Machining a reflector on a Mini-Lathe?

Nubo

Enlightened
Joined
Dec 23, 2004
Messages
461
My last post was about using spin-casting to form a parabolic mold. While I still think it's an interesting approach but the devil is in the details. As I've been reading I'm not sure that setting up a home foundry for casting aluminum is going to save me all that much work /ubbthreads/images/graemlins/crazy.gif, and I might still end up with quality issues. And, it doesn't take care of my other fabrication needs, especially forming bodies, and threads.

So, I am wondering if it's feasible to form flashlight-sized reflectors with one of the Mini-Lathes. Not under CNC control, but manually? I have this idea of forming a finely-stepped surface, and then finishing the parabola either by hand or perhaps with a metal template (parabolic cross-section) as a tool guide? I don't have much experience with this sort of thing so looking for opinions on this.

Thanks!
 

HarryN

Flashlight Enthusiast
Joined
Jan 22, 2004
Messages
3,977
Location
Pleasanton (Bay Area), CA, USA
I assume that this is for fun as their are a lot of easier and cheaper ways to get a reflector. Manually grinding out a reflector is not so easy, but perhaps rewarding.

If I wanted a very custom reflector, and really understood what I wanted, I would either contract with a commercial reflector maker, or hire a local shop with a cnc lathe to make it. Of course, I always am looking for ways to justify new machines and tools myself, so maybe that is your goal ? /ubbthreads/images/graemlins/smile.gif
 

Nubo

Enlightened
Joined
Dec 23, 2004
Messages
461
HarryN, Thanks. Yes, for enjoyment. The reflector is really like the heart of the device to me. The idea of making a truly custom light makes me want to be able to produce that feature by my own methods. Sort of like amateur telescope builders who go to the trouble of grinding their own mirrors. I'm also envisioning the reflector as an integral piece of the body rather than a discrete component, and would like to size the apparatus to goals that aren't dictated by commercially available reflector sizes or depths.
 

Trader55

Newly Enlightened
Joined
Dec 2, 2004
Messages
14
Nubo

If you're really going to go to that much trouble, why not use silver? It is the best reflective material known. You will undoubtedly have to coat the aluminum with a clear coat to prevent oxidation, so it goes with silver also.

Possibly a local college or High School jewelry (shop class) could put your wax mold into a centrifuge to cast it. Grinding and polishing it would be the fun part on almost any type lathe. And you could make your own forming tool for the reflector by simply grinding to shape, an old file to get your parabolic curve.(For cutting tool)

Trader55

May not be much help.....
 

XtremePyro

Newly Enlightened
Joined
Nov 30, 2003
Messages
81
Ive machined a couple of 2" reflectors using a series (300 or so)of steps and then sanding and polishing. Not really that much machine time especially if you calculate the rough taper of the parabolic curve and remove that material first. What takes the longest amount of time is calculating all the steps.
Playing human CNC is actually kinda fun, unless you dont have any patience ;-)

Tim
 

Nubo

Enlightened
Joined
Dec 23, 2004
Messages
461
Thanks XtremePyro, you give me hope!

Did you use any optical methods to fine-tune the sanding/polishing, or just aiming for a smooth shiny surface? I guess with 300 steps it was already pretty close!

How was the beam quality? And, did you find it necessary to coat the reflector?

Trader55, I had thought of grinding a parabolic cutter but wasn't sure how long it wold take before it would wear down to the point of becoming inaccurate? Hmmm... add "grinder" to to-buy list.... /ubbthreads/images/graemlins/smile.gif
 

Anglepoise

Flashlight Enthusiast
Joined
Nov 4, 2004
Messages
1,554
Location
Pacific Northwest
Nubo.
I am with you and firmly believe that the heart of any light is the reflector.
I am amazed when I see $300 lights that use a $2.00 plastic reflectors.

I have made 3 reflectors on my lathe. Just personal stuff. Would take far too long to duplicate unless a full CNC was available.

