I recently challenged myself with a project that required milling on two sides of the parts in the design. Doing this requires removing the stock, in this case a mahogany board, and flipping it over so the mill can reach the back side. The difficulty is in maintaining a precise known position relative to the side that has already been milled. While there are a number of approaches to doing this I'll be showing a method utilizing two dowels for X-Y positioning and a facing job to establish a precise stock Z size.
I also made the decision to avoid tabs or other work holding features that would require sanding to remove later. While I will mention this in the post please feel free to ignore it if you just want to know about two sided CAM. You will need to have some method of holding and you can design small cylinders (or other small objects) that intersect your stock and part or use tabs generated from contour passes by controlling the cut depth.
The dowels I used were simply cut from some aluminum rod. In the future I'll order a proper precision set but these worked OK for my purposes here.
Here is the setup for the first set of cuts on the bottom side.
The inner hole of the gray cylinders is sized for a tight slip fit on the dowels. The location of these cylinders sets the dowel locations relative to the other parts. Try to position the dowels as far apart as is reasonable for best accuracy.
When deciding which side to start with consider that you will likely only be cutting all the way through on the second job. I think it's best to do the most aggressive or detailed work first but this is something that will change from situation to situation.
The Z zero position is set on the surface of the spoil board (bottom of the stock). The models are all aligned so their flat surfaces are in plane and located to the top of the stock to assist with holding later but this is not required and the parts could be centered in Z as well. It is however important that they are centered in the X and Y stock positions.
The stock itself is a 3/4" mahogany board. It's actual average thickness is about 19.4mm but I set the stock thickness in the CAM setup to 18.75mm. When I run those initial facing operations it will be milled down to a flat surface 18.75mm above the spoil board. Since the spoil board is our zero point this gives us our precise Z positioning for both sides. I added a couple of centimeters over the setup Y length to the actual stock size to allow room for the screws I'll be using to hold down the board. These areas won't be touched by the surfacing operations and will need to be removed once this side is done.
There are a lot of operations in this setup but the only critical aspect to them is to leave enough stock behind to avoid completely cutting out any of the pieces. For me I raised the bottom height to 3mm above the stock bottom but you should consider your own models and think about how you will be milling them from the reverse side when deciding how much material to leave behind to keep them in position. I don't like sanding things and I'm not a production shop where time is critical so I tend to program long finish passes at tiny step overs to leave the best possible surface quality.
Here are the results of that first run:
Notice that nothing is cut all the way through and the dowel positions have been milled out. You can also see that there are raised areas between the screws where the stock is still at it's original height. When I removed this piece from the table I cut those extra raised areas off leaving only the flat surface behind.
Here is the cam setup for the other "Top" side:
Most importantly the stock size is exactly the same as the other setup. Next is that the model position is the same or centered in X & Y. Since I used the top of the stock as the model position in the first job I now need to set it to bottom to keep the same relative Z position in the material. If you are not depending on the model faces being flush with the stock you can use the centered Z position for both passes.
The next thing to do is mill out the dowel positions in the spoil board. This is the ONLY operation that cuts below Z zero. You can choose any starting X&Y zero you like as long as things will fit. Once set all other cuts on this side will be relative to this point.
With the pins in place relative to a known zero position I can now place my original work-piece with the previously milled side down.
At this point I have the stock located relative to the same X&Y zero point the dowels used and it has been milled to a precise Z height relative to the spoil board. I will use this zero position for all remaining passes.
I mentioned that I was avoiding the use of tabs in this job. I wanted to use small screws to hold the parts in place while they were being cut out of the stock. To do this I added some holes for screws in strategic places and made sure that all the flat sides were aligned with the bottom of the stock. Before putting those screws in place, however, I needed to run all passes that would otherwise collide with those screw positions.
Here those first cuts are complete and the hold down screws are in position:
Now I know that when I run the remaining passes and cut those parts free the screws will keep them in place.
Here are those parts fresh off the machine with no sanding or finish work:
I hope this helps other people who are trying to understand how to approach two sided machining. Please feel free to like or share, and post any questions you may have in the comments.