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help with ronchi 8in f2.6 pyrex mirror

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#26 macleod

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Posted 17 July 2019 - 08:35 PM

Thanks Danny thats what I did after your "oblate correction".

My lap is Gugolz 64, and its VERY hard. I,ve done 6hours+ with it and channels have NOT moved ! Always have to hot press, score/ brush surface to get contact.

I,ve just imported G55 and maybe a new lap ? Temp in workroom is 50 -60F - its mid winter here. Oh, and I made lap squares 1/2in square hoping they conform better to steep curve - f2.6 . It,s fun !  By the way Foucault shows absolutely blank surface , even when I try to cut in knife.I cannot accurately determine COC this way.Surface looks flat. Yes TDE bright ring on knife side. Macleod.



#27 Lognic04

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Posted 17 July 2019 - 09:02 PM

is that why you have stig on your 16... uhm.

don't think so... straight off fixed post.



#28 dave brock

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Posted 17 July 2019 - 09:19 PM

 Yes TDE bright ring on knife side. Macleod.

You might want to check that.



#29 MKV

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Posted 17 July 2019 - 09:22 PM

use a ke to judge the edge

waytogo.gif



#30 macleod

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Posted 17 July 2019 - 09:29 PM

Thank you, in foucault think edge ok. Macleod



#31 Pinbout

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Posted 17 July 2019 - 09:32 PM

You want a balanced edge. But you can clean that up later



#32 Pinbout

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Posted 17 July 2019 - 09:34 PM

don't think so... straight off fixed post.

Then it’s the rear support



#33 dave brock

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Posted 17 July 2019 - 09:48 PM

With the Foucault test the mirror looks as if it is illuminated at a glancing angle from the opposite

direction that the knife cuts in so a TDE will show a bright ring on the edge opposite the knife and

will be dark on the knife side. That's why I suggested you check :-  "Yes TDE bright ring on knife side."


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#34 macleod

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Posted 17 July 2019 - 10:22 PM

Hey Dave a contradiction there ? TDE show bright ring on opposite the knife V Yes TDE bright ring knife side. Thats where I,m seeing it. But this is near sphere as NO detail on Foucault irrespective of KE. Macleod



#35 macleod

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Posted 17 July 2019 - 11:47 PM

Did Pinbout shuffle TOT for 3/4hr to correct TDE. Looks much worse to me ! Lap was well adapted, short quick "shuffling strokes as per Pinbouts video. Macleod.     post-233319-0-67530000-1563425210_thumb.

 

                                                                                                                                                    

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#36 macleod

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Posted 18 July 2019 - 02:18 AM

Any bets on what it is ? Don,t be shy !!

Macleod



#37 dave brock

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Posted 18 July 2019 - 02:25 AM

If that's outside then it's TDE which is more likely than up being so narrow.
I think it's a lot better overall than your previous pic though. I would do more of what you've been doing. I think you just haven't pushed the turn right off the edge yet.

Edited by dave brock, 18 July 2019 - 06:38 AM.

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#38 macleod

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Posted 18 July 2019 - 05:10 AM

Thanks Dave  its OUTSIDE ROC .May not be able to do much for a day or two - got problem with leg after my walk today ! Macleod.



#39 dogbiscuit

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Posted 18 July 2019 - 06:16 AM

Good sphere. Bad TDE.  Get a sphere that good (only hint of oblate) and edge turned it's a fair bet the tde is from slightly undersized lap.

 

I suspect the lap is beveled at the edge so that it's surface diameter is .5" smaller than the mirror diameter.  That leaves .25" to turn down (each end of the 8" mirror diameter).

 

Cure is don't bevel. Cut the edge of the lap vertical, or just don't cut it.  Let the pitch spread to fill in the bevel so the lap is equal to or larger than mirror diameter. 


Edited by dogbiscuit, 18 July 2019 - 06:19 AM.


#40 dave brock

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Posted 18 July 2019 - 06:38 AM

Thanks Dave its OUTSIDE ROC .


DOH, I meant outside. I've corrected my post.

#41 macleod

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Posted 18 July 2019 - 04:40 PM

dogbiscuit - your right on the money in ALL respects !!! Trimmed lap vertical, then suddenly had a brain fade and remembered Stellafane put 45deg bevel on edge, so away I went. Yes its 1/4 smaller all around ! I remember that Mel Bartels on his fast mirrors makes slightly oversize laps to avoid TDE , but not sure how to do this correctly.

SOOO - a new lap needed ? The present one is Ggolz 64 , and is extremely hard in my winter temps -  50-60f in shed - so I,ve got G55 and will use that. After 8hrs polishing the present lap channel and edge have NOT moved . I have to warm press every time I start , then  cool the lap/mirror in tap water. So what to do please ?? Macleod.



#42 Pinbout

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Posted 18 July 2019 - 06:36 PM

<<Cure is don't bevel>>

 

we stopped beveling at delmarva years ago cause a lot of people were coming out of polish with tde.

