Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: Davek0974 on October 23, 2016, 04:50:53 AM

Title: Just when you think you are finally getting there....
Post by: Davek0974 on October 23, 2016, 04:50:53 AM
It kicks you in the wotsits again....

I thought i'd try engraving on the bridgeport conversion today, got a little job coming up, knocked up a simple job - just the word 'Test' in V-Carve, 2mm deep 30deg 0.2mm tool.

Worked fine on my mini-mill before.

Every time it hits line N2230 i get the damn ePid fault and everything stops. :(

I tried 50% feed-rate, no change.

The motion seems very harsh, its lots of little moves but the machine shakes when it does it.

Acceleration too high? It's currently set on 400mm/s/s and speed is 3500mm/min and seems pretty low i think??

I have 0.05mm backlash comp set at 10% speed.

Its been working well up to now.

Any ideas???
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on October 23, 2016, 06:16:12 AM
Well, i lowered the acceleration to 350 and increased the max following error in Pid tuning to 400 from 300 on the X axis - this was the one that was faulting.

Seems to be ok now, it did the test run ok anyway.
Title: Re: Just when you think you are finally getting there....
Post by: robertspark on October 23, 2016, 06:24:40 AM
Google search epid cismo turned up a few hits

Here's one, few theories, mach3 version changed fixed it for at least one person...

Cismo problem...https://www.machsupport.com/forum/index.php?topic=29032.0

Tried their support

What versions of everything are you running (m3, cismo plugin, how much ram, graphics card ram (is it dedicated)... seems a long way down the line numbers and strange to always hit at the same place when all they are is x Y motion (no noise induced tasks... spindle speed change, coolant, air, whatever...)
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on October 23, 2016, 09:02:30 AM
Running Mach 062, CS_Labs reckon 022 but have been told that one has many other issues that make it not a good choice.

The PC is just a standard Dell Optiplex GX520 no idea on specs but have them running Mach on 3 machines reliably.

Plugin version is the one delivered with system, not sure on ver but dated 27th Jan 2016.

Its the X axis that was faulting, maybe i just had the max following error setting too tight?
Title: Re: Just when you think you are finally getting there....
Post by: robertspark on October 23, 2016, 09:10:46 AM
Sorry Dave, can't help more.
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on October 23, 2016, 09:20:35 AM
No worries, its working now thats the main thing, it may have just been me being too tight with the settings.

I think the 300 limit i had equates to 0.15mm following error before faulting, too tight for rapid reversals?
Title: Re: Just when you think you are finally getting there....
Post by: Ford Prefect on October 23, 2016, 10:21:11 AM
I find its better to start slow and increase both feedrates and not so much spindle speeds, and the smaller the bit size the lower the speed but hey this is teaching old dogs new tricks

regards Ford
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 27, 2016, 06:41:32 AM
Ok, so what's not right here?

Cut some 5mm Alu on the high-speed spindle yesterday and it looked ok i thought, confidence was restored :)

Went out and did another part today, same file, but results looked poor and scrutinising yesterdays work closer it seems the fault was there too but not quite as pronounced.

it looks like the toolpath on the finish cut is pulling away from the corners too soon - corner rounding??

The finish tool is a 2mm single flute carbide at 23000rpm, 1000mm/min feed.

Not sure why todays was worse than yesterday although the machine was slightly warmer yesterday as this was the second job run.

Great care was taken when doing the CAD&CAM to get the tool right into that corner to give a 1mm radius

The surface is actually smooth - the rough look is swarf stuck with cutting fluid;)

I did do some major servo tuning but have not altered speeds or acceleration in motor tuning.

What could do this, any suggestions?
These parts need to be accurate :)
Title: Re: Just when you think you are finally getting there....
Post by: mc on November 27, 2016, 07:43:42 AM
I'm going to guess that's around where the axis speeds starts/finishes changing speed for/after the corner?
Title: Re: Just when you think you are finally getting there....
Post by: robertspark on November 27, 2016, 07:44:30 AM
What acceleration are you using?

