surface milling and the arduous process
before you attempt surface milling, much less 2 sided surface milling, you should be aware of the process and what you are getting yourself into. There are several steps in the process:
stock (laminating mdf) back to top
knowing that i wanted to explore 2 sided surface milling, i knew i was going to need material thicker than provided. So i made several sets of laminated mdf to use as stock. Mdf is a good material for testing the process; it's not so great at looking good as a final product.
modelling back to top
there are two main factors that you should keep in mind when you are creating your model:
when doing 2 sided milling there will be a point where the two mills meet. This area will always have some amount of imperfection. no matter how hard you try, you should always factor this in. the easiest way of doing this is by planning a vertical edge tangency.
doing a 2 sided mill is, to some extent, allowing you to do under cuts that a 3 axis machine cannot do otherwise. there are significant limitation still the what can be done. undercuts are areas where a vertical bit cannot cut because there is material above it that cannot be remove because it is part of the final product. undercuts should be avoided unless you have a way to reach that area when cutting on the opposite side.
tool selection back to top
tool selection and modeling is a "chicken or the egg" scenario. You cannot fully complete one without know the other. This inherently becomes a back and forth process that, over time, one can gain an intuitive sense for. But until that time here are a few pointers to help understand the decisions that can be affected by the tools chosen.
first the amount of milling time is directly proportional to the tool diameter. the wider the tool the more material can be removed at a time. So why not use the widest tool every time? big tools are bulky and cannot fit in tight areas or provide high levels of detail (depending on your surface type). this is where modeling and tool selection meet. there are two primary types of milling tools.
with hundreds of others that can do specific tasks. if your surfaces are flat, then obviously a flat end mill will do everything you need. but if you have 3 dimensional surfaces then you will ultimately use a ball end mill to finish the surface.
tool paths back to top
it gets even more complicated still. its more of a "chicken or the egg or the ...something else that came before the chicken but not the egg" scenario. in order to select your tools you have to keep in mind several things.
before you can get the surface you have designed, it is important that you carve away to bulk excess material first using a larger bit. this saves time and life of finer, more delicate bits. you most also consider that when you do your finishing cuts the bit should not cut very deep at all. you risk either breaking the bit or scarring the surface of the material. so it is important to plan out your roughing paths such that what ever your final cutting tool is, you are only finishing the surface, not removing excess material.
the other important part that i haven't mentioned but is crucial is the program you create your tool paths in. I am using Rhinocam, a plugin for rhino, but there are several to chose from and it is important to make sure the which ever one you choose it can post process the tool paths into a language your CNC will understand.
jig back to top
this is a very crucial part of the process. if you are going to do a 2 sided mill, you need a mechanism to allow for the maximum amount of alignment after flipping the piece. dimensions before incredibly important. for every fraction you are off when you flip the piece to do the 2nd mill you have doubled your error. so what seems like a small amount of play can actually ruin the piece.
there are any number of methods for creating a jig. the method i used in this process was to use a rectangular stock and create a frame the it fit into snugly. this way when i flipped the piece i could put it back into the jig and know that my 'x' and 'y' zeros are as accurate as possible. this worked because i did not remove a large amount of the top surface in the 1st mill leaving ample surface area for the stock to rest on.
touch-up back to top
now if everything went as planned in the milling process, you will probably need to do a little clean up. depending on the material it may require a little sanding or filing to make the edge where the two mills meet look as good as possible.