[awerby]

The stiffeners help, but I'd advise lengthening the base to provide support to the moving table. The way you have it set up currently, it has to cantilever way out into space in order to cut the back of your part. This will become a big vibrating "tongue" sticking out, which will adversely affect the cut quality. With more base, you can space your trucks further apart, which will give you more stability. The footprint of the machine won't increase, although I suppose it will be a little less portable.

Andrew Werby
www.computersculpture.com

[dmalicky]

It's looking good and stout. Agreed with Andrew on the base length--that's the downside of fixed bridge machines. Or add some tall steel tubing under the table to make it very stiff in that bending mode (like a milling machine, though they use thick cast iron).

[Michaelofburien]

More great advice and experience. Thanks folks. I redesigned again to shorten the table (about 1/2 the depth of the whole machine) and relocate the base versus the bottom of the table. I'd post images of the result but the image tool on this forum seem to not work for me.

[Michaelofburien]

Hmmm... Now I'm thinking that IF I bag the fixed gantry idea I get a lot of table back and use a lot less material. Back to Sketchup I go...

[Trotline]

Moving gantry machines do take up a bit less operating space, but the savings in material, if any, will be minimal, and the complexity increases somewhat. I'm not trying to scare you off. My machine, and many, many others are moving gantry designs. It's not going to be an adaptation of your work so far, though. You'll save time and headaches by starting fresh, and exploring the new design from the beginning, as you have the first one.

Luke

[Michaelofburien]

Moving gantry machines do take up a bit less operating space, but the savings in material, if any, will be minimal, and the complexity increases somewhat. I'm not trying to scare you off. My machine, and many, many others are moving gantry designs. It's not going to be an adaptation of your work so far, though. You'll save time and headaches by starting fresh, and exploring the new design from the beginning, as you have the first one.

Luke

That is exactly what I'm doing. I like 3D modeling so I don't mind redoing the work.

On a side note. What is the deal with the Picture posting tool on this site? I was able to post sucessfully for a while. Now it seems like no images show up from the site. No matter the thread.

[Trotline]

The File Upload Manager is a bit funky at the moment. I got it to work by uploading a file, then closing the window and re-opening it with the "Attachments" button. All that after "Going Advanced" with the post, of course.



Luke

[dmalicky]

Fixed vs moving gantries are indeed very different machines, each with ads and disads.

For moving gantry: It's harder to make it as stiff, but it can be stiff enough with long and braced 'feet', and the bearings placed a little above the table height. Usually the gantry is driven by 2 motors and screws for best racking stiffness. 1 screw under the table can be done but the table is harder to make stiff, and the bearing aspect ratio (in plan view) needs to be higher, which cuts travel. Plywood is a better material choice since it's both lighter and stiffer than MDF.

For fixed gantry: The footprint is normally (2*TableTravel) x (GantryCarTravel + GantryCarWidth + GantryDeadLength). Because of that 2* factor, the most space efficient layout is to have the gantry car move much more than the table, and let that car fully travel to each end, like this:
http://northwoodmachine.com/images/5...APID-DRILL.jpg
And that's easy since the gantry can be made massive to make up for its longer length.

(Technically, all "gantry" machines have moving gantries since that's the definition of a gantry. "Fixed gantry" is properly called "fixed bridge", "moving table", or similar. But we all know what we mean. )

[Michaelofburien]

Aaaannnddd... after a few days of trying to work out a Moving Gantry design (I'm happy with) I decided to go back to the Moving Table design I started out on. Way easier to make stiff and sturdy. I have some nice steel tubing (3" x 3.5" 1/8" wall) I can add to the interior of the gantry to stiffen even more and add weight. If I get really creative I'll cut and weld some to the riser area too to make a big horse shoe shape inside the grantry and riser.