re:OK - you convinced me - was "Automated "Manual CNC"

Brian,
-answers interspersed below

This setup would probably be excellent for situations where cutting
does not take place while the table was moving - like drilling an
index plate - move to XY, drill down, drill up, move to XY, etc.

-Yes,exactly. And that's how it was used early in the NC lifespan.

But my problem is now: can I achieve full CNC with plain linear
encoders, plain leadscrews, and a CNC program like one of those
listed below?

-qualified Yes. encoders ok.cnc program ok. leadscrews ok,but...

-If you are moving slowly(hand milling speeds)and can deal with the
marks backlash WILL put on your parts, using leadscrews is fine.

-The problem(and others on the list will surely agree)is that once
you have CNC, you start using it... Then you REALLY start using it...
Then you see how easy it would be to do this part, the one that you
just never did before you had CNC(because it WAS so hard). Next thing
you know, you're pushing beyond the limits of your present setup, and
you start looking for ballscrews, or at least anti-backlash nuts for
the screws you have(see previous posts re:moglice, Ron Ginger is a
resident expert)

-The other thing is that you find yourself pushing the feedrate up.
Yes,even hobbiests who "swore" they'd NEVER need to go that fast!
The original screws in most machines are designed for hand-cranked
speeds and wear. THIS is what gets ya!

-Here let me say again,Ballscrews are NOT the ONLY answer! Kerk
leadscrews with a/b nuts ARE an alternative. Having said that, if I
was doing a bridgeport, I'd use ballscrews.

A) I would like to use a plain DC motor coupled to the lead screw
through a gear reduction. They would be interfaced with the PC
through a driver where the speed could be varied, accelerated,
decelerated, etc. Is this do-able or do I have to buy full fledged
servo motors?

-A servo motor IS a plain DC motor.(can be AC, I know) It's like I
said in an electronics thread earlier today. Many things are easy to
do, harder to do WELL. "full-fledged" servo motors are optimised for
efficiency and accel/decel, so they will work (much)better than a
plain ol' motor. BUT,MANY on this list will tell you they have spent
next to nothing for the servos(real ones) driving their machine(s)!
Check some previous posts to re-assure yourself THIS is NOT a big
deal! Pretty typical reductions are 2:1 or 3:1 Usually a timing belt
drive is used. I MUCH prefer the HTD style tooth to the USA classic
HL"trapezoid". Again, check thru the back msg's.

B)I would like to use linear encoders like that of digital readouts.
I know the update frequency is not quite up to speed, but if I adjust
my accel/decel & top speed to compensate for this, could I still use
them?

-Yes. And depending on your scales, you may find you don't have the
limits to speed. Post what you have(or have in mind) to the list.
Someone's prob'ly using 'em now!

I would have to build and interface between them and the PC. Is
this do-able or do I have to buy faster encoders?

-Not necessarily. There are at least a couple of "encoder reader"
boards available and mentioned on this mail list.


C) What are your thoughts on not converting to ballscrews with table
feedback?
-Some of what I put in the last post(and written above)still applies,
but I'll add:

-IF you use a good one-shot lubricator(often), IF you keep your
speeds down, IF you don't use the machine very often, IF you don't
run for long periods of time(causes heat build up and dimensional
problems),IF you can live with the backlash and don't mind re-
adjusting the nuts for wear OFTEN,IF you understand you'll need
larger motors and drives because of the friction,IF you REALLY can't
afford it now...

-Hopefully you can see that the switch to ballscrews isn't really
just for accuracy. The OTHER benefits can be MORE important

-Really, using the leadscrews can be OK, but MOST of the time people
going this route wish they had just done it all at once. Like I keep
saying, what are YOUR GOALS for this "CNC thing". In the homebuilt
aircraft arena, there are BUILDERS, and there are FLYERS. More rare,
is the BUILDER_FLYER. Which are you? What makes you happy?

-You can take one of two common approaches.

-Do a GREAT job on the mechanicals, and figure on upgrading the
electricals/software as you can. Advantages here are the "tightening
up" of the machine, less-expensive motors/drives can be used to get
started, this part is usually harder (effort and time wise); you can
throw money at the s/w & electricals more easily than you can pay
someone to come work on your machine.

-or

-Really go after the S/W and electricals, and figure you'll upgrade
the machine later. Advantages here are using s/w and electronics to
overcome mechanical limitations, Usually a faster "time from start to
chips" (performance may be lower, but enthusiasm is increased, making
it easier to do whatever's next)

-See, it really depends on YOU...


D) Would EMC be a likely contender with this setup?

-I would have to say, especially with the zipslack 10 minute CD's
appearing to work for a number of people(see previous posts and
http://www.linuxcnc.org ) that EMC is a likely contender for anybody
with 2 caveats:

-1. The lookahead currently does not work well for files of "lotsa
short lines" although Jon Elson has said recently he's gotten some
decent results from engraving(That WAS you,Jon, wasn't it?)FAST
computer, I think.

-2. The gcode program is not pre-processed, so jumps forward and
back, (broken tool)re-start, and subroutines, loops, and parametric
programs are difficult to impossible.

-The price is right. The code is free to modify. People are working
(HARD)on it so improvement(s) are likely.

PERSONALLY, I find something to LOVE(and something to HATE) in nearly
all the CNC controls I've ever used. So I use a number of them
(depends on what I'm doin'). Works for me, but others have more
success REALLY learning the ins and outs of ONE program.

Hope this helps, and I'm not banned from the list for filling
mailboxes with long posts.

Ballendo