[wizard]

While it would be nice to have a 5-axis machine, it is not viable. Some of the molds would be on a huge movable mold rack - something I can't put on a 5-axis.

Why not? Your extra axis could be on the end of the Z which is a pretty common arrangement for a machine like this.

FYI - I already have a very nice CAM and CAD package

That is nice. I might suggest though some help from fellow students doing structural engineering. You really want to look into what the minimal beam size would be for the span In question. Since saging under its own weight will be an issue you may need a more advanced design that just a single box beam. It really comes down to what you are expecting performance wise.

The idea of a moving table design is interesting but it would lead to a very large machine. For your purposes though it might be a smart move especially if you can build upon a concrete foundation. You can eliminate a huge amount of structural material with everything built close to the ground on a concrete foundation. This is a "cheap" approach but for foam ( really light work) and light duty usage it could be a smart move.

[OneWound]

For your first response, machine crashes are something I have not considered. It is something to look into when I start formally designing this and get into all the nitty gritty details. Technically I can put the mold racks on a 5 axis setup, but the mold racks are VERY heavy (it takes about 4 guys to move it on level concrete). To me, this would mean motors would be very expensive. You are right, a moving table might be very wise. You have have a very small span in one axis. Maybe it would be worth a thought to think about moving on axis on a gantry and one axis on a table.

[CaptainVee]

Years back I saw a cnc on the web, bigger then what you mention.

Carbon fibre space frame is all I remember, and I think it was built up to 15x50 metres or something like that, for the same purpose as yours.
E
dit: Darn, it was the one in the link above...

[alan_3301]

This is the kind of 5 axis mentioned. The workpiece doesn't need to be supported by anything different than a regular 3 axis setup.

[OneWound]

Ahhhh, my apologies than. I had the thought stuck in my head that the table would rotate about the spindle head, not the spindle head about the table. My apologies again!

[UA_Iron]

There's a build thread here dating back to 2009 I believe where Mariss Freimanis (the gecko driver guy) was a part of - in this thread they built a massive Gantry CNC for cutting out giant foam shapes. I tried to search and find it, but I couldn't. They used massive nema42 stepper motors which worked well.

I am a fan of closed loop systems myself - linuxCNC will allow you to use a dual closed loop system - an encoder on the motor for velocity feedback, and a linear encoder for position feedback. See this thread:
http://www.cnczone.com/forums/diy-cn...-software.html

The other option is to use drivers that will accomplish the same thing and use Mach3/Mach4. Example of driver:
Copley Controls - Accelnet - Servo Drive - Servo Amplifier for Brushless & Brush Motors - CANopen - CAN bus - EtherCAT

Dual close loop position/velocity control allows one to handle crashes and stalls much better than a typical open loop system. This is why everyone is saying "go with servo" but really, the requirement is the ability to handle these situations in a timely manner to avoid breaking your machine. This can be done with stepper motors with the copley controller or similar.

Linear encoders allow you to choose drive mechanics with less precision (less $$$) and still get reasonable repeatability out of your system. Magnetic linear encoders are coming down in price, including the magnetic strips. You can get encoder strips that span the entire length you plan on cutting, and expect deviations of ±40um per meter (±0.0015748in) over 20 feet that's within your spec limit of ±0.05

If you're already in the business of making carbon fiber parts, then that is an excellent material to make your gantry out of. Mass does a lot for machine dampening, but carbon fiber also has considerable dampening properties - they use it on a lot of super rigid high-speed wafer handling applications because of this. Maybe wrap a layer or two of kevlar over the exterior of the gantry beam so in the event of a collision, carbon fiber shrapnel doesn't shoot everywhere

All my comments above are really trivialized summaries of what this will really take - this is quite the undertaking!

[OneWound]

It is. I haven't even gotten into properly thinking about what controllers to use. Gulp!

[steve123]

Here is another video of the HSM modal mill cutting foam. The gantry structure is interesting.
https://www.youtube.com/watch?v=BMYJB5Ikb3Y