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Author Topic: .300" overshoot at 40"/min when using feed hold  (Read 7373 times)

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Offline dresda

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.300" overshoot at 40"/min when using feed hold
« on: February 11, 2011, 08:55:59 PM »
As the title says. Using DSPMC, bridgeport style mill, 30inch/lb servo motors with encoders and tach feedback.
Spoke with Rufi and he said the acc/dec was fixed and hopes to make it the same as Mach3, I'm lost for words...
I can jog at 120inch/min in rapid and stop on a dime. I can press the program stop in program and it stops instantly(it would be nice if I could restart, but it will not ) but in feed hold at 40"/min it overshoots  .300" before it stops and that's not a complex move with lots of data.
G01 f40. x-20.
I suck at integrating pc's to machine tools , but with over 30 years in the CNC machine tool business I know there is a problem, but with what?
I would love to get some feed back.
Ray.
Re: .300" overshoot at 40"/min when using feed hold
« Reply #1 on: February 11, 2011, 09:25:04 PM »
I don't understand what the problem is....  If you want it to stop RIGHT NOW, you press Stop.  But, if you do so, and it's moving fast, you may, or may not, lose position, as Mach3 simply stops outputting step pulses - there is no deceleration at all.  With steppers, you're pretty much guaranteed to lose position.  With servos, you may, or may not lose position, depending on how fast the machine was moving, how much following error your servo drives allow before faulting, and the servo tuning.  If you press FeedHold, the machine does NOT stop immediately.  It stops when it's convenient for Mach3 to stop.  This means it's likely to complete the current segment, and it will decelerate to a stop, so it WILL move some distance before all motion stops.  The faster you're moving, the further it will move.  This is not a bug, it is how it is designed to work.

Regards,
Ray L.
Regards,
Ray L.

Offline dresda

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Re: .300" overshoot at 40"/min when using feed hold
« Reply #2 on: February 11, 2011, 09:57:15 PM »
So if I move at 120"/min in jog and remove my finger from the jog botton it stops dead and I don't loose postion why would it loose position at 40"/min? I have a milltronics with a 386sx computer with servo's & analogue feedback that can stop dead in feed hold.
I think Mach3 is great value and has lots to offer but I need the closed loop system and don't want to loose pulses that's why I went with the DSPMC. I have high hopes for this system but you could not use it in an industrial application.
Is there any way I can use the program stop and then restart the program, i know I will not loose postion.
Imagine approaching your job at just 40"/min and pressing feed hold, at .300" overshoot the damage is done.
Ray.
Re: .300" overshoot at 40"/min when using feed hold
« Reply #3 on: February 12, 2011, 03:29:39 AM »
So if I move at 120"/min in jog and remove my finger from the jog botton it stops dead and I don't loose postion why would it loose position at 40"/min? I have a milltronics with a 386sx computer with servo's & analogue feedback that can stop dead in feed hold.
I think Mach3 is great value and has lots to offer but I need the closed loop system and don't want to loose pulses that's why I went with the DSPMC. I have high hopes for this system but you could not use it in an industrial application.
Is there any way I can use the program stop and then restart the program, i know I will not loose postion.
Imagine approaching your job at just 40"/min and pressing feed hold, at .300" overshoot the damage is done.
Ray.

hello Ray,

I use feed hold and yes, Mach does not stop that instant, but the position control is maintained -
I use mach in an industrial application, it has produced many 1000s of components and never lost position once using feed hold.
The only time I have lost position is when crashing / breaking 10mm carbide cutters - done it twice.
Pressing STOP is another matter, you can be in a rapid or feed move, pressing stop halts the pulse train so the table, motors etc are expected to stop in an instant - my steppers can't handle this - I doubt if any industrial system can - OK with a closed loop system it will go out of position (by a much lesser degree) and then recover position automatically.

So you have a 0.300" over run but you are still in position - is that correct? Your job shoud still be OK, as the tool is still on the correct tool path?
It's only scrapped if the tool goes off the desired path - or am I misiunderstanding the problem?

