Hi TK, and zsas,( and AGX who asked similar by pm)
Thanks for the queries.
I see there are more DIY projects of LF in wood. MF in metal is not so common and requires more accuracy.

Lathe is a South Bend model 9A which was delivered to its original owner in Detroit in 1939.
I acquired it in 2011 as second owner with a comprehensive set of tooling and attachments.
That included a small machinist's surface plate which is ideal for camera measurements.
The lathe was used in the original owner's residence as a hobby lathe and it is in excellent condition.
On it I have made, from solid Al 6031 billet, lens adaptors from Micro 4/3 up to the Pentax 6X7 bayonet and threaded adaptors too.

I have just purchased (from Australia, for reasons rather too lengthy to explain here) a compound gear set to allow machining of metric camera threads.
The Leica M39 mount is actually a 39mm Whitworth thread with 26 Turns Per Inch , so I purchased a set or Whitworth threading tools from UK.

The "elementary" mill drill is a work in progress.
It is based on a Delta Shopmaster DP200 pedestal drill purchased new in about 2005.
I recently added a generic cast iron "Made In China" XY table to convert the drill for limited milling duty.
There are issues in doing such a conversion, including safety, which are documented in the machinists' fora (forums) perhaps with varying degrees of veracity.
I have made some weldments (in mild steel by oxy acetylene bronze welding) to bolster the rigidity of the thing and just today (when I got tired of yard work) I spent a couple of hours on what I hope is the final weldment to make the head much more rigid. I reckon I have a few more sessions to go on that project.

I am aiming for a consistent TIR of 0.05 mm (0.002 inch) on say, a 6X9 frame in aluminum.
I am using 4 fluted spiral side/end milling cutters in range 4 ~ 7mm, dry on Al.

To answer your question about the back more specifically:
The Graflex backs here are an RH/8 and a Singer RH/10. Both at least 30 years old and well used.
Both have the pin rollers that are documented as necessary for the modern 120 film (I have no experience on this)

I measured carefully and decided that the distance from the emulsion side of the film (contacting the pin rollers) to the contacting front of the back
was 5.080 mm , 0.200 inch, considering wear.
here are my actual measurements in the 4 corners [inch]:
RH/8 0.180, 0.190, 0.202, 0.200
RH/10 0.175, 0.191, 0.179, 0.180
I assumed that the RH/10 was a bit more worn than the RH/8, also my calipers pull the pin rollers to the front of their spindles which the film may not do.

The Graflex Singer back is bolted to the Eddystone body by 4 clamp plates which were actually quite difficult to make first time. Next time will be easier.
You can see them on the rear view in the op link.