Having been called out, I’ll try to describe my procedure as best I can. I slowly refined this technique and right now I’m getting fairly consistent results on my Nomad 3. But, to begin with, an error of 0.25mm is fairly small! I think I managed to get to around 0.1mm or slightly less (see pictures below), and I don’t think it’s possible to do much better, especially with wood.
Here’s how I approach two-sided machining. There are two CAM setups, but I zero the machine only once. I use the tiger claws and stops to hold my work, forming a square of sorts using gauge blocks. These are important! They help greatly with several things: aligning work, setting zero, and preventing the stops from biting into your work, which can ruin the positioning.
The first CAM setup machines the top side of the model, but leaves a frame around it. Two sides of that frame are important. These get a 12mm deep cut (12mm so that I clear the gauge blocks). The other two sides are left untouched, we don’t care about them. I also make sure the top gets surfaced.
After this step, we have the top side of our model and the frame. This is what it looks like (and note the position of the zero for the next setup!), the stock still hasn’t been flipped over:
The second setup has the zero positioned at the corner that we machined in step 1. The nice thing about this approach is that we don’t care about the stock, what we use for reference is a corner that we ourselves machined. So, we proceed with step 2 (this shows the stock already flipped).
I use Fusion 360, which makes this significantly easier. I model the thing itself and sometimes add tabs as well, but the frame is created in a “manufacturing model” (sometimes it’s easier to add tabs in the manufacturing model, too). This means I don’t have to redo everything from scratch if my model changes.
So, that’s the CAM part. Now let’s look at the physical side of things.
Here’s how this setup looks physically on my Nomad (I only have pictures taken at the end of step 2):
If you look closely, you will see that there is a “parting line” of sorts around the model, but it’s so tiny it’s almost negligible (also, two of my supports/tabs broke).
And here is a finished example in softwood:
I’m pretty happy with those results. But it took a number of tries and I had to eliminate some sources of errors.
On the CNC, you want to set the zero at the XY corner of your square with Z zero exactly at table height. And you want that zero to be very precise. This was my major problem, I couldn’t get the zero to be precise enough. I tried using the BitZero, placing it on a square forced against those gauge blocks:
That did not work too well. The XY error was significant. So I then tried setting the XY zero manually, by sliding out one of the gauge blocks, moving the probing pin so that the block couldn’t slide in, then moving the X until I could slide the block back in, and then setting the zero offsetting by 1.5875mm (half the diameter of the dowel pin) (repeat for Y). This worked very well for XY and reduced the error to almost nothing. I then tried measuring Z using the BitZero placed on the table, but again, the results were inconsistent. So I settled on zeroing Z using the same method: try to slide a block under the probing pin, if it doesn’t fit, lift the pin increasing Z, until it fits.
The fully manual zeroing method gives me good (and consistent) results. I’m not sure how precise the BitZero is intended to be, but it doesn’t work too well for me.
The other things I learned to watch out for: you need to clean the table before the flip, because any debris/leftovers will not allow your model to rest on the table and will increase your Z error. Also, machining those two reference sides of your frame is really critical, so use an endmill that is known to be precise (e.g. not Carbide #101) and go slow. Some of my failures were because the endmill diameter was not what I expected, and some were caused by going too fast, where deflection ruined my precision.
Overall, I’m pretty happy with the results I’m getting, but I feel there is still room for improvement. I really wish Nomad shipped with a better table: I’m losing not only Z, but also lots of XY space needlessly. The Nomad table could easily extend to the left and slightly towards the front, and have a way to set up a precise square corner exactly for this kind of work. Also, the sizes of the tiger claws mean that sometimes you end up with stock that you can’t clamp without using extra material, because you end up in between two clamping ranges.
The nice thing about this approach is that it lets you cut models of varying sizes (the flip jig restricts you in what you can machine).
I hope this helps!