Machining finish question

[rorschach]

Using a good quality CNC lathe machining a paraboloid, (section of a parablolic arc) how fine a finish could be typically be obtained? Would say a 3 microinch RMS finish be out of the question?

 

[sreid]

Out of the question. 3 micro inch is a borderline optical surface. You might be able to do it by diamond turning on a high precision lathe.

 

[rorschach]

Thanks for the reply, just out of curiosity, how good a finish could you get?

 

[tripleZ]

What material and what hardness? Finally, what's the profile tolerance?

With a fresh set of inserts, a good series of roughing profiles followed by the right series of finish passes, maybe a 10-12? With the softer stuff, maybe smooth it out with an abrasive cloth followed by some scotch-brite to blend it in (by hand on the last stuff). A combo of the right honing stones & oil will work in a similar fashion. All depends on how much time/effort/skill you want to put into it.

Do you need to do multiples, or is this small batch work?

 

[tripleZ]

On second thought, 10-12 is probably too fine. Maybe 16-20. The proper stoning technique will get you closer to the 10-12 finish.

 

[rorschach]

the material is a controlled expansion alloy (iron/nickel). Ihe goal is to get as close to an optical quality concave paraboloid, (approximately 1-3 uIn RMS final finish), before polishing, preferably in the same chucking. The intent is to start with smaller batches but building to assembly line work as demand grows. if specialized hardware is required to achieve that, I'd like to know what it is for project planning.

 

[BillPSU]

Precision air spindle lathe with temperature controlled environment. Ultra precision radius diamond turning tools. CNC control with .000001 stepping motors. Vibration isolation. Such equipment exists in military supplier plants.
You not only want to control the finish but also the spherical shape of the mirror.

Finish is controlled by the dynamic motion between cutting tool to the turning workpiece.

 

[Chappy2]

I've had excellent results using Roller burnishing on lathes as well as machining centers.
There are several manufacturers of this type of tooling.
Cogsdill comes to mind.
Google them , they have an informative site.
You will have to finesse speeds and feeds.

 

[rorschach]

If I can get the finish down around 4-6 uIn, then electropolishing should get me the rest of the way.

 

[sreid]

One cannot diamond turn iron or steel. But you can diamond turn a nickel plating.

 

[sreid]

You might want to contact Colorado Precision.

http://www.coloradoprecision.com/

 

[tripleZ]

50-50 for the Fe-Ni combo? We mold, sinter, and coin/forge such material. My experience with it is that it's really gummy, meaning you're probably not going to like the chip you're throwing. The roller burnishing idea might be something to try on some prototypes. I personally haven't ever tried it. It might enable you to smear the material some to give it an acceptable surface finish, at which point you can polish.

Cu-W electrodes were like this. We had to play with special coolants and high speeds, coupled with fine steps/cuts, to get the surface finish just right. And it was hell on cutters!

We've occasionally electropolished parts for appearance and it hasn't been cheap (compared to other methods). Depending on the complexity/tolerancing of the form, Extrude Hone Corp. in Irwin, PA, makes some interesting machines which use media polishing. Although Fe-Ni may not be a great material to consider. In any case, it's a potential pursuit. Sounds like a fun project!

 

[rorschach]

tripleZ, I suspected it would be pretty gummy, the nickel alloys I've worked with before aren't exactly a machinist's friend. But I've never worked with any of the alloys in the Ivar/Kovar family before so I'm not sure if the much higher iron content changes much.

sreid, I know this is going to be a stupid question and I think I might even know the answer, but why can't you diamond turn iron/steel? Does the diamond try to alloy with the iron or something at the extreme temp/pressures involved? what about CZ? could it be used in it's place? or does it not have the same lubricity?

 

[sreid]

rorschach,

Yes, the carbon wants to diffuse into the steel resulting in extremely fast tool wear. Other material for the cutting tool can be used but these are not typically available lapped to the precision needed for turning to optical accuracy.