[pippin88]

Z Axis
You can't just design this later

Moving cars vs moving rails impacts the rest of the machine

With moving rails, you cannot get the Z axis above the bottom car (which is usually near the bottom of the gantry)
You have to know what height work piece and what TOOL LENGTH you might work with
People often forget the tool
This determines how high the gantry needs to be
Gantry height = workpiece height + tool length + distance from spindle nose to bottom Z car

Moving cars - you can get the spindle up above the gantry.
Gantry clearance sets you max work piece height

[GeckoSub]

Z Axis
You can't just design this later

Moving cars vs moving rails impacts the rest of the machine

With moving rails, you cannot get the Z axis above the bottom car (which is usually near the bottom of the gantry)
You have to know what height work piece and what TOOL LENGTH you might work with
People often forget the tool
This determines how high the gantry needs to be
Gantry height = workpiece height + tool length + distance from spindle nose to bottom Z car

Moving cars - you can get the spindle up above the gantry.
Gantry clearance sets you max work piece height

Yes, working on it these days. Also, you are right, I did forget the tool for a little while but once I had the rotary on the table and a rough Z-assembly with a moving gantry, it was obvious I was missing something. For that design, I did actually still have enough clearance in terms of being able to lift the tool above the part. The issue was the Z-outer plate collide easily with the rotary.

Still early days but let me share a few pics.
First of all, am I right in designing a moving rails assembly so that the bottom of the collet is flush with the bottom of the plate and rails?
Like this:


I am fairly sure the rotary will be around this height and quite a lot of work will be done on that rotary. Parts wont be huge, maybe 50mm in diameter. The tool stickout is 50mm here though:

But as you can tell, if I want the end mill closer to the chuck, that Z-plate will collide with the rotary housing. Unless, I place the rotary along the Y with the part and chuck facing away from me, which wouldn't be very user friendly. So, I guess that's one use case where the moving cars would be better(?).

And actually, as it is right now, that Z can't even reach the bottom (again, 50mm stick out on the end mill):


It can kinda reach a low profile vise. But of course at a loss of rigidity down there:


So maybe better to stick to a taller vise:


Honestly, the Z is not easy for me cuz I find I need to design them first and then move stuff around to visualize how it would all play out.

[GeckoSub]

Z Axis
You can't just design this later

Moving cars vs moving rails impacts the rest of the machine

With moving rails, you cannot get the Z axis above the bottom car (which is usually near the bottom of the gantry)
You have to know what height work piece and what TOOL LENGTH you might work with
People often forget the tool
This determines how high the gantry needs to be
Gantry height = workpiece height + tool length + distance from spindle nose to bottom Z car

Moving cars - you can get the spindle up above the gantry.
Gantry clearance sets you max work piece height

OK, so let me share the most extreme part I see myself trying to make in terms of height (let's not get into the challenges of chip evac, work holding and long skinny tools now, haha).

I am hoping I can cut some 18650 battery holders but it would an extreme case. The batteries are 65mm tall, let's add 15mm for a few mm of a solid bottom and some allowance at the top for a cap or connector of sorts. So, 80mm in total. So, 80mm stick out of the tool then double so the tool can actually pull out of the part, right? That's 160mm.
My clearance above the fixture plate in the current design is 235mm leaving 75mm. If I put the part in a low profile vise, I can retract the tool plenty:

Even the same with a tall vise like this:


Not sure what the takeaway is, though? Does this mean, I should go with the moving rails as it's supposedly stiffer and I can clear the tallest part I ever intend to make?