Re: [CAD_CAM_EDM_DRO] Re: robotic workcell design

skykotech wrote:

>The main reason I suggested linear rails and carriages is because you
>can bolt them directly to the extrusion for a very easy motion
>solution. My shizuoka uses box ways...I don't see how box ways could
>be cheaper than linear rails/carriages. Also, with box ways, if you
>lose lubrication things get nasty pretty fast. When I visualized
>this gantry, I pictured something that could be transported by two
>individuals and did not require a lubrication pump :-)
>

This is not always a problem. We have a whole bunch of machines at
work, some using linear bearings and others using old fashion ways that
run on grease. The problem with linear rails, the ones with balls, is
that you can not run them without lubrication. For certain opertions
that lubrication can be a problem. It is in my mind a question of
what you are targeting the machine at, for some materials and usages the
linear rails with the ball cars are a problem.

>
>The dead cheapest solution would be 3/4" ground shafting (about $15 a
>foot from MSC) and Thomson Frelon 3/4" bearings (about $11 each).
>Those bearings don't require lubrication and can actually operate
>with *heavier* loads than the ball bearing type. They are maybe not
>as accurate as the ball bearing type. Things get much harder if you
>want to support the shaft. I was amazed at how much an unsupported
>18" section of solid 3/4 inch shafting deflects when force is applied
>in the center. Still, for a light duty machine, maybe?
>

Round shafting and frelon bearings are certianly suitable for some parts
of the machine. For some parts of the machine adjustable bearings are
in order. As you have noted it doesn't take much to deflect the
shafting, but again the importance of that issue that depends on the
machines intended usage. Ideally though your shafting should be
supported by a more robust base or frame. Instead of using full
diameter bearings you could go for the partial wrap bearings.

In my estimation the selection of a plain bearing will result in a more
versatile machine which may be good for the intended users. If
someone is to use the machine to work on wood or fiberglass parts for
example you would be better able to deal with the gunk that forms. On
a machine this size the loss in performance on other materials would not
be significant.

>
>I would not mind coming up with a 3 axis servo controller that
>plugged into a PCI slot. There are very nice PCI bridge chips out
>there now, and there is even a $199 card with a bridge and FPGA that
>includes a core for a servo controller. The problem is that it would
>be a lot of work and worthy of a series of articles by itself. :-)
>

Ahh an electrical destraction in an otherwise mechanical problem. AS
you note a whole article in and of itself, but don't let that stop
you. The market can always use fresh hardware, especially hardware
that solves problems cheaply.

>
>I hate steppers, but maybe for a first run...
>

Properly applied steppers are a very good deal. It is certainly true
that servos have come a very long way relative to price and performance,
but servos introduce their own problems into a machines design. In
any event steppers are still the cheap choice for the smaller machines.

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>Circuit Cellar is another decent magazine that might be interested in
>this type of article.
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