I actually did some testing last weekend now that I have my vac table working to try and determine "optimal" cut depths.
On through cuts I don't want to cut deeper than I need to since it just puts more wear on the waste board, but I want to make sure I get full cuts. 6.5 feels like overkill since the foam is only ~4.5mm thick...but it varies so a bit of leeway is always going to be necessary (I've seen foam sheets over 5mm thick and barely 3mm thick - in the same freshly opened carton!) I've done 5.5 in the past but found sometimes I didn't get a full cut across the full sheet. 6 has been a good compromise but I wonder if I can do better.
Score cuts I usually used 2.5 but lately I've been doing 3. If things are setup well there's little danger of going too deep at 3 and it makes for an easier to break score.
For shallower score cuts (where the foam doesn't have to open but you want to be able to bend it on the line) I use 1mm now.
For marking cuts along bevels (to help guide me when I cut the bevel) I use 0.75mm. Deep enough to make sure it goes through the paper but not so deep it causes a weak spot.
For most marking cuts I'll use .5mm - but I've been experimenting with 0.25 lately which if things are dialed in well is really nice for marking cuts since it will sometimes only dent the paper and not actually cut through it. And if I can get that working consistently it would be ideal for marking cuts since then they'd truly only mark and not risk the integrity of the foam at all.
However....
I'm still trying to determine what it is about my machine that's causing my Z to vary over the surface of the workspace since my outer rails are all level but I still have about a 1mm variation in certain areas. (I suspect one of my gantry rails is warped slightly...but I also noticed one of my rollers may have issues as it doesn't "clamp" with all of the bearings touching the rail, at some points one of them stops touching the rail so something is off there but I'm not sure if it's the cause or a symptom.)
So I made up a test in estlcam, I just enabled a 1/4" grid and then used the engraving tool to draw "freehand" snapping to the grid. I made a series of lines a few inches long: 6, 5.5, 5, 4.5, 4, 3.5, 3, 2.5, 2, 1.5, 1, .75, .5, .25 (IIRC - I don't have the files here at work and shockingly didn't take any photos yet.) I then drew a box at 6.5mm deep going through the middle of the lines and extending 1/4" past them on one side.
Next I selected all of that and grouped it, then stamped it - once in each corner and once in the middle.
Basically I'm cutting 5 combs with each "tooth" of the comb cut to a different depth so I can see how consistent they are.
The results were interesting. Turns out the low points aren't where I expected them to be - I thought the variation was only left to right but it turns out there's a bit of variation front to back as well so it's more complex than I expected.
6mm was always a great cut.
5.5mm went through in 4 out of 5 places (IIRC) and almost went through in the 5th. 5.75 as max depth is probably safe on my setup.
I need to look at it more closely to determine which cut was the deepest that didn't go through in any position - as that should give me a safe max score cut.
The marking cuts - .25 just broke the paper in some spots but in others was barely visible...the Z variation in my machine is too much for it to be reliable. .5 however at least marked everywhere.
I'll post more details and photos later but I've been meaning to talk about this for almost a week and keep forgetting so figured I should get something down now while I'm thinking about it
The big thing bugging me is still why I've got that Z variation. The conduit I used for my gantry rails has always bugged me...it was from a different store than my side rails and not quite as smooth and I had a harder time finding straight pieces. So that's where my suspicion is focused. But that one roller acting goofy has me concerned again...it's the only one printed out of that particular brand of PETG so it may be slightly different than the rest.
I really need to stop slacking and make some calls about SS tubing to see if I can afford enough to finish the low-rider AND redo my MPCNC with 525 parts and SS tubing or if I'll just go SS on the low rider and stick with conduit when I print 525 parts.