Re: [CAD_CAM_EDM_DRO] re:OK - you convinced me - was "Automated "Manual CNC"

Howdy All! Just took a quick look on eBay for "servo motors" and turned
up several. I also thought as Brian did, that servos would be
outrageously expensive. But once more, the collected wisdom of this
group shines through.

Rick Dulas

http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=455612817
http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=451878305
http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=450494469
*http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=457342979

*for the really big project

ballendo@... wrote:

> 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
>
>
>
>
>
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