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chuckknigh
09-11-2004, 11:44 PM
As most of you know, my first attempt at a CNC machine had a few nice little innovations. Well, I'm in the process of designing a new one to replace it, and have a question.

On many machines on this site (7th sojourn inspired, I think) it looks like the moving table is unrestrained. It simply "sits" on the rails, and is held on by gravity...racking is controlled by a second set of bearings. But, the bottom line is that only gravity holds it on.

My first machine had the bearings located such that I could turn it upside down and the axes would stay in place...but this created a different set of issues...outward thrust and the flexibility of the rails.

In an attempt to further simplify, I'm thinking of running unrestrained rollers over a fully supported track, but with only gravity holding down a moving table. Is this a good solution, in practice? Seems like it should work.

What other issues would it create? Lift from upward spiraling bits, perhaps? Like I said...I did it differently, last time...and I'm just trying to simplify.

-- Chuck Knight

balsaman
09-12-2004, 09:00 AM
It works for light loads....as you could probably guess. Figure out what you plan on doing with your machine, and then go from there.

Eric

ynneb
09-12-2004, 10:02 AM
.
An un-restrained axe.

http://www.weyburnreview.com/News/2002/2002_25/130-head-chop-rg.jpg




Sorry Chuck but I couldnt help recycle an old joke. :)

chuckknigh
09-12-2004, 03:04 PM
Hey, be nice! I had to look up the plural form of "axis."

I still can't find anyone to tell me, definitively, the plural of "computer mouse." I presume it's mice, but I've heard many say it as mouses (to distinguish from rodents, I presume) and after today's little problem with my pastor's mouse, I can honestly say that I hates meeses to pieces.

:-)

Thanks, Eric -- yours is the perfect answer.

It's just a little bit harder to build it so that it's constrained...not a major problem, but ways to simplify are always worth examining.

-- Chuck Knight

sol
09-12-2004, 08:55 PM
I have never seen wood remain flat unless forced to do so; daily and seasonal humidity changes invariably cause warping of some sort and if the table is rectangular (rather than triangular) the slightest cupping will cause one bearing to lift off of the track... and there goes any hope of stability.
I posted an image in your Corian thread a while back as a bit of a joke, of a simple bearing truck that allows rail support while preventing uplift. The truck has also been made of Al and works well...there is a recently posted pic of that too somewhere here.
Not trying to push a bearing design on you, just saying it is so cheap and easy to make an assembly that prevents uplift I cannot imagine not building it into the system unless the table has to be easily removable.

And GuruNet says it is mouses :D

jgro
09-13-2004, 02:52 PM
I ended up going with this type of design for my second machine. I made the table out of 1x1 steel tubing welded together. I did this after trying in vain to find a flat piece of MDF or wood. The table weighs in at around 20 lbs, so lift should not be a factor. You can also figure that the lead screw does help to keep the table down, depending on what you are using for a coupler at the stepper side. I guess I'll learn more as time goes on as to whether this is going to work or not.

jgro