I have found that the main problem is getting the basic shape
and once you have got a design that works for you, it is almost impossible to repeat.( I don't have XtremePyro's patience )

I have been very happy with how long the reflector remains polished. People talk about aluminum oxidizing but in my limited experience, this does not seem to be a huge problem.

However as all tinkerers know, making personal stuff is allot different from any sort of production.

Good luck.....
 

Nubo

Enlightened
Joined
Dec 23, 2004
Messages
461
Anglepoise

Thanks. The oxidizing issue is interesting. For telescope mirrors this doesn't seem to be a problem, reportedly because aluminum oxide itself is transparent. But perhaps this only applies to very pure aluminum. What kind of alloy did you use?
 

Anglepoise

Flashlight Enthusiast
Joined
Nov 4, 2004
Messages
1,554
Location
Pacific Northwest
[ QUOTE ]
Nubo said:


What kind of alloy did you use?

[/ QUOTE ]

I was afraid someone would ask that. I have a raw metal bin that I keep stocked with surplus stuff I find out and about. No idea the grade.

However I forgot to mention that I apply a coat of auto wax
after the final polish. I use the same applicator as the final rouge polish and suspect the wax helps keep the oxidization away.
 

XtremePyro

Newly Enlightened
Joined
Nov 30, 2003
Messages
81
Actually even with 300 steps the parabolic curve was still visably stepped. I used corse sandpaper to sand smooth the steps and then used progressivly finer papers,100,350,600,1200, 2000 to smooth it on up. Then turn the lathe up on high speed and use 2000 with light machine oil and a light touch. This brings it to a finish that you can see yourself in. Then I melted some jewlers rouge buffing compound in spots onto a cotton cloth. Again running the lathe at high speed use the spots on the cotton cloth to buff the aluminum. must change the dots frequently as aluminum will build up on them. This produces a mirror surface that you can easily see the details of your face in but objects say 4-5 feet behind you will be slightly blurred.As the aluminum is exposed to air the oxide will slightly degrade the reflectivness of the reflector. If say you were looking at a brand new maglight reflector and it was a 10, you can produce an 8.5 by polishing aluminum, and after it oxides about an 8. that make any sense.
Also the 2 reflectors I have made so far were from aluminum that I cast, thinking that the purer softer aluminum would be more reflective. However I am now inclined that I may try to machine one from 6061 T651, since other parts I have machined, I have been able to produce a very high lustre on.
Oh and on both of the ones I made the beam they produced was very similar to a maglight reflector on tight focus.
Hope this helps some.

Tim
 

IsaacHayes

Flashlight Enthusiast
Joined
Jan 30, 2003
Messages
5,876
Location
Missouri
Would keeping a thin layer of oil on it help it not oxidize? Or would that hinder reflectiveness?

Also, could you boil it in water to "seal" the aluminum like others have suggested, or would that also mess up the shiny surface?
 

NewBie

*Retired*
Joined
Feb 18, 2004
Messages
4,944
Location
Oregon- United States of America
[ QUOTE ]
Nubo said:
Anglepoise

Thanks. The oxidizing issue is interesting. For telescope mirrors this doesn't seem to be a problem, reportedly because aluminum oxide itself is transparent. But perhaps this only applies to very pure aluminum. What kind of alloy did you use?

[/ QUOTE ]


Aluminum oxide is not as transparent as you may think.

Here, do this. Take two pieces of aluminum sheet metal. Polish each to a perfect mirror finish. Assure you remove any traces of polish, rouge, or Brasso. Then let them sit for two weeks. Then take one and polish it again.

Compare something reflected in the two, something you can see the reflection of side by side. Do it in the dark, after your eyes have adjusted for half an hour, and pick something that isn't really bright for the reflection.

In reading about reflectors, take care NOT do confuse solar reflectance with visible light reflectance, as there is alot of IR and some UV in sunlight, which are different critters than visible light.

Most aluminum reflectors are overcoated with Disilicon Trioxide or similar material like Silicon Dioxide, around 275 nanometers thick.

