Machining finish question
Machining finish question
(OP)
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?





RE: Machining finish question
RE: Machining finish question
RE: Machining finish question
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?
RE: Machining finish question
RE: Machining finish question
RE: Machining finish question
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.
RE: Machining finish question
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.
RE: Machining finish question
RE: Machining finish question
RE: Machining finish question
http://www.coloradoprecision.com/
RE: Machining finish question
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!
RE: Machining finish question
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?
RE: Machining finish question
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.