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Author Topic: Limitations of PC Parrallel Port  (Read 6675 times)

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Offline Hood

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Re: Limitations of PC Parrallel Port
« Reply #10 on: June 19, 2009, 06:47:19 AM »
Another reason that fast accell is probably better  is because if you had slow accell you may be bothered with corner rounding whilst in Constant Velocity mode. If that was the case then you would be using Exact Stop which would put more strain on the hardware.
 Its probably not really worth bothering about but I would certainly prefer to compromise my  rapids to get faster accell rather than the other way around, even if its just from a machining point of view, and you will probably find the faster accell/slower rapids would make the cycle time faster than the other way around.

Hood
Re: Limitations of PC Parrallel Port
« Reply #11 on: June 19, 2009, 12:37:11 PM »
Thanks guys! 

Great feedback, but, guess what?  It generates more questions.

Back to my intention for this machine.  I'm looking for the greatest accuracy I can get for my pool cue inlays.  The more precise the fit, the less likely it will be that you will see the "glue lines" around the inlay.  In the research I've done so far, I read that Constant Velocity mode may create "rounded corners".  This would be unacceptable, so I was thinking I would use Exact Stop mode.  However, I agree with simpson36 that this would potentially cause "burning" due to the zero chip load during the exact stop.  In addition, Hood makes the statement "using Exact Stop which would put more strain on the hardware".  So now I'm torn on what mode to use.

So here's my questions.  Can you elaborate on the "strain on the hardware" statement when using Exact Stop mode.  I can understand the strain on the hardware during high speed start/stop operation.  Since cycle time is not a priority for me, would this statement still be true if I tuned my motors for a relatively slow rapid rate?  Conversely, can I eliminate the "corner rounding" aspect of Constant Velocity mode by setting my acceleration as high as possible?
Bill (the Cat) Shubert

Offline Hood

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Re: Limitations of PC Parrallel Port
« Reply #12 on: June 19, 2009, 01:55:31 PM »
If the machining speed was fast as is often the case in materials such as wood then exact stop can often cause very jerky movements, especially if the code is made up of lots of small segments. The machine itself will play a part in how things are affected, if its a very rigid machine then its unlikely to be an issue, if its a light fast machine then rigidity will be less and problems may be seen.
 As I said above its probably nothing really to worry about, you can always tune the motors differently as you see how your machine performs. Steppers will never have ultra fast accelleration but the better it is then the better it will perform in CV.
 I suppose in a less rigid machine even having a fast accelleration could cause problems so best not to think too much about it and go get the machine finished ;D

Hood

Offline simpson36

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Re: Limitations of PC Parrallel Port
« Reply #13 on: June 19, 2009, 02:16:31 PM »
I can point you to a feature in Mach that you will then need to study up on and experiment with.

The CV feature does have some controls that you have access to for tailoring the behavior for specific needs/jobs. I am no expert in these settings and while I have been known to occasionally cut the cheese, I don't cut wood so I'll have no specific advice for you.

You can define within CV, as I recall,  at what angle the CV will disengage. With this control, theoretically, you can have CV active to avoid the shaky movements that Hood described while still having the ability to, within reason, keep your corners sharp.

I cut a lot of aluminum with small (sub 3/8") end mills at 7,000 RPM so I have similar concerns to people cutting wood at say 20,000 or more, and thus far I have only encountered on precision shaped pocket that required expressly invoking exact stop mode.

CV does not round off corners randomly. Is is proportional to the feedrate. Sort of like driving your car; the faster you are travelling, the sooner you need to start turning and the arc you can sustain is going to be bigger the faster you are going. So IF you are not able to obtain a completely satisfactory result with the CV controls, you can still 'tune' a particular cut by slowing down the feed rate with a proportional reduction in spindle speed to prevent burning at problem spots in the process.

The above is not an ideal solution, but then again there is no logic in dissecting too closely a problem that may not even exist, so as Hood suggested, there is a time to talk and a time to just get in there and see what she'll do for you. Methinks you are at the latter.





« Last Edit: June 19, 2009, 02:19:42 PM by simpson36 »
Re: Limitations of PC Parrallel Port
« Reply #14 on: June 19, 2009, 04:35:11 PM »
I was not aware that you could tailor CV mode.  I will have to search on that subject to see what I can find out.  My guess at this point is that I WILL use CV mode, but keep my feedrates low to minimize it's affects.

You're both correct.  At some point I'll have to "just do it".  I just wanted to take advantage of the knowledge and experience in this forum and not "reinvent the wheel".

Thanks again to both of you!!!!!!!
Bill (the Cat) Shubert

Offline Hood

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Re: Limitations of PC Parrallel Port
« Reply #15 on: June 19, 2009, 04:40:41 PM »
Best thing is to try and make your machine as rigid as possible, that way you will have less issues.
Hood

Offline RICH

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Re: Limitations of PC Parrallel Port
« Reply #16 on: June 19, 2009, 06:51:53 PM »
Hi Bill,
You may want to send Bill a message. Maybe he'll share with you some experience on how he achieves his great inlay work. Here is a link to his posting on cue sticks.

http://www.machsupport.com/forum/index.php/topic,3348.msg25656.html#msg25656

After you get your machine done, you may want to try and find a reliable setting where velocity
equals acceleration setting and experiment some with your machine.

RICH
Re: Limitations of PC Parrallel Port
« Reply #17 on: June 20, 2009, 10:22:49 AM »
Thanks for the link Rich!  I'll definitely be getting in touch with Bill!
Bill (the Cat) Shubert