The 22" mapstar mirror

[Ajohn]

It might be working Damian but not in the way you expect - trouble with laps. Say you use a chordal stroke  with the edge of the lap just running just past the centre of the mirror. Stroke length so that the max overlap at each end is say 1in.  This would scarcely work the edge at all. If things are working efficiently it should produce a hill in the middle that runs out gently to the cord the lap ran on and then rise up towards the edge leaving your turned down edge. Extend the stroke so that the max overhang is about 1 1/2 and eventually the edge should start cleaning up. You need to work up to that. There's a need for a sense of humour and determination to meet the challenges.

Ronchi isn't something I am very familiar with but I'd guess you need to adjust to see 5 lines with one over the centre of the mirror. That way you may see that you are making progress as outlined above. In fact you can probably look at the lines and guess how things will turn out if you continue. The shape of the lines will relate to errors but wont measure them unfortunately.

Small laps can make big changes as some have mentioned but yours has a pretty large area really. It isn't going to work a 22 in mirror as quickly as say a 15in.  In your case you can get round the problems by doing some centre over centre from time to time with no overlap at all to try and keep the curve that is being worked out spherical. That can be aided by mixing in some mild w's again with no overlap. Eventually when the edge has gone or staring to go that is the sort of stroke that will be needed but this time  with some overlap. Only testing can indicate when you should do this. You might find 1/2hr work takes longer to clean up or a few mins. The time spent correcting can be balance to suit.

I'd guess you already know that it's very important to spin the lap a bit regularly and alter the amount as randomly as possible. Every stroke wont hurt.

If there isn't any efficiency I would wonder about the polishing mix being too thick and in any case add weight to the lap.

However you do it - try it for sufficient time to see what it's doing, clean up and test. At this stage I don't think you need worry about letting things cool. That comes when you are worrying about wavelength sized adjustments. Here you need to lower a substantial area of glass. Waite's video might well be correcting 1/4 wave or less over correction - that really does need doing slowly and carefully. The problem with people like him is that he has done so much of it that he knows how much work will be needed to do what ever he is trying to do - us poor sods have to learn. If the work doesn't do what we want at least we know how long it took to make the change. If there is no change and it should be possible to detect it - something some where is very decidedly wrong.  Some people make notes.

John

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Edited April 4, 2015 by Ajohn

[Soupy]

Looks like some more progress has been made Damian.

[barkis]

It might be working Damian but not in the way you expect - trouble with laps. Say you use a chordal stroke  with the edge of the lap just running just past the centre of the mirror. Stroke length so that the max overlap at each end is say 1in.  This would scarcely work the edge at all. If things are working efficiently it should produce a hill in the middle that runs out gently to the cord the lap ran on and then rise up towards the edge leaving your turned down edge. Extend the stroke so that the max overhang is about 1 1/2 and eventually the edge should start cleaning up. You need to work up to that. There's a need for a sense of humour and determination to meet the challenges.

Ronchi isn't something I am very familiar with but I'd guess you need to adjust to see 5 lines with one over the centre of the mirror. That way you may see that you are making progress as outlined above. In fact you can probably look at the lines and guess how things will turn out if you continue. The shape of the lines will relate to errors but wont measure them unfortunately.

Small laps can make big changes as some have mentioned but yours has a pretty large area really. It isn't going to work a 22 in mirror as quickly as say a 15in.  In your case you can get round the problems by doing some centre over centre from time to time with no overlap at all to try and keep the curve that is being worked out spherical. That can be aided by mixing in some mild w's again with no overlap. Eventually when the edge has gone or staring to go that is the sort of stroke that will be needed but this time  with some overlap. Only testing can indicate when you should do this. You might find 1/2hr work takes longer to clean up or a few mins. The time spent correcting can be balance to suit.

I'd guess you already know that it's very important to spin the lap a bit regularly and alter the amount as randomly as possible. Every stroke wont hurt.

If there isn't any efficiency I would wonder about the polishing mix being too thick and in any case add weight to the lap.

