[richard-at-home]

Anyone ever built a gantry machine with the z axis rails and ballscrews mounted at the ends of the gantry and the spindle only moving in x along the gantry? (The whole gantry moving in Z)

I know it doubles up the Z axis parts, but I wondered if it would be more rigid and maybe allow deeper Z?

[1Jumper10]

Its been brought up before and then generally dismissed as not a very good idea.
A couple reasons that come to mind is it would make the moving parts of your Z axis much heavier than necessary. And it would be highly complex in execution to get the same rigidity that one would from a standard gantry with standard Z axis. A 3axis machine would likely require 5 servo/stepper motors instead of 4 or 3.

[ger21]

I've seen pictures here of a machine like this, but have no idea how to find the thread.

[manofmanyhats]

[louieatienza]

I've seen pictures here of a machine like this, but have no idea how to find the thread.

I believe it was pippin's build... There are a few on YouTube. used a rotating nut as well...

[richard-at-home]

Very interesting! Thanks.

That design looks like it only has one rail on each side for the Z axis.

I think I would have joined the uprights together across the top of the gantry to make it more rigid.

[pippin88]

I believe it was pippin's build... There are a few on YouTube. used a rotating nut as well...

Different pippin? Cause it wasn't me.

I've toyed with the idea for a mill though. I would connect the top of the pillars together rigidly.

[louieatienza]

Different pippin? Cause it wasn't me.

I've toyed with the idea for a mill though. I would connect the top of the pillars together rigidly.

Sorry pippin, mistaken identity. I was close however, it was phife:

http://www.cnczone.com/forums/diy-cn...ing-nut-2.html

[wizard]

Ive been thinking of such an apptoach but it is a more intensive build than the normal approach here. My computer broke down so im a bit limited in what i can type here.

Pros:
1. Stffier
2. Fewer moving parts. Im talking controls here on a moving table machine. You would only need a drag chain to the saddle on the gantry beam.
3. Ends up being a Planer Mill like machine.
4. Potentially easier to manage dual Y (Gantry) axis saddles.

Cons
1. Variable Z clearance
2. More machining intensive. Access to a machine shop likely required.
3. More materials required
4. The need for dual servos on the Z or a mechanical solution between the two lead screws.
5. Moving the beam up and down is energy intensive this the need to carefull consider drive sizing. The machine may not be ideal for operations dealling with a lot of Z movement.

In any event that is a start. As for being a good idea that depends upon your usage.

[Goemon]

I was thinking about this for my machine but I dismissed it because I though it would put too much axial force on the Z-axis rails. Some of that could be addressed by mounting the z rails facing forward on the front of the riser columns but.... ultimately, the goal was to increase rigidity and I concluded that this was better achieved with a fixed gantry.

Still, a z-axis protruding below the gantry can't be great for stiffness. I am still wondering if I should have made the table move up and down for the Z-axis instead.

[peteeng]

Hi Goemon - I know this is old but...How did this work out for you? I'm planning a rising Z axis mill at the moment. I see it as a great solution for increasing rigidity. To add to Wizards list:

Pros :
1) Stiffer much much stiffer then a Z axis that hangs way down below the gantry. Plus its relative rigidity to the gantry is constant no matter where it is
2) No saddle which places the spindle closer to the gantry ie less leverge = stiffer

Cons
1) I don't think this matters unless you are doing deep cup shaped objects. You still need a tool long enough to go from part top to part bottom height and this is the beauty of the rising Z the columns can be elephant legs if needed
2) Not sure why, just as much machining in conventional solutions
3) No actual less - other then another motor and drive screw all other stuff is the same
4) No difference to dual axis on Y axis
5 ) Drive screws generally have heaps of grunt so Z weight is not significant, back driving may happen so put a brake on the motor..

I think this config could become the std config for high z axis machines. Peter

[davida1234]

Hi Peter,

I have one with an active gantry.

It was my first version with the "Brygga" name. Designed and built in 2015. Using it since then as one of my main cnc's without any problems. It is using V-bearings as at that time linear rails were too expensive.

It has 14" of z travel and the gantry is fully balanced by springs. It uses Nema23 motors with a chopper board.

Turned out that the heavy gantry as the z-axis provides for much greater steadiness and accuracy, much more so than a 1/2" thick sheet of aluminum attached to closely spaced linear blocks (which I have on another machine as well).

I have a bunch of pics and movies if you are interested. I am uploading one for you and will post once it is done.

I gave up on the design because, as good as it performs, it was too complicated and expensive to manufacture.

[davida1234]

Here are couple pics and a somewhat lengthy introduction video I made at that time.

Attachment 443668

Attachment 443670

[nlancaster]

davida1234 any videos of that machine actually cutting?

[peteeng]

Hi David - Can see the love that went into that machine. Unfortunately people won't pay for that stuff but we do move on and its great to hear that its been a successful machine. There are many large scale rising Z axis machines, but there does not seem to be a common name for it. Once it has a commonly recognized name it will become more popular or vis versa. Chicken and egg problem...Maybe a "dual action double moving column with sprinkles" DADMCWS is catchy? Peter

Active gantry is good too. Lets start a naming contest...

[davida1234]

nlancaster, yes I do have a few.

Here is a 1/8" aluminum in single pass one:

[davida1234]

Peter, the concept does provide a few advantages.

One of them is that the spindle can be very close to the gantry beam.

Then the mass of the gantry as stated.

The immensely available Z-travel without the problems of tall mounted fixed gantries.
If you go low, your accuracy goes up.
If you go high, your accuracy drops.
On tall mounted fixed gantries, you have low accuracy when high to start with
and it gets worse if you go low from there.

The main disadvantage is the necessary weight support system.

Two pairs of slaved motors for a total of 5-motor system.

And the mechanical column extensions on the side of the machine.

[davida1234]

Here is one showing the machine cutting the top of a 14" cube:

[davida1234]

there does not seem to be a common name for it.
Active gantry is good too. Lets start a naming contest...

Peter, I actually called it a drawbridge machine. Resembled a drawbridge to me.

Swedish name for a drawbridge is Brygga, Hence the name.

[peteeng]

Hi David - To me a drawbridge swings up/down from the castle side Here's some clews for names... Peter

https://en.wikipedia.org/wiki/Moveable_bridge

https://en.wikipedia.org/wiki/Vertical-lift_bridge