Hi Folks,
   I'd like to build a router/mill for wood as well as light metal work so it will be a bit beefy.
I have a steel table, 3' x 5' with a Blanchard ground 1" thick top. Should be a good platform.
From what I,ve seen, Bridge type machines seem to be more robust and rigid, require only 1 screw under the table but have a reduced capacity due to the fixed bridge.
   The Gantry type usually requires 2 screws to move the gantry and doesn't look to be supported as well as a fixed Bridge.
Any other major differences, pros/cons of either that I should be aware of ?
Your thought/opinions appreciated.
Thanks,
 

Hello,
I dont think a bridge type machine has a reduced capacity due to the fixed bridge. Both Machines "could" have the same capacity if designed correctly. The only thing is that as a ficed bridge requires a moving table, the machine requires a larger footprint to accommodate the movement of the whole table.

For smallish travel, Id go for a fixed bridge, since it could be built a lot more rigid than a gantry machine.
Regards
Fernando

Howdy Fer.,
  Yes, what I meant to say was for a given machine footprint and available floor space.
I'm just not certain that I'd want to sacrifice the larger capacity of the gantry design just to assure the better rigidity of the bridge type.
After all, the overall size will be 3' x 5'.
Bridge would yeild about 24" x 30",
Gantry           "             24" x 50" estimated.
Thanks for your input Fer.,
Russ

Another thing to consider would be moving the weight of the table+any tooling+material if you went with a fixed head. That table alone is over 600 pounds. Add whatever your working on, plus any vises, fixture plates, etc... You would need allot more motor ($$$) to move something like that compared to a fixed table. I would guess that the acceleration/deceleration times would be considerably longer.
On a side note...I've built quiet a few industrial jigs with flat plate. I've also seen many "cheap piece of junk" units that were not designed properly. These "junk machines" usually made it into our shop for repair. It was not uncommon for a machine less than a year or two old to be useless, because of lack of proper support, resulting in that nice flat blanchard ground plate to look like a banana from sagging under its own weight. My personal advise would be to make absolutely, positively, SURE that you have enough support to distribute the weight evenly, across more than just four corner legs. Add some center legs and adjustment pads on the legs. A ground table is useless if its not flat I'm sure you've got it all figured out already, but hopefully no harm in me expressing my words of caution.
Hope to see some pics soon!!

Hi Sam,
   The table is in storage right now, will get a pic soon. The table is a complete weldment, 3' x 5' steel welded to cross members and legs of square tubing. It was a machine base that I stripped.
   The 1" plate on the table is the base to mount the gantry/bridge to and the rails for the sliding table (if I go with the bridge). The moving plate would be about 24x 30 ...not sure what I would use there, I have some 1.25" Mic6 alu tool and jig plate but would rather have steel.
Another advantage I see for the bridge, the rails are underneath the table ... better protected and easier to shield. (?)
Thanks Sam,
Russ

Oh Now I get it,
you want to use the plate as a base, not as the actual worksurface. Then your assumptions of a smaller work envelope are right if you go with a moving table.
Have fun
Fernando