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6" F3 Mirror and Scope Build

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#151 dogbiscuit

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Posted 20 January 2020 - 05:37 PM

Stellafane shows some W strokes.

 

 

I usually define W strokes by stroke length and width.  

To show you what I mean this drawing shows strokes of 1/3 diameter on the top row and 1/2 diameter on the bottom row.

There can be other stroke lengths, but these two are suitable for what you are doing now.

Each row shows various widths from COC to 5/8 D.  COC can be thought of a W with no width. Widths between those shown are ok too.

So when I say wide W I mean somewhere to the right side of the row.

Red zigzag shows path of center of top disk traces on the bottom disk.

NarrowWideW.jpg


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#152 Aaron_tragle

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Posted 05 February 2020 - 09:22 PM

Finally after two weeks I finally got a tube ring that I liked, turns out cutting a circle is extremely hard. Learned alot about woodworking from this project, now to just finish the second one and I'll be in business. https://www.youtube....h?v=c8HYttOOjk0


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#153 Aaron_tragle

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Posted 26 February 2020 - 07:34 PM

Sorry for the lack of updates. I haven't been able to work on the mirrors in the last few weeks due to a combination of school taking up so much of my time and some serious sleep issues where I am basically feeling exhausted 100% of the time with zero energy left for mirror grinding after homework is done. Luckily I have found time at school after finishing assignments to design some of the 3d printed components for the OTAs, first print I did as a test was the end caps for the tube. These are mainly for cosmetics, but they also serve the purpose of preventing the cardboard tube from fraying on the edge as I anticipate I will be using these scopes for many years to come. As you can see, they look absolutely amazing and I anticipate they will look even better with the custom rings and carbon fibre wrapping. Luckily I have a few days of no school coming up where I will be able to work on the mirrors a little bit more.

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#154 Aaron_tragle

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Posted 19 March 2020 - 05:13 PM

I’m back! This project is NOT dead, I’ve resolved my health issues and thanks to Coronavirus I now have plenty of time to work on the mirrors. I will be grinding through the work over the next month I have out of school (pun intended).
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#155 Aaron_tragle

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Posted 21 March 2020 - 03:11 PM

F3 mirror is through 80 grit! F5 still has a bit of work on 80, but I hope to be on 120 by the end of the day.


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#156 Aaron_tragle

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Posted 22 March 2020 - 08:13 PM

Alright I put a lot of time into the mirrors today with very little progress sadly. I spent 3 hours grinding the F5 mirror using the penny tool with MOT. I tried 1 hour of CoC grinding with less than 3 thou change throughout the hour, next I tried a Wide W stroke that dogbiscuit mentioned for a hour and same story. I am honestly stumped, I am almost out of 80 grit and will be forced to move to 120 in about 6 wets. Hoping that I will find a way to get the sagitta to the 75 thou mark during 120 grit.


Edited by Aaron_tragle, 22 March 2020 - 08:17 PM.

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#157 Aaron_tragle

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Posted 22 March 2020 - 08:31 PM

I think it may be worth trying chordal with the rest of my 80 grit tomorrow on the F5 mirror.


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#158 Aaron_tragle

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Posted 23 March 2020 - 04:21 PM

I hope this video explains the issue I am having better. This stuff is difficult to convey through text. EDIT: Here is the video with sound https://youtu.be/FDu8psxAd4s

Edited by Aaron_tragle, 23 March 2020 - 05:39 PM.

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#159 Dale Eason

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Posted 23 March 2020 - 04:48 PM

Aaron,   I don't get any sound with your video.  So I don't know what you are trying to show.  However at 80 grit you are just trying to get the depth about right and not looking to have a sphere at that stage.  That is what the next stages are for.  That is to spherize the surface as well as to get finer and finer surface.

 

Dale


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#160 Steve Dodds

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Posted 23 March 2020 - 04:56 PM

Alright I put a lot of time into the mirrors today with very little progress sadly. I spent 3 hours grinding the F5 mirror using the penny tool with MOT. I tried 1 hour of CoC grinding with less than 3 thou change throughout the hour, next I tried a Wide W stroke that dogbiscuit mentioned for a hour and same story. I am honestly stumped, I am almost out of 80 grit and will be forced to move to 120 in about 6 wets. Hoping that I will find a way to get the sagitta to the 75 thou mark during 120 grit.

