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Author Topic: Projecting to plane?  (Read 8155 times)

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Offline Keith

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Projecting to plane?
« on: January 07, 2006, 10:33:12 AM »
Hi,any hints/short cuts on milling a 2 D carving onto a spherical surface without too much distortion and without too much depth variance(the sphere is hollow)?I have an image I'm rasterizing and would like to just make that impression(negative) on a small area of a plastic sphere.The sphere is about 4" diameter and the area with the image is negotiable but around 2", centered. I actually once had this done many years ago before I did any milling and I believe the guy that did it for me actually calculated the different heights and offset Z with these numbers but there's got to be a better way(curve interpolation?).Thanks,Keith

Vmax

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Re: Projecting to plane?
« Reply #1 on: January 07, 2006, 01:14:15 PM »
HI keith If you go to the yahoo mach support group and look up starting at
Message 43730 that issue is discussed.  Thanks Terry

Offline Keith

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Re: Projecting to plane?
« Reply #2 on: January 07, 2006, 01:52:08 PM »
Well yes that seems pretty straight forward and I'm familiar with axis swapping to accomplish this but my brain is having a hard time  extropolating this idea to a sphere(the 'message you mention deals with a cylinder).Thanks Terry,maybe I'll have an 'Eureka' moment after reading those posts ,over.-Keith

Vmax

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Re: Projecting to plane?
« Reply #3 on: January 07, 2006, 01:57:07 PM »
Keith what your are wanting to do is reverse ortho rectification from a flat plane to a sphere. I don't know the answer but hopefully the discussions will get you going in the right direction.   (:~)= Terry

Offline Graham Waterworth

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Re: Projecting to plane?
« Reply #4 on: January 08, 2006, 06:11:56 AM »
Hi Keith,

to do this properly you need a 5 axis machine,  if you try it on a 3 or 4 axis machine you will get overcutting as you move down the sphere.  The correct way is to keep the cutting edge at 90 degrees to the work surface at all times.  The nearest you can get with 3/4 axis is to use a lollypop shaped tool so the back edge of the tool is not cutting into the work surface.

Most CAM systems will wrap a design round a ball using surface project, to get a negative you pocket out the background.

Hope this helps

Graham Waterworth.
Without engineers the world stops

Offline Keith

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Re: Projecting to plane?
« Reply #5 on: January 08, 2006, 08:38:44 AM »
Thanks Graham,I wasn't thinking so much about the overcutting so much since I was going to use the tiniest of tools and the image I want is very 'high contrast' so I'm really looking more for what on paper would look either black or white/on or off,all cuts being the same depth, but I can imagine this still to be a concern.What I was concerned with is doing it without a CAM.I was pretty sure a CAM package could do it.I was more wondering about the feasibility of roughly mapping the Z offset of a sphere since the image would easily be rasterized.I'm going to contact the guy who once did this for me,maybe I'm wrong but this guy was so 'old school' ,I don't even think he ever used a CAM package.Thanks,Keith

Vmax

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Re: Projecting to plane?
« Reply #6 on: January 08, 2006, 09:50:52 AM »
Keith I have seen it done with 5 axis many times. I also have seen it done with 4 axis. Here is a thought.

If you use a 4axis setup with A:  being polar and angle A: to 1/2 the Rad off of vertical. That will lessen the correction needed, then use a cone shape section of your picture to cut as a linear cut around the sphere, it would be fairly close. Not perfect but close. It will depend on the thickness of the sphere surface. Do you have a 4 axis or 3 axis machine.

There are cam packages out there that can do the sphere cut with no problem but they are a little pricy. What you might want to try is a demo package that will write a limited GCODE. Try BITCAM.COM

I'll ask my son they work in 5 axis 4d all the time.

Might think of using a spiralled helix cut as a cone if you only have 3 axis. Or just bite the bullet and sit down and map out the Z corrections from a flat plane as your friend did.

Please let us know how it ends up, Interesting project.


(:~)= Terry

Offline Keith

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Re: Projecting to plane?
« Reply #7 on: January 08, 2006, 09:56:33 AM »
Oh right,you brought me a little closer with that one...think polar.
I have a 4-axis.I'm going to chew on this for a bit as a back of the brain distraction.When I do something mentionable,I will.Also,if that guy tells me what he did to achieve it,that did it for me in the past,I'll pass it on to the group.Thanks for your thoughts Terry.-Keith

Vmax

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Re: Projecting to plane?
« Reply #8 on: January 08, 2006, 10:48:04 AM »
Keith do yo have a digitizing probe???? and an example of the object. Nah that would be too easy.  (:~)= Terry

Offline Graham Waterworth

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Re: Projecting to plane?
« Reply #9 on: January 08, 2006, 10:50:52 AM »
Hi Keith,

the way we did it before CAM systems was to work in concentric circles from the centre of the logo/image on a flat drawing and machine one circle at a time working out the drop in Z on the sphere at that diameter, then replace all the Zs in the code with the original Z+the drop.

Hope that makes sense.

Graham
Without engineers the world stops