Re: bubble cavity between mirror blank and tool

You want to work COC, not the W stroke.

IF you dug a real hole hogging out to depth, then you will need to obtain contact with 80 grit; 120 will take a very long time to get there, especially on an F4 or faster. Go back to the 80 grit, work centre over centre, 1/3 stroke until contact is obtained. You can return to work MOT or TOT as required to control the sag once you obtain a spherical curve.

hope this helps,
dan

Robert,

I suspect there may be nothing wrong at all. If you had good contact once, you shouldn't have lost it with a 1/3 stroke, COC. Any possibility you are using a little too much water and trapping an air bubble that can't escape?

I would work a few good wets of the 120, MOT. For each wet, once you've done a few strokes, pause the grinding and attempt to slide the bubble off the edge by carefully sliding the mirror sideways. Once the bubble breaks, rotate the mirror to reduce the bubble and return to grinding. Maintain the 1/3 COC stroke.

Once the mirror starts to form a good sphere the bubble shouldn't form up as quickly and it should certainly be smaller. You may need to experiment with the volume of grit and water to get the consistency right but it shouldn't take long. Work MOT, spread about 1/2 teaspoon of 120 in the centre of the tool, add a few drops of water, and start the grinding. Adjust as needed to get smooth grinding that doesn't mud up too quickly or wash the grit out too much.

You can tell the feel of a mirror with poor contact to the tool. It binds and grabs as you move it through the stroke. With good contact, the mirror moves much more smoothly without any jerkiness at all and it doesn't take a lot of pressure to complete a stroke.

keep at it.

dan

I agree with Dan. Keep at 120 grit because you should not loose contact that quickly although F4 is a good challenge. What are you using to support the mirror on the barrel? It's not rocking at all is it? Also how are you holding and pushing the tool? You want the downward pressure to be from the center not the edges?

Grind more worry less.

Good luck Dave