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help with ronchi 8in f2.6 pyrex mirror

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#51 dave brock

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Posted 18 July 2019 - 11:23 PM

Well, that was clear as mud!  grin.gif

Basically, a lot of turned edges are not caused by the lap plowing over the edge and wearing it down 

but instead are caused by shortening the roc of the rest of the mirror while leaving the edge as it is. The

edge remains longer so in effect becomes turned by default. A lap that is slightly undersized combined with 

coc strokes is the main culprit.


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#52 macleod

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Posted 19 July 2019 - 01:26 AM

After 2+ hours , this is what I have - OUTSIDE roc.

The surface is troubling tho ! One half mirror is visually a "perfect looking surface, BUT the rest has what looks exactly like " water marks" - dried fluid swirly curls and loops ! It wont come off washing with detergent , or alcohol or acetone. They are NOT scratches or ripple, it is a visual surface effect.  Almost looks like staining.I have been using " red cerium oxide" ( from the 80,s  I believe ) , and Its given a wonderful polish. The marks were not on the glass this morning. I,ll give it 10min wth white Ceox from  "Firsthanddiscovery" and see if it goes.

                                                   so OUTSIDE ROC :-

                                                                                           post-233319-0-09024000-1563517594_thumb.

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#53 dogbiscuit

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Posted 19 July 2019 - 05:10 AM

Magic! The edge looks better.  Much better.  And didn't take much time. 

Was anything done about the beveled lap?  Must have done something to get the results that fast.

 

 

I assume the water marks are the cause of the anomalies seen on the surface to left and bottom.

Maybe it's some dried slurry.  Whatever polishing agent you use, if it were at fault it would affect all of the surface.

10 minutes with the "red cerium oxide" would remove the marks as well as the white cerox.

 

At the end of a work session don't let slurry dry on the mirror.

If slurry and swarf dries on the mirror it can be difficult to get off.

More polishing is the way to do it



#54 BGRE

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Posted 19 July 2019 - 05:59 AM

Dissolved minerals can also be deposited on the glass when water is left to evaporate. Rinsing with deionised or distilled water followed by an isopropanol rinse can help.

An ultrasonic cleaner may help if a large enough one is available.



#55 macleod

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Posted 19 July 2019 - 06:00 AM

Ready to parabolise ??? Macleod



#56 Pinbout

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Posted 19 July 2019 - 06:42 AM

Of course

 

draw a 30% circle on the back and don’t cross the edge of the tool with that line while doing 5 stroke w’s

 

later You may need to make that circle smaller but 4 now... have at it

 

you can buy a sharpie right? lol.gif tongue2.gif


Edited by Pinbout, 19 July 2019 - 06:44 AM.


#57 Pinbout

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Posted 19 July 2019 - 06:50 AM

https://youtu.be/erF2jO34pgg

 

watch the cerox  from time to time.

 

use a brush and lift it out of the channels when you don’t see the xerox flowing well.


Edited by Pinbout, 19 July 2019 - 06:21 PM.


#58 tim53

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Posted 19 July 2019 - 09:39 AM

Basically, a lot of turned edges are not caused by the lap plowing over the edge and wearing it down 

but instead are caused by shortening the roc of the rest of the mirror while leaving the edge as it is. The

edge remains longer so in effect becomes turned by default. A lap that is slightly undersized combined with 

coc strokes is the main culprit.

I just got that explanation from a friend this morning.  Makes perfect sense, stated that way,  



#59 macleod

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Posted 19 July 2019 - 03:20 PM

Thanks Danny, it took 2+ hrs to scrub away TDE just as you predicted . Sharpies ?  no probs - we invented them !  Macleod.


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#60 Pinbout

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Posted 19 July 2019 - 05:03 PM

Do you have the program diffract? It helps judging correction before you do the ke or other final testing methods. 

 

Message me with an email if you don’t, I’ll send you a copy.



#61 Lognic04

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Posted 19 July 2019 - 06:16 PM

Guys, sorry to butt in here but which side of the knife edge shows a TDE? Danny seems to disagree with others, but I imagine it can't be that hard...

#62 Pinbout

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Posted 19 July 2019 - 06:20 PM

Guys, sorry to butt in here but which side of the knife edge shows a TDE? Danny seems to disagree with others, but I imagine it can't be that hard...

 

No it ain’t like that little man

 

i miss read his post - bad labeling.

 

what ke

 

he fixed it

 

outside RoC shows how the mirror is - lines turn out - tde 


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#63 macleod

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Posted 19 July 2019 - 10:26 PM

Danny, at this early stage of parabolising how often to test ? How long for the wide W stroke should I be looking at? It sure looks a bit odd after 1hr ! Don,t want to dig a hole. Macleod



#64 macleod

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Posted 20 July 2019 - 03:30 AM

Danny after 2hrs this is where I am at - OUTSIDE ROC :-  post-233319-0-34782800-1563611370_thumb.

