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Maximum processing speed?
« on: February 10, 2015, 05:55:05 AM »
I am doing some raster 3D carving, and noting that the speed seems limited in ways that can't be explained by the actual machine.

I have a 6"x6" relief with a 1/16" ballnose doing 20% stepover.  So 2880 linear inches.  Feedrate 400ipm.   By itself, 2880/400= 7.2 min.  Which, I  understand, decelerating to a stop to lift up over a feature with an abrupt change in Z-height takes extra time.  And I watched it, on flat areas it rips right across but really slows to a crawl across detail with Z-movement.

The machine's XY axes were set at 600 ipm 40 in/sec/sec accel, z set at 200/50.  This got 37:30 runtime.  So I play with possible more aggressive numbers for acceleration and get only modest gains.  

For information purposes I went in and entered the max acceleration Mach3 allowed, which was 187.  Well it lowered the runtime some, to 22:44.    Tried kicking Z up to 400 ipm and still no improvement.  Kicked XY down to 400 ipm max, which opened up acceleration to be set to 281 in/sec/sec.  Still no significant change in runtime.  

Well if acceleration was the limiting factor, those crazy high acceleration numbers should have brought it close to 7.2 min.  But no dice.
 
Had a theory "maybe the Ethernet Smoothstepper has a processing limit" so I changed to a profile with Parallel Port.  No difference. 

Loaded Mach3 on a different PC just in case somehow it was looking at processor speed and slowing down the run if the PC couldn't calculate that fast.  No difference.

Changed Kernel Speed and restarted.  No difference.

Doubled the ipm in the file, no effect. 

So it seems like motor performance is not the reason time cannot be brought below 22:44.  Mach3 is introducing some sort of speed limit across detail that has nothing to do with acceleration- what is it??
« Last Edit: February 10, 2015, 06:01:13 AM by MechanoMan »

Offline Hood

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Re: Maximum processing speed?
« Reply #1 on: February 10, 2015, 07:12:38 AM »
Are you in exact stop mode?
What is your look ahead set to?

Hood

Offline ger21

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Re: Maximum processing speed?
« Reply #2 on: February 10, 2015, 07:42:18 AM »
A friend of mine makes guitar necks. With Mach3 and a UC100, his programs took 1hr 5 minutes. When he switched to the UCCNC control software (which controls the UC100), the same program would run in 45 minutes. Roughly 30% speed increase, and more accuracy. It appears that there are issues with the trajectory planner in Mach3.

One thing you can try is to increase the lookahead to a higher number. My froend says it increases the speed, but results in Mach3 occasionally rounding some corners.
Gerry

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Re: Maximum processing speed?
« Reply #3 on: February 10, 2015, 01:17:58 PM »
Switching to Exact Stop, it doubled the runtime, so it definitely wasn't using it before.  So I went back to CV mode.

Lookahead was 20.  I changed to 40 and I changed to 5.  Nothing made a difference.

I did watch the actual feedrate during the physical run.  It changed so much you couldn't make anything of it, but I saw like 120- 70 when it went into the detail work.  Adjusting the Feedrate % slider had no effect.  The Feedrate was already set to 400 ipm but it wasn't ever able to do that so setting to 200% didn't change anything.  I'm suring lowering it to 25% (100 ipm max) would start to slow it down, of course.
« Last Edit: February 10, 2015, 01:28:56 PM by MechanoMan »

Offline ger21

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Re: Maximum processing speed?
« Reply #4 on: February 10, 2015, 01:46:16 PM »
Change the lookahead to 200 and see if it makes any difference.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: Maximum processing speed?
« Reply #5 on: February 10, 2015, 01:54:03 PM »
Nope, no difference.
Re: Maximum processing speed?
« Reply #6 on: February 10, 2015, 10:03:41 PM »
Think about this:  If acceleration is set for 281 in/sec/sec the equation is D=1/2AT^2 and V=AT so to get to 400ipm or 6.666 inches per second in 0.023 seconds in a distance of .079" and to stop will take another 0.079" and for a total move required of 0.158" in.  0.046 seconds to start and stop while reaching 400ipm.  Note that at a constant velocity of 400ipm the same distance 0.158" is traversed in half the time or 0.023 seconds.  This means that the best you could accomplish with moves of 0.158" starting and stopping is twice as long as the total distance at a constant 400 ipm.  Shorter moves make the time longer, longer moves reduce the time.

Offline BR549

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Re: Maximum processing speed?
« Reply #7 on: February 10, 2015, 11:03:42 PM »
Several things happening here. The Z cannot move any faster than the Acceleration values will let it in. AND the XY cannot move any faster than the Z will let them.

To do 3d cnc well with Micro moves it takes a very light  fast excellerating machine to do it fast.  TOP speed means very little. It is ALL about how FAST you can excellerate .

When your machine runs I bet it gets nowhere near the Feedrate you have set.

Just a thought, (;-) TP
Re: Maximum processing speed?
« Reply #8 on: February 11, 2015, 10:47:35 AM »
Could you post the gcode?
Re: Maximum processing speed?
« Reply #9 on: February 11, 2015, 01:54:35 PM »
Think about this:  If acceleration is set for 281 in/sec/sec the equation is D=1/2AT^2 and V=AT so to get to 400ipm or 6.666 inches per second in 0.023 seconds in a distance of .079" and to stop will take another 0.079" and for a total move required of 0.158" in.  0.046 seconds to start and stop while reaching 400ipm.  Note that at a constant velocity of 400ipm the same distance 0.158" is traversed in half the time or 0.023 seconds.  This means that the best you could accomplish with moves of 0.158" starting and stopping is twice as long as the total distance at a constant 400 ipm.  Shorter moves make the time longer, longer moves reduce the time.


But the numbers disprove that.  Feedrate time would be 7.2 minutes.  Of course you don't actually get that with 3D carving.  It's taking 33.5 min, which is 26.3 min overhead- but can this be explained as acceleration delays?  Let's see.

Well, time spent in acceleration would be easily tested because it should be fundamentally proportional to the reciprocal of the acceleration parameters.  If you double the acceleration parameters for all axes, that would halve the acceleration delays so that overhead time would be halved.  That should be 13.15min overhead, plus the 7.2 min, is 20.35 min.  But I get 25.13 min, so there's still 17.9 min of overhead.  So a full 4.75 min of runtime can't be explained.


So next I change acceleration by 10x, so it should be 7.2 min+ 2.26 min =9.5min.  But no, it's 19.9 minutes, it's still got a 12.7 min overhead, of which 10.1 min is unexplained. 

Well then that CAN'T be caused by acceleration delays, because the overhead does NOT scale with the reciprocal of acceleration.  Something additional is going on inside Mach3 that has nothing to do with acceleration delays.