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mapstar

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Everything posted by mapstar

  1. Such a shame Ron after all that work, I have been at this stage as my next post will reveal
  2. Hi there, the blanks way back then cost £480 but have since gone up to about the £550 area? In financial terms the blank is only one part, you should also figure in the amount of grinding supplies below Grinding table Various grades of carborundum grit 60, 80, 220, 400 Aluminium oxide abrasive 600 grit Cement Tiles Cerium oxide Pitch Plywood for laps and transport box Test equipment Postage Mirror coating And last but not least the time you have to put in to make it happen.
  3. Turned edge blue's!

    1. Uplooker

      Uplooker

      Oh No :-( Sorry to hear that How long do you reckon it will take you to get back on track?

    2. mapstar

      mapstar

      It's actually a rolled edge so not as bad as I first thought. Not sure how long it will take Ian

  4. Cheers Ian I've missed the Galloway gathering too could've done with getting up there!
  5. On to the next stage of the grind The rough curve had been generated and a quick measure on the edge showed the blank to be down to just over 41mm. This curve is non spherical so we have to make a tile tool to bring this to a sphere. The tile tool was to be 18" in diameter (about 3/4 diameter of mirror). It consisted of a cement base with mosaic tiles glued on the front face for the grinding. Sound a little strange at first, for a smaller mirror you would cast the tool the same size but this would be too unwieldy at this size for me to use so 3/4 diameter. I made a former to cast the cement disc which would fit on top of the mirror blank. Here's a few picture's of the progress from me making the former to casting the cement backing I'd put packaging tape around the inside to help the cement release better. The former sit's on the greaseproof covered blank and when the cement is poured it forms to the rough curve that has been generated on the blank by the sub diameter tool (11" barbelll weight) After scree-ding the excess off and giving the former a few taps with a block of wood to release any trapped air bubbles I left it to set. I used 15kg of cement for this. After I'd separated it from the former I sealed it with a coat of fibre glass resin The mosaic tiles had arrived so it was the job of attaching them with yet more fibre glass resin. The convex surface of the tiles can be seen clearly Once they were well and truly stuck it was out with the angle grinder and diamond disc to trim them up and put a nice edge bevel on them. So onto the grinding with the tile tool, the theory being to work two surfaces together one a rough convex surface and the other a concave surface of the same radius to eventually produce a spherical surface. My understanding of the mosaic tile's is that they create facets (gaps) between them which carry the grit across the surface each tile being a cutting surface to grind the glass away (Apologies if this an amateurish way of putting it!) First of all the surfaces need to mate together and the first few wet's (carborundum and water) show on the tile tool how well these are mated together So the contact was not very good to start with. As things progressed it steadily got better. I used a variety of strokes at this point ranging from pushing the tool across the blank in a straight line through the centre to a W stroke across the blank in order to get the tool and blank to contact fully. So finally all in contact so onto what is the pencil test. A grid is drawn on the surface of the blank to see if even contact was being maintained. A trip then around the table and I inspected the lines to see if they were all grinding off at the same rate. I did this several times until I knew that I'd good contact between the two by all the lines grinding off equally (took 5 hours). At this stage I must've been around this table a thousand times between the rough and tile tool grind. More later but I was quite happy to this point Damian
  6. Thanks again for all the encouragement guys. I'm not sure about the awe bit as the how mad must some people be? Damian
  7. Still working on the turned edge!

  8. Thanks for the encouragement guys. Yes I hand chamfered the edge's and developed what's commonly known as Mirror bevellers elbow! Hard work
  9. Yeah blank is how your mind ends up doing the hogging. If I ever do another I'll get it pre-generated. P.s. the blank lost weight a lot faster than I did
  10. Cheers Guys I made a start on the front face by yet again flattening it making sure it was the same thickness all round. It was better doing this side as the ridges were no where near as pronounced as the other side. This time I started with 1Kg of 60 grit moving onto 80 grit to finish with. As can be seen it was a messy business!! This took 5 hours in total, the 60 grit cutting a lot faster. I then renewed the bevel on both sides which was another hour Once the front was flat, I started the hogging out progress after preparing the 10Kg 11" barbell weight by grinding the middle off to leave the ring around the outer edge I'd worked out that the sagitta for F3.9 was going to end up at roughly 9mm so a lot of glass to remove. I used a centre through stroke testing with a straight edge as I went. I needed to take the sagitta deeper as the barbell would create a non spherical curve. A tile tool is used then to bring it to a spherical curve and would grind the edge down (in theory) so lots more grinding later and another 1kg of 60 grit, 9 kg of 80 grit 12 gallons of water and I was at the depth I required. Here's a few photo's of it progressing. A total of 41 hours later I had a depth of 12mm. Just out of interest I weighed it at 18.7Kg. Wish dieting was as easy! Just to say hogging has to be one of the most boring things in the world!!!!!!!!!!!!! I re-bevelled several time's during this process So next I was ready for the tile tool Damian
  11. Cheers Faulksy just wish it was finished like the scope was before writing.
  12. Hopefully, although it's not finished before putting a thread together unlike the scope build. I've been on with this whilst doing the scope builds so its been a bit of a slog! Yet to start the scope wood work but that's another thread although won't be as long as the last one ;-)
  13. Cheers Stu, hopefully you'll get to see it by the end of this year!
  14. Fingers crossed, Unfortunately Rich I'm battling a turned edge? It's taken a week up to now and its steadily moved outward although I think I'm hitting a bit of a brick wall as it doesn't seem to be changing now? Not sure if it's the slight warp on the table thats caused it?
  15. Is the turned edge changing???????

