The 20 " grind has begun

[mapstar]

Coming along Rick and thanks for the update.

Are you still working with the full size lap? Or have I missed you've changed back to the 3/4 size lap? 

[JRM]

its coming along Damian, slow and steady, hope not to mess it up again, still using the fullsize 20" lap with M.O.T., it is a real workout when

things are in full contact, took a break for 5 days and lost contact so tonight I have the mirror dry pressing with mesh in between for micro

fauceting with 45 pounds on top, will let this press for an hour and once this is done it should be back in good contact so I can have a session

afterwards, then clean, let dry and put the glass on test stand for testing in the morning and go from there...

Rick M

[JRM]

Hi all,

in case anyone is wondering I have been on a bit of a break, with youngest daughters graduation and such filling in my spare time I have

not worked mirror since 10th of June,but it is pressing out now with mesh, should be back at it this evening,will keep you posted.

Rick M

[JRM]

Hi All,

 just to update, since July I have been returning to spherical after messing up the edge once again, and

as you can tell I did not work the glass in a steady fashion, July till now to work a total of 15 hours correcting

the edge and getting back to the sphere but it is real close to being ready for next figuring session.

 

The surface is a bit rough and needs to be smoothed some and the full size pitch lap is thin so it is now in

the freezer, will strip it off and remake again so I can smooth the surface and set the lap before I start the

next figuring session.

This time I will once again make the 15" lap and start with it, will also be keeping a detailed log of each session

to keep track of the changes so I don't get so bent out of shape this time around, (I hope), will also be making a

10 and a 3 inch lap to have on hand in case they are needed.

will need more polishing compound if this session does not go right, so here's to hoping...

I have all the parts to build the telescope except for the plywood but that I can get when need be, will build

with baltic birch, just need to finish this mirror first, so hope to be figuring again this week.

will keep you posted.

 

Rick M

[JRM]

Well, I got all the pitch off the 20" tool I was using for my pitch lap and in the process I wacked it a tad to hard well

removing the pitch that it broke, so the 20" grinding tool is gone, done for, hope I don't get a scratch and have to go

back to grinding as I will have to make a new grinding tool, I know before ya say it, "that is why you don't use the

grinding tool as a pitch lap base", but I did , it's broke, will cross that bridge if need be but certainly hope to never

have too.

have decided to get the machine back up and running so I will be revamping the turntable and completing the

rest of it so I can use it for fixed post work. will let ya know how it goes...

Rick M

[mapstar]

Hi Rick

Are there any updates on your grind? Will you be posting a few details of the table upgrade? 

Here's a pic of a recent dob mob outing to the Isle of Skye. Maybe spur you on a little

 

 

Damian

[estwing]

keep the updates coming Rick....

[JRM]

Hey guys& gals,

          Nothing new to report, life has taken over again, to much to do and no time to do it, winter has finally arrived in the form of a snow storm, the

jolly ole fella in the red suit is less then three weeks away and my wrist is acting up again, but on a positive note my place of work is finally starting to

slow down ( thank goodness).

just realised it was three years ago yesterday that I started this project, where does the time go, 133 ish hours in so far,it is not over yet but hope to

see that light at the end of the road soon.

Thanks for the phot Damian, it is a very inspiring image, my they look so nice.

Rick M

[faulksy]

nice one rick, keep it coming buddy

[James Vincent]

Hi Rick

I have made quite a few large mirrors up to 24 inches  and i may be able to help you.

where  are you with the mirror ? is it spherical , almost fully corrected or some were in between ?

but most importantly hows the edge ? 

Jim

[Timebandit]

On 06/12/2016 at 02:48, JRM said:

Hey guys& gals,

          Nothing new to report, life has taken over again, to much to do and no time to do it, winter has finally arrived in the form of a snow storm, the

jolly ole fella in the red suit is less then three weeks away and my wrist is acting up again, but on a positive note my place of work is finally starting to

slow down ( thank goodness).

just realised it was three years ago yesterday that I started this project, where does the time go, 133 ish hours in so far,it is not over yet but hope to

see that light at the end of the road soon.

Thanks for the phot Damian, it is a very inspiring image, my they look so nice.

Rick M

 

Hi Rick. I see 3 years and still going to get your Big mirror. You certainly deserve to get to the end of the project with a nice reward of a great mirror for a big Dob. Best of luck buddy.

 

10 hours ago, James Vincent said:

Hi Rick

I have made quite a few large mirrors up to 24 inches  and i may be able to help you.

where  are you with the mirror ? is it spherical , almost fully corrected or some were in between ?

but most importantly hows the edge ? 

Jim

 

 

Hi Jim. After reading Damian and Rick mirror grinding experience. Then I certainly am having second thoughts of doing a bit of my own grinding?. I see you have done a few of your own mirrors Jim , handy to know. I may need your number at some point and be  in the queue for a Big mirror ,if I still have that itch to go Large . ☺

[James Vincent]

I have a mirror making section on my web site  http://www.astropictures.co.uk/ if you are interested.

Big Blue (22 inch dob) has one of my mirrors in it.

Jim

 

[Astrobits]

Nice to see another mirror maker here to help Rick.

I don't try to get a spherical surface before I figure, I see that as a complete waste of time, materials and effort. After polishing I look at the figure that I have got and remove glass from the "high" spots to reach the parabola. Any "turned edge" simply means that there is glass to be removed from everywhere except the edge.

Nigel

[mapstar]

39 minutes ago, Astrobits said:

Nice to see another mirror maker here to help Rick.

