The Giant Binocular

[savarin]

Well, that took way longer than I anticipated, approx 15 hours but I did get there.
So I decided to try a different approach for the second disk.
I glued a sheet of 80 grit wet and dry to the tool face,


Draw a heap of check marks to see the high points once we start.

I moved the tool towards the rear and added a side to side movement of 3"
This seemed to be very effective as 3 mins grinding produced this

and within 30 mins it was down to this

Now this is where a giant amount of luck is required.
You will note the wide black line on the top of the disk, This is where the pendulum grinder missed because the disk wasnt quite centralised on the turntable.
Looking closely at the edge of it it is wearing down nicely so I have huge hopes it will become spherical all over. A high low spot in the centre and 1/3rd out is also reducing as the rest meet up to it.

I had to stop here as the first sheet of wet and dry has worn out.

 

[extropic]

I would have guessed you would have cut the Wet-or-Dry into a "spider" like form (multiple tapered arms) to get it to conform more uniformly to the tool. A flat sheet glued to a convex surface is bound to result in puckers/pleats. I'm very impressed with all I've seen you do on this project. If my suggestion is off base, please explain.

 

[savarin]

excellent idea, thanks, I never thought of that.
I should have as I have some initial polishing pads produced in that shape to stick to the tool.

 

[savarin]

.
Its getting there, the step in black is barely 0.002".
The curve is around 0.74mm which is a bit deep.
Thats why I have resorted to side to side grinding to help flatten the curve.
The red mark on the tool and the yellow/green tape is to check if the tool covers the disk in a different spot on each rotation so it doesnt wear a path.
Theres also a drip feed of water as the 80 grit wet and dry can rotate without constant supervision.

 

[savarin]

The wet and dry wears out on the rings of the tool after a very short while.
I got cheesed of standing there adding grit and water so thought a change of tasks was required.
I will be needing 2x M10x0,5 mm by 100mm long countersunk screws in stainless.
I had a thought that an allan key would be hardenable so used an end piece to make a broaching hex cutter.
After annealing it I did the usual grinding to shape etc etc, got it red hot and dropped it into a pot of water.
Cleaned up and polished, counter bored the end of some stainless rod, off set the tail stock and drove the cutter into the stainless.
Except it didnt. The stainless took all the corners of the hex.
Oh well, it was worth a try. I will try again with some drill rod/silver steel.

 

[Downunder Bob]

The wet and dry wears out on the rings of the tool after a very short while.
I got cheesed of standing there adding grit and water so thought a change of tasks was required.
I will be needing 2x M10x0,5 mm by 100mm long countersunk screws in stainless.
I had a thought that an allan key would be hardenable so used an end piece to make a broaching hex cutter.
After annealing it I did the usual grinding to shape etc etc, got it red hot and dropped it into a pot of water.
Cleaned up and polished, counter bored the end of some stainless rod, off set the tail stock and drove the cutter into the stainless.
Except it didnt. The stainless took all the corners of the hex.
Oh well, it was worth a try. I will try again with some drill rod/silver steel.

Some high quality hex head screws, such as "unbrako" SHCS are very hardenable infact they often quite hard when you buy them, but you may need to talk to a fastener specialist, you won't get them in a big box store, good luck.

 

[savarin]

Hi Bob, its not the screws I need to be hard, they will have very little pressure on them.
They are .5mm pitch for the tension adjustment on the mirror backs.
I just had the idea of broaching a hex hole in the stainless to make my own.

 

[pdentrem]

As mentioned cutting the sand paper into a open flower petal pattern does help it to conform to the tool.
Pierre

 

[savarin]

I did that Pierre and it did conform nicely but no better than the straight circle but it does wear out too fast to much actual work.
I'm finding that continuous spooning of the wet grit from the drip tray back onto the part is working the fastest.
Not all the grit is worn down so its a mix of full size 80 grit and smaller.
Its still slow though.

 

[Flyinfool]

Good excuse to make or buy a wobble broach setup.

Allen keys are case hardened, If they were hard all the way thru they would snap off all the time. That is why you can twist without breaking them in a very stuck fastener. Once you start grinding the case is gone.