  16. Cheers Calv Notice you added the word "when"! I'm getting there but it's slow going as I'll get to in the thread
  17. Hi Ron I was initially aiming for F3.9 but as I'll be detailing later this changed to nearer F3.7 It's an ambitious goal but with time and dedication its steadily progressed. There have been plenty of ups and downs
  18. Hi all, I began this journey way back in Oct 2013 so it has taken a while to get to where I am at present but thought it about time I put my experiences down. Wanting a larger scope than the 8" I had and after discussing with my good friend Rich (Crashtestdummy) we both decided to find out about mirror making. There is a mine field of information out there on the subject some of which is useful but not much in the way of practical experience except what I'd read about on here (Rustysplit 14" and Rick M 20") So when a seminar was advertised on SGL by Glasspusher we both decided to book in and attend. As many know Glasspusher is John Nichol and he took us through a little bit of history on himself and mirror making in this country which is sadly not as prolific as it used to be, his experienced friend Jack also popped in to talk of the many years he'd spent at Grubb Parsons ( ) . After an informative and enjoyable few hours with John we left with thoughts of big bits of glass and lots of work ahead (I mean lots) On returning it took us a couple of weeks to decide what size we were going to go for. Hunting for a glass supplier in this country leads to Spectraglass in Scotland and Rich ordered the two 22" blanks. Here's one with a cup for size! The blank was 44mm thick and weighed in at 22kg! There was a small defect near the edge which I decided would be the back Whilst waiting I'd put together a couple of grinding tables so we could make a start To make a start I had to bevel at 45 degrees the edges of the blank to about 5mm with a carborundum stone. Each side took an hour! On a blank this size it is important when you turn it over you lift it completely or roll it on something flat. I didn't initially and caused some shelling of the surface. This would later be ground off but valuable lesson learnt. Next was to flatten the back with the 80 grit carborundum. This was done using a steel ring about 12" in diameter. It was quite nerve racking due to the blank having concentric rings on the face I was flattening so each time it skipped over these really does not sound nice This took 11 hours in total so quite a while. I tested as I went with a straight edge and finally finished with 220grit although I think John is convinced I used 400grit as it's really smooth Total grit usage up to this point was 2.5kg 80 grit and 0.1kg of 220grit. The blank had also lost weight, just under 1/2 kilo and 2mm from it's thickness so was 42mm. Turning the blank over I then made a start on the curve generation using a barbell weight of approx 11". Damian
  19. Getting closer to a sphere!

  20. Good to read an update and looking forward to seeing the new lap. Damian
  21. Completed my new lap today Crash so hopefully it will polish the centre out after a good press. Damian
  22. Nice to have another entry from you Rich. You've had a bit of a rough ride with the health lately. I'm looking for definite at a new lap as it's down to less than 2mm on the edges. I'm at about 12 hours polish but due to the lap thinning the contact has been poor so it is hazy in the middle but getting better. Looks like I'll be making the trip up to see John to hopefully acquire some more pitch just hope he's got some? On another note the aluminium for the scope truss poles is sat in my lounge ready for the scope builds. I have also ordered the focuser so tin itch to start the builds is getting nearer Just wish I was a little nearer the end of the mirror Hopefully your MoJo and health will return soon as mine as you know has had it's big ups and downs Damian
  23. Still very much alive and being taught by those that don't want our history to disappear
  24. I had the 25, 15, 8 and 5mm BST's and for the balance of price and views I think they're very hard to beat and a fantastic upgrade to the standard eye pieces that come with most scope's. Well designed and as you say extremely comfortable to use. I was using them in an 8" F5 newt. Some have said the 25mm is the weakest of the range I found it nice to use. Highly recommended from me
  25. Shame that Rich. I need to send some Ronchi images to John see how he thinks mine should progress? I am thinking of ordering the ply and truss poles for delivery next week fella so will be able to make a start on the dual build so I'm sure you'll have the mirror sorted before its finished! Damian
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