I measured several items with 3/8'' attachments. LedZilla: 6mm socket (when off it's mounting screw!). Jag-35 DSL-plate: 5mm screw. Inexpensive ball-head: 5mm screw.

My measurements below, all relating to the 1/4-20 UNC mount standard: (bear in mind that I didn't have my caliper at hand, so the measurements may be off by ~0.5mm)

Cameras / dovetails:

Panasonic Lumix GH4: ~7mm deep
Canon 550D: ~6mm deep
Allstar dovetail (Red One acc.): ~6mm deep

Plates:

Industry standard Manfrotto quick release plate: ~6mm protrusion ( - 1mm rubber)
Wooden Camera Quick Cage: ~6mm protrusion (no rubber padding)
Random q/r plate (Chinese made): ~6mm protrusion ( - 0.5mm rubber)
Zachuto Gorilla Plate: ~5mm protrusion ( - 0.5mm rubber)
Old Libec tripod plate: ~7mm protrusion ( - 0.5mm rubber)

As one would expect the only combination of items on this list that do not work together are the Libec plate attached to the Canon 550D.

My initial conclusion is that the AXIOM Beta should have a 7mm deep 1/4-20 UNC mount hole.

It should be a 3/8 standard coarse thread female, not 1/4 20.

The depth varies with manufacturer or item, with some plates threading through and some terminating with a mere four threads for the securing bolt.

Three threads of grab would be a threshold for too shallow.

Industry standard is not 1/4 20 as it is a stills unit convention.

What about one 3/8" and one 1/4" thread ?

One 3/8'' and one 1/4'' would be nice. Is often the case with various mounts.

I have seen a tandem before on prosumer (hate that word) plates, but industry plates are universally 3/8s, hence the discussion from yore requesting several around the body for mounting.

Top plates and side are often cheese plated with 3/8ths and 1/4 alternating, as 1/4 can be common for smaller accessory arms.

I would add that typically there are two 3/8s for the addition of standard quick release plates to Ronford / Baker style quick release clasps.

File sourced by @atagunov

Is anybody else bothered by the horrible mixture of metric and imperial dimensions in that document?

If NASA has thought us anything it is that we should never mix imperial and metric on the same project.

Is there a possibility to go full metric or is the industry too entrenched in legacy to attempt any changes?

@HAL-1999: I'm all for keeping things metric, but in the case of 1/4", 3/8" mount standards you've got to think of those as universally recognized names, not imperial units. The 1/4" 20 UNC mount standard uses a 0.635cm wide hole, yes, but that does not mean we can simply call it the 0.635cm UNC mount standard.. Replace <1/4"> and <3/8"> with <smallhole> and <bighole> in this thread and you'll find we're sticking with metric for all our measurements.

True. We should make a list of protected features that need to remain imperial because it would break compatibility otherwise. Something like

| imperial dimension    | Precedent           | Amount of compatibility broken if changed  |
|-----------------------+---------------------+--------------------------------------------|
| UNC 1/4 20 Tripod mnt | well established    |  All tripod mounts                         |
| 1" hole spacing       | Arri accsories      |  Some uncommon accesories                  |

After we make our list we at leas know that there are X amout if imperial dimensions on the design and we take care not to add any more to them.

The threaded holes are not a big deal but the spacings between these holes are very difficult if you are trying to make something modular. For example if you have modules that stack in 30mm increments but you need to maintain hole spacing of 25.4mm for compatibility.