mapstar

What is chordal ?

Derek

As AJohn says it's the chord of a circle. Working the edge at present I'm only doing a short chordal stroke.

The centre of the lap does the most work so where ever you concentrate this it has the most effect.

Moving it nearer the edge works that part more although caution is required as too near the edge as you can appreciate will just accentuate the edge problem.

John had said that it may be a case of going round in circle's (ha ha) between working the edge then the middle to rectify the problem I have

As AJohn states maintaining a press area for the Lap is essential as a raised centre will press the Lap hollow so not working the edge when I return to it.

Damian

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

Cheers soupy.

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

Very interesting read and great sketches and well put together.

Galaxy hunting is my favourite bit of the hobby and the thrill of looking at a star chart and hopping to find a galaxy give's me great pleasure.

So many to see, and as you say visit and then re visit especially the ones that get a wow in my notes.

Thanks for putting a lot of effort into that post, it take's a lot to write up your thoughts

Damian

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!

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?

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

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

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

That's the spirit sir!

Cheers Calv

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

Forgive this question Damian, it might seem a little impudent.

Have you looked at the edge to make sure that it is polished all the way?

Just a thought that crossed my mind. I'm sure it will be, just being nosey is all.

Ron.

That made me chuckle Ron, and you said David Sinden was good at joke's   

Definitely polished out, especially after I think nearly 30 hours extra now after the initial polish. I wish it was that simple 

The image's make it look like you could see it with the naked eye it seem's that big!

I worked today with the lap centre about 2" from the edge and then about 6" from the edge. Doesn't seem to have worked 

Damian

I can imagine how messy the Rouge is Ron as everything else about mirror making seems to be messy 

I have done another 2 hours today mainly edge work but very little change to be fair

Inside ROC

Outside ROC

I did 10mins and then cold pressed for the same repeating in about 1 hour sessions. I have roughed up the centre and created a little mound in there.

When I finished today I cleaned and re-bevelled the lap.  This turned edge is nearly as stubborn as what I am 

Damian

Damian,

There is no rush to answer this, I know you are somewhat pushed for time. I'm just filling in mine waiting for gear to arrive for imaging. I'm in the middle of thought experiments. At times very boring others my brain hurts.

Just how much are you trying to remove, maybe a difficult question I know. It is a question possibly answered in time or microns. From the bit of the process that I have watched and seen on other videos, it seems that as mirror makers you try for a slice of a circle first then change this into a parabola by using a different stroke type. ( if all goes well). As everything I have seem so far suggests, it is more of an art with careful thought than an scientific exercise. I know that the bit removed will be small but by what amount? You told me that the solution you use is made up from the powder you buy. You also mentioned the grade was not as fine, sorry wrong wording there, but cheaper than that used by another mirror  maker. Would it make sense  in the last bit to use the finer cerium for the very last few laps, thus saving money?

Can't help wondering. Goodness........ so many questions!

Just keep going got to finish before Galloway you know........

Derek

Hi Derek

It is difficult to know but I think Nigel may have mentioned something in the thread a while ago which I'll have to check back on. You are quite right with the technique's and there is a lot of careful thinking that goes off on the next polishing/correcting session  

I totally agree that it may be better to finish with some finer cerium although the average particle size that is stated on the packet is 2micron  where as I think the stuff from Lapmaster is 1micron 

I'm sure Galloway will be the best gathering I've ever attended this year (wonder why that will be  )

Damian

Hi. John. That video is very  informative, and that TDE correction method is indeed a great way to correct it. Short centre over centre with a hard lap.

An Oblate centre is the usual penalty for such an action, as the gentleman in the demonstration explained. That fortunately, is easily corrected, again adequately explained.

Unfortunately, the mirror being worked in the Demo, looks like a 150mm dia disc to me, worked with a full size lap.

Damian's disc is 22", and a full sized lap is going to be a beast to work on a mirror of those dimensions.

As much as I personally dislike sub diameter corrective laps, they are necessary on Damians project.

Many instances of TDE are a legacy from the fine grinding process, and in severe cases, reverting to fine grinding is the only sensible way to reverse it.