It's real hard to change the radius with a metal tool, you should be using tile or glass.  Pennies once you get there.


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#161 Aaron_tragle

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Posted 23 March 2020 - 05:21 PM

Sorry just realized the audio didn’t work, I’m going to reupload
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#162 Aaron_tragle

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Posted 23 March 2020 - 05:41 PM

Aaron, I don't get any sound with your video. So I don't know what you are trying to show. However at 80 grit you are just trying to get the depth about right and not looking to have a sphere at that stage. That is what the next stages are for. That is to spherize the surface as well as to get finer and finer surface.

Dale


Just updated the link, the issue is that I can’t seem to get close to the desired depth. It’s almost like I’ve hit a wall at F6 and the mirror just won’t grind further.

#163 Dale Eason

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Posted 23 March 2020 - 05:54 PM

 I can’t seem to get close to the desired depth. It’s almost like I’ve hit a wall at F6 and the mirror just won’t grind further.

There you go that is a good description I'm sure people can advise better.  I don't have much experience but I bet a small ring tool less less than 3 inches would work better than a full size tool for hogging that out. Working tool on top in that case.  However I'm sure there is a good technique for mirror on top as well that some will tell you.

 

Dale


Edited by Dale Eason, 23 March 2020 - 05:54 PM.


#164 davidc135

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Posted 23 March 2020 - 06:11 PM

P3231083a.jpg

 

Here's a 4'' aluminium ring used to control curvature of a near flat 8'' surface. 50% diameter seems good. A heftier ring of steel might be better for your purpose. But you had a 4'' pipe flange earlier in the thread? Maybe 3'' would be better.

I use a rotary motion, the ring describing small circles as it travels around the disc. In the photo it extends well over the edge to push the surface convex. Barely touching the edge results in concave travel.

 

David


Edited by davidc135, 23 March 2020 - 06:18 PM.


#165 Pinbout

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Posted 23 March 2020 - 06:20 PM

i like to add lots of weight to hogging deep curves. the deeper, the heavier.  grin.gif

 

https://www.youtube....W8rBwA&index=74

 

 

but the fastest I've got to an f3 something was on a drill press with a sine table and a diamond cup drill hole saw.


Edited by Pinbout, 23 March 2020 - 06:25 PM.

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#166 dogbiscuit

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Posted 23 March 2020 - 09:29 PM

I hope this video explains the issue I am having better. This stuff is difficult to convey through text. EDIT: Here is the video with sound https://youtu.be/FDu8psxAd4s

Youtube says the video has been removed.

 

You mention the penny tool but I haven't seen a post saying how large it is, but I'm guessing it is full size because you say you worked COC MOT.

 

If you want to change the ROC,  that is the wrong stroke.

The big W will work slowly and is good if you want only a small change because it won't make the curve deviate from a sphere as much as a chordal stroke.  W stroke length can be longer than those 1/3 and 1/2d shown in post 151.  3/4 d stroke length and 3/4 d wide W will go faster.

 

Chordal stroke far offset from mirror center should work fast.

 

Not sure you should trust your method of measuring sagitta.

First might be good to measure ROC by reflection off the wet mirror, such as Foucault.

 

Making 2 mirrors at the same time means making the same mistakes twice when only once should serve to learn the lesson.

 

To go from f:6 to f:5 should be easy and shouldn't be any trouble getting there with 120 grit.

 


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#167 davidc135

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Posted 24 March 2020 - 02:27 AM

But has anyone had the experience of a full size coin or other metal tool deepening within a reasonable time? I haven't tried any other metal but the ratio of wear between glass and aluminium was huge; at least 10:1. 

Perhaps a brittle metal like cast iron might behave like a ceramic and wear by surface shattering but plastic/elastic material such as aluminium seems to mostly soak up the punishment and just wear a little by scraping.

 

David


Edited by davidc135, 24 March 2020 - 02:39 AM.


#168 Pinbout

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Posted 24 March 2020 - 06:32 AM

 

But has anyone had the experience of a full size coin or other metal tool deepening within a reasonable time?

sure, but not from flat, and having the curve started....

 

a subdia nut tool with lots of weight will get him there 


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#169 Aaron_tragle

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Posted 24 March 2020 - 08:02 AM

Youtube says the video has been removed.