ATB
Derek
You can "chop it off" but can't "chop it on"

Offline Jeff_Birt

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Re: .300" overshoot at 40"/min when using feed hold
« Reply #4 on: February 12, 2011, 01:13:02 PM »
Quote
So if I move at 120"/min in jog and remove my finger from the jog botton it stops dead and I don't loose postion why would it loose position at 40"/min? I have a milltronics with a 386sx computer with servo's & analogue feedback that can stop dead in feed hold.

When you press feed hold Mach looks at what it is currently doing and then decides when to stop based on what GCode is currently executing. If you are near the program block Mach will probably pause there. If you are in the middle of a long move Mach will plan on pausing at some point that makes sense to it (some point from which it can start right back up.) When you are jogging and release the button you are in essence ending the command to move so all Mach has to do is decelerate, it does not have to look at where it is at in a large GCode file to decide where the best place to pause is.
Happy machining , Jeff Birt
 

Offline dresda

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Re: .300" overshoot at 40"/min when using feed hold
« Reply #5 on: February 12, 2011, 05:25:52 PM »
I understand what you are saying. I don't have a problem with this but I know my customers will. If you watch a really good CNC operator lets say on a lathe, he only uses the cycle start, feed hold and FRO. Imagine coming up to a shoulder, he would be using the cycle start and feed hold and may have the FRO set below max rapid, he needs to know that the will stop when he wants it to, not overrun. I am using a closed loop system and I'm not loosing postion when in rapid and I stop dead, I don't get a following error at 120"/min.
I did some more tests. Now, for some reason I can press program stop and then cycle start again (couldn't do that yesterday) and what happens is It must look at the look ahead blocks (lets say 100 blocks) restarts ignores all the blocks between the stop point and starts a 100 blocks ahead like nothing happened except the spindle turned off, and I don't loose postion.
I know It's not a big ticket item like Fanuc and that's why I am testing it, big price difference and I think it's great.
I have a milltronics with servos and encoders and a 386SX computer and stops on a dime in feed hold.
I am not knocking it, but I have never seen a control PC or otherwise stop like this.
Ray.

Offline dresda

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Re: .300" overshoot at 40"/min when using feed hold
« Reply #6 on: February 13, 2011, 11:47:23 PM »
So should it be allowed to restart after pressing program stop and make a B line to the look ahead point and run from there like nothing happened.
Ray.

Offline Jeff_Birt

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Re: .300" overshoot at 40"/min when using feed hold
« Reply #7 on: February 14, 2011, 11:28:34 AM »
If you press 'Feed Hold' you can press 'Start' and have no worries. If you press 'Feed Hold', let it stop, then press 'Stop', you should also be fine. If you press 'Stop' without pressing 'Feed Hold' first, and letting the machine come to a stop then you could have problems. Stop stops things immediately with no deceleration so the machine may be out of position.
Happy machining , Jeff Birt
 
Re: .300" overshoot at 40"/min when using feed hold
« Reply #8 on: February 14, 2011, 12:44:12 PM »
Ray,

If you stop a run before the end,(I always pause then stop as Jeff said) you'll need to scroll to the last block executed (or any number of blocks (lines) before) and select "run from here"
mach will do some preparation moves (take care here !!) and then the run will continue.

If you press pause without then pressing stop, the tool will stop on the tool path and then continue along its merry way after you press start again.

If you press pause, stop, the axis movement stops as well as the spindle, if you press start again, Mach will obay the gcode as written but as there is no M03 spindle start instruction, mach will try cutting without the spindle running.
You migt be able to manually start the spindle (F5) or by clicking the spindle on icon, then press run Start and away you go.

I always use the run from here method.

ATB
Derek

So should it be allowed to restart after pressing program stop and make a B line to the look ahead point and run from there like nothing happened.
Ray.
You can "chop it off" but can't "chop it on"