grinding down an aluminium ring for telescope? stuck

hello everyone, my name is michael ,from south africa

im currently trying to make a primary mirror cell for a telescope

i have been given an aluminium ring (flat not a round bar ring),
lets say OD is 10" , ID is 8"
see pic..

im trying to reduce the OD from 10" to 9.5"



any ideas how i can do this?
i don't have a lathe or milling manchine
i do have a bandsaw, grinders, grinder wheels, and a dog named misty

Need a picture of the dog.
What kind of grinder? Surface, cylindrical, angle, bench?
What kind of accuracy do you require?

i have a normal angle grinder and a small bench grinder..
the dog is black..curly tail...you know

accuracy will be around +_ 0.5mm

was thinking making a wooden 8" insert, somehow wedge it in the circle

then a rod in the middle, and put it in my drillpress...

What kind of telescope?

Aluminum is not a very good material for your holder as it takes a long time to cool down and not produce turbulence in the light path of the mirror....



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almost all primary mirrors are made from aluminium

actually for a mirror cell it doesn't matter, althought the TCE of alu is
about 20x that of pyrex, the mirror is lifted abouve the aluminium cell
in a floating fashion, it is therefore relatively unaffected by the expansion differences

the cell will have a fan at the back,and plenty of cooling before use

my scope is a reflector in a dobsonian mount

the main problem isn't tube turbulence since i have a truss setup

the main problem is thermal expansion differences, but even that can be
negated with a proper design

For the accuracy needed, just scribe a line, bandsaw to within a mm and file the rest. Check side squareness periodically with a small square. You should be able to get a pretty fair circumference as your hand will be able to feel hills and valleys that are hard to see.

Another way would be to glue the ring to a MDF or plywood backplate. First drill a pivot hole at the center of the backplate. True the ring to the hole and glue on the ring using cyanoacrylate glue. Set up your grinder in a vise or clamps with the wheel vertical to the supporting table. Turn the wheel/ring slowly on the pivot and gradually reduce the diameter. You could debulk the amount of grinding by using the pivot to rough cut on the bandsaw the ring prior to finishing with the angle grinder as above. If you had access to a typical disc sander that would save the setup on the angle grinder. I've made plenty of discs that way prior to my now having more suitable equipment.

Denis

Back in 1989, 1990 or 1991 I designed a mirror cell that was designed for low expansion, fast cool off and easily alignment.

All my scopes were designed for elimination of turbulence and heat sources that affect collimation and seeing. I was looking for a picture of one of the cells but it has been a long time, 27 years, many moves and many computers If I find one I'll post it..

I think there are pictures of one of my scopes, with that feature, in Sky and Telescope's January 1990 issue.

I controlled heat by using fans, materials that cooled off fast and even portable a/c units vented into the scopes to keep the interior components near expected night time temperatures. I was somewhat a fanatic about cooling and alignment at the time... I'm feeling much better now ...

Almost all primary mirrors are made from glass with a very thin film coating of aluminum on the first surface.

The best material I ever found, for mirror cells, was cloth phenolic. 1 inch thick was great because I could mill away all the material not needed for the support points and still have a very strong cell. Less material ment less thermal mass to control and left the back side of the mirror open so that the cooling fans were more effective.

thanks dennis, that sounds like a good idea..will try it

Swatkins..

as a member of CN (cloudy nights) most of all the guys there (Don Pensack, Nils Olof Carlin, Vic Menard etc)
have done extensive -and often trial and error- research into almost anything optical.

Most guys use ply or aluminium for their prim. mirror cell, and it seems that tube turbulence
can be controlled by various different measures -as you said-

yes -true- the most optimal material would be the one which has the same thermal
expansion coefficient as pyrex (in my case)-which- turns out to be carbon fibre.

for collimation purposes I don't think the TEC (or TCE) matters a great deal as long
as the mirror can freely float -or move- in all three directions, the main problem
encountered is collimation shift when moving towards -or away- from the zenith.

in any case, I'm goin ahead with my build...once I'm suitable in progress I wqill post
some pictures !

thanks everyone for the responses

Since you have lots to do, make a circle-grinding jig for your bandsaw. You can find pictures on the web. Make a wooden plate that holds the rings snugly, with a hole in the middle, that spins on the circle-grinding jig. Tune the diameter until you are happy, then cut them all to size. To smooth them make another wooden plate that holds a ring and spins on your drill press. Then hold a file or sanding block against the spinning ring to smooth out the saw marks.

well...

i'm going to try...my neighbour offered to put it on their
big lathe and machine it to size...so yes...i'll do that..

but still...i would like to do it myself...

will try again...but im grateful for your input...

i hate handing over my work to someone else, but in this case
precision and expertise is paramount...

i will practise on other things first, and build some experience
and confidence first

it's no good trying only to find out it didn't work

swatkins..

you seem (or sound like) a well established telescope maker...

you sure know what you're talking about

i am intrigued about your 'low turbulence' setup

shall we call it the 'Low Internal Diameter Thermal Reynolds Number Scope'? (LIDTRS)
-couldnt think of a better acronym-

could you post me some ideas maybe?

The main thing to watch out for if you grind aluminum is not to do it on a grinding wheel meant for sharpening steel tools. The wheel would load up, heat up, and eventually might fracture violently. I've seen the aftermath of such an incident and it wasn't pretty.