The problem with mach3 is the CV mode tradjectory following is not tunable.... Like it is in some if the newer cnc software
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 27, 2016, 08:00:30 AM
Its set at 3500mm/min and 350mm/s/s on X&Y with 2000mm/min and 750mm/s/s on Z
Title: Re: Just when you think you are finally getting there....
Post by: robertspark on November 27, 2016, 08:21:35 AM
linear distance travelled during accel or deceleration -  0.5 x V^2 / a = 0.5 x (1000/60)^2 / 350 = 0.4mm

centripetal acceleration radius = V^2 / acc = (1000 / 60)^2 / 350 = 0.79mm

hence at a feedrate of 1000mm/min and with an accel (+decel) of 250mm/s/s, you are likley to end up with a 0.8mm radius on inside and outside corners ..... without consideration of tool offset which may affect this  given a 2mm dia tool is nt going to end up with a 0.8mm insider radius in the corners .... you may be looking at 1mm + 0.8 etc

then again all of the above could be bull!   (just something I'm reading up on at the moment.... I've not been under the hood with mach3 to figure out the tradectory planner following error )
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 27, 2016, 08:31:50 AM
Bull or not, it's above my math grade ;)

But what does it all mean?

Surely if the mathematical rounding is 0.8mm and my physical rad is 1mm i should be good to go as machine rad < cutter rad ? If i had a machine rad of say 5mm and a cutter rad of 1mm then i would expect to see a 5mm radius regardless of tool dia??

Clearly i have something not set right?

I doubt the servo tuning has any input here as the following errors are lower now than before.
Title: Re: Just when you think you are finally getting there....
Post by: robertspark on November 27, 2016, 09:07:17 AM
https://www.khanacademy.org/science/physics/centripetal-force-and-gravitation/centripetal-acceleration-tutoria/a/what-is-centripetal-acceleration

If you had a 5mm tool, with tool compensation, at those feed and accel rates, I'd still expect a 0.8mm error as one axis must slow down and the other speed up to maintain velocity
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 27, 2016, 09:32:12 AM
Ok, thats complex. But what does it mean to my issue?

Do I just need to whack the acceleration up, retune the servos and test?

What sort of figure would I need to aim for?
Title: Re: Just when you think you are finally getting there....
Post by: robertspark on November 27, 2016, 10:41:29 AM
Given acceleration is generally fixed as high as it can be, I would suggest lowering the feedrate in the corners using your post processor (sheetcam can do it, no idea about vectric).

Basically lowering your feedrate 1mm before and after the corner to 60% will probably sort it ....

Sorry out and about at present, no pc access
Title: Re: Just when you think you are finally getting there....
Post by: Hood on November 27, 2016, 11:07:36 AM
Could well be a CV issue but I am also wondering if you have backlash comp enabled and maybe that is also a factor.
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 27, 2016, 12:33:01 PM
No, BC is turned off on this setup.

I don't think Vectric can do the trickery sheet cam can.
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 27, 2016, 05:05:29 PM
What settings should the mill have for CV distance and stop cv on angle boxes??

At present CV distance is turned off in my screen set and there is no cv angle box at all (2010 screen set) but i guess it still has a value set somewhere?
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 28, 2016, 07:54:41 AM
I stumbled upon this little gem from Art this morning, excellent analogy of CV, explains a lot....


::Why doesnt CV run at commanded feedrate, or why is it code dependent.?

CV ( constant Velocity ) is an ideal, not a reality. The laws of physics show it cannot be done. I mean just jump in your car and try to drive a square path around the block at a constant speed. You cannot do it under any circumstances without tearing off your tires, or rounding the corner when you spin the wheel. ( or stopping at the corners and having someone pick up the car and turn it. :) )

CV is really the name of an option that ATTEMPTS to do a constant velocity of the path the Gcode calls for. In truth its name should be "Best-Speed Optimization" There are many methods of trying to do it, some better than others. It depends not only on the code itself, but on the lookahead. It depends on so many things in a planner that each type of planner has its own issues with it and its own variation of how the feedrate reacts to Gcode.

Mach3 is a bang-bang accelerative trapezoidal velocity planner, and a very good one for almost all code types. Its CV is created by the simple rule of beginning the acceleration of the next motion when the acceleration of the current move goes negative. ( deceleration phase). This means you are literally adding one move to the next , allowing for the speed to increase. Im sure many have read this before and can picture it in the context of two motions where the second is added to the first , thus making the deceleration phase virtually invisable ( thus constant velocity ), but what I , or Brian, rarely mention is that it operates in full lookahead, this means that in tiny segment code, the code of a line may be passed over by the time sequence of the planner as it progresses, but that motions speed must still be added in to the total of the motions distance.

