Re: Introduction/Ideas/Questions

stratton@... wrote:

> Once I get tired of "manual numerical control" I really want to put a
> drive on at least the crossfeed.

I made an ellipsoidal mirror this way! VERY tedious!

> Alas the crossfeed screw has tons of backlash due to a worn nut. I'm
> thinking of doing a conversion to 5/8-5 rolled ballscrews, but I
> haven't yet decided if the nut will fit in the channel in the slide
> (there is not much extra iron there to allow for machining it larger).

This is a tricky problem, but there are several solutions. One is to mount
the whole ballscrew assembly BEHIND the carriage, and have a stiff
link (like 1/2" steel rod) to the crossslide upper part.

Another (more complicated) approach is to make a new crossslide
upper part. Knowing that there will be no need for a compound slide,
you can make the upper part 'taller' to accomodate the ballscrew
inside it, and make the top compatible with a toolpost.

I may go with the first option, and see how it does.

> For my own experiments, I'm thinking of buying a prototype card that
> comes with address decode in a pal, 8255 for parallel I/O, 8254 timer,
> etc and then using that as a base for the Ls7266 encoder chips and
> possible servo DAC's... in fact, I wouldn't be suprised if that's how
> the servo-to-go developed. But my first bit of hardware (to be built
> in the next 24 hours) will probably be a parallel port interface for
> the 7266, to get basic DRO functionality without any need to open the
> computer case.

You might look into some of the FPGA/CPLD chips that have a library
of PCI interface code available.The ISA bus is somewhat of a bottleneck
for the real-time functions.

> Computer:
>
> Current plan is to use a P120 ultimately under rtlinux/EMC, but
> probably under dos/qbasic for initial DRO experiments. (We have linux
> running a house network, and I did a test install of EMC on another
> machine yesterday). I'm going to put it on the opposite side of a
> partition wall behind the lathe with long keyboard/mouse/video cables.
> Monitor is likley to be a little monoVGA that does only 640x480 - I
> can buy a better one if needed, but it seems like that is the best to
> sacrifice to the shop enviornment. I'll build a bracket on the wall
> to hold it over the lathe headstock.

I have a 100 MHz Pentium I built into a rack-mount case, dedicated
to machine control. It is in a rack with the servo amps, power supplies,
and space left over for a drawer of measuring instruments and small
tools used on the machines. It is generally behind me when I'm working
on the machine, and I have a desk-mount keyboard/monitor arm adapted
to the side of the mill for a 'workstation'.

> Keyboards/Mice:
>
> It hardly seems worth it to spend 20 bucks on a type-through cover to
> protect a $5 keyboard... Perhaps I'll experiment with clear plastic
> bags. Or just buy lots of $5 keyboards. Mice may be an issue, too.

I bought one of the $20 kbd covers because I was worried about what
sort of crash I might get if a chip shorted out the guts of the keyboard,
and it read it as a key press on some key like 'jog'. And, EMC tends
to be blinded by too many keys pressed, so that holding down any
key that has a continued action prevents any other key (like E-stop)
from being recognized. (This may have been corrected in a later
version than I'm running.) So, I thought it was worth it to reduce the
likelyhood of such an event. Also, I have a big, mushroom E-stop
manual switch right on the video monitor, that bypasses the computer
entirely. It has been useful a couple of times.

>
> The MIT 3d printer had a standard mac mouse restrained with a wire tie
> and sitting on a little fold out half-mousepad platform on the side of

The particular video monitor I used has a large flat area on top of the
case, just big enough for a mouse pad. Someday, maybe I'll swap a
trackball in its place.

Jon