Damian is persevering in correcting the  anomaly  using his sub diameter tool, and it does appear to be responding. It's a condition mirror makers dread,

and either one gives up, and decide to mask it off, or prepare himself for the difficult job of Eradicatiing  it, which is Damian's goal I'm sure.

I think Damian is very pleased with all the attention his project has attracted, and all of us watching, and sometimes adding our owns tips and suggestions

to the mix, maybe getting a little tiresome for him at times. He is probably too much of a gentleman to say as much, and in fact I might be wrong in suggesting such a thing.

However, at such a critical time in his progress, he might wish to enter the last phase of the job with less comment from us, and just let him alone for a while.

He knows what is required, and if he needs qualified help and advice, he has John Nichol to turn to, and in better hands he couldn't be.

I hope no one is offended by what I've said, and indeed, if Damian himself doesn't agree, he is perfectly at liberty to say so, and I won't mind one bit.

Ron.

Edit] My apologies. The mirror in the Vid. is  a 200mm  Diameter Disc.

Thank you Ron for the very thoughtful post, written with sympathy, experience and dignity from a true Gent. The Jaffa's will get me to the end

I have returned to it today after a session yesterday morning for 2 hour's. I will be working solely on the edge today to see what results I can come up with so I shall feedback later with some image's 

Damian

The reason is usually because the central areas of the lap tends to collapse due to the constant friction. There is no respite in the middle.

The softer pitch unsupported on the mirror due to the sagging, causes the lap to plough into the edge zones, resulting in turning the edge over, and can get pretty bad if not recognised soon enough

The mirror also turns Oblate, because the central areas are not being worked by the lap, due to the same sagging condition.

Hard pitched laps are an insurance against TDE, but can result sleeks on the mirror. Not as bad as scratches though, and can be polished out.

Lots of pressing is also mandatory to maintain good contact all over.

Ron.

Well explained Ron

Hi Damian,

In that video of John Dobson he talked about the sound changing to squeaking when it became time to replenish the cerium polishing solution. I noticed he just spread a bit cerium by spoon on the surface and wet it. Then used his finger to spread it about. From what you have said you make a solution. So I guess you think that is better. Is it because of consistency of mix?

Derek

I have been making a solution of cerium oxide up the day before using. I've been using in the proportions of half a teaspoon to about 300ml of water. It looks like milk to be fair but you can vary this to what you need. Thicker slurry works faster, thinner is more for polishing.

I'm working by hand so don't get to the squealing stage

Small update

After an hours polishing the table is working lovely and the drainage system is a revelation.

May be psychological but it feels that little bit different

Cheers Ron

There isn't the other half to worry about as I live alone except when my daughters come to visit. So all clear

I hope too you are right on the mirror behaving itself now we shall see

I have this morning added a drain hole and length of tubing so that's all sorted

Mentioning recutting the channels I have also done that too, cleaned the lap and inspected it before a bit of warming and now it's pressing ready for a couple of hours work.

Results I shall post later as I have work this aft which will give the mirror chance to settle

More bits for the scope build also arrived this am which was a surprise ;-)

Damian

Just checked the table before turning in for an early start

All is looking good and with the new batches of cerium polishing solution made battle will re commence in a few hours

;-)

My fault, as you say banter, just trying to keep spirits up.

Yes Derek behave or you'll be sentenced to 18 months mirror making!

looking good Damian, should make for a great glass working table.

Rick M

Thanks Rick

Will be getting it's first run tomorrow. Wish I'd done it a while ago especially when I moved into the house.

If it works well I shall be passing all the info on to Rich (crashtestdummy) to modify his table

Hope the wrist is well on the mend now

Damian

More whipping tomorrow Ooooops I mean working ;-)

No I'm not Derek, although I think I'm into self punishment doing this ha ha

Just where are you doing the work Damian? You seem to be in the house somewhere.

The Kitchen maybe? Either there, or you have a very posh Garage/Workshop  .

Apologies if you've explained this elsewhere in your thread.

Ron.

Hi Ron

I have been working in the kitchen as you quite rightly point out. The reason being that during the winter the pitch was just too cold to work right in the garage. I've just stayed in the kitchen ever since and there is also much less chance of contamination as my garage is quite a mess and needs a tidy out

Damian