Gantry or Bridge?

[jeffs555]

Most of the machines described in this forum seem to have either a fixed gantry with moving table or a gantry which moves on rails. Either way, the X rails usually seem to be below the work surface, with the gantry rising way above it. I was wondering what the advantages or disadvantages of these designs are compared to a CadCut type machine (http://www.inshorepowerboats.com/cnc.../CNCROUTER.htm) which has overhead X rails with a more or less strait bridge over the rails. It seems like the overhead rails would be sturdier, with a lower moving weight, and the only disadvantage I can see is less access to the work surface. Does anyone know of any other advantages of one design or the other?

[ESjaavik]

Add to the advantages that it is easier to keep the rails free from dirt from the cutter and coolant when metal cutting. It will be heavier but I guess that also counts as an advantage in most cases.

It will be interesting to follow the opinions on this one.

[Patrick2by4]

well, I guess it would take more material to fashion a raised x platform for the gantry. So the whole machine would be heavier. Other than that, it is probably more stable (assuming the x axis is sturdy).

[samualt]

My machine, being built, has overhead x-rails. It is a little like the CadCut but more like the Practical CNC machine.

I chose the overhead x-rails design because:
1. Gantry is more sturdy, no wobbly doughnut shape.
2. Gantry is easier to build, simple bridge.
3. Gantry is smaller and lighter (lower moving weight), no doughnut shape to go under table.
5. Nothing is under the table so it's easier to add a vacuum table later.
6. If I get a mechanical malfunction it is easier to spot, no hidden machinery under table.
7. This is probably particulcar to just me, but it would be easier to disassemble and reassemble.

If your building a small machine then the above advantages are much less significant. But when you get to the 4'x8' size machines then it all becomes much more important.

JMHO. No offence to the heavy, complicated, wobbly doughnut shaped type machines!

[DLMACHINE]

I built mine with the rails under the table for the loading ease.8 hours of loading and unloading on those machines you have linked to and your back will be begging for the under rails. 3/4 MDF is approx 100 lbs a sheet loading ease is crucial. Also trying to move material over those sides in the photos can easily destroy laminants or verneers. Just my single reason for one style over the other but ease of use is always tops for me.

[Hobbiest]

Some of the commercial machines have the gantry come up from the table height, so that materials can be loaded with a forklift. Very good idea, if you have a forklift! It would be much easier however, to manually load materials on this type of machine as well. If you don't have a "wobbly doughnut" gantry, and your machine is good size, you probably ought to consider dual drive on the X axis, to eliminate the skewing that can happen on single side drive. I think it just all boils down to personal opinion, and whatever seems the easiest and most logical to build.

[jeffs555]

I am just getting started so maybe this is done all the time, but the mention of a vacuum table, and all the talk about difficulty loading got me to thinking. If you had a vacuum holddown table would it be possible to reverse the airflow and use it like an air hockey table so you could easily slide a sheet in from the end?

[High Seas]

Like samualt - I chose the overhead design - and for may of the reasons cited.

I went from a more-or-less- traditional style gantry to the bridge in my "reconfigure of System1 to System2 (photos in gallery for details). BTW as Patrick2by noted - sturdy IS good! I moved from a "legged" system similar to the PracticalCNC system and mounted on a workbench, but maintained the overhead x rails.

I am interested in maintaining as much flexibility and having the rails above the work space means I can always "lower the work surface - or "raise the Bridge" to gain more z axis!

I'm driving one side of the x axis and am on a 24 inch wide bridge - with no skewing or binding problems. WHen I go larger up to 4x8 work surface - I'll keep the bridge and may go to dual drive on x axis. Well see.

I like the bridge design - it keeps the mass of the router and all the working bits centered up and less opportunity for error induced by the z axis "wobbling about" on its up and down motion. I have a lot of Z axis potential and am able to drive the router high enough to change bits easily - which could be a concern on a bridge design.

So many choices - at some point you just gotta pick one and start building! Best of luck! Cheers - Jim

[IndHobby]

I too have been thinking about making a CNC router, now of course mine is to sell, not to keep, so I have slightly different design issues in mind. Also I do this for a living so I have some advantages there.

I’ve been looking at a gantry design with the X screws below the work surface. 2 Screws for the X, 1 for the Y and Z. In order to keep the rotational inertia of the screw from being too high I’m looking at a spinning nut design with the encoder mounted on the nut.

It offers the advantages of ballscrew accuracy, very fast and easy to work, and the design can accommodate ANY length table you want, you just need to get a screw a bit longer.

Anyone looking for a 24 foot long CNC router?

Or just as easy it could be a standard 8 foot design.

The Y axis can be handled in a similar manner and it would be able to handle just about any length, sans rigidity issues.

The thing about a rack design is junk can get in the teeth and screw everything up, also when you’re going really fast (300 IPM +) you’re gonna get deflection because of the tooth profile and meshing. Of course there are ways to design these issues out. Putting the rails up high is one of them, but then you got rails way up there.

You’re welcome to contact me offline and we’ll discuss design issues and stuff like that, no selling.

Lastly, of course, everything I said is my opinion and I don’t want to take anything away from the folks who made their machine differently. Every design has pros and cons, its just picking the ones you want that counts.

Although I’ve never tried it, I hear, there is more than one way to skin a cat.