Accurate Strong Gantry

[Ross77]

Ok, I'm a bit confused. You keep saying fixed gantry but then you show a bed with rails on the side.

Is this the bit that will be moving? If so I would have said that it is not the best way to do it. Moving beds should have the rails underneath and at approx. third points or a quater in.

That way the rails are supporting the load better and are closer together so you can get away with one ballscrew.

As you have drawn it the bed can bend in the middle under load.

[vargai]

Ok, I'm a bit confused. You keep saying fixed gantry but then you show a bed with rails on the side.

Sorry for being double meaning.
I have started the thread with the moving gantry principle and as the design and details developed according to the useful comment from the site two days ago especially after having Dean's #41 post I have changed my mind .
My intention was not to leave my earlier ne'er-do-well concept and correct it.
This is the case when the design concept turn to other way so I put this moving gantry into a drawer now and finish this chapter.
I will start a new thread with it when it is actual again.
So I am giving thanks everybody and I am at full pelt with the fix gantry from now on.

[Boyan Silyavski]

Nevertheless keep this thread alive, cause that's one of the most interesting things, somebody starts with one idea that evolves in something quite different sometimes.

[vargai]

Chapter #2 Fixed gantry

I am back again after aging some thoughts however I could not go too far. I cannot find too much examples for the ideal fixed gantry frame.
Probably I am making the typical mistake and try to reform something that is good as it is.
My viewpoints are:
using hot rolled profile -do not know but think it is better vibration damper than cold formed tubes (SQ or RECT)
using bigger cross sections- to eliminate the bracing and welding as far as possible
setup with 1700 mm total length-just slightly longer than moving gantry

My concern are:

-bolted connection as figure shows-is it is good enough- instead of welding?

-The spindle overhang is quite big -though is fixed in one direction . Is there a better setup to eliminate?

-I would balance the moving bridge on both side with pneumatic cylinders or gas spring that I red here and like this option.
As the spindle moves in Y direction the Center of Gravity moves with it -that seems not too beneficial for the balancing

Figure below shows mainly principles first to make a good machine setup



Thank you for your comments and advice in advance.

[EddyCurrent]

My concern are:
-The spindle overhang is quite big -though is fixed in one direction . Is there a better setup to eliminate?

Double gantry or move the uprights to the end of the gantry, just some thoughts but I realise it might make the machine wider.

[vargai]

Double gantry or move the uprights to the end of the gantry, just some thoughts but I realise it might make the machine wider.

Neither do I would make it wider.
The right figure with protruding plates or so with overlap the post could be good.
I do not assume real rigidity loos at Y axis when the rail ends are bolted only onto a strong plate.
I will make some detailing.
Thanks

[Ross77]

To get the best rigidity you need to keep the spindle inline with the support bearings so no overhang. You could rotate the rails so they are mounted end on and in the middle of the beam.

The vertical moving gantry beam looks a bit slender with regards to the end bearing spacing. I would guess its 200mm deep and 1000mm wide? Thats is not a good ratio for racking and I would make the most of the gantry height and increase the bearings to maximum so the moving beam is like 'H' shape.

That would naturally reduce the racking and take the pressure off the screws and rams.

Just how I would do it anyway :0)