The 20 " grind has begun

[mapstar]

Looking good there Rick

Going to prepare the pitch lap base this aft so I should be casting my lap sometime this weekend ready for the cutting of the facets.

Damian

[crashtestdummy]

Nice work.what were you using to make the channels?I tried a saw and it's just stick stick stick.I've just stripped my lap again and going to try again this weekend and add a few oz of beeswax

[starman345]

Crash, don't let it get you down, I'm doing a 10" and I've made and re-made my lap at least 10 times probably closer to 15. I learn something each time I do it. I used a cheap short hand saw to cut channels, I tried with my longer hand saw but it seemed to flex too easily, each time it flexed I would get pitch chip-out.  The short saw seems to work much better. I can only make two or three strokes then have to stop, clean the teeth and let the saw cool down or it will clog up and start chipping. Gordon Waite has a great YouTube channel with lots of mirror making videos. This one:

shows how he channels his laps. I think he's doing a 15" in that video. 

You certainly have your hands full with that 22" chunk of glass but I'm sure you can do it. You mentioned John Nichol, he has also helped me, its great you have him to guide you when the going gets tough.

[JRM]

Thanks guys,

 I used a box cutter Rich, with the blade you break off as it gets dull, had 10 blades when I started and only 2 are left.

 if you look at the photo of the my pitch lap you can see it chipped quite a bit, hot cutting with a wood burning tool work

really good, did my 10" lap that way years ago, tilt the lap somewhat and start at the bottom working up the channel if

I remember correctly. I am going to cold press soon and maybe get some polishing time in today, will keep you posted.

Rick M

[crashtestdummy]

Starman-that video makes me feel a lot more confident that I'm not going far wrong and have just been unlucky.I think I will just take it steady and not try and rush things too much.I still think got cutting some channels could be easier though

[Peter Drew]

If you use the "squares" method you don't have to cut channels.    

[crashtestdummy]

I know but if I go that route I will end up having to cast a full sized plaster tool and that will take much more energy to push across the mirror for 20+ hours so I want to try this meathod one last time.

[JRM]

Hi All,

Yesterday I cold pressed for about 1 hour with 36 lbs of weight on top,unloading weights every 10 minutes to rotate the lap 

and insure it did not stick to the glass,I then proceeded to polishing with a 10 lbs weight on top which worked really good with

the shorter strokes but not as well with longer strokes I continued to polish for 30 minutes then called it a day, after seperating

the lap from the glass I checked the pitch squares and noticed the inner 6 inches do not have as much polish inbedded into

them so they are not in full contact, will do a longer cold press today with more weight on top to rectify the situation and achieve

full contact and polish more, till then..

Rick M

[starman345]

Rick,

I can't imagine pushing that 20" lap, I find my 10" hard enough. Just curious, you didn't  mention warm pressing after the lap was channeled, I found I had to do that to get in good contact, maybe my pitch is harder or my work room is cooler. If I ever do another mirror (if i ever get done with this one) I think I might try those polishing pads, it sounds like it gives you quite a head start on polishing.

[JRM]

Hey Brian,

  Pushing the fullsize lap is a real workout, its not easy but then again its not as tough as I thought but I do grunt between push

and pull at times and you work up a sweat real good too, nothing here that is really big enough to warm the lap in, I could place

a heat lamp and do it that way but my pitch is gugloz 64 and the temp runs 68 or so, a longer press with more weight will work

out the contact issues, I did warm press during my 10 inch as well, it helps for sure.

The polishing pads really do a good quick start for polishing, I highly recommend them.

Having a Grad party for our oldest daughter tomorrow, so much going on here today no time for polishing, will try pressing again

tomorrow evening, till then...

Rick M

[crashtestdummy]

How wide have you done your channels Rick?

[JRM]

channels are roughly 3/16 of an inch Rich, they will close up during pressing somewhat but I will try to keep them that size throughout.

Rick M

[crashtestdummy]

Cheers fella.3rd time lucky for me.my lap was cast at 3pm so will leave it overnight then have a look tomorrow night after work and cut the excess off then decide on what I will do for the channels

[mapstar]

Hope it come's out better this time Crash. I've gave it a miss today as I was late home for work. Done some sending and vanishing though so not a total waste of time.

