How to Set up Supports for 3D Printing.
In order to get good results, and properly scaled supports, you need to start by using the ‘Edit -> Transform‘ and ‘Analysis -> Units/Scale‘ tools to get the print properly oriented on the bed, and scaled to the finished print size. If you don’t do this, and instead scale the print afterwards in your slicer, the support pillars and particularly the tips of those pillars won’t be the right size, and may not print well, or be too small to offer effective support – or too large to remove easily.
When you first go in to the Analysis -> Overhangs mode in MeshMixer, you can adjust the overhang angle on a slider, and it will highlight the critical overhangs in red. Set the Overhangs to the following settings:
Millimeter to Inch conversion Calculator: https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=millimeters%20to%20inches
Equally, I find I get best results if I make the support pillars quite thick, and not too far from vertical. You want the supports to print cleanly, and not fail – because if they break or come loose, not only will your main print not have the support it needs, but also you will end up printing the support structure itself into the air, possibly for many layers, and that can result in a tangled mess getting dragged around and potentially stuck to your print. So, I generally chose a 3mm diameter for the support structures, and don’t let the pillars lean less than 60° from horizontal. I have the pillars start with a 6mm diameter base, and taper to a point that is just 1mm across to provide the anchor point for the supported part of the print. I also prefer to lower the density setting of the posts and the structs that support and tie the posts together, so that the program generates a basic essential set of supports
Once you’re done tweaking the support structure, you can save the project in a native ‘.mix’ format file that will let you reopen it later, and make further tweaks, or indeed totally remove the support and start over. You can also save a combined mesh file that includes the original object and all the support struts, in several different formats, including .obj, .ply, .dae, .amf and, of course, as an STL file.
Here is the full article if you want more information: http://www.extrudable.me/2013/12/28/meshmixer-2-0-best-newcomer-in-a-supporting-role/