You mention the penny tool but I haven't seen a post saying how large it is, but I'm guessing it is full size because you say you worked COC MOT.

If you want to change the ROC, that is the wrong stroke.
The big W will work slowly and is good if you want only a small change because it won't make the curve deviate from a sphere as much as a chordal stroke. W stroke length can be longer than those 1/3 and 1/2d shown in post 151. 3/4 d stroke length and 3/4 d wide W will go faster.

Chordal stroke far offset from mirror center should work fast.

Not sure you should trust your method of measuring sagitta.
First might be good to measure ROC by reflection off the wet mirror, such as Foucault.

Making 2 mirrors at the same time means making the same mistakes twice when only once should serve to learn the lesson.

To go from f:6 to f:5 should be easy and shouldn't be any trouble getting there with 120 grit.


https://youtu.be/vScSre6T3oY

#170 dogbiscuit

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Posted 24 March 2020 - 01:41 PM

pennytool.jpg

 

I don't like the distribution of pennies on the tool. 

Sparse near the edge.  Red circles in picture mark "missing pennies".  Two problems with that.

 


The outer zones have less grinding surface than the more densely populated central zones. Might be why your central area is longer radius than outer zone. With COC strokes central zones grind faster because of more tool surface area there, grinding deeper but not much change of ROC in the center.  Less contact in outer zone causes edge to grind slowly and it becomes  relatively higher with shorter ROC than center.  It is possible that you have reached your desired sagitta but the center being flatter than the edge you don't see it across the 3.56" diameter of your spherometer.

SpherometerFlatcenter.jpg

 

Take a measurement across the full diameter of the mirror. You might have your target depth but really only need to make the surface spherical.

 

 

There is not much support for the mirror when doing a chordal stroke with mirror center offset to the 65% - 70% zone causing a danger of tipping over the edge when mirror center is over one on the "missing pennies", so safe offset is limited, and you want that offset for shortening the ROC.

 

ChangeROC.png

 

 

Your tool is what it is now.  I wouldn't recommend adding pennies in the spaces now. They will be too high.

It might work to removed a couple of pennies in the central part of the tool so that overall distribution of pennies is more uniform.  I don't know if the coin tool as is will work well.  I think it might be too much trouble because of too many unknowns.  My recommendation would be to cast a normal plaster and ceramic tile tool fully populated with tiles. 

Cast on the mirror so tiles match mirror curve well and then it will only take a little while to grind into contact with 120 grit and make it spherical.  Then you continue through the various grits in a normal manner.

 

You would do better using basic commonly used methods.  Small tools, penny tools, and other oddball tools work much better if you have experience to know what is a good result and that particular tool can get to that result. I doubt there is anyone experienced at using a penny tool with a distribution of coins as on your tool, and that means no one can tell you from experience if you will be able to make a good sphere at each grade of fine grit so that you will be able to polish in a reasonable time.  Your penny tool might work ok, or it might be a colossal waste of time.

 

 

Measure depth across the full 6" diameter and post your result.

If you put a straight edge across the mirror and measure depth at the center of the mirror.

Drill bits make a good gauge.  Here are some sizes around your desired sagitta.  Any would be close enough.

Put the back end under the straight edge just far enough under the straight edge to have full diameter under it.

 

#49       0.0730"
1.9 mm 0.0748"
#48       0.0760"
5/64"     0.0781"


Edited by dogbiscuit, 24 March 2020 - 01:52 PM.

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#171 Aaron_tragle

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Posted 24 March 2020 - 03:10 PM

pennytool.jpg

I don't like the distribution of pennies on the tool.
Sparse near the edge. Red circles in picture mark "missing pennies". Two problems with that.


The outer zones have less grinding surface than the more densely populated central zones. Might be why your central area is longer radius than outer zone. With COC strokes central zones grind faster because of more tool surface area there, grinding deeper but not much change of ROC in the center. Less contact in outer zone causes edge to grind slowly and it becomes relatively higher with shorter ROC than center. It is possible that you have reached your desired sagitta but the center being flatter than the edge you don't see it across the 3.56" diameter of your spherometer.
SpherometerFlatcenter.jpg

Take a measurement across the full diameter of the mirror. You might have your target depth but really only need to make the surface spherical.