However you do it - try it for sufficient time to see what it's doing, clean up and test. At this stage I don't think you need worry about letting things cool. That comes when you are worrying about wavelength sized adjustments. Here you need to lower a substantial area of glass. Waite's video might well be correcting 1/4 wave or less over correction - that really does need doing slowly and carefully. The problem with people like him is that he has done so much of it that he knows how much work will be needed to do what ever he is trying to do - us poor sods have to learn. If the work doesn't do what we want at least we know how long it took to make the change. If there is no change and it should be possible to detect it - something some where is very decidedly wrong.  Some people make notes.

John

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Some very sound advice in your post John. I hope when Damian surfaces again, he has some positive news. Goodness knows, he deserves it.

Ron.

[Ajohn]

This might help Damian inject some science into lapping  or at least provide a bit of  light relief now and again. I found it yesterday, written by some one who is interested in computerised figuring. If I remember correctly that idea was started by a person  called Zambutu who uses a whole series of laps to figure mirrors getting gradually smaller towards the centre. It's here

http://martin-cibulski.de/atm/polishing_simulator/

Despite the comments it does react to changes in lap and mirror sizes. They can be changed from the setup menu  I tried the stroke  I suggested out of curiosity but find I can't attach a video file so I have uploaded it here. Filebin is a useful service at times

http://filebin.net/s9con6ib13

It shows the wear pattern it should achieve and concentrates the glass removal where it needs to be so should do it fairly quickly providing the turned edge isn't too deep.

John

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[Demon Barber]

I might be showing my age but this video reminds me of a kids toy ,Spirograph,which i had as a youngster,as i remember it produced loads of stroke patterns 

Keep the faith, will launch tickets to the Mapster Mirror Tutorial a little nearer Galloway             

Hope your keeping well Damian,

Regards

Mike

Edited April 7, 2015 by Demon Barber

[mapstar]

Hi all,

Apologies for not posting anything for a few day's and thanks for all the comments and help 

What size is the polisher you are using, and how are you working it on the mirror,

When you said 2" and 6", is  the centre of the lap traversing that variation in vertical strokes across the centre of the mirror.

or are they  chordal?

 Ron, I hope you didn't take offence to the joke comment about the polished edge. The lap is 10" in diameter and after a discussion with John I am now moving in a different direction and will update the thread later on with the result's hopefully more positive we shall only see?

I have also ordered more Cerium as unfortunately I am again running low

I might be showing my age but this video reminds me of a kids toy ,Spirograph,which i had as a youngster,as i remember it produced loads of stroke patterns 

Keep the faith, will launch tickets to the Mapster Mirror Tutorial a little nearer Galloway             

Hope your keeping well Damian,

Regards

Mike

Steady on Mike it may be a tutorial about how to polish forever on a turned down edge! have you and Derek worked out the price of the refreshment's? 

Right it's time to get stuck in and get this beat

Damian

[estwing]

That's the spirit sir!

[mapstar]

That's the spirit sir!

Cheers Calv

[barkis]

Hi all,

Apologies for not posting anything for a few day's and thanks for all the comments and help 

 Ron, I hope you didn't take offence to the joke comment about the polished edge. The lap is 10" in diameter and after a discussion with John I am now moving in a different direction and will update the thread later on with the result's hopefully more positive we shall only see?

I have also ordered more Cerium as unfortunately I am again running low

Damian

Certainly not me lad, I don't offend easily anyway.

I've taken time off myself for a little while, trying to come to terms with a very persistent cough I've inherited from a recent chest infection.

It has wearied me somewhat, and still no end in sight. Maybe it's waiting until your mirror is figured and ready for the Vacuum  :grin:.

(Now that is meant to be a lighthearted jest    ).

I sincerely hope you are on the last lap. (Whoops!, a pun now too.)  .

Good Luck, you deserve some.

Ron.

[mapstar]

Evening all,

Over the last two days I've put in another 5 hours work. After discussing with John it was time to dispense with the softly approach as it just wasn't bringing it to heel and go for something more aggressive.

Using the 10" lap it was back to the edge working with the centre 1"-1 1/2" inside the edge with short chordal strokes and pressing at regular intervals (Nigel also suggested the same). The results are below

Inside ROC

Outside ROC

Sorry the pics aren't my best efforts but I think you can see there is finally movement there   although the middle is a complete mess   but as John said this can be sorted later.

