Need Help!- Confused About Machine Design Geometry

[Brenck]

Hi everyone, im about to build another CNC machine, but now i want to make something really tuff!!! Im thinking on use 1'' thick ( or more ) plates of 7075 Alum. On every single part of the structure.

I want to use it to machine Iron, Aluminium, Steel and maybe once in a while Stainless Steel and hard woods...

Im confused about what kinda geometry to use, like, fixed table with all moving structure ( Like thoose really big wood routers that machine a entire MDF sheet at once, the ones that the table doesnt move at all and its the spindle that goes x,y and z )

Or fixed Y axis, so the Y axis carry the Z and the X axis moves just the table foward and backwards.

Or fixed Z axis, so the Z its attached to a collumn and below you have a x,y table...


I wanted to do it fixed table. Im already used to build like that...

But in terms of structure, precision and tuffness i dont know if its the best choice....

I want to know even if i build really solid and well structured machien with the fixed table, would it still be better to build with moving z and moving x table or x,y table and fixed z? Because I have never seen a metal working machine with fixed table...

Thanks for the attention everyone

[Sieg]

The big downside of a moving table is that it about doubles the machine's footprint. The upside is that it is much more rigid.

That is why most woodworking machines have a fixed table: they tend to cut whole sheets of MDF at once.

Metal working machines however, tend to cut much smaller jobs. They are used to machine solid parts from blocks of metal. They don't cut very large sheets of metal. Therefor their work area doesn't have to be very large, and so footprint is not an issue.

Cutting large sheets of metal is done on plasma cutters or waterjets. These don't have to deal with high friction and vibrations, so they use fixed tables.

I assume you are looking to build a metalworking mill? Then definately go for a moving table. Moving x AND y is the best. Why? because you need to absorb the spindle's vibrations the best you can. A fixed Z means you almost directly brace the spindle to the ground.


Look at it this way:

Fixed Z:
You are holding a large powered jackhammer and are drilling away at a rock wall. You brace yourself with your feet on the ground.

Fixed Y:
Same thing but now you are on rollerskates....

Fixed table:
Same thing but now you are on rollerskates ontop of a skateboard!

[Gir]

I want to know even if i build really solid and well structured machien with the fixed table, would it still be better to build with moving z and moving x table or x,y table and fixed z? Because I have never seen a metal working machine with fixed table...

I don't really have any knowledge of this, but I work as an intern at NASA's Wallops Flight Facility down here in Virginia and we have one of the largest machine shops on the East coast. We have one large machine with ~3'x6'x4' cutting area (yes, 4 foot Z), and it's a fixed table. Given, this beast is BIG. I mean REALLY BIG. Excluding the table, everything driving it is housed in a 4'x8'x10' box. BIG. However, we also have a number of machines that use moving xy tables with the spindle being the z. These machines are a good bit smaller and have a much smaller cutting area (2'x3'x1').

Given, all these machines are meant for rockets, which usually run in sections no wider than 3' and no longer than 4' and are circular (Good God you should see the lathes. About 20' long! No idea why since only the first 4' are used). Also, aluminum is what's usually being milled, only the occasional steel.

Just thought I'd let you know either can be done and done accurately.

[esm]

yea, if you've got the space I'd go with moving table and fixed spindle. Next best would be Z on the spindle with Y and X in the table.

With the beefy aluminum you're planning to use, along with proper bracing, you could probably do whatever you want.