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Feed slowing due to data bottleneck?

Posted by stevempotter 
Feed slowing due to data bottleneck?
August 10, 2019 07:43AM
I am carving complex 3D terrain. I am using F360's 3D Parallel toolpath, and find that no matter what feed rate I set, it slows way down whenever it has to do a lot of Z movement. Is there some setting I can use to speed things up? I get a lot of burning because the cutter is traveling through the wood too slowly at times. I have turned the RPM all they way down to 1 to reduce this problem. I have the Adventure Edition with 1.6.2 Fabmo.
Thanks, Steve Potter
Re: Feed slowing due to data bottleneck?
August 11, 2019 02:50AM
what is your plunge rate set to?
Re: Feed slowing due to data bottleneck?
August 11, 2019 02:55PM
Feed was set to 2000mm/min (33mm/sec, 1.3"/sec), plunge to 1000mm/min (16mm/sec 0.7"/sec). Those should be really fast. But it oozes along at about 5mm/sec when it is doing a lot of X,Y and Z at the same time.
Re: Feed slowing due to data bottleneck?
August 11, 2019 03:24PM

G2 (the motion control system that underlies fabmo) uses "S-Shaped Ramping" when changing speeds. This means that on a 3D terrain carving, the when the Z Axis reverses from plunging to lifting up, the speed change (-0.7ips to 0.7ips) is governed by this "ramping" strategy. Typically I keep my Z axis ramping set to 25 inches/s^3 -- that is inches per second per second per second (the rate of change of acceleration) -- this is a safe value and keeps me from pushing the Z motor too hard when it is trying to slow down or speed up to high speeds.

However, in your situation, your Z axis is never going to get to high speeds because it is constantly reversing between up and down motions. In this situation, you could afford to use higher jerk (ramping) values for Z. These settings are found in the configuration menu under "axes" -- try setting the jerk values for X, Y and Z to 4x their current value (yours will be in metric so the numbers will be different from what I referenced above).

Also, you'll want to put cap on your feedrate -- the tool won't be trying to move that fast in the 3D carving, but when it jogs into place for the cut, it can get up to high speeds. Limit that by setting your feedrate and jog maximums to 2000mm/min for X and Y and 1000mm/min for Z.

That should keep the tool under control and allow you to fly through the 3D moves a lot faster.

Re: Feed slowing due to data bottleneck?
August 12, 2019 10:00PM

I made a video of a handibot doing a topographic map cut with different jerk values. [youtu.be]
In each example clip -- the handibot speed is set to the same value (6 inches per second). The only thing that is changed is the Jerk for the X Y and Z axes -- it gives a pretty good idea of how much Jerk can limit movement when there are lots of directional changes in a file. It also shows what happens when the jerk is set too high!

Re: Feed slowing due to data bottleneck?
August 14, 2019 03:40AM
Thanks, Brian! That video is exciting because it suggests that I may not have to carve for 3 hours to do a decent 150x200mm terrain carving.
Thanks for explaining about the s-curves and jerk settings. I am glad it was not data starving (bumping up against the limits of the microprocessor speed)
I will make the adjustments you suggested for my next carving!
Re: Feed slowing due to data bottleneck?
August 14, 2019 07:30AM
OK, for those of you who have their machine set to mm by default:

I empirically set the max jerk for X, Y to 2000 mm/min^3 and
Z to 4000 mm/min^3. This seems to produce pretty fast motion carving terrain.
Since the Z axis never travels very far it does not accelerate to as high a speed and therefore can tolerate higher jerk than X & Y without missing steps.

I have not tested this