Design: overhead gantry

[Gizmot]

Looking at some large overhead gantry systems, got some questions that maybe you guys can answer. I'm refering to the large picture posted below.

1. legs: some even larger machines use what looks like concrete legs (along the Y axis on the pic). I understand that steel legs can be moved later on, but wont concrete creep a bit over time, affecting accuraty? It would be quite easier to build with concrete...

2. speaking of axis: is there a standard for axis? most machines I have seen from DMS (see http://www.5axiscncrouter.com) got the gantry moving along the Y (and it seems like the Y is always longer). Must be easier to have a shorter gantry right?

3. Rack and pinion: Since you CAN'T have for exemple a ballscrew in the middle of an axis, how to do slave accuratly to make sure both racks and pinions of the same axis always move in tandem?

4. Z Column: here, the YZ axis plate has the rails for the Z axis (while the blocks could be bolted to the plate and rails to the Z axis). I understand that this allows you to have a counterweight for the Z. But, that adds lot of weight. counterweight or not?

5. Motors with rack and pinion: looks like the motors on the Y axis are direct drive. Is it possible?

6. ballscrew over X: is it me or it looks like their is a 10+' ballscrew over the gantry? would accuraty and backslash suffer a lot to go with belt or chain?

thanks again guys

[Al_The_Man]

The coordinate configuration appears to be the same as it would be on a standard Mill.
The Y axis motors may have planetary gear boxes on them, and controllers of this type use electronic gearing, i.e. One motor is master and the other is slaved/geared off the master encoder.
Al.

[Gizmot]

Originally Posted by Al_The_Man The coordinate configuration appears to be the same as it would be on a Gantry Mill.

for exemple on a 4X8 table, the gantry is going to be on the 4 foot axis, not the 8 axis unlike those guys. But it can be a good thing with a moving table...

Originally Posted by Al_The_Man The Y axis motors may have planetary gear boxes on them, and controllers of this type use electronic gearing, i.e. One motor is master and the other is slaved/geared off the master encoder. Al.

oh yeah, maybe that the blue and silver thing just below the motor....

[Gizmot]

found the answer to all my questions, thanks anyway

[The Blight]

Well I'm a bit curious as to what all the other answers were. Could you please share them with us?

[Gizmot]

Originally Posted by The Blight Well I'm a bit curious as to what all the other answers were. Could you please share them with us?

only because you are danish...

1. legs: some even larger machines use what looks like concrete legs (along the Y axis on the pic). I understand that steel legs can be moved later on, but wont concrete creep a bit over time, affecting accuraty? It would be quite easier to build with concrete...

-They do not use concrete. They use steel tubing that is filled with what looks like a mix of epoxy and fiberglass matting. This helps to dampen vibrations.


2. speaking of axis: is there a standard for axis? most machines I have seen from DMS (see http://www.5axiscncrouter.com) got the gantry moving along the Y (and it seems like the Y is always longer). Must be easier to have a shorter gantry right?

-2 types of gantry: short and long bridge. On a long bridge access is easier to the work piece(s). Also, if you use a moving table, you can move the shorter axis (smaller footprint). check http://www.f-zimmermann.com/ lot of awesome **** on there


3. Rack and pinion: Since you CAN'T have for exemple a ballscrew in the middle of an axis, how to do slave accuratly to make sure both racks and pinions of the same axis always move in tandem?

-must be a way to slave correctly 2 engines. Still, it is possible (in my case), to use 2 belts or chain hanging above linked by a steel shaft for synchro.

4. Z Column: here, the YZ axis plate has the rails for the Z axis (while the blocks could be bolted to the plate and rails to the Z axis). I understand that this allows you to have a counterweight for the Z. But, that adds lot of weight. counterweight or not?

if you check http://www.f-zimmermann.com/ again, you see that the heavy machines use a counterweight while the lighter dont. Still havnt checked yet, but I assume std engines are strong enough for a lighter Z.

5. Motors with rack and pinion: looks like the motors on the Y axis are direct drive. Is it possible?

-havnt really checked std motors. Maybe their is a planetary gear. Or maybe gearing is just right for weight and speed... not really an engineering problem.

6. ballscrew over X: is it me or it looks like their is a 10+' ballscrew over the gantry? would accuraty and backslash suffer a lot to go with belt or chain?

-Maybe it is the nut that is turning. on a short bridge (5-6', it would be possible to use a ballscrew, or even easier, a chain)

[RICHARD ZASTROW]

Ref: counterbalance. Pneumatic counterbalance with cylinders and reservoirs don't weigh all that much. Just a thought.

Dick Z

[Gizmot]

Originally Posted by RICHARD ZASTROW Ref: counterbalance. Pneumatic counterbalance with cylinders and reservoirs don't weigh all that much. Just a thought. Dick Z

hummmmmmmm....................

the force is strong with this one

[The Blight]

"only because you are danish..."

Haha! That one made me laugh! And to be honest, I'm Norwegian, but I moved to Denmark a couple years ago.

Thanks for your answers, and I think I fell in love with this (FZ 37-5 M3ABC) machine. 8000mm long table. Thats a bit of inspiration for my next project. Will be making it of concrete like this swedish guy did (http://www.elektronikforumet.com/for...er=asc&start=0)

Cheers!

[Gizmot]

Originally Posted by The Blight "only because you are danish..." Haha! That one made me laugh! And to be honest, I'm Norwegian, but I moved to Denmark a couple years ago. Thanks for your answers, and I think I fell in love with this (FZ 37-5 M3ABC) machine. 8000mm long table. Thats a bit of inspiration for my next project. Will be making it of concrete like this swedish guy did (http://www.elektronikforumet.com/for...er=asc&start=0) Cheers!

oh thats fine, norwegian chicks are even better. Those danish girls are dangerous. Once in New-Zealand, 2 tall blond danish chicks tried to sexor me. What s frightening is that they were both taller than me... and I'm 6'3....

btw, that FZ 37 doesnt have any table, thats a gantry portal (while the swedish guys concrete table is a fixed gantry). I'm personnaly designing something that looks like that FZ 15 with 10x5x4' work area (plus space for 5 axis head)

I also tough alot about using concrete but I really dont think its going to be stable enough on the long term.

[Al_The_Man]

Originally Posted by Gizmot I also tough alot about using concrete but I really dont think its going to be stable enough on the long term.

There are some manuf here (Cincinnati )that install their machines on a concrete base for stability, I wouldn't swear to it, but I believe it is similar to Ferro-Cement, the stuff they make the boats out of.
Al.

[Gizmot]

Originally Posted by Al_The_Man There are some manuf here (Cincinnati )that install their machines on a concrete base for stability, I wouldn't swear to it, but I believe it is similar to Ferro-Cement, the stuff they make the boats out of. Al.

well, there is a couple of patents about concrete machine tools.... and I would love to do it!
but on the other side. People such as prof Slocum are against using concrete

While I love the idea of epoxy/granite, i dont think it is cost effective for large machines. Need to look a bit more into it