I’m trying to understand two sided milling from ‘first principles’ before I start experimenting. To that end, I have watched tons of youtube videos, and read all the forum posts I can find.
As near as I can figure, the whole point is to make sure the machine knows precisely where the workpiece is for milling operations on both sides. For side one this is not usually an issue, you set your zero and go about your business as if you were only milling one side.
Side two is where things get tricky. The orientation of the workpiece relative to the machine and the first side operations must somehow me indicated as accurately as possible. It seems that holes with alignment pins are the most common method to do this, as they are milled into the workpiece and the spoilboard (or fixture) before flipping, so the position is ‘known’ after flipping.
OK, fine. Then I came across the following videos:
And:
So it seems that both of these guys are using methods that ensure alignment with fixtures external to the workpiece, no holes or alignment pins in the workpiece itself are needed. Is there a catch I am not understanding? I guess the software used to design the toolpaths needs to support different zero points for the different sides? Are these methods somehow less accurate?
I know this question have been asked before, but it still hasn’t ‘clicked’ with me yet.
I know you deleted your post about sizing your workpiece, but I wanted to respond with a question, because I think that the Corbin Dunn video I linked above kind of addresses that in a sneaky way?
It seems that while cutting the first side he also cuts the piece to a very precise width with a profile cut, and then indexes off that on the flip?
The theory being that the CNC can cut sizes more accurately than any stable saw, jointer, planer, etc.?
The method Corbin shows is solid, and it’s one that we’ve used a lot. Once you flip, you cannot assume the factory edges of the stock are flat or parallel to anything. So, you recut the perimeter and the top surface as part of OP 1, and you’ve got perfect edges once you flip the part.
Ohhhhh, that makes sense. I thought he was making that cut for width, but he was making it to be true parallel to the original side against the dogs in the spoilboard, which he knew to be square to the X axis?
That’s exactly correct, for the times I’ve done it. You also need to resurface the top, so that there’s no “tilt” to the stock once you flip the part over.
When I’ve done this and needed everything to be absolutely perfect, I’ve put down a square of MDF after OP 1 and then cut an L into it, so I can fit the flipped part into that corner at a known location on the machine, with perfect alignment (nothing is “perfect”, but it’s close).
I have an MDF L-frame permanently anchored to the front left corner of my spoil board, and I do about 90% of my work using it as my main fixture. When I’m running a two-sided project—like a picture frame—I always start by cutting the back side first. That lets me carve out the recesses for the glass, photo, and backing. Once I’ve got that dialed in to the exact glass size, I use a separate front-side file in Carbide Create Pro. From there, I just select “mirror horizontal” to match the front layout perfectly, flipped for proper orientation. When the back side is done, I flip the piece, lock it back into the L-frame, and run the front. Simple as that—easy peasy, lemon squeezy.
In the video he says that the origin is at a slightly different location presumably because of the insets he does. For me that would be an issue because I need to make a mirror image of the first side of the object. I need the second side to be exactly the same. Am I misunderstanding something? (PS: using Fusion 360)