Gantry VS. C frame for metal mill

[Tristar500]

I've settled on a steel tube filled with a special concrete mixture for the structure of my mill project. I'm using THK rail and ballscrews and will be either servo or stepper driven, (haven't decided yet)

I just can't decide between a gantry style machine or a C frame (Bridgeport style). Both have their merits and pitfalls.

Anyone care to comment?

Lawrence

[JPMach]

A gantry is basically a double C-frame. Being that its a home build I would lean toward a gantry style. It should be the easiest way to make a rigid machine (assuming not too large). Sure it could cost a bit more if you put two screws and motors on the X but... I would definetly go with Servo driven closed loop control.

Another thought for ya.. Why not build a Horizontal. Put the Z axis in the "table" and the X and Y in the Head. The Head can move in Y in basically a very tightly spaced Gantry. Would also be easy to add a 4th axis but thats another thing to mess with. Horizontals are typically always more rigid than verticals and theres the whole gravity affected chip thing too.....

JP

[ZipSnipe]

Hey Tristar, I,m not far off from starting my Gantry mill which is in the early planning stages. I,m really leaning towards gantry, especially the overhead gantry, which appeals to me the most logical. The table would be stationary while the rails and xyz are all placed above the table. This seems a very logical choice as it helps to keep chips off your axis's. This mill will be mainly for cutting steel and will be made of steel. Keep us posted.

[jerber]

I've chosen the gantry style.
I like the idea of a stationary table. As ZipSnipe says it will keep the chips away from my spindles.

Since I use alluminium for the frame it's also a plus that I can use shorter parts for the frame then when I would have if I had chosen for a C-style frame.
Wich I think is good for rigidity.

Here's my work in progress : http://www.cnczone.com/forums/showthread.php?t=23180

[ZipSnipe]

One of the designs I,m looking at http://www.cnczone.com/gallery/showp...o/2165/cat/505
My Photo Gallery - Proposed CNC Router - Powered by PhotoPost

Except I plan to beef up the corner support posts and run 1/2 x 6 steel plates around the upper sections.

[Tristar500]

I've gone with the C frame and concrete derivative to dampen harmonics and increase rigidity.

See… http://www.cnczone.com/forums/showthread.php?t=26854

[digits]

Originally Posted by ZipSnipe Hey Tristar, I,m not far off from starting my Gantry mill which is in the early planning stages. I,m really leaning towards gantry, especially the overhead gantry, which appeals to me the most logical. The table would be stationary while the rails and xyz are all placed above the table. This seems a very logical choice as it helps to keep chips off your axis's. This mill will be mainly for cutting steel and will be made of steel. Keep us posted.

I like the look of your design ZipSnipe, I am considering building something very similar - do you mind if I ask you a few questions about it?

Are the linear rails only on the top most of the X support beams, or do all 4 X-beams have rails and trucks?

Also, what sort of X-Y deflection do you expect to see on the end of your Z-axis due to cutting loads? I am trying to design a aluminium machine for cutting aluminium, and the 80/20 deflection calculator has me worried about having my spindle on the end of a relatively long cantilevered beam.

Is there a good reason not to have both Y-rails at the same Z like the X-rails? I was thinking that a flat horizontal Y-bridge would allow me to put the Z-axis symetrically between all my Y-axis trucks.

Cheers.