rough grinding trouble
Posted 09 December 2012 - 11:48 PM
Posted 09 December 2012 - 11:52 PM
Posted 10 December 2012 - 04:25 AM
I'm now using a full sized plaster tool with glass tiles, using center over center strokes. The last grinding session I did didn't change the high zones much. May be a little.
How did you make the tool, that is did you match the curve to the mirror?
How long was the session that didn't change things much?
I suggest you just carry on with the glass tile tool. It will come right sooner or later. There may be (or may have been) high spots on the tool that have to wear down before real progress is made. Are all glass tiles showing wear? If you change back to the hogging tool you'll have to match the larger one later anyway.
Posted 10 December 2012 - 08:41 AM
Posted 10 December 2012 - 09:38 AM
Posted 10 December 2012 - 07:01 PM
Posted 10 December 2012 - 11:41 PM
If you have a sphereometer you can actually find out how much the curve changes for even a single wet, but it will be "not much." This takes a long time, and you "have to work like a caveman" (John Dobson.)
You are probably appreciating that those guys are trying all sorts of shortcuts -like spincasting and slumping to bypass this stage of grinding out all that glass.
Posted 11 December 2012 - 12:07 AM
The rubber expands while tightening, therefor snugging the blank...... Would it be useful to do a sharpie test at this stage?
I'm a little worried with the term "snugging the blank". You should really have some room for the blank to move to avoid astigmatism.
At the risk of starting a riot here, I think the sharpie test is overrated and never use it. It will likely fail early in coarse grinding and later, with the finer grades you can "feel" when the mirror and tool match and are spherical.
Posted 12 December 2012 - 10:39 AM
Posted 13 December 2012 - 11:58 PM
Posted 13 December 2012 - 11:59 PM
Posted 14 December 2012 - 05:46 AM
Roc was at 74, needing 84. FL was 55 needing 42.
Not sure what you mean here. FL is 1/2 ROC. With that test you are finding ROC so at 74" the FL is 37". How did you get 55" FL?
Posted 14 December 2012 - 03:18 PM
Grinding the way you are with such precise steps isn't how you're going to get that excess glass out of the way but when you get out to the edge you have a good regimen for accuracy.
Do you see any grinding machines that use straight strokes? The reason grinding the glass with grit works is because a spherical surface happens by default.
Use the sun for your focus testing. It's at the right distance and all you need is a convenient wall or piece of paper on a stick. I think its exciting to see that round perfect circle become sharper-edged and brighter. But right now any kind of focus test tells you nothing unless the mirror is ready for it. It won't be until you get all the way out to the edge.
Posted 14 December 2012 - 11:22 PM
Posted 14 December 2012 - 11:38 PM
Posted 14 December 2012 - 11:47 PM
Posted 15 December 2012 - 08:23 AM
Have you tried a W stroke? centre over centre will get there but I find a W quicker, and it evens out any lumps/bumps. A tangential or W also lets you work on the edges more. Whatever stroke suits, each to his own.
I prefer a rigid base for the tool, I find any give or slop can lead to rocking at the ends of the strokes and edge problems.
Posted 15 December 2012 - 08:29 AM
Posted 19 December 2012 - 12:12 AM
Posted 19 December 2012 - 02:48 PM
The image you posted starting the thread shows what you typically have as a result from using chordal strokes when roughing out the blank. A hole in the middle and flat edges. You can correct a bit with wider TOT strokes, high pressure and adding grit each 10 strokes or so.
Once you get to approximately the right depth, put saran wrap on the blank, make a rim all around, lay out a mosaic of tiles, and pour plaster over it. Then I take #120 grit to cut in the new tool and get the shape spherical. Sharpie is indeed your tool here.
Once you're spherical up to the very edge, you start inspecting pits and move down through finer grits when the current larger pits are gone.
Posted 19 December 2012 - 08:23 PM
Posted 20 December 2012 - 01:31 PM
Posted 20 December 2012 - 01:43 PM
Tool looked like heck when I was done, but it worked. By the way, all my plaster tools are a 1 to 6 ratio.
Posted 21 December 2012 - 12:03 AM
Posted 21 December 2012 - 12:18 AM