There is some information on bare aluminum here, note the rapid degradation comment:
http://www.rmico.com/optics/opticCatalog.asp?mainID=10&subID=&coatID=29

At the bottom, check out their protected aluminum:
http://www.rmico.com/optics/opticCatalog.asp?mainID=10&subID=&coatID=31

Enhanced Aluminum, check out the 95-97.5% reflectance:
http://www.rmico.com/optics/opticCatalog.asp?mainID=10&subID=&coatID=32

See also Enhanced Silver (which is protected), at 97-99.0% reflectance:
http://www.rmico.com/optics/opticCatalog.asp?mainID=10&subID=&coatID=34


You will also note quite a number of telescope reflector coating shops will not even sell unprotected Aluminum coatings:
http://www.majestic-coatings.com/


These folks use an enhanced coating to make a 750W lamp put out more light than a 1,000W lamp (while using the same bulb):
http://www.premier-lighting.com/sales/pars.htm

As far as telescope mirrors, look down at the bottom here:

Coating

Aluminum coating is done by Aluminum Coating in a high vacuum tank. Aluminum metal is evaporated from a wire near the bottom of the tank and coats the mirror which is being rotated at the top of the tank. A very thin SiO transparent coating is coated over the aluminum coating. The mirror never becomes hot. A recoated mirror looks just as good as new.
http://home.covad.net/~alcoat/al_2.htm

Similar situation here:
http://www.spectrum-coatings.com/telescop.htm

At the bottom:
http://www.rfroyce.com/stand_mirrors.htm

For mirrors larger than 1 meter, they sometimes do not overcoat, some have the mirror in an inert atomsphere, such as nitrogen (rather easy since it is heavier than air)
Vague reference here:
http://www.hs.uni-hamburg.de/EN/Oef/Stw/Alu-anlage/index.html

On the large telescopes, they often will build a custom aluminum coating plant next to the facility, where the mirror degrades to a point that you really have to strip and recoat it at least once every two years.

Here are some photos of one method of mirror coating with aluminum:
http://researchnews.osu.edu/archive/mirror.htm

Some places have to recoat once to twice a year:
http://www.sofia.usra.edu/News/news_2000/news_nov_2000.html
 

gadget_lover

Flashaholic
Joined
Oct 7, 2003
Messages
7,148
Location
Near Silicon Valley (too near)
The reflector in my UK 4aa eLED is not parabolic, but you can plainly see the marks where the mold for the reflector was made with a lathe type device. There are visible concentric rings in the reflective surface.

Making flood reflectors does not always require a curve. I made one for an AAA sized 1 watt low dome by simply cutting a 60 degree bevel on my lathe and boring a hole for the LED in the middle. The surface was polished, then sprayed with acrylic clear coat.

Daniel
 

Nubo

Enlightened
Joined
Dec 23, 2004
Messages
461
[ QUOTE ]
NewBie said:
Here, do this. Take two pieces of aluminum sheet metal. Polish each to a perfect mirror finish. Assure you remove any traces of polish, rouge, or Brasso. Then let them sit for two weeks. Then take one and polish it again.

Compare something reflected in the two, something you can see the reflection of side by side. Do it in the dark, after your eyes have adjusted for half an hour, and pick something that isn't really bright for the reflection.

[/ QUOTE ]

Thanks for all the info. As I go along my initial idea for a "hardware scrounge light" keeps evolving into ever-more expensive permutations /ubbthreads/images/graemlins/broke.gif But it's a beauty. Hope you guys get to see it one day /ubbthreads/images/graemlins/smile.gif

I noticed the following on the Carley site:

Hand polishing reflectors forms a fine coating of aluminum oxide on the reflector face and this protects the reflector, maintaining its bright surface.

Any insight into that? I wonder if they have some proprietary polishing compound. You don't suppose they actually use their hands (fingertips)? /ubbthreads/images/graemlins/smile.gif
 
Top