Moving gantry Z axis

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.

There are a few DIY builds that have done this, but not many.

Some of the biggest CNC Mills use this design

https://www.cnczone.com/forums/verti...14840-cnc.html

The uprights should be joined by a bridge at or near the top.

I think a 'lifting gantry' design works particularly well for a moving table machine.

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

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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.

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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.

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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.

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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.

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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.

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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.!!

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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.

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Why compromise when can have the best of both worlds at virtually no extra cost.?

https://www.youtube.com/watch?v=-BeYWDfEgS4

https://www.youtube.com/watch?v=cH9zw4oCCw0

https://www.youtube.com/watch?v=M-hbeujeuRg

https://www.youtube.com/shorts/u5HuW9AqwpU

The blue machine was a work of art, nice features and very smooth motion too. I vaguely remember someone building something in that style many years ago (I think he was German?) but can't find the reference in my files. I think you did a much better job though and well thought out.

Any chance of a peak in the control cabinet? Not sure I saw a door so if it's an effort to remove covers etc. (or not tidy yet!) then no problem.

My only thoughts are would the chips go into the vertical slots in the side members which contain the Z ballscrews? Would that need some brushes on it which still allow the bracket to move up and down but keep the worst out?

Jazz, those are very nice machines, especially the blue one.

You are right about the benefits, but I was thinking about moving gantry type machines, which are more popular and common among DIY builds. The ones you are showing are moving table type, and no doubt that moving the whole beam up and down, instead of just the spindle, would have a rigidity benefit there, as well as it can be made with more throat (Z to table) clearance also, without losing rigidity. For my moving table type of DIY machine, I also considered this type of solution, but then in the end, abandoned the idea for some reason, probably because it was my first build and was not that confident. Today I'd go for that solution, because for a moving table type of machine, it definitely is a good idea.

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Originally Posted by A_Camera

You are right about the benefits, but I was thinking about moving gantry type machines, which are more popular and common among DIY builds. The ones you are showing are moving table type, and no doubt that moving the whole beam up and down, instead of just the spindle, would have a rigidity benefit there, as well as it can be made with more throat (Z to table) clearance also, without losing rigidity.

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Yes, those machines are both fixed gantry but the same can be done with a moving gantry machine and still have all the benefits be it slightly less ridged than a fixed gantry, however, still significantly stiffer than a typical Z-axis at full extension if say had a 300mm + z-travel.
This type of lifting gantry setup is common on very large industrial machines, although they often have a short Z-axis as well due to the size and weight of the lifting gantry.

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Originally Posted by routercnc

Any chance of a peak in the control cabinet? Not sure I saw a door so if it's an effort to remove covers etc. (or not tidy yet!) then no problem.

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The electronics are shoehorned into the back of the machine as we didn't want a separate case, it was a tight fit getting everything in there but worked out ok. This is the only picture I have.

Attachment 31177

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Originally Posted by routercnc

My only thoughts are would the chips go into the vertical slots in the side members which contain the Z ballscrews? Would that need some brushes on it which still allow the bracket to move up and down but keep the worst out?

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The video doesn't show it but the screws are offset to the side and out of the way of any chips that might get inside, also the chips just drop through a cut-out in the bottom and into the pan below. I did think about fitting brushes but they are messy things that wear away and in practice, the chips that do get inside or near the screws are a fraction compared to conventional z-axis so it's no big issue really.

Nicely done on the control cabinet,

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Originally Posted by JAZZCNC

Why compromise when can have the best of both worlds at virtually no extra cost.?

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Well exactly Dean, making a machine like that doesn't really require much more in the way of materials or motors etc. Thanks for the info. + videos, you've confirmed my hunch about this design strategy. Your comment on slaving motors on the Z axis is interesting though, my current machine has slaved motors on the Y axis and I had no problems setting that up - is there something funny on the Z in UCCNC then??

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Originally Posted by Voicecoil

Your comment on slaving motors on the Z axis is interesting though, my current machine has slaved motors on the Y axis and I had no problems setting that up - is there something funny on the Z in UCCNC then??

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Regards slaving there's no issue, it works just fine and is exactly like the Y or X axis. The issue we have is with probing, it looks like there is a bug only when slaving the Z-axis, which obviously doesn't often get slaved compared to the A or B axis so hasn't come to light before. I am in contact with Balaz at CNCDrive and they have found the bug but it's deep in the system so not a simple fix so it will be corrected on the next major update hopefully.!