Driving the gantry on Z axis?

[Skipsoaring]

I understand that one of the areas most critical for cutting stability and repeatability is the rigidity of the spindle on the z axis. Has anyone tried to drive the gantry vertically?
This would leave the spindle more rigidly attached to the gantry and reduce the leverage.
Are there any major pitfalls in my thinking?
2 hiwin slides on each gantry post 90deg opposed and ball screw driven.
Any thoughts?
Clayt

[Robin Hewitt]

Sounds needlessly complicated. Could it be you have no clear idea of what this proposed machine will be designed to cut so you are trying to make it universal?

[Skipsoaring]

Thanks Robin. Possibly true, although I do know that I want to create dished moulds in Aluminium (possibly stainless if it's achievable) block up to 2000mm long and no more than 1000mm wide. The depth of cut will be no more than 100mm.
I believe that ideally I would want the spindle stationalry and move the bed so was thinking that removing one axis movement from the spindle would help with strength.
I'm not sure I would have the space to accommodate a 4m+ machine (moveable bed).
I suppose one thing in my favour is that I don't want much width with the length?

[Clive S]

I understand that one of the areas most critical for cutting stability and repeatability is the rigidity of the spindle on the z axis. Has anyone tried to drive the gantry vertically?
This would leave the spindle more rigidly attached to the gantry and reduce the leverage.
Are there any major pitfalls in my thinking?
2 hiwin slides on each gantry post 90deg opposed and ball screw driven.
Any thoughts?
ClaytI am sure that I have seen some pics on here of what you are looking for. If I am not mistaken Dean (Jazzcnc) put them up. The Y gantry acted like a Z with twin screws. ..Clive

[Skipsoaring]

Thanks Clive. That sounds like it!
There is sooo much info here!
I'll keep searching and see what I can find. TA
Clayt

[Boyan Silyavski]

I have quite mixed feeling with this.
It looks that it will be stronger but after analyzing more carefully the idea, i remember i found something that actually did not like.

Its difficult to explain but i saw that sometimes if everything is done normally and is compact, could be stronger that going the deceptively stronger way.
The fact that the forces actuate on shorter distances and cut resonances to transmit along frame, was the thing that stopped me when starting to make similar design. Still not proven though, may be i am wrong here. Have to redraw everything to make sure.

I still feel that the as short as possible overhang of spindle and correct spacing of rails, especially gantry and the way the gantry rails cross with Z blocks, is the key to making stronger machine. What i mean is that if done correctly and using 2 spindle brackets, it could be done so that the spindle itself at all times is strengthening the z plate, so actually correct design there could make brutally strong Z, which looks normal, but is superior.

However weak spot in video is the hold down, as could be heard from second video. I made with first machine same mistake, not making a strong enough hold down table from the beginning. If you look somewhere in my second build you will find comparing all possible table beds ideas, for a strong bed.


https://www.youtube.com/watch?v=4Bj6er322Dg


https://www.youtube.com/watch?v=MhfdxuS2vRY

[Skipsoaring]

Cheers for that Silyavski. Some food for thought. Having the bed move on the short axis could work and the spindle fixed to the gantry. The gantry would then have to traverse the long axis and be able to be raised and lowered. Could this cause significant weakness/flex/give?
i think i understand what you are saying regarding spacing and bracing.
I don't think I will be relying on wood screws to hold my work. Unless it is a work-around to make something more secure ;-)
Clayt

[Boyan Silyavski]

Maybe all comes from fact that he holds the spindle with only 1 bracket and is not using water cooled spindle with steel body. But you could definitely hear on second video that when digs in material, something vibrates. Or if he is using short hiwin blocks the overhang of all is a bit too much, or mixed effect of all this.

See below, i try to illustrate what i was talking about, using a drawing i am still working on:
-if there is a second spindle bracket the spindle itself strengthens the Z
-weak spot is in yellow, where spindle pushes at first, its so short and in between 2 pairs of long bearing blocks so its irrelevant, so all force transmits to the upper bearing pair, instead of the plate itself.
I also have a feeling that the initial push at that short yellow distance eats all vibrations instead transmitting them to gantry

- then spacing of gantry rails must be same as when spindle is at its lowest point, so it balances the arm of force, see black line

So seemingly very simple thing could be very strong if well designed. Somebody could say it needs further bracing, but not. This design is tested by me in first build and works like a charm routing aluminum fully extended without problem. See video at end of first build where 0.8kw spindle makes 1.5mm pass fully extended without a glitch. See that there is a moment of scream when digging downwards, that because the lack at that moment of sturdy table bed

http://www.mycncuk.com/attachment.php?attachmentid=15500&stc=1

[Skipsoaring]

Aaahhh... pictures are 1000 words etc!
I'll check out the vids again, how does that design cope with lateral loads (gantry side to gantry side)?
Would you consider Stainless steel with that fully extended?
My gut feel is that you wouldn't?
I did read your 1st build and was very impressed (I'm sure I have watched the video). I'm sure I'll read it again (along with many others).

[Skipsoaring]

The only drawings I am going to be contributing at the moment will be on paper and a mobile pic taken. My computer died earlier this week, these things are meant to test us!

[Boyan Silyavski]

Stainless steel with moving gantry :beaten:, may be , if the gantry was 2 tons, the spindle 20HP BT50 , ballscrews were 30 size and bearing blocks 35 size roller bearing. :hysterical:


These computers were testing me too. First broke my main one before 3 weeks. Changed disc to SSD. Great upgrade in speed and all. Then the one at the workshop learned and it also broke. Yesterday fitted it also with SSD . Good that my laptop has already SSD. Needless to say that after previous crashes learned my lessons so all CNC drawings are on mirrored discs + cloud storage

[Skipsoaring]

Stainless steel with moving gantry :beaten:, may be , if the gantry was 2 tons, the spindle 20HP BT50 , ballscrews were 30 size and bearing blocks 35 size........
Okeydokey!! Aluminium it is then!

I'll also be using cloud storage. Just gotta get a new computer

[Skipsoaring]

Just had a read of this thread and found some of the fog lifting: http://www.mycncuk.com/threads/7112-Need-help-buy-or-build-to-cut-steel-stainless-steel-titanium-etc/page1
I will be trying to build an extremely rigid machine, just won't be considering stainless as I feel it would be too restrictive.

[toomast]

Hi,
I had similar "vertical fixy" idea.
http://www.mycncuk.com/threads/7321-Mill-480x500x180-RIGID
Did not finished it though :( I also go with Silyavskis design.
Good luck!

[Skipsoaring]

Thanks for that Toomast.
Fixed gantry looks an idea.
When I think of a fixed gantry, I automatically 'see' the gantry fixed across the narrowest axis requiring a very long bed travel, 4m in my case! If built correctly am I right in thinking that there is no reason I can't have the gantry spanning the longest axis? This way my machine would be a smidge over 2x2m allowing for a cutting area of 2x1 or thereabouts with the bed moving across the shorter axis.
It was thinking about the size of the machine which got me thinking about having the gantry move as the z axis (as per the video in your thread).

[Skipsoaring]

...Just gotta get a new computer

Got a new computer today, went out to get a cheapy and came home with a mac, ooopps!

Before I get too carried away thinking about my second machine, I need to start my first machine (hot wire) so that I can get the money for the second machine (and somewhere to build it)