[OneWound]

Hi All,

I'm here to ask for a proof-of-idea. I am looking to build a huge CNC router table to be able to cut foam for carbon fiber molds. When I say huge, I mean 25'x25' (yes feet). In my mind, it would be just large aluminum extrusions with overtly-large brackets (which I can make no problem). My only concern would be the stepper motor themselves. Since I am going with such a big X & Y travel, is there something powerful enough that can be had for a few hundred bucks?

P.S. If this is do-able, it will be modeled thoroughly and such. I do have the experience and knowledge to machine any custom parts (I'm guessing out of aluminum since there will not be a lot of force when cutting foam...ie nothing like cutting aluminum)

[OneWound]

I take that back, the size would roughly be 15' by 18'

[pippin88]

Where do you get foam stock that big?

[OneWound]

Pippin, most of it is donated (we are a student organization at a university). We glue the foam together to get the correct Z height we want for the molds.

[jueston]

because of the weight of the gantry you will need massive servos not something that can be had for a couple hundred dollars.

that being said, on yourtube you can find pictures of massive CNC machines, so your not the first. one that was several hundred meters long by 15 or 20 meters....

i think you might also find that aluminum extrusions will flex under there own weight at that span, i would recommend some kind of truss design for the gantry, and with the prices of aluminum, i would look at steal...

[jueston]

http://www.wired.com/2012/06/cnc-mil...build-a-house/

MEET HSM-MODAL. THIS modular and customizable milling machine can expand into a 41-foot-wide, 14-foot-tall, and 495-foot-long giant

[OneWound]

jueston,

Would the spindle be that heavy (as well as the aluminum t-slots..)? If I am just looking to cut foam...I don't see I need that high of spindle RPM (although I need a large end mill, but that's a different issue). I see what you're saying about the truss idea and agree.

And what kind of servos would I need? Any kind of recommendations?

[ger21]

You select the motors after the machine is designed, based on the specs of the machine, and the performance goals.
Without knowing how much weight your moving, and how fast you want to move it, it's impossible to suggest what will work.
Also, what's your budget? Expect to spend at least $2000 for the servos, and that's for chinese ones.

[OneWound]

This is what I was looking for.. I am still confused as to where this massive weight will come from if I just use aluminum t-slotted extrusion? as well, how could the spindle be so heavy? I don't mind if its a small spindle that will take a while to machine.

[jueston]

i don't know much about servos because my machine uses steppers, but i think your going about the whole thing backwords. figuring out what motor you will use and then designing the gantry is like picking out a motor and then choosing weather your going to build a fiat or a mac truck.

you have to design the gantry and know the weight and speed requirements, then figure out that kind power it will take to drive it. i would expect the gantry to weight a few thousand lbs depending on design, next you have to figure out how fast you want it to move? then your drive system? then lastly calculate how much force you need to get it moving, and how much force it to would take to stop it once it is moving, and then you will arrive at a motor...

the process is the same as making a 2'by2' machine except the people making small machines don't care about the weight because its a given that a standard nema23 stepper will be plenty, so they don't waste there time considering these things.

the weight of the spindle will be much less then the weight of the gantry itself.

[jueston]

I am still confused as to where this massive weight will come from if I just use aluminum t-slotted extrusion?

unless you have some aluminum extrusion that the rest of us have never seen, you can't really use "just t-slotted extrusion" for a 15 foot span, i bet it will droop a few inches before you put any weight on it.

if you did a truss you have 2 sticks and some webs in the middle, your looking at around 2000lbs in aluminum alone.

[OneWound]

Didn't see a forementioned post

[OneWound]

I guess I have some CAD modeling to do, oh joy!

[DonKes]

First, I have not heard a word about tolerances.

My 5 second design of the gantry includes 5 2" aluminum seh 40 pipe 20' long @25# each (1 or the ends) and 4 1" for bracing @12# equals less than 300 pounds for the gantry.

After 10 more seconds of engineering, maybe channel for the structure for the Y support side and you are still less than 500# for the gantry. At $4 a pound that is $2000 for material.

I am exhausted.

Don

[OneWound]

To address tolerances, I'm assuming I couldn't hold a +/- 0.001 tolerance. I was kind of expecting +/- 0.050, and if you oversize the mold...and sand it down to the final finish...

[pippin88]

Can you cut parts of the mold on a standard size router and glue them together after? You could have holes or other features to index / align the parts for gluing.

[OneWound]

You can, yes. I have actually done that for slightly large molds that I couldn't machine out of one piece of aluminum. Although, the molds we use are on two huge mold racks that are roughly 150 square feet each. (FYI - each mold gets machined on the mold rack it is on...). As well, I may be able to find out how much motors cost due a professor I work for having a machine that has the same general dimensions.

[awerby]

Since this is for cutting foam, it seems that a fixed-bridge moving table design would be better than one based on a moving gantry. The table needs to be able to handle the weight of the material being cut, but if it's foam that's pretty negligible. The advantage is that you can build your bridge out of steel I-beams or whatever you want without having to worry about its weight. It will also save you a fortune in aluminum extrusions. The disadvantage is that it requires more floor space. I agree that servos would be better than steppers for this application, probably to run pinion gears against a rack. You'll want to make this a 5-axis machine, since the cutters would otherwise need to be unrealistically long. Save some money in your budget for the 5-axis CAM software you'll need.

[OneWound]

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.

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

[wizard]

This is what I was looking for.. I am still confused as to where this massive weight will come from if I just use aluminum t-slotted extrusion? as well, how could the spindle be so heavy? I don't mind if its a small spindle that will take a while to machine.

Generally I don't recommend t-slotted extrusions for even smaller machines. On a large machine likely out are suggesting I honestly think it would be crazy to try. I would qualify this a bit because it is possible to get structural aluminum extrusions but then you run into a huge cost problem. Even a suitably stiff steel beam won't be cheap but it should be far cheaper that a structurally equivalent aluminum beam.

As for the spindle well the problem here is what is your Z axis like. If you are expecting a massive Z clearance, which is often the case on these big machines, then you have to consider the torque that a long lever arm will apply to your beam. You could easily design a machine that will twist the gantry beam on the first crash. In other words you really need to consider sound mechanical engineering when designing a large machine like this. It isn't cutting forces that will damage your machine but rather the forces that result from a crash. A gantry that is too lightly built could easily be damaged resulting in a huge waste of money.