Re: [CAD_CAM_EDM_DRO] Question No. 1

sutherland_oz@... wrote:

> Hi all in the group, I'm planning to build a small home workshop cnc
> mill, lots of questions to come but where possible I'll try to find
> answers on previous posts (so many) I've been following the recent
> posts on limit switches & would like some clarification on what is
> required on each axis, I think I need a hard limit switch at each end
> of travel to switch off stepper power in the event of an "H bridge"
> failure, (this would require a full program restart) inside these I

> would need a switch at such a position that the axis could decelerate
> from rapid to stop without tripping the hard limits (program able to
> continue) I think that these can be software switches in the program,
> also at one end of the movement I would require a hard switch for
> home position (needs to be very repeatable). Am I close or way off
> the track?

You don't say whether you are wanting to use steppers or servo motors.
A failure of a transistor in a stepper driver will cause the motor to stop,
in most cases. It will buzz and vibrate, but not move more than the
equivalent
of 2 steps. If the transistor failed while the motor was moving at high
speeds,
it might be able to continue on one phase until the computer commanded a
slower speed, then it would stall. There is a possibility for some stepper
drivers with sophisticated retiming (like the Gecko and other microstepping
drives) to run away. There is also a possibility for any closed-loop
stepper or servo system to run away if the position feedback information
is lost (loose connection, loose shaft coupling, failed encoder, software,
firmware or logic problem). Unless you are working with large, truly lethal

machinery, so much attention to limits is not really needed. (On gigantic
machines like room-sized bed mills, the peak velocities and the size of
the equipment make safety a much more serious concern, as thousand
pound workpieces hurled through a wall at 50 Miles/hour could kill people.)

> I have some "Omron" photo electric limit switches type No.
> EE-SX308-P1 (no data yet) which have a LED & photo diode in a plastic
> mount such that an opaque vane can be inserted between them, may be
> suitable for home switches & perhaps the vane could have a mechanical
> advantage of 4 to 1 or such to improve their accuracy. Any comments
> greatly appreciated.

The only problem with optical switches is that they can be fouled by metal
chips and/or coolant and oil. If you can be pretty sure to keep chips out
of the sensor and the vane, they should work fine. You can use another
optical switch reading a disc with one slot on the stepper motor to refine
the coarse home position. Combine the two signals with an and gate, so
the computer only senses home when the coarse switch sees the vane
and the fine one sees the slot. (There are other ways to do this, too.)

> Also what cnc program should I look at to begin
> with, & is something available on the net that I could start
> with, "Dancad" perhaps?

I use Bobcad/CAM, mostly because it integrates both CAD and CAM
functions in one package. I had heard so many horror stories of
this CAM package not properly accepting files from that CAD
package, that I wanted to be sure to avoid such a mess.
Bobcad/CAM works, is not terribly expensive, but does have some
deficiencies. Still, it gets the more complex work done for me. The
simpler work (boring a hole here, a square slot there, circle of holes
around there) I do with some short C programs I cooked up, because
they are very efficient on my time.

Jon