Design Considerations...Basic!

[Nonoriginal]

This seems like a very basic question, and I can't find a decent answer.

What would be my design considerations when choosing a;
"Moving table" over a "moving bridge" or vice versa?

Please correct me if I'm wrong, but I think I have found examples of each;

Bob Campbell "moving table" : http://www.campbelldesigns.com/

Data-cut "moving bridge" : http://www.data-cut.com/Page2.html

Thanks!

Michael

[2muchstuff]

Yep, thats it.

[CNCadmin]

The movable gantry to me is a bit more complex than a sliding table, the sliding tables disadvantage is the extra foot space, needed to allow for table movement. The fixed gantry would make for a very ridge machine. The movable gantry is the most popular and can me made so many ways. Their are many great examples here on the site of many machine that will give you many ides.

[Nonoriginal]

Please let me know if the following is accurate;

A movable Gantry or bridge is more complex, less sturdy, but has a smaller footprint than a fixed gantry or bridge. The fixed gantry or bridge requires more space, is very sturdy, and less popular. Hmmm....

Is there a major price difference between the two designs other than the components one would choose to go with Ie. Steppers, rails, lead screw vs ball screw, etc. I would think a movable gantry would require more components to construct? but then again I'm no pro when it comes to CNC routers (if you couldn't tell... )

Where would one go to gander at how all these systems work? I have been reading a lot in regards to these sweet machines. However, I'm still not sure how all the components "fit" to complete the "rolling chassis", if you will. I guess one could say that I'm a visual (picture) learner.

I appreciate the help.

[Torsten]

Originally Posted by Nonoriginal Please let me know if the following is accurate; A movable Gantry or bridge is more complex, less sturdy, but has a smaller footprint than a fixed gantry or bridge. The fixed gantry or bridge requires more space, is very sturdy, and less popular. Hmmm.... Is there a major price difference between the two designs other than the components one would choose to go with Ie. Steppers, rails, lead screw vs ball screw, etc. I would think a movable gantry would require more components to construct? but then again I'm no pro when it comes to CNC routers (if you couldn't tell... ) Where would one go to gander at how all these systems work? I have been reading a lot in regards to these sweet machines. However, I'm still not sure how all the components "fit" to complete the "rolling chassis", if you will. I guess one could say that I'm a visual (picture) learner. I appreciate the help.

First part of you question is mostly incorrect exept for the added space
requirement.
Both systems are allmost identical in there requirement sturdyness and cost.
Think out of the box, forget about moving or not moving gantry and concentrate your efforts on the things that matter.
Transfer of power from the table to your motormounts, use the most efficient
sturdy structure between those components as your requirement calls for.
In the end if you have a moving bridge or a fixed gantry will simply depend
on where you decide to mount the legs onto.
Good Luck

[ger21]

A moveable gantry can be made just as stiff as a fixed gantry. It may not be as easy to do, but it can be done. Most people go with the moving gantry because you only need about 60% of the length for a given cutting length. You'll also need a longer linear bearing setup for the moving table, which could cost a lot more if using high quality components.

[WoodSnarfer]

IMO, if you've got a requirement for having a rigid, accurate machine then both the moving gantry or moving table designs give ample opportunity for problems

Personally, I like the idea of having a moving gantry and a fixed table mainly because I can picture down the road wanting to swap out features on the table (like a vacuum system, clamping fixtures, etc) and it seems to me that it would be easier to do that sort of thing when dealing with a 'passive' table.

Also, on a larger router the idea of accelerating and decelerating a big, heavy table with another couple hundred pounds of wood or metal on top isn't too appealing.

Finally, as ger21 stated, getting the cutting envelope covered by a moving table on the x-axis means that you need a set of linear rails that are 2x the length of the table. For anything over a 24" table, the cost of those rails (even on eBay) is going to be tremendous. Plus the cost of the ballscrew.

-Chris

[CNCadmin]

Some of the better professional wood machines I have seen and operated were all movable tables, they will accelerate much faster than a gantry type, they only have to move a table not the gantry motor etc. With a fixed gantry you can build that so stiff and heavy that you can reduce vibration that would only travel though the components that make make up a movable gantry. For small hobby machines movable gantry is what I would go with.

http://www.thermwood.com/twood_site/...nc_routers.htm

[Torsten]

Originally Posted by ger21 A moveable gantry can be made just as stiff as a fixed gantry. It may not be as easy to do, but it can be done. Most people go with the moving gantry because you only need about 60% of the length for a given cutting length. You'll also need a longer linear bearing setup for the moving table, which could cost a lot more if using high quality components.

Hi ger21, I just read your post from a different thread, looked
at the link you left there.

http://www.k2cnc.com/machines/KG-3925_detail.html

Nice Machine pictured on the Page.
This Picture pretty much makes the point I was trying to make
it shows a basic machine structure without any legs mounted.
A very good design I might add.
But the machine still can be used ether way depending on where
you chose to mount the legs to.
Mount then on the ends of the Table and you have a moving gantry.
Mount then spread out to the gantry itself and you got yourself a
fixed gantry with a moving table.
The components of the machine structure itself remains absolutly
itentical.

[Nonoriginal]

Thanks for the replies!

Ok, here is another question;

What are the pros and cons of a pillow block bearing system on rails vs. a linear slide system like THK, for example?

Do I lose anything signifigant besides static load capacity?

The reason I ask is due to my visit at the liquidators this morning. I have 4 Thompson super twin 12 open pillow block bearings (mounted on 36" SS rails) on hold until tomorrow morning. I'm not going to say the price until they are in my grubby hands.

thanks,
michael

[Hobbiest]

If the rails are supported, they should be a good buy. Well...it would be a good buy if they cost less than $25 for the whole kit.

I'm kidding!!! If you get a good buy on them, they sound good. If they are supported. Sounds like you are getting a pretty good handle though. Still plenty of hurdles left, keep at it...BTW, if they are supported...

[Nonoriginal]

They come with a shaft that is pre-tapped for a guide rail. I see no way of supporting it, other than milling out a section to mount them in Aluminum. I see why most use a linear sytem. Its much simpler.

Do you have any ideas to make it work?