A new round

[lum]

Hello

Long time ago I was about to start my first CNC-router project. I did some light research started to save up some budgetary means and time went on.
Then. Short story. Life came in between.

Now I'm here again. And in a much better seat than the last time. This time I have some monetery resources available from the start AND a-ok from the lady!

Anyhow. I have been lurking around now and then during the years and have gathered some info. Also a decent idea of a machine in my mind. Funny enough it would seem like several people have started similar projects lately.

Basically I'm thinking the old moving table, stiff gantry deluxe. Medium sized with solid engineering. I even started to do some light CAD-work but after seeing:
Help and guidence with new desing
and:
NEW CNC BUILD! Brilliant Minds needed within!
I kind of lost my CAD muse...

Besides they both incorporated most of my own ideas anyway...

Here goes: I'm thinking along Sturmer's design but haven't still decided if the gantry should be long and the table short or the other way around. I'm thinking 500 x 800 mm working area. I'm thinking relatively big steppers (900/500/300oz? 1.8/1.2/0.9/0.45deg?) and probably g540(?). I'm thinking modular where I start with cheap components and a wood/polymer frame and upgrade when the wallet allows and so on.

Well one thing led to another and here I am with a small... list of topics that I'd love to get some input for:

1: The goal. Well I'd like to end up with medium speed but good accuracy. Closer to 0.01mm than 0.1 preferably. As far as materials go: wood, alu, copper, composites incl. PCB, polymers and maybe light steel...yeah only that...doable?

2: The frame. As stated I will be starting of with wood or some polymer. maybe PolyCarb or Plexi? Suggestions are welcome. But later o I wish to move onto a metal frame. I'm thinking solid alu sheet 1/2-2" or ext alu profiles filed with some kind of dampening stuff (Sand, cement, epoxy mixture?). On the other hand maybe go all the way to steel or filed square steel profiles? Also what about the table orientation? Is really 1000mm gantry with so much worse that 700mm stiffness wise? A note worth mentioning is that I want to be able to fix a fourth axle or drainage or a vacuum table in the future if I feel I need it.

3: The screws. I was thinking ground zero backlash c5-c3 ball screws but can't afford it now at the beginning. So for now I'm mostly choosing between C7 ball screw from:
http://cgi.ebay.com/1-backlash-balls...2#ht_600wt_911
and the oh so popular ACME with a good nut. How do these compare? I'm more interested in precision than speed. What's a good pitch for a machine like this? 5mm? Also are the polymer ACME nuts good?

4: The Rails. Now in the long run I'd love to have some good linear square rails. But for now I'll probably go with 20/16/12 (y/x/z) from linearmotionbearings on ebay. Then I saw this: http://www.pbclinear.com/Metric-Open-Bearing-(FMN)
Will they work with the said fully supported round rails? Are they good? Expencive? and do they work in the SBRxxUU blocks directly or do I need custom blocks?

5: The motors. In a machine this size how big motors should I run. I'd like them to be future-proof, so when I upgrade the rest they should still play nicely. I'm thinking big for the two stationary ones. 300oz or maybe 900oz(these ones only get to about 500oz with the g540 I hear). Thinking 200-300oz on z. But then I find these other steppers with 1.2/0.9 and even 0.45 deg true steps! But they are also weaker. Some 200oz for the stronger 0.9 deg ones and around half that for the 0.45 deg ones. Can the precisiongain be worth it? Could 200oz be enough on X and Y? 100oz on Z? Mostly looking here:
http://www.linengineering.com/site/products/5709.html
And would they play nice with the gecko?

6: The Software. This one is short. At least for now. EMC2 or Eagle3? I hear eagle has great microstepping but can ECM match it?'

If you red all that or left some input then THANK YOU!

Now enough of this for now

[lum]

Few simple sketches.

















Most brackets and small details are missing.

The white working area is 800 x 500 mm. Z travle is 100 mm.

[lum]

Do you guys think the bottom of the gantry should be reinforced even more? For rigidity that is?

And is that kind of x-axis rail support as rigid as the front facing type seen on this?



The height of the gantry is adjustable by +-100mm. That's why the sides are so tall. Still working on those side brackets for the gantry. Aiming for three bolt-in positions to make it quick and easy to change.

It's also pretty likely I'll go with an alu construction from the start. Found some extra credits available and it's cheaper in the long run.