[mapstar]

Hope it come's out better this time Crash. I've gave it a miss today as I was late home for work. Done some sending and vanishing though so not a total waste of time.

Sanding and vanishing

[crashtestdummy]

I'm assuming that's varnishing

The base lap has come out much better.just the channelling to do high is where it usually goes wrong.I added 1oz of beeswax this time.

[mapstar]

Yeah varnishing, glad the lap has come out better you'll be polishing by the weekend

[crashtestdummy]

Assuming I get time to do the channelling.the new job shifts are all over the place until I finish my maritime training but hopefully get some time in tomorrow afternoon.still need to make a test stand for my mirror yet so until I do that there's no big rush to polish.

[JRM]

cold pressed for an hour, then 30 minutes of polishing with 16 lbs of weight on top of lap,starting to work much better now, I am thinking

my room is a little cold or pitch is a tad to hard but easy fix, will leave light on tomorrow afternoon and that will bring temp up a couple

degrees, will let you know tomorrow how it worked out..

Rick M

[crashtestdummy]

Bet that's a hard push with 16lbs of weight on it!!

My pitch almost feels too soft bit it's been really warm down here the last couple of weeks.hoping tomorrow to trim and bevel the lap and start cutting channels

[JRM]

not really Rich, I mean it is a forceful push and pull for sure but my pitchlap is on the tile tool which I made with hydrostone and plywood

to reinforce it,and it weighs a lot less then the 2 inch glass, probably 20 lbs difference so it makes it more like using two full thickness glass

disks and besides I am betting it will polish faster with the extra weight so long as I can put it through the motions,I did another half hour

again this evening, hope to get longer shifts this weekend, but every half hour shift still gets me closer to the goal...

Rick M

[JRM]

got another hour and a half in over the last few days, slow and steady,been switching up trips around with center over center for a trip around or

two and then a very narrow W stroke for a couple,should help to prevent zonal problems, the push continues...

Rick M

[Astrobits]

Keep an eye on the pattern of the pits. An even distribution over the whole surface is what you are after. If that is so then you are using a good stroke combo. If there are more pits towards the edge then lengthen the stroke and use more W. If more pits in the centre of the mirror use shorter strokes and C over C and make sure of good contact.

With the tool on top work is concentrated on the center of the tool and this can tend to flatten the tool loosing central contact and resulting in an over polished edge. The oposite can happen with the mirror on top but that is easier to see as the blackness of the facets varies from centre to edge.

Nigel

[JRM]

good advice Nigel, as such this afternoon I placed a heater down stairs and let it heat all afternoon, when I got home

it was 73.5 degrees F so I pressed with tool on top for 15 minutes with 36 lbs weight then 1/2 hour with  51 lbs, after this
I turned them over and placed mirror on top with no extra weight for 1/2 hour and proceeded to polish mirror on top for

a total of 1 hour,contact was good, the extra 6 degrees F made a lot of difference, will plug in the heater at noon and

warm things up to 73-75 degrees F from now on, will polishing 1 more hour mirror on top then switch each hour after that

between mirror on top and tool on top, if I had to choose between m.o.t. or t.o.t. I would say I like m.o.t. much better.

was just thinking Nigel.. when I did the 10"  I could not really see the pits but if I looked through the back of the glass I could

easily see where the polishing was taking place, it was fuzzy out towards the edge so I knew it needed more work out there,

I will try that with this big one, only issue is the back was ground flat and its not possible to see through from the bottom like

the 10" was but I guess if I wet the back it should work just the same, will check it out after and let you know..

Rick M

[JRM]

first off, the wet back peek through did not work, think I would need to polish the back to see through well enough to look for grayish areas,

will have to use the loupe and the lazer pointer..

today I opened some channels that were closing up some, then pressed the lap M.O.T.  for two 15 minute stints then polished for a total

of 1 hour and 10 minutes M.O.T. all C.O.C strokes, contact was good, I worked up a real good sweat too, more tomorrow..

Rick M