses the edge - those dots should be uniformly spread out.  stroboscope would be better?

On 11/15/2019 3:11 AM, Rien wrote:

Again, I have to thank you Bill. To me this is like opening another door of which I did not even know it existed.
I will study both the site (yubagold) and the pdf. I am however not yet in a knowledge state that would allow me to make sensible comments on it.

Btw, I have pretty much decided to make the CNC machine -that I was planning anythow- sturdy enough to also handle grinding/polishing forces. That does not impact my schedule too much and at least gives me the option to start experimenting with CNC grinding/polishing whenever I feel ready.

<StrokeSetup.pdf><log_22s.xlsx>