Aluminum for Diamond turning op.

[1MadMax]

I have an aluminum mirror that has very high surface irregularity and roughness requirements which is being diamond turned. We are trying to determine if there is an aluminum available in an 11" or 12" stock diameter that could be substituded for 6061 T6 (our current mat'l of choice) to improve our D.T. machined results ... any suggestions?? Stability is of greater concern than strength.

thank you, Max

 

[btrueblood]

Mic 6 I believe is one brand of precision-cast aluminum plate; sold in the cast+aged+stress relieved condition. Others are Alca Plus, K-100S...

 

[sreid]

I've been told that an Alumiplate aluminum plating prior to the final finish turning greatly improves the surface finish on an aluminum mirror.

http://www.alumiplate.com/

 

[unclesyd]

6061 with various tempers is the nominal choice for making mirrors.
Are you using single point tool?

I would give TCI a call with your problem. They should be able to help with both the material and the machining.

http://www.tciprecision.com/affiliations.php

http://www.alcoa.com/global/en/prod...&Region=&Category=&Query=tooling plate&page=0

Here is a little more help from two perspectives, finishing and machining.

http://ipp.gsfc.nasa.gov/ft-tech-super-alum-polishing.html

http://www.techbriefs.com/content/view/479/32/

You also might be interested in this product

https://www.micro-surface.com/default.cfm?page_id=200

 

[1MadMax]

Thank you for the quick replies! My understanding of the cast materials was that they would have an undesirable porous finish and is also susceptible to inconsistant surface quality. I'll check out all the suggestions provided ... thanks again.
- more info - Yes we use a single point for D.T. Unclesyd, you bring up another aspect there, may be machining techniques to improve the surface quality?
We found no trouble turning a 4" mirror. I believe the stock for the small mirror is cold rolled and the 12"dia stock is extruded. Does anyone know if it is possible to get 12" dia cold rolled stock?

 

[sreid]

If cast material is the problem, could you start with rolled plate instead?

 

[1MadMax]

We haven't used cast because of the before mentioned concerns, only 6061 T6 has been used. The thought was that alloy aluminum would provide the best surface form and finish ... is this correct? We measure surface irregularity by Lambda (wave front error).

 

[unclesyd]

Looking at the Goddard process they turn to 30-80 angstrom finish and polish to 5.

If you look at the Goddard page you can see that you can license the process. I would check this out to see if you can get a nonexclusive license for their process. At one time this was not a very expensive proposition.

http://ipp.gsfc.nasa.gov/lic-partnerships.html

The plate carried by TCI is the Alcoa tooling plate material which is 6061 T651. It has been my experience that this material is inherently stable due to the mill processing. I believe that you can use the stability treatment from the Tech Brief article to further enhance material during preliminary machining.

A barb from the blue or off the wall.
I've never used SP diamond tools on Al only tool steel and SS so I can't say this will work or not.
We had to finish turn 30" disks of 356 Al to the smoothest possible finish. We used a very fine single point tool bit. To make a long story short we tried every thing to improve the finish. From a local machine shop I picked up on a lubricant they used to machining AL. The lubricant was equal volumes of Lard (cooking) and kerosene. Using this mixture made an immediate quantum improvement in the surface finish. It is still being used today.

 

[1MadMax]

unclesyd - thanks for the reply. I reviewed each of the links you supplied earlier and on this 12" mirror 30-80 angstroms would be acceptable even without the post machining polishing. I need to make every effort to improve the machining process before I can justify an additional process. I was also thinking about the coolant as an opportunity for improvement. I have used mineral spirits mixed with tapping oil to greatly improve a boring finish (another single point tooling appli.)- the resulting smoke was likely toxic though!!
The Alcoa "tooling plate" is that not a cast aluminum? I am checking into 12"x12"x2"thk. piece of cold drawn 6061 T651 to see how much different it is from the current 6061 round stock that is extruded. I'm thinking the machining process might be a contributor than the material... your thoughts?
regards, Max

 

[sreid]

You say you maeasure irregularity using Wave Front. Dosen't this tend to show the surface "Figure" rather than the micro surface roughness?

 

[1MadMax]

sreid - I'm not sure about the term "figure"? The mirror is parabolic. The requirements I'm trying to meet are specified as "surface irregularity <0.7 Lambda" (this relates to control of the form)and "surface roughness <10nm RMS" (this relates to control of the surface finish) The later is the control we're trying to improve, that would be your "micro surface roughness" ...correct?

