Advantages of Acrylic vs Wood vs Aluminum for the gantry? (1st build)

[pswayze]

Hey everyone, I'm in the preliminary planning stages of my first CNC Router. I haven't decided on what model to build yet because I'm working on material selection first. I plan on buying a kit from hobby CNC to take the guesswork and witch-doctoring out of that aspect. I'm trying to decide what material to use to construct the gantry and I thought someone here might have some suggestions.
I'm looking at acrylic, wood, and aluminum but if you have some super material I've never heard of, please let me know.

[awerby]

Acrylic? Because you want to be able to see through it? There's a reason most people choose aluminum for a moving gantry; it's light, stiff, strong, and straight while being relatively inexpensive and easy to work with. Titanium would be another good choice, but it's a lot less affordable, and it's not available in as many handy shapes. People who are really poor (or cheap) sometimes use MDF - it's not very strong, or particularly light, but it's fairly stiff and more dimensionally stable than real wood, and you can cut it pretty easily. But I've never heard of anybody making a CNC gantry out of acrylic; it's floppy, heavy for its strength, hard to cut and drill, and expensive to boot. But maybe your uncle has a warehouse full of it which he's giving you for free, and you're a whizz at working with the stuff - in that case, what the heck - maybe you'll be the first to construct a see-through router. It would probably be interesting to watch it work, anyway...

Andrew Werby
www.computersculpture.com

[wildwestpat]

Hi

Of the three material you have listed the Acrylic has the potential to fracture if stressed and whilst you could build a very attractive machine you would be better served with selecting a more suitable material. If you must use a clear see through plastic polycarbonate would be a better choice. There are reasons why most people use metal or wood and these are to do with the stiffness under dynamic loads impossed by the cutter. Plastics in general are not as stiff weight for weight.

Hope this helps but above all think about the safety not only of yourself but any onlookers.

Pat

[pswayze]

Thanks a lot for the replies. From your input, I've certainly ruled out acrylic. I don't have any experience working with it, I just thought it would be novel. I am, in fact, very cheap and am considering the MDF route only because my intuition says a comparably sized gantry would be an order of magintude more expensive (aluminum vs wood). Thoughts?

[martinw]

Dear pswayze,

Please do not even consider acrylic. It is not easy to machine without specialist tooling, is liable to stress fractures, and will creep under load. Also, the coefficient of thermal expansion is about double (?) that of aluminium ( from memory).

If you have the money, aluminium might be the best bet.

Good luck,

Best wishes,

Martin

[FandZ]

I wouldn't be so quick to jump on the MDF wagon. Yes it can be very strong but will it ever be as strong as or wear better than thicker steel tubes or aluminum extrusions? Really aluminum extrusions is the way to go. You can bolt them together, tap the ends, mount rails etc, with none to very little modifications. For every 1 foot of aluminum extrusion you use, it will probably saves you 5 hours of actual work time in building your machine. At least that's how it would be for me.

If you shop around on the 80/20 ebay store you can find some great deals. I'm sure you could have your framing done for under 150 if you shop carefully. I just ordered 4 pieces for around 50 shipped a few weeks back and each one has a length of 27.5 inches. I'm useing it on my second build. You'll find a lot of people that use 80/20 on their second build. And most of those are because they went the cheap route the first time around.

http://stores.shop.ebay.com/8020-Inc...34Q2ec0Q2em322

[pswayze]

Thanks for all the input. I think everyone made it pretty clear aluminum is the way to go. I'll be sure to keep an eye on the 80/20 store.

[CarbonKevin]

Consider using Baltic Birch plywood. It's advantages over MDF are many - it's multi-ply construction results in an overall stiffer structure. It holds screws extremely well, resists splitting, and will deal with humidity fluctuations better than MDF, too. And it still is extremely easy to work with. Price is above MDF, but I expect it to be below aluminum extrusion.

I don't know if resonant vibrations are ever a problem in CNC machines, but BB ply will be far superior to aluminum in that respect.

I'm building a 28x24x10 unit out of 3/4" BB Ply, and will be posting pics and info once it's complete...

Good luck, either way!

[wildwestpat]

Hi

Yes good point vibration and and ridgidity are important factors. There is no substitute for making all the beams used as stiff as possible for the weight of material used. This means cross bracing and the use of channel sections. I have seen some very impressive machines built from well seasoned wood and quality ply. Over here in England quality ply is expensive so I would go with alloy sections and sheet metal gusets.

While you are considering the basics also think about how you are going to drive each axis. Again solid mounting of all drive components is easier to build in rather than bolt on. For example the gantry coyld have a common flange that stiffens the gantry and also provides a mount for the motor.

Have fun - you have so many options may I suggest you jot down the dream machines spec and think about the sort of things you are going to use it to make. This will help sort out some of the compromises / choices.

Pat

[bajaru]

Hi,

For what it's worth, I've just completed building an MDF router. I chose MDF because it's fairly inexpensive, easy to work with and widely available here in the US. I've only cut wood and MDF on it and my tolerance requirements aren't super tight, as +/- 1/64" is plenty fine for me at the moment. Things are working out quite well. However, the next thing I plan make is a copy of my machine in some kind of high grade plywood, to add some stiffness all the way round which will tighten up the tolerances, perhaps considerably AND allow more aggressive cutting. The cost of a sheet of good cabinet grade plywood starts at about $70 versus $27 for MDF. I don't think starting out with MDF was a waste of time or money at all. MDF was a great way to get started and I did screw up some pieces in the process, making those learnable moments relatively cheap. I didn't know anything when I started out, but if you already know what you're doing, starting a step or two above MDF might be the way to go.

-BrianB

[wildwestpat]

Just a point to note with any laminated product the glue used is way more important than surface finish for structural use.

Reagrds

Pat