Double sided milling with dowels for alignment

Looking to do some double sided milling and reading up on how to do it with good alignment. I think I have a handle on most of it but a little unsure on the origin point. My plan is to drill part way through for the location holes on the top (not drilling through the part and into the fixture). Then finish the top milling. Then mill the holes in the fixture piece, install the dowels and flip the part onto the dowels. I could take a break between the first side and milling the holes in the fixture without loosing accurate location of the pins which is a plus.

My main question is what to use for the origin. I know it needs to have the same x and y for both sides but Z may change. Lots of examples use the center of the part. Couldn't I use a corner or an end pin location? I think this should work but may not be as convenient. I am used to using a corner for the origin but maybe for this the center would work better. What are the trade offs?

Same plan should work for all materials but if it matters the final items will be steel.

If you are going to use dowels, why not use one of the dowels as the 0,0. It really doesn't matter where the 0,0 is as long as you and the machine knows where it is.

Another trick I use is to pocket out a piece of MDF, with the profile of the finished side, to hold the part after machining the first side. I normally go about 1/4'' deep. This way you know exactly where the 0,0 is. MDF is OK with coolant for 1 or 2 parts.

Here is an example
First side finished
https://www.cnczone.com/forums/attac...d=466786&stc=1

And fixture for the other side, 0,0 is the bottom left corner. You don't have to worry about the 0,0, it can be anywhere you choose as long as the CAM setup and the machine are in agreement.

https://www.cnczone.com/forums/attac...d=466792&stc=1

And the part in place
https://www.cnczone.com/forums/attac...d=466794&stc=1

And finished

https://www.cnczone.com/forums/attac...d=466796&stc=1

Thanks Jim,

I am doing some drawing and punching dies so I think dowels into steel or aluminum will give the needed precision better than MDF although I can see where MDF would be great for a couple of parts of slightly less precision. A dowel location as zero would work well.

When I'm doing this, I typically set the Y zero origin at the center, and set the dowel pins in the center as well, which gives me a zero point in the middle of the dowel as Jim recommends. Z is set in the middle in the CAD/CAM to avoid a discrepancy when flipping, but I touch off to the top of the stock and set the Z at the tool tip to half the stock thickness.