Planning Stage 8 x 4

[alembic]

Hi currently at the planning stage of a cnc router that will take 8 x 4 sheets.
This DIY machine will primarily be used for moulding mdf wardrobe doors and panels in a non commercial aspect.

From what I have seen and read, fully supported rails and rack and pinion drive are the way to go for a machine of this size.

At the moment the base works out at about 2900 x 1400 but I'm unsure about which way to run the x-axis and have been toying with the idea of running a long x-axis as per this modified Joes model here: http://www.youtube.com/watch?v=PoRKGFPmRn0&feature=channel&list=UL]Homemade CNC X axis explained - YouTube
The reason I like this layout is that it seems easier to level and true 2 short rails as opposed to 2 long rails.

So the question is what are the pro's and cons of such a long x axis?

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[PaulRowntree]

At the moment the base works out at about 2900 x 1400 but I'm unsure about which way to run the x-axis and have been toying with the idea of running a long x-axis as per this modified Joes model here:
The reason I like this layout is that it seems easier to level and true 2 short rails as opposed to 2 long rails.

So the question is what are the pro's and cons of such a long x axis?

Ease of access to the work table is the clear plus for me, and flex in a long beam is the clear downside. Can you do the deflection calculations for a 20 kg router+carriage in the middle of the span? Then add ~80N of cutting forces. It would be an interesting result to share.

Cheers!

[alembic]

I have read about the effects of deflection but as someone who prefers to think on their feet, will try to overcome this during the stages of the build by making modifications as I go along.
With many materials from so many different sources involved, can measurement of deflection every be that accurate pre-build?

Are there any other pros/cons of a long x-axis.

Thanks

[PaulRowntree]

I have read about the effects of deflection but as someone who prefers to think on their feet, will try to overcome this during the stages of the build by making modifications as I go along.
With many materials from so many different sources involved, can measurement of deflection every be that accurate pre-build?

I too thought I would figure out this type of thing on the fly, but to be honest, I think it was a fairly large mistake on my part. It is so easy to overbuild, which means unnecessary weight and poor performance, or to underbuild, because most of us don't worry about deflections measured in thousandths of an inch in our day-to-day work. I thought that 2.5"x3" I beams would be stiff enough to use as main supports, but nope; a disappointing surprise. Of course, your intuition could be much better than mine, but your gantry idea is also much less conventional.

You can't size your motors without a considering the weights and forces, and you can't get the weights without thinking of structure and stiffness. Plus, the calcs are pretty easy to do (download free program BeamBoy and you are away).

Deflection calculations are at the heart of most mechanical and civil engineering projects, so I would say the theory is well in hand.

YMMV.

Cheers!

[ger21]

I agree with Paul.
Easy to load sheets, easy to get quite a bit of deflection in the beam.

[louieatienza]

Check out CR Onsrud's Extreme Series CNC routers. They use the gantry for the long axis, although it is fixed, with the table moving in the short axis. Granted, the bridge ismade of cast iron, but maybe it will give you an idea as far as design.

[alembic]

Thanks all
Decided to go the more traditional route of short x and long y.

Probably have a million other questions throughtout the build.