Wiki glossary 2D/2.5D/3D

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Wiki glossary 2D/2.5D/3D

Postby WillAdams » Tue Oct 13, 2015 4:28 pm

added entries for this:

==#==

;2D
:(in terms of CAM) Cuts are all made to the same depth along a single plane (typically XY).[https://www.reddit.com/r/CNC/comments/3oj8id/difference_between_25d_and_3d/]

;2.5D
:(in terms of CAM) Cuts are all made in planes parallel to one another (typically XY), but may be made to different depths. Usually no more than two axes will move at once.[https://www.reddit.com/r/CNC/comments/3oj8id/difference_between_25d_and_3d/]

;3D
:(in terms of CAM) Cuts can be made so as to describe an arbitrary 3D shape (typically w/o overhangs), and the cutter will need to move in all possible planes so as to make a smooth finish. On a 3-axis machines overhangs or carving fully in the round will require either special tooling or flip jigs (or both).[https://www.reddit.com/r/CNC/comments/3oj8id/difference_between_25d_and_3d/]

Seem okay?
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Re: Wiki glossary 2D/2.5D/3D

Postby twforeman » Tue Oct 13, 2015 5:25 pm

Sounds okay to me.

You could add that laser cutters are (usually) 2D.

Most CNC mills are 2.5D at a minimum.
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Re: Wiki glossary 2D/2.5D/3D

Postby WillAdams » Tue Oct 13, 2015 5:31 pm

Yes, but if one made nothing but through cuts on a particular job, that would be described as being 2D, right?
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Re: Wiki glossary 2D/2.5D/3D

Postby TomDChi » Tue Oct 13, 2015 7:14 pm

A few months ago, when I was 100% new to all this, I found "2.5d" to be a pretty confusing concept in particular, and "which machines can do 2d vs 3d" unclear in general. I think that your description is technically accurate, and that some people who are new to CNC and looking for an explanation will understand immediately. I'm pretty sure though, that without more explanation (and probably illustrations) me of a few months ago would not have fully grasped the concept from that text alone.

I'd suggest fleshing it out a bit more:

2d: As Tim commented, laser cutting is an example of 2d, as is CNC plasma and waterjet cutting. Theoretically, you could have a CNC milling system that was purely 2d (mill plows into the workpiece from the side for every cut - don't know if that exists in reality) or is effectively 2d - all cuts start with the mill plunging to a fixed depth, then cutting the path and retracting.

2.5d: All the milling/cutting is done in a series of horizontal paths on different planes. Once the end mill plunges vertically down to a particular level, the cutting path moves in only X and Y, a "2d" operation, but combining several such 2d operations on different levels, you can machine a more complex part. (I'm trying to think of how to put this in the least "mumbo jumbo" wording possible to make it as clear as possible to someone who is new to CNC.)

I think the key idea with 2.5d is that it's lots of (or a few) horizontal 2d paths, which combine in sequence to leave a potentially complex resulting part, but with some limitations such as (not sure how to word it)

I think that for noobs, it would be ideal to explain that 2.5d vs 3d cutting is generally more of a limitation of the software generating the g code, and that many "modern" controllers (such as those running grbl) can drive the CNC mill to physically do 3d cutting - it isn't a limitation of the hardware, typically.

In part, you're answering the noob's questions such as "why isn't everything 3d?" and "why do I care? What can't I do with only 2.5d?"

Then to really flesh it out, this topic blends right into what a 3d/3axis system can and can't do, versus 4 axis and 5 axis, but that's a whole additional can of worms, and would ideally have a lot of illustration/photos/video to explain it clearly.
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Re: Wiki glossary 2D/2.5D/3D

Postby AnonymousPerson » Tue Oct 13, 2015 8:05 pm

Out of curiosity, what does 6-axis give over 5-axis?

A 4th axis commonly seems to mean rotation of the workpiece around (say) the X axis. 5th axis seems to be rotation around either Y or Z. Trying to think where the 6th axis would be useful though, and I'm coming up blank...

Any ideas? :)
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Re: Wiki glossary 2D/2.5D/3D

Postby cvoinescu » Tue Oct 13, 2015 8:07 pm

Engraving to a fixed depth is also a 2D operation: it doesn't have to be all the way through to be 2D.

The way I see it, if all cuts are to the same depth (including all the way through), that's 2D. A 2D part can be described by one 2D drawing.

If the part has elements at various depths, but each element has a constant depth (either cut all the way through, or cut with a flat bottom), that's 2.5D. A 2.5D part can be described by a small number of 2D drawings, one for each depth, or a single 2D drawing plus depth information for each element. In other words, a finite number of 2D operations, each at a different depth, make a 2.5D part.

2D and 2.5D do not necessarily mean the tool moves only in the X-Y plane; they just mean the end result has a constant depth. For instance, a hole with a flat bottom can be milled with a helical movement involving all three axes, followed by a circle to finish cutting the bottom. Profiles often use ramp-ins, even in 2D cuts.

Another way of looking at it: parts without overhangs can be described by 2D images with depth encoded as the grey level. 2D milling needs a black and white image, 2.5D milling needs an indexed image with a small palette, and 3D milling needs a greyscale image with continuous variation.
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Re: Wiki glossary 2D/2.5D/3D

Postby WillAdams » Tue Oct 13, 2015 8:34 pm

Thanks!

Tweaked it a bit and we now have:

==#==

;2D
:(in terms of CAM) Cuts are all made to the same depth along a single plane (typically XY) and may be described by a single monochrome (b/w) 2D drawing.[http://www.shapeoko.com/forum/viewtopic.php?f=17&t=7007&p=55173#p55173] Plasma cutters and lasercutters which are not doing engraving are limited to 2D, as are engraving machines which cut to only a single depth. The simplest form of CAM, any program should be able to create such a file.

;2.5D
:(in terms of CAM) Cuts are all made in planes parallel to one another (typically XY), but may be made to different depths, any given area will have a flat bottom and the appearance will be like to a ziggurat or stepped pyramid, and may be described as a series of drawings each depicting a different depth or layer (a series of onionskin drawings each stacked on top of the other). Usually no more than two axes will move at once.[http://www.shapeoko.com/forum/viewtopic.php?f=17&t=7007&p=55173#p55168] Most programs are able to create files for cutting such shapes, usually by allowing one to define a given region, shape or path, and assigning to it a cut to a particular depth.

;3D
:(in terms of CAM) Cuts can be made so as to describe an arbitrary 3D shape (typically w/o overhangs), and the cutter will need to move in all possible planes so as to make a smooth finish. On a 3-axis machines overhangs or carving fully in the round will require either special tooling or flip jigs (or both). When described as a drawing, a full 3D design would require a continuous variation in darkness, like to a photograph.[https://www.reddit.com/r/CNC/comments/3oj8id/difference_between_25d_and_3d/][http://www.shapeoko.com/forum/viewtopic.php?f=17&t=7007&p=55173#p55173] Typically the domain of specialty CAD and CAM programs, they require the depiction of a form in full 3D, usually using a 3D-specific file format such as .stl.
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