It can sometimes be a lot easier to move the workpiece.  Since you are
not worried about the workpiece lifting like on a milling machine you
can have a table that rolls on a flat surface or uses two sets of rails
or drawer slides.  With a gantry table you are moving something heavy on
three axis.

Brandon Fosdick wrote:

>Alexandre Guimarces wrote:
>
>>Hi,
>>
>>>I've seen the discussions about moving arms / printer
>>>mechanisms for the CNC.
>>>But is it not easier to move your PCB on the X and Y
>>>axis and let the pick and place part just move X and Z
>>>axis to get parts ????
>>>
>>    I am unable to make a mental picture of that !!! Could you sketch it for
>>me ? The board moving X and Y is ok but where do you get your parts from ?
>>Would not you end up with 5 axis to control ?
>>
>
>I was getting ready to suggest the same thing. My lab has a CNC mill
>that basically does this. The head only moves in the Z (up,down)
>direction while the table, which is long and narrow, moves in the X and
>Y (horizontal) directions. The tool caddy is at one end of the workspace
>and can move independantly of the table. To change out a tool the
>part-in-progress is moved out of the way while the tool caddy moves into
>place to do the change out. You probably don't need the complexity of a
>free moving part caddy, but you could easily make the work space large
>enough to hold both the board and the parts pick-up area.
>
>I'm not sure if this method is really applicable to this case though.
>AFAIK mills do this because they need everything to be rock solid stable
>to get high enough tolerances. The tradeoff is that the moving
>components are much more massive (which is why they're so stable under
>milling loads) and therefore slower. For a mill, accuracy is more
>important than speed. A PnP machine doesnt need anywhere near the same
>level of accuracy but does need speed. I this case I think it really
>depends on what's less-massive, the workspace or the head.
>
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