[Coco] George's CNC Machine (WAS: Something else)

[Gene Heskett]

On Wednesday 24 September 2008, George Ramsower wrote:
>> On Wednesday 24 September 2008, Chuck Youse wrote:
>>>On Tue, 2008-09-23 at 22:24 -0600, Ron Bihler wrote:
>>>> The speed all depends on how fast you want it to interpret the moves and
>>>> how accurate you need this to be.  Trade off, accuracy requires more
>>>> math
>>>> and processor time hence slower movements.
>>>
>>>Is it not possible to precompute the math?  I.e., do something akin to
>>>compilation - take the time up front to do all the calculations, before
>>>the CNC machine gets any commands at all?  Or does it require feedback
>>>from the machine?
>
>And Gene Heskett replied with this:
>> The output files used as cache would be quite large, and the bandwidth
>> from
>> the disk to the driver interface could still limit things.  The main loop
>> in
>> my emc install, runs at a period of no slower than 50 microseconds per
>> pass
>> through the loop.  The loop itself takes about 15 microseconds on that
>> box,
>> and emc runs its other, not so time critical control loops run in that
>> spare
>> time and do the rest of the calculations on the fly.  There is also a
>> display
>> loop that runs at about 10 hertz to keep the realtime display reasonably
>> well
>> up to date.
>>
>> In my present configuration, there are 2, 8 bit wide outputs to the
>> parport,
>> and at least one read, just to monitor whatever the machine may be
>> reporting.
>>
>> That is 40k a second going out the parport, and 20k coming back in.  I
>> don't
>> think the coco can manage that data rate since its best bandwidth is 11
>> seconds for a megabyte moved.  Bear in mind it has to keep the data
>> flowing
>> even when it is also refilling the output buffer, no pause in the data
>> flow,
>> or even a bit of jitter in the timings can be allowed else the max
>> attainable
>> speed must be reduced until it can manage a steady data flow AND do the
>> housekeeping too.
>>
>> With steppers, there is no feedback to the program, so if the stepper
>> misses a
>> step, it doesn't know about it, and the part may well be wrecked if the
>> operator doesn't hear it.  Usually, if it misses a step, it will stop, and
>> is
>> not able to accelerate enough to get back to speed, so its stalled, and
>> the
>> operator can't miss hearing that unless the spindle is screaming too loud.
>>
>> I think George is doing amazingly well, given the limitations of the
>> coco's
>> speed.  And he is doing it for the sheer love of doing it with a coco.
>> And
>> that folks, IS what its all about.
>
> Thanks you, Gene.
>
> It's been quite an adventure and an amazing chance to re-learn old rules
>and knowledge.
>
> This afternoon, after struggling with duplicating a darned S for four days,
>it occurred to me to fall back on my old training in a college class I took
>many years ago in mechanical drawing.

We have a utility called potrace, which takes a text string and a truetype 
font name & size, and coverts the text string into G-Code that carves the 
characters outlines into whatever is on the table.  I have used it to make 
brass nameplates, one on our mailbox, and one under the doorbell button.

> All I have to do is make a mechanical drawing using my drafting tools (I
>haven't used them in years) to accurately draw the letter. The measurements
>used in making that drawing can then be used to transpose onto my program in
>the coco.
> Doh! I was approximating using a printout from this PC. The letters aren't
>fine lined and I had to just guess at where the centerline of the wide
>printing path is.

potrace does not follow the centerline of the character, it follows the 
outline, up one side and down the other.  With a fixed size endmill, this 
doesn't always look good, so a v cutter is better.

> Mechanical drawings give a much more accurate account of exactly where the
>line is supposed to be. I just started this so the results aren't in, yet.
>I'll keep ya' posted. Later, I'm going to put more information on the my
>Coco CNC website to show more details on the machine, the electronics and
>the programs.
>
> Folks, there is no external circuitry to step those motors other than the
>power transistors to drive the coils.

And how many bits are you using per axis?  One simplification I would make 
would be to through some money into a driver board, which needs only 
direction and step, 2 bits to run one axis, so a single 8 bit wide port can 
run 4 axis's.  I don't think you could go wrong with the xylotex board, which 
among other things can be programmed for the number of steps it microsteps to 
make one whole step.  At the higher step speeds, the lack of the step and 
jiggle due to its much smoother movement is a major dampener of speed 
resonances that can stall a motor that isn't running anywhere near its 
maximum speed.  This board also chopper regulates the motor currant so the 
flat part of the torque curve extends to the theoretical limits, while 
regulating the current at low speeds to prevent overheating the motors.
 
>The coco is doing the actual stepping 
>as would a circuit that takes input from a parallel port in a PC. The coco
>is doing it all. If I used one of those circuits on my Coco CNC, it could
>speed it up somewhat, but not much. The external circuit would keep track of
>which coil the stepper is on and this little detail would not be an issue
>for the coco any longer. This is a small detail that takes little time or
>memory.
> I think I could emulate this to see what difference it would make. However,
>right now I'm working feverishly trying to finish up the alpha characters I
>need to engrave aluminum ID plates for the equipment we have at the rental
>company I work for.
> This effort may not work out for me as they are so tight with money, they
>may not wish to spend a few dollars for a professional looking ID tag but,
>choose to just stamp numbers into them by hand at work.
> I'm hoping to make a buck or two on this project.
>
> Yeah, the coco is slow. But, while it's doing it's thing, I can do other
>things. If I had to make a bunch of parts in one evening, then this may be
>an issue. However, I don't expect to ever have to do mass production with
>it.
> Should this evolve into something like that, I garonteeya that a PC and
>other circuitry will be forthcoming.
> I already have the Linux stuff to do it. Been accumilating things for
>several months.
>
> Ah, the old Boy Scout training...... "Be Prepaired"

:)

>George
>
>
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-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
The world is moving so fast these days that the man who says it can't be
done is generally interrupted by someone doing it.
		-- E. Hubbard