I don't try to get a spherical surface before I figure, I see that as a complete waste of time, materials and effort. After polishing I look at the figure that I have got and remove glass from the "high" spots to reach the parabola. Any "turned edge" simply means that there is glass to be removed from everywhere except the edge.

Nigel

Easier said than done though Nigel but makes sense. Been that soldier 

[Astrobits]

Agree Damian.

Figuring a mirror, especially a big one, is always easier said than done. All you have to do is remove the glass you don't want. There, that took me less than 15 seconds

There is an awful lot of glass to be removed with those big mirrors as they are usually very fast and consequently the parabola is a long way from a sphere.

Nigel

[James Vincent]

Mirror makers all have their own way of doing things, i tend to try to polish the surface to a good sphere with no turn down edge if possible.

then check the surface for astigmatism before going any further.

Astigmatism is one of the greatest problems with a thin mirror.

A Turned edge can be dealt with by effectively making the focal length of the mirror longer and then blending it in with a small polisher.

Jim

 

[Astrobits]

Yup. First test: astigmatism.  Sort it out first.

Nigel

[JRM]

Thanks for the response guys,

Jim, I am currently on a break from working the glass as my wrist is acting up again, but in the mean time I plan to revamp my turntable

and complete the top post section for future use.

The edge of mirror is fine as of last testing, some roughness on surface but that is an easy cure once I get back to it. my plan this time

around is to take an image via ronchi after every session and keep track of every change as it occurs, and not to dig out the center as

I have been doing in last three figuring sessions. will let you know when I restart, Thanks.

 

Mike,

thanks and will do.

 

Timebandit,

yup three years in time but the total amount of time put into it so far is a tad over three weeks,so not really alot for a first time attempt at a

20"

 had lots of time off in the past three years, nearly broke my wrist, that cost me eight months alone, plenty of life issues took me away as well,

but thats life, we are not here forever as I have sadly come to learn far to many times over these last few years, but life must go on and I want

to get this done before it is my turn lol

 

Nigel,

I can say with confidence there is no astigma in the glass at this time, bit always cautous and checking that for sure.

 

on a final note,

I have a treadmill down stairs with a motor that runs the treadmill anywhere from one up to 6 mph, I wonder if

this would be a better set up for my grinding/polishing station, it is a 2.25 hp motor, any opinions, might help me

 to do away with some pulleys and such, any advice appreciated.

Thanks All

Rick M

[James Vincent]

Hi Rick

You do not require  large powerful motors to make a grinding/polishing machine,  1/2Hp is plenty.

lots of electric motors and car parts ( Mondeo rear stub axle very handy for turntable)  on E bay.

When testing the mirror use the Foucault test and a Couder mask and input the data in to a program like figure XP . You can not figure a mirror using the Ronchi test on its own.

Hope the wrist gets better soon.

[Astrobits]

Hi Rick,

My comment on sorting out astigmatism was a reinforcement of Jim's previous post.  Jim mentions astigmatism as a problem with thin mirrors but I got astigmatism on one of my 20" mirrors that was 40mm thick. Maybe some will class that as a thin mirror.

Jim is also right in that fractional HP motors are all that is needed for polishing and figuring. The largest tool that I have used for my 20" mirrors has been 13".  Never a full-sized lap in sight!

Personally, I have never used a Ronchi screen but I do move the knife edge across the beam inside and outside of focus and can see the profiles that a Ronchi screen would produce. For the final figuring I use a null test.

Nigel

[JRM]

no worries Nigel, sometimes typed words may seem harsh compared to what was meant, I only mentioned it because astigma can be an 

issue, so I am constantly on guard for it.

I test with both ronch screen and knife edge, ronchi gives you a quick assessment of where things are I find, but knife edge gives you a more 

precise measure.

My grinding machine does have a 1/2 hp furnace motor  currently, but I was thinking the variable speed motor would give me a better

means to control the speed of the turn table, and I could remove some of the pulleys and shaft as well, that is my reasoning for

wanting to switch it out, will have to look into it more.

Question is, can you figure a mirror on a top post machine?

Thanks

Rick M

 

[James Vincent]

Not sure what you mean by top post machine, but a driven table rotating at a few rpm would be would make life easy.

Using a 50% sub Dia tool  tool with the  tool on top pushing from one side of the mirror to the other with a 20% overhang both sides will deepen the center.

This stroke should be off set so that it dos not go through the center of the mirror, reduce the amount of overhang if there is a turned down edge.

3 phase Motor speed controllers  are great they will allow you to remove some of the pulleys but not all of them because the motors lose torque at very low speeds.

The controller will also allow a three phase motor to be controlled from a domestic  single phase supply, i use these on my machines.

[JRM]

James,

 My mistake, it should be fixed post, heres a link to Gordon Waites  youtube site 

https://youtu.be/PSGVzPTocWE

if I have to use an moving arm for figuring I can do that as well, just wondered about parabolizing with a fixed post.

Thanks

Rick M

 

[mapstar]

I did think you meant a fixed post machine Rick.

I jnow they are good for polishing but I'm not sure you'll be able to figure on it. When I worked with John on his machines it had the oscillating arm with an adjustable throw length.

This was positioned to deepen the centre ocassionally moving back to the neutral 70% position to keep everthing in check. My memory is a bit vague on this so will bow to better knowledge.

Keep at it 

 

 

[Astrobits]

The bulk of figuring on my large mirrors was done on the fixed post type of machine. My machines had the oscillating arm powered from the same motor that turned the turntable. Offset was fixed but adjustable. Only the very last bits of narrow zone touch-ups was done by hand.

As you don't need to remove ANY glass from the very edge I minimised the amount of overhang starting with zero but increasing it when testing indicates that was needed.

Nigel