May get back to it Thursday but at least a breakthrough 

Damian

[Physopto]

Nope don't use that word,try progress!!!

Got to be honest Damian, I'm finding it difficult to see what the Progress is. Untrained eye ya know.

[mapstar]

Nope don't use that word,try progress!!!

Got to be honest Damian, I'm finding it difficult to see what the Progress is. Untrained eye ya know.

Derek

It's easier if you actually have the outside image's side by side, the differences are very subtle to see.

To see it look at the lines either side of the middle one. The top and bottom are bent outwards but in the new image the bent line is smaller and beginning to turn back towards being straight. Also the middle vertical line has less spread on the end.

Hope that's as clear as mud for you

Damian

[mapstar]

Derek

It's easier if you actually have the outside image's side by side, the differences are very subtle to see.

To see it look at the lines either side of the middle one. The top and bottom are bent outwards but in the new image the bent line is smaller and beginning to turn back towards being straight. Also the middle vertical line has less spread on the end.

Hope that's as clear as mud for you

Damian

I've actually posted the wrong image as it doesn't tally up with the others having too many lines

Damian

[Demon Barber]

Damian,it`s not just his sight going          

Mike

[Physopto]

Thanks Mike,

Friends Huh?

And thanks Damian. I tried opening two instances of I.E. Side by side, earlier on today, didn't help much. Ah wus still puzzled. Maybe it's an age thing!

I'll try again when on the main computer. This iPad is rubbish.

Keep trucking.

Just remember Progress.......

Derek

[mapstar]

Thanks Mike,

Friends Huh?

And thanks Damian. I tried opening two instances of I.E. Side by side, earlier on today, didn't help much. Ah wus still puzzled. Maybe it's an age thing!

I'll try again when on the main computer. This iPad is rubbish.

Keep trucking.

Just remember Progress.......

Derek

Thought you might be able to save image as and put it in my pictures?

[gooseholla]

Man all these images are like staring at a Zebra's backside daily. I just hope you understand what they mean, because that is all I see when I look at them!

John

 

[mapstar]

Man all these images are like staring at a Zebra's backside daily. I just hope you understand what they mean, because that is all I see when I look at them!

John

Cheers John

Yeah I wanna put a flashing orange light at either end as they do make you cross!

[JRM]

Nice to see you moved towards the edge Damian, see... things are starting to turn for the good,I still

say slow and steady though, and beware of digging to deep a ring.

they say inside roc is a more sensitive test and the edge is looking way better inside now so things

are moving in the right direction.

can you try a shot with less lines outside focus, say five so you have same inside and outside to

compare. just a thought.

keep up the good work my friend.

Rick M

[mapstar]

Nice to see you moved towards the edge Damian, see... things are starting to turn for the good,I still

say slow and steady though, and beware of digging to deep a ring.

they say inside roc is a more sensitive test and the edge is looking way better inside now so things

are moving in the right direction.

can you try a shot with less lines outside focus, say five so you have same inside and outside to

compare. just a thought.

keep up the good work my friend.

Rick M

Hi Rick

Yes it wasn't responding to the soft approach and I think it was about time I saw some movement for morale at least.

I will try to get some image's posted as I have plenty on the pc at home with less lines but it doesn't take much to get more.

I'm wary of going too deep with it, hopefully it should start to flatten again and move back towards more of a sphere?

Back to it Thursday so I shall update with hopefully more progress

Thanks

Damian

[Soupy]

Nice work Damian... Progress indeed and the Middle can wait. Well done buddy

[mapstar]

Nice work Damian... Progress indeed and the Middle can wait. Well done buddy

Cheers soupy.

[Mark62]

Wow what a change Damian. Hopefully ur on the right track and things won't be long in coming to fruition. Keep it up

Mark

[Ajohn]

I'd still suggest you spend some time cleaning up the centre as you go rather than leaving it all to later.  Just over centre and some mild W and NO overhang until that looks like it will work the edge. It should even out any irregularities the chordal work leaves and give you an area to press the lap on. As the area being worked is smaller it should correct a lot more quickly than the edge. How long - you would need to find out.

Numbers of lines - every  time I see examples shown there are usually around 5 but this gets more tricky as the F ratio gets faster.

John

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[Physopto]

What is chordal ?

Derek