This means its an iterative process, where up to 100 lines or more may be actively added together in terms of their deceleration phases being added to the next motions acceleration phase. This can be hard to wrap the head around. Each Gcode line has to be considered fully in the iterative calculations involved. Imagine tiny segment code where the angle of the next motion is constantly shifting ( typical 3d work ), and the processor is trying to keep track of the real speed thats possible. Since the consistancy of the speed is a function of the additive properties of each motions deceleration phase, it means the max speed an individual motion can do is important in the functions total speed accumulation.

I know, still hard to wrap your head around. So back to the car analogy, you hop in your car and try to CV your way at 30 MPH around the block ( 4 lines of Gcode), you can probably do it, but the corners will be rounded and slower than the straights and the faster you go, the more rounded the corners, but the speed will be fairly constant.. bit slower at the corners...

OK so far, but in 3d GCode, that 4 line trip around the block is replaced with 1,000 small 1 foot motions around the block,AND you dont know your actually driving around the block. With 20 line lookahead, and 1 foot moves, you get to know only that you need to drive 20 feet ahead at a time. SO naturally, you will be light on the gas pedal, and will drive much slower because for all you know, the wife may yell STOP!! after any 20 foot section. You wont get up to the speed you did when the block was 4 lines of Gcode..
Now after 10 feet, your wife deigns to tell you that your going at least 20 more feet.. you still wont stomp the gas, after all you MAY need to stop soon.. this situation repeats till your all the way around the block.

This is the crux of 3d slowdowns and feedrate limitations in CV modes. Even if we increase the efficiency of your wifes glasses so she can see 100 feet ahead ( 100 lines of lookahead ), youll still need to drive slowly.. though a bit faster than with the old glasses. And thats with straight lines. Now if the wife is constantly saying " Move an inch to the left..now right..now left..", youll slow down even more. A computer is nowhere near as smart as you are.. ( well, most of you. :) ) , so it too will drive pretty slow, the sole advantage in fact is the computer has endless patience and wont end up smacking the wife about the head and ears for the back seat driving..

Yes, ( I can smell the oil burning. :) ), it IS possible to look further ahead, recalculate current motion speed based on upcoming traffic and conditions, and then recalculate all motion based on the future events, but this has ramifications in other more basic calculations the planners need to do in their uSecond to uSecond activities. Work is being done in that area, but we're still a ways off from finally cracking that nut to allow CV to operate in a more mathmatically pure way. For one thing, the actual mass effects need to be taken into account to let planners know the true maximum speed that should be allowed for any motion to motion joining process, properly taken into account that would be the basis for the next generation of planners to start the CV process. Conformity plays an import part as well, Mach3 is famous for driving almost any machine, so next generation planners have to take into account that not all systems are created equal, this means that they must be more systematic in how they apply calculations and must base them on physics engines that better reflect the actual world they live in.

Hopefully, this explains a bit about the challenges of CV modes ( or better yet "Best Speed Optimizers" ). Its hard to convey whats really going on under the hood and the limitations each planner type has, but Ive been deeply dipped in the theory of planners, and I can tell you from an inside view that you assist your CV by two methods, first, never raise the feedrate commanded thinking this will help you go faster, youll find best speed is obtained by commanding a speed the code can actually do. What I typically do is run the code in the air for a few minutes, see what it does, and then change the F-Word to match what you actually get. For example if you command F1000 and get 62.5 , I then reset the system F70. This actually helps the system go a bit faster. ( due to some peculiarities in the way such algorithms work.), and second, set the lookahead (LA) as large as you dare for that code. 200 lines LA will work much faster than 20 line LA, just like buying your wife better glasses.

For those that dream of a system that takes corners at the commanded speed, forget it, aint gonna happen, never will, and if someone offers you such a system...back slowly away.. :-) , their breaking the laws of physics, and Hawking has a mean left hook.

It IS possible for some manufacturures to have software that does CV much better, but they do suffer from other ailments as a result, and quite often will drive only the machine or drive mechanisms they were written for, this makes those solutions bad for the general case. Its all a matter of the tradeoffs one selects in their planner design and operations, and how the machines vary in the general population. Like backlash, its a theoretical way of approaching a perfection that is provable to be impossible. Its my hope that some day tiny code will CV exactly as large code does, but like the world we live in , things get annoyingly hard as you go smaller and smaller.. just ask Einstein...

Thanks,
Art


So, it seems the solution to ALL CV rounding issues is simply more acceleration - make it so the change in one axis slowing as the next is accelerating is so small that it's pretty much invisible. Of course slowing the code down has the same effect but this can damage the work and/or cutter to some degree so not really an option in many cases - plasma will burn a hole if too slow, routers will burn the wood and mills will eat cutters due to rubbing instead of cutting unless spindle speed is lowered to match.