There is not much support for the mirror when doing a chordal stroke with mirror center offset to the 65% - 70% zone causing a danger of tipping over the edge when mirror center is over one on the "missing pennies", so safe offset is limited, and you want that offset for shortening the ROC.

ChangeROC.png


Your tool is what it is now. I wouldn't recommend adding pennies in the spaces now. They will be too high.
It might work to removed a couple of pennies in the central part of the tool so that overall distribution of pennies is more uniform. I don't know if the coin tool as is will work well. I think it might be too much trouble because of too many unknowns. My recommendation would be to cast a normal plaster and ceramic tile tool fully populated with tiles.
Cast on the mirror so tiles match mirror curve well and then it will only take a little while to grind into contact with 120 grit and make it spherical. Then you continue through the various grits in a normal manner.

You would do better using basic commonly used methods. Small tools, penny tools, and other oddball tools work much better if you have experience to know what is a good result and that particular tool can get to that result. I doubt there is anyone experienced at using a penny tool with a distribution of coins as on your tool, and that means no one can tell you from experience if you will be able to make a good sphere at each grade of fine grit so that you will be able to polish in a reasonable time. Your penny tool might work ok, or it might be a colossal waste of time.


Measure depth across the full 6" diameter and post your result.
If you put a straight edge across the mirror and measure depth at the center of the mirror.
Drill bits make a good gauge. Here are some sizes around your desired sagitta. Any would be close enough.
Put the back end under the straight edge just far enough under the straight edge to have full diameter under it.

#49 0.0730"
1.9 mm 0.0748"
#48 0.0760"
5/64" 0.0781"

I was worried about the pennies being an issue, I’ve lost quite a few of them during 80 grit. They just pop off, the F3 tool has even more pennies missing. I measured the sag using a straight edge and drill bits. The F5 mirror sag is slightly less than a 5/64th bit, straight edge can just barely wobble. If I had to guess it’s around 70 thou. For the F3 I used a 1/8” bit and it is much more wobble, to be expected. The F3 is around 114 thou leaving 10 thou from 1/8”. Would it be worth it to make a full diameter disk for the spherometer? I have the ability to make a disk with my 3D printer.

Edited by Aaron_tragle, 24 March 2020 - 03:19 PM.


#172 Aaron_tragle

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Posted 24 March 2020 - 03:43 PM

I have some good news, when I originally casted my disks for the penny tools I casted two extras. They were originally intended for the pitch laps, but I think I can use them for tile tools. What type of tile will I need exactly.



#173 dogbiscuit

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Posted 24 March 2020 - 05:48 PM

If the pennies have been popping off, it shouldn't be difficult to knock them all off.

probably could put the edge of a screwdriver on the edge of the penny and just a light tap with hammer to knock them off.  Don't dig the screwdriver under the penny, just want to knock it sideways.

 

The tile should be a solid tile, not porous, not glazed, about 1" across, about 10 mm thickness, square, hexagon or round.

 

You might find some a little larger or smaller than 1", like say 7/8" or 1 1/8".  You might have to take what you can find, but larger tiles would take longer to grind to full contact.

 

Look at posts 60 and 61 here

https://www.cloudyni...3#entry10028109

 

I also see some tiles online at homedepot but can't be sure if they are solid and not glazed.  You could look there in person and see if they have something suitable.

 

You want floor tiles.  They are usually harder than wall tiles. 



#174 Aaron_tragle

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Posted 24 March 2020 - 06:31 PM

Well, I am stuck in a rut. I have no choice but to continue with the penny tool, I can't leave the house for 6 weeks. State just issued a SIP order and due to previously mentioned medical issues I am in a "high risk" group for the virus. I am moving to 120 grit, I think the slight gaps on the edge of the tools where pennies have been lost can be mitigated by frequent, random rotations of the mirror and tool. I am going to add some little bits of epoxy on the edges of the pennies to ensure that I don't lose more. I know this isn't the advised course of action, but it is either this or not finishing the mirrors before I head off to college.


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#175 dogbiscuit

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Posted 24 March 2020 - 10:19 PM

Yep... Best not go out looking for tiles.

You might try using 120 grit with 1/3 COC for a while and see if you get a sphere where the spherometer is reading is the same in the center and when you move it off center.


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