 

[unclesyd]

No, the Alcoa tooling plate is a wrought product from a state of the art rolling mill so states Alcoa.
I think the tooling plate is worth a try. I would try to get a couple of smaller pieces to establish a method as a 6" would have the same physical properties as a 12".
TCI at one time had circular drops at very good prices.

If you saw an improvement by using a cut tapping oil with your SP tooling I think using the Lard/kerosene would be beneficial. The smoke would be no worst than eating a biscuit by a kerosene lamp after a hurricane. As I posted this combination worked exceeding well with our SP tool.
Our tooling is essentially a pointed 1mm dia needle with a half diameter flat at the point.

You will notice that I like the lard/kerosene lubricant for Al.

 

[IRstuff]

Surface figure is the gross shape and the specific parabola or hyperbola coefficients.

Wavefront error is usually defined as the localized RMS error relative to the ideal surface and usually measures roughness.

In some cases, the measurements can get quite contorted. We had, once, a diamond turned mirror that resulted in "ruts" that each focused to a different point in space. No matter how you wanted to quantify it, it was BAD...

TTFN

FAQ731-376

 

[1MadMax]

unclesyd - thank you for clarifying the "tooling plate" material, I had in my head something like ALCA Plus tooling plate which is a cast aluminum. I've sent a RFQ to TCI for a chunk of round stock. I neglected to mention the earlier concoction was used to bore steel and tool steel with a s.p. carbide on a milling machine.
IRstuff - Sorry, I am not familiar with the "surface figure" terminology, thank you for providing the definition! Our results aren't nearly like your example (thankfully), we're getting a "hazing" when we image.

 

[unclesyd]

How close are you to your desired shape and finish?

Do you think you can polish out the haze producer?

You might want to contact someone like these people to get a perspective on an electropolish since you state that the 30-80 finish would work.

http://www.ep-systems.com/aboutme1.htm

Thinking again.
Here is a very good Al polish, Simichrome. I have always wanted to add something like a .5 micron friable diamond grit to this polish.

http://www.competitionchemicals.com/simichrome/simichro.html

 

[IRstuff]

Depending on what you mean by "haze", that could be an indication of surface roughness. Think of it as an ideal mirror, with little mirrors superimposed, but not always parallel to the ideal surface. In effect, it would be like my "rut" example, except that each little mirror is randomly oriented, resulting in a cloud of faint images surrounding the true image. But, that could also arise from an incorrectly turned surface figure, ala Hubble, when it was first launched.

If the former, then hand post-polish would be the way to go. There are supposedly diamond turners that can give you hand-polizhed surface quality, but I've not personally seen any. The ones I have seen required hand polishing.

A surface profilometer can give you a measure of the surface roughness directly, so it might be worthwhile getting that measurement done.

TTFN

FAQ731-376

 

[1MadMax]

You are both correct in suggesting polishing to improve the quality. However my goal is to get to the "30-80 angstrom" range by machining, we're currently measuring between 130-320. I'm chasing 2 parallel paths, material and machining both of which I believe are contributing to these results. The material for the 12" mirror is extruded, the 4" mirror is cold rolled. The small mirror exceeds requirements, any evidence to the extruded causing poorer results? The NASA link mentioned a preference for "forged" material?
thanks for your insight ... Max

 

[unclesyd]

One would normally start with some very clean material for a high quality forging. Another would be by orientating the forging you could get a favorable grain orientation for subsequent machining. With the right material and a good hard squeeze you will also get part equal to one that has been Hipped, no cracks or voids.

Here is another perspective on achieving a smooth surface on Al mirrors. The company mention in the paper doesn't have website yet.

http://www.techbriefs.com/content/view/2021/34/

http://optics.nasa.gov/tech_days/te...nd Materials Ultra Smooth Diamond Tooling.pdf

You state that the smaller mirror is okay while your larger one is giving problems in machining. This is kinda redundant but have you checked the rigidity of the you machine, tool holder, and chucking device. One thing we encountered quite a bit was that certain weights and speeds caused some vibration problems, small but troublesome still. This would be corrected by slight change in speed.

 

[1MadMax]

Set-up is critical for sure, the mirror has potential for some vibration due to the mounting design which has characteristics to eliminate localize stress at the mounting holes (pulling distortion on the mirror's face). We are using adhesives and packing to dampen any vibration. What speed correction are you thinking of? 10%-15% slower? The article is quite interesting, its a bit like burnishing I think?
Max

 

[unclesyd]

As I recall the speed change that quieted the vibration was in the order of 5-10% max.

One thing I forgot to mention was the tool stick-out changed when changing sizes of mirrors?

Does the problem with haze over the entire surface?

I've seen this cause problems with very small changes in stick-out.