CNC mechanics, linear rods

[Yaro]

Hi all, I'm going to build a CNC 40w laser. Dimensions i've choose for the laser was 1000mm x 1000mm , and the material are stainless chromed rods of 12 mm diameter. But, someone said me that, a long and heavy rod like this(or in general, rods longher then 300mm) tents to bend under its own weight of less then 1mm in the middle and if I add on it something like a laser head, the rod will have a significant bend. Someone can confirm that's true? And with laser application where we have acrylic sheets from 2mm to 5mm will influence a lot the focus?

If it's true I tought to reduce to 500mm or 400mm chromed rods and use, for fixed X axis, a long rod support that need to carry the more heavy movable Y axis. But the problem is how to choose materials for Y axis, i tought 2 or 3 rods, but they are heavy. Someone can say me the ideal diamater i should use with 2 rods on Y axis without any rod support? And if i use a chromed or a regular aluminium tube?, they will be more light than full rods and will offer the same resistance.

Or this consideration will not influence a short rod of 500mm or 400mm?

Thank you.

[Yaro]

I add another question. If i use SBR/TBR linear bearings, how much i need to use of these per rod? if I use one SBR/TBR per rod there can be a possible vibration on himself?

[TLHarrell]

For something that size you will need to use some sort of supported linear rail/bearing system. Aluminum extrusion with a THK type rail is best for the application. Makerslide with v-wheel carriage is also a decent option, especially for light weight applications like a laser carriage. Unsupported round shaft is not recommended.

[Yaro]

Thank you for your advice. THK is something like this?
http://i00.i.aliimg.com/wsphoto/v0/4998 ... n-font.jpg

But seeing his form seems it need to be attached to something looking his hole.

So THK on friction coefficient and mechanical resistance is better than makerslide?

Can you advice me how max lenght i can accomplish without problems with THK and Makerslide? Considering i'm a weight like this: http://i00.i.aliimg.com/wsphoto/v0/1727 ... 20x220.jpg about 100g

[Yaro]

if i use a T slot like this:
http://www.cnczone.com/forums/attachmen ... ds-_12-jpg

It's 20mm x 20mm(small but seems resistant) of 400mm lenght, i will be sure that won't flex on 100g moving weight? So, do you advice me to use an MGN attached on it or a chrome rod with support attached on it? and 1 or 2 carts on a single MGN/chrome rod linear bearing?

[TLHarrell]

Check the structural calculators at the Misumi and 8020 sites to verify your application. Addition of the THK rail will add to the rigidity of the gantry beam, and is extremely well constrained. MakerSlide is far less expensive, and is essentially a 2040 extrusion with rail already attached. For 1000mm of travel, you will need around 1100-1200mm of rail depending on design of the carriage and ends.