Moving gantry Z axis

**JAZZCNC** · 11-08-2022

Originally Posted by Voicecoil

I'm thinking of doing some improvements to my CNC, one of the things I've noticed is that I do the majority of machining with reasonably thin material and hence the Z axis is at near maximum extension and there at it's least rigid position. Having the spindle fixed at a small extension from the gantry and moving the whole gantry up and down whilst being more complicated would seem to give better rigidity when machining "low" stuff, has anyone built a machine like this? I seem to remember seeing a pic of one somewhere but can't find it.

I have built several machines just like this which lift the whole gantry up, the gantry is effectively the Z axis, and they are very ridged at all heights by the nature of design keeping the overhang and tool stick out to the very minimum, however, they are built very different from a standard router and not something you could easily upgrade an existing machine to. My advice would be to build a new machine with this design. This design suits fixed gantry best but can easily work just as well on a moving gantry setup.

Originally Posted by A_Camera

I think it would just complicate things. It works for 3D printers, but I don't think it has any benefits for CNC router.

That's not the case actually, they have several benefits over a standard router, even mills if built strong enough because of the cutting area vs footprint which can be gained while having the same strength qualities.
They allow maximum strength at any cutting height as the spindle is mounted directly to the back plate and supported at all times by the gantry and its bearings meaning no extending Z axis and a typical lever setup like that of a standard router, the distance from tool tip to back plate/spindle support never changes so maximum strength and least amount of chatter at all heights. Much much stronger than your typical router with far superior surface finishes and longer tool life due to less chatter.

One of the machines I have built has 400mm Z-axis travel and can do fine engraving work on 1mm material on the bed surface just as well as on the surface of a 400mm piece of material.

Originally Posted by A_Camera

Also, if you will have two motors, one on each side, then you should zero both because eventually they will be out of sync, and that will cause problems.

Again not an issue, I have built this design using a single motor with screws connected with timing belts and also with motors at each side, both work equally well. Twin motors are simpler to implement than a single motor setup with belts but have cost implications. The only slight difference between standard routers is that the motors need brakes to stop the gantry from falling when not under power (or counterbalance), but this is also true of any router with a heavy spindle on a Z-axis.!

Yes, the twin motors each need their own Home switch but that is no different from a router that uses twin screws on say the Y-axis, and when setup works perfectly fine and accurately. That said my preferred method is the single motor with a timing belt as it's cheaper and less to go wrong. There's also another reason which relates to UCCNC, slaving the Z-axis and probing issues that are currently driving me nuts but won't get into that here.!!

Originally Posted by A_Camera

If I knew that I don't need the gantry clearance I'd make the sides lower and would reduce the clearance that way, or would raise the table to be able to reduce the need of moving the gantry down to it's maximum extension.