I will revisit my setup and tune the servos after setting a far higher acceleration, see what that does.
Title: Re: Just when you think you are finally getting there....
Post by: robertspark on November 28, 2016, 08:37:01 AM
... centripetal acceleration....

Increase acceleration or decrease feedrate in the corners to get LEDs following error
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 28, 2016, 08:39:57 AM
As far as i know, its only SheetCam that can do whacky, useful things with feed-rates on demand.

I'll tune her up a notch and see what happens.
Title: Re: Just when you think you are finally getting there....
Post by: TOTALLYRC on November 28, 2016, 11:39:06 AM
Onecnc XR7 has just added corner feedrate adjustment. Only helpful if you have the program of course.
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 28, 2016, 01:26:09 PM
I've doubled the acceleration to 700mm/s/s and checked the servo pid settings - following error did not alter and is still sub 250 on X&Y.

Air cutting the same code seems ok, you can feel it is a lot sharper when direction changes but will need to do some test cuts to verify.
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 29, 2016, 12:47:19 PM
Instant improvement, same code as before, same feeds etc just the increased acceleration in motor tuning.

Nice corners, no sign of rounding at all. :)

At least it now works up to 1000mm/min feed rates, no idea how much beyond that as i have no job that needs it.
Title: Re: Just when you think you are finally getting there....
Post by: Hood on November 29, 2016, 01:51:46 PM
Good news, never realised you originally had such slow acceleration but good its sorted now.
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 29, 2016, 02:03:50 PM
Seems so, without any firm figures i just plonked it somewhere it seemed happy at - seems it was ok at the lower speeds but then i upped the feed rate to book specs because i was seeing premature tool wear and thats when it started rounding corners.

It probably will do it again but unless i do wood or plastics on her, I can't see much work needing to go faster than 1000mm/min. - this is on aluminium with a 2mm carbide tool so pretty much the fastest CAM needed i think.
Title: Re: Just when you think you are finally getting there....
Post by: robertspark on November 29, 2016, 02:07:03 PM
Good to hear Dave.

Looks like it really is all about acceleration.
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 29, 2016, 02:14:44 PM
It does, I was just babying her i think ;) Looking at the setup with it's 32mm ball screws and proper A/C thrust bearings in there now, I am confident that it will be ok.

Would be interesting to see what sort of figures are in use on the big boys vmc's.
Title: Re: Just when you think you are finally getting there....
Post by: robertspark on November 29, 2016, 02:38:05 PM
It seems like few want to post their acceleration and velocity settings, I was look for numbers on the plasma, and managed to find Jim Colt stating 40miliG as a starting point target, and some hyperthem literature as 10miliG as a plasma minimum, hence why I was reading about centripetal acceleration

 40 miliG = 0.04 x G (~9.81m/sec) => 392.4mm/s/sec.

But not found an idiot's guide explaining the importance of acceleration in cnc world (yet)
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 29, 2016, 03:43:47 PM
Just been trawling, and yes its pretty well hidden :) Lots of speed figures, found this nice little machine, would look good in my shed....

http://www.sodick.com/products/highspeedmilling/hs650l.htm

boasts a nice fast spindle and 1G of acceleration at pretty insane table speeds :)
Title: Re: Just when you think you are finally getting there....
Post by: robertspark on November 29, 2016, 03:49:41 PM
haha.... think that would like nice in everyones shed  .... whats the address of your shed ;)

.... think I'd end up having to sell the house and living in the shed to aford it... (or when SWMBO finds out)...
Title: Re: Just when you think you are finally getting there....
Post by: Hood on November 29, 2016, 05:03:21 PM
ppfffft 1G, that's a hobby machine, new versions of my Chiron are 75m/min rapids and either 10, 15 or 20m/s/s accel  :o
Title: Re: Just when you think you are finally getting there....
Post by: Davek0974 on November 30, 2016, 02:09:24 AM
Wow, is that servo or linear motor?

And no i haven't looked up what linear motors are yet, i just stumbled upon them during last nights trawling session drooling over machine *********. ;) Seems they are the next step up for axis motion.
Title: Re: Just when you think you are finally getting there....
Post by: Hood on November 30, 2016, 02:40:15 AM
I would imagine at those kind of rapids and accel they would be linear motors but not looked to see.