                                                                                                         

                                                                                                     

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#65 Lognic04

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Posted 20 July 2019 - 03:42 AM

Hi MacLeod, not Danny but I had a similar issue with my 6 inch. You basically add more correction out to however big, then it cuts off. Your center is flat, then transition, then flat. I think at this point you need to dig out the center more, and especially the outer 70 or 80% zone to smooth out the kink. I used accented pressure. Funny you got the same issue, using a full size lap. I didn't get any huge issues with subdiameter on the other hand...

#66 Pinbout

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Posted 20 July 2019 - 07:43 AM

Danny after 2hrs this is where I am at - OUTSIDE ROC :-  post-233319-0-34782800-1563611370_thumb.

Longer strokes  in the middle 

 

but if you want to try

 

draw a circle for the kink on the back of the mirror. I actually set it up and draw it on the face while on the stand checking it with the ronchi then trace it on the back. With a good polish the sharpie will just wipe right off the face

 

mot zigzag from just inside the kink out to the edge and back.

2x’s around and test - if your going in the right direction repeat...


Edited by Pinbout, 20 July 2019 - 02:27 PM.


#67 Pinbout

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Posted 20 July 2019 - 10:07 AM

Normally you work tot to work the outside and mot to work the inside but you don’t want to raise you center yet.

 

also swayze told me once he gets to a certain stage he like to do mostly mot... cause tot can mess up the center.

 

so.... mot 



#68 DAVIDG

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Posted 20 July 2019 - 12:37 PM

 The way I parabolize these very fast mirrors is to put the TOT and the place the center lap over  the 70% of the mirror . I do a W stroke so the center of the lap goes just about to the edge of the mirror  and then back so the center of the lap goes  just about the center of the mirror The stroke does not cross over the center of the mirror so you just working one side   Turn the lap  a little,  go about 15 degrees around the mirror and do then W. What your doing is slowing deepening the 70% in a controlled manner.  If you need more wear in the center move the center of the lap toward the center, more wear toward the edge move the center closer to 80 or 90% zone.

 

                  - Dave 



#69 macleod

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Posted 20 July 2019 - 03:00 PM

Thank you all. I am  HAND PARABOLISING  but on a MOTOR  TURNTABLE  at 6rpm. Should I stop it and walk around instead ? Macleod.



#70 Pinbout

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Posted 20 July 2019 - 03:09 PM

Now you tell us.

 

thats a different animal.

 

I’d do subdia on a table - similar to what Dave said.

 

fullsize for walking around.

 

thats how I do it 


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#71 macleod

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Posted 20 July 2019 - 04:46 PM

OK , I am going to do MANUAL FIGURING ONLY !!  I used the  turntable  to get back to a sphere, got " over excited" about figuring - my age yer know - and started with  turntable

on.

            Far prefer manual , problems don,t get out of control so fast - forgot I used  manual  on my 10in f3.4 job in January. Macleod.



#72 macleod

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Posted 21 July 2019 - 07:01 PM

Danny you advised me in post #56 to draw 30% circle and work from there out. See " dogleg " in post #64 I measured on Ronchi screen and those curves ae right at the 30% mark you advised. My mirror is 200mm diameter so 30% is a central circle of 60mm diameter - right at the kinks ! Macleod.

NB hand parab only !



#73 Pinbout

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Posted 21 July 2019 - 07:17 PM

That looks more like 70% which is 5.5” dia

30% is 2.4” dia

 

anyway

 

two ways 

 

use edge of the tool to push out the hole to the edge either tot or mot - as I said earlier. 

 

or as Dave said center the tool on the .7 zone and zigzag toward the center

 

either way your adding correction to the outer zone

 

 

when doing the w’s the wide stroke out on the sides is short but in the middle long.

At the moment if you did w’s you’d do more strokes in the center longer maybe a 1.5” circle not to cross with middle strokes and out toward the edge use the .3

 

Every time you switch stokes do 2x’s around and test. 


Edited by Pinbout, 21 July 2019 - 07:21 PM.


#74 Pinbout

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Posted 21 July 2019 - 07:24 PM

Longer strokes  in the middle 

 

but if you want to try

 

draw a circle for the kink on the back of the mirror. I actually set it up and draw it on the face while on the stand checking it with the ronchi then trace it on the back. With a good polish the sharpie will just wipe right off the face

 

mot zigzag from just inside the kink out to the edge and back.

2x’s around and test - if your going in the right direction repeat...

This with slight thumb pressure just inside the edge of the mirror. Thought I had a vid but... still looking


Edited by Pinbout, 21 July 2019 - 07:26 PM.


#75 Pinbout

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Posted 21 July 2019 - 07:39 PM

Like this but from inside the kink out to the edge

 

https://youtu.be/Vr5c3NL9yR4

 

watching this reminds me of an important habit 

 

mark the mirror, a sharpie line, that points at you when you start, and when you stop have the sharpie line almost point to you, not pointing at you. That’s once around.

 

if you start and stop with it pointing at you you’ll polish in stig.




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