design decision

[Varske]

Hi,
I was reading this forum for a while and finally I am going to build my own cnc router. (I wish I would discover this forum before buying anything!)
The good thing is that I already purchased major parts. and the bad thing is that these purchases are limiting my creativity and the whole design.
working area is around 175x60x20cm
Frame will be build of 20x80 t-slot profiles (literally 20x80mm) because I got them for a very good price. Most likely they will be used as 40x80 sandwiches or 'H' beams
Linear rails are Chinese TRH20, some cheaper 'hiwin' copies
got 2x 195mm and 2x 105mm with 12 carriage blocks.
Also took same amount of SBR16 rail (just to re-sell and get back some shipping expenses)
SFU1605 ball screws:
1x2000mm, 1x 700mm, 1x 300mm (didn't think about speed before buying)
steppers: 3x nema23 312 oz-in, with tb6600 drivers (yes, I know they are crap )
bosch router will be used as spindle for a while.
first I was thinking about to build a moving gantry machine, but as long as I am limited with 3 screws and not going to buy the second 2m length, that long 'floating' table is scaring me because of it's flexibility and possible vibrations.
and the plan B is to build fixed gantry/moving table machine(110mm SBR16 rails with 450mm bearing spacing for table, 1m distance between rails). but because of wrongly chosen rail length I will have to use round rails either for moving table or gantry's top rail. Not going to buy another set. at least not now.
Here are examples of designs I am considering between:

these are only 'sketches'.
I know that round rail's performance is lower than square ones, but wondering if they can be used for table without facing too much of their disadvantages? 312 oz-in morots shouldn't create big forces any way. My biggest concern is vibration and surface finishing.
Another idea is to use one full length round rail on the top of long fixed gantry. it would have to compensate router's cutting force created torque.


Router will be used for wood cutting, also would be nice to upgrade it with water cooled spindle and steel frame in order to cut aluminum sheets... some day

Would be nice to hear a good advise, opinion for one or another design. how about table racking? any experience with these rails spread so far apart? for moving gantry: is it possible to have 2m floating table made of 20x80 profiles without worrying about flexibility too much? should I consider in size reduction (like 60x120mm)?

Thanks!

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[awerby]

Moving table/ fixed bridge designs are generally more rigid. It's easier to build a table that won't flex than a moving gantry that's rigid. But they take a lot more floorspace, which is why the moving gantry designs are more popular.

As any component gets larger in a linear dimension, it needs to be built more heavily. So you can build a smaller machine a lot more cheaply than a larger one. If you already have the materials, you need to decide whether you want a smaller, tighter build or a larger looser one. The further apart the X axis trucks are spaced, the more susceptible it will be to racking, if driven from the middle with a single screw. People wanting to avoid that use dual motors on the long axis, and either dual screws or racks and pinions.

[Varske]

Thanks for reply,
These general rules are known and makes a lot of sense. I want to believe that 45cm distance between round rail bearings with 50cm distance from rail to screw should eliminate/reduce racking chance. I would like to know if anyone had racking problems with these bearings and what distances between bearings and rail to screw were used.