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CNC Mirror figuring

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#1 mconnelley

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Posted 14 January 2018 - 02:34 AM

Hello:

  

  I realized today that I think I have the parts that I need to try CNC mirror figuring.  I made a Bath interferometer, so I can generate an error map of the mirror.  At work here we have a Hurco CNC mill.  The challenge is to program the CNC mill based on the error map.  I've heard of G-code, which we've used to determine the path that a 3D printer should take.  The Hurco has software that will determine the path of the tool based on the part design.  So we have software that is close to what I want, but not exactly.  Rather than control the Z-axis of the mill, we need to tell the mill to pass over a high spot more often.  It would also be good to incorporate the profile of the polishing tool.  I imagine using a petal shaped sub-diameter tool to smooth out the edges of tool's path.  

 

   Has anyone here tried to do this?  Is there software that exists that we can use to convert the error map to program the CNC mill?

Cheers

Mike



#2 BGRE

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Posted 14 January 2018 - 02:41 AM

Hello:
  
  I realized today that I think I have the parts that I need to try CNC mirror figuring.  I made a Bath interferometer, so I can generate an error map of the mirror.  At work here we have a Hurco CNC mill.  The challenge is to program the CNC mill based on the error map.  I've heard of G-code, which we've used to determine the path that a 3D printer should take.  The Hurco has software that will determine the path of the tool based on the part design.  So we have software that is close to what I want, but not exactly.  Rather than control the Z-axis of the mill, we need to tell the mill to pass over a high spot more often.  It would also be good to incorporate the profile of the polishing tool.  I imagine using a petal shaped sub-diameter tool to smooth out the edges of tool's path.  
 
   Has anyone here tried to do this?  Is there software that exists that we can use to convert the error map to program the CNC mill?
Cheers
Mike

\have a look at the posting on fluid jet polishing its much simpler and more predictable.

#3 BGRE

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Posted 14 January 2018 - 03:52 AM

For a small tool its best that the wear profile on the workpiece is gaussian or close too it.
This was worked out some decades ago together with the software that computes the tool dwell profile.
A fluid jet polishing can achieve this far more simply than a conventional small polishing tool. Fluid pressure is quite low at around 5 bar.

#4 synfinatic

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Posted 14 January 2018 - 02:09 PM

Just a FYI, but G-Code for 3d printing isn't the same thing G-Code as CNC mills/lathes.  Basically the exact same command means different things which can have very unpredictable and dangerous results if you don't know the differences.

 

That said, never heard of any software which does exactly what you want.  The Hurco software you're talking about is what is know as "CAM" software.  You've probably heard of "CAD" software (computer aided design which is used to design parts), well CAM takes those designs and generates tool paths for the mill or lathe.  Sorta like what a "slicer" does for 3d printers.

 

Anyways, g-code isn't too hard to learn, it's just not very intuitive.  So you in theory could manually program the mill yourself without relying in special CAM software to do it.



#5 laixiaolue

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Posted 14 January 2018 - 06:18 PM

I really don't think a CNC mill has enough accuracy/resolution for mirror figuring purpose. You will end up ruin your mirror. Forget about it.



#6 BGRE

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Posted 14 January 2018 - 06:54 PM

I really don't think a CNC mill has enough accuracy/resolution for mirror figuring purpose. You will end up ruin your mirror. Forget about it.

Really? without any calculations???

In fact it will be fine as long as the tool isn't too small.
Its not a machining process where the surface accuracy is determined in part by the vertical setting error of the machine.
The amount removed is determined by the pressure of the fluid or the force on the tool and the dwell time over each 'point" on the surface.
A transverse positioning error of 10 microns or so depending on tool size is perfectly adequate.

When figuring by hand one doesnt achieve lateral positioning errors of anywhere near 10 microns yet done properly the process works well.

The CNC isnt being used for SPDT to generate a nanometer accurate surface directly rather a lapping process is being used to remove the high spots.
Neither Zeiss (NTT), nor REOSC (VLT), nor tne Finns (NOT), found it necessray when figuring to achieve nanometer accuracy in lateral positioning of the tools used.

#7 steveastrouk

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Posted 14 January 2018 - 07:59 PM

G code is what CNC machines use. The Hurco ALSO has an excellent "conversational" programming mode which allows you to directly input a model and mill it, but underlying it is G code. As has been noted, there are flavours of G code for different types of machine, but G is actually failrly standard.

 

If you really understand G code FOR THE HURCO, you could actually write the "program" yourself. Far eaier these days is to use a CAM program to generate the G code for you, but CAM programs also take a lot of time to learn to use well. There is an inexpensive CAM tool called CAMBAM , which has a very active and knowledgeable user forum.



#8 BGRE

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Posted 14 January 2018 - 08:10 PM

One question is how to translate from the error map to a dwell time map and thence to G code?
Except for a simple semi regular form error map doing this manually is impractical.


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