 

we trim with the glass on top and cut straight down.

 

if the lap is too hard lightly brush olive oil on the pitch. It will soften. I like to leave it on over night press next day.

 

that tde don’t scare me, just more work

 

work that edge. You got to work... slow feet don’t eat.


Edited by Pinbout, 18 July 2019 - 06:42 PM.

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#43 tim53

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Posted 18 July 2019 - 06:42 PM

How does a 45 degree bevel cause TDE?  Seems it would be the other way around - that if the lap is as big as the mirror, it spends more time at the edge than if were smaller.  And if it were bigger than the mirror, it would always be hanging over the edge and turn it down.  What am I missing?


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#44 Pinbout

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Posted 18 July 2019 - 06:44 PM

Keep the lap cut straight down. After several pressings cut it again.



#45 macleod

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Posted 18 July 2019 - 06:56 PM

Danny should I make a new lap ? or after correcting can I still use this one ? Macleod.



#46 Pinbout

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Posted 18 July 2019 - 07:04 PM

Your smooth so keep it



#47 dogbiscuit

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Posted 18 July 2019 - 07:14 PM

At your temperature range I would choose Gugolz 55 but at least for now getting a sphere you seem to be doing good with the Gugolz 64.

As well as can be seen in a single Ronchi, your pretty good sphere, only slightly oblate looks smooth.  If your lap is thick enough it would not be unreasonable to press the current lap to fill in the beveled area and bring it into good contact, retrim the channels and continue working fix the edge.  If I'm right about the lap size causing the TDE, it will not take long for it to get very good with the lap equal to the mirror's diameter.  In most cases while working to remove the tde, the cause of the tde remains active, so the fix can take a long time.  When you have removed the cause of the tde, it will go away quickly.

 

For figuring the f:2.6 I'm sure Gugolz 55 will do better.  And maybe you intend to use sub diameter laps for that too.

 

My tools are usually the same diameter as the mirror and trimming the edge of the lap vertically leaves the surface of the lap equal to the mirror diameter.  Frequently as the work continues, the lap bulges at the edge and curls slightly around the mirrors edge while pressing.  After pressing I will stroke mirror a series of short strokes forward back left right and the edge of the mirror shaves the curl off, leaving the surface of the lap a mm or two larger than the mirror diameter. Without lifting the mirror off of the lap (or lap off of the mirror) I wipe the pitch chips off the edge of the mirror and lap with my finger, and continue work as normal.

 

I have been assured by some of those who have not done this method of dressing the laps edge that letting the pitch curl around the edge and shaving it off with the edge of the mirror will turn the edge down, and the chips will scratch the mirror.  Neither has ever happened to me.

Pitch chips will not fit between lap surface and mirror unless you separate the mirror and lap.  If you do that and chips get between them, pitch will not scratch the mirror. I have only rarely had pitch chips get between mirror and lap and it is easy enough to make a stroke taking the edge of the mirror past the chip and the next strokes push it into a channel or off the edge. Sure... if you have chips between lap and mirror and you can't take care of it by stroking the edge of the mirror past it,  stop and clean the chips off.  Just describing how to handle chips makes it seem to be a much bigger problem than it is.    I have only done this method with Gugolz pitch and it does have a different fracturing nature than some other types of pitch.  I think it would work ok with most other pitch types, but I can't be sure.


Edited by dogbiscuit, 18 July 2019 - 08:25 PM.

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#48 dogbiscuit

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Posted 18 July 2019 - 08:04 PM

How does a 45 degree bevel cause TDE?  Seems it would be the other way around - that if the lap is as big as the mirror, it spends more time at the edge than if were smaller.  And if it were bigger than the mirror, it would always be hanging over the edge and turn it down.  What am I missing?

Yes, to many, I'll say to most It seems that way and in fact in some literature it is described that way.

So many have had persistent problems with tde because it seemed that way to them and many still have problems because it still seems that way to them. 

 

At the same time some of those will tell others that tde comes from changing all but a narrow zone at the edge to a shorter ROC and it is too little work at the edge causing tde.  But  then they don't carry that concept to the size of the lap, and stick by the misconception that removing pitch at the edge of the making the lap smaller than the mirror lessens the chance of tde.  They bevel the lap and the turn down is as wide as the bevel.  With work the pitch spreads and the turn down gets narrower.  They trim the lap and bevel it again, and the turn down gets wider again.  They resist recognizing the obvious.

It is obvious it is not the way it seems to them.


Edited by dogbiscuit, 18 July 2019 - 08:28 PM.


#49 tim53

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Posted 18 July 2019 - 08:46 PM

Well, that was clear as mud!  grin.gif



#50 dogbiscuit

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Posted 18 July 2019 - 08:57 PM

Well, that was clear as mud!  grin.gif

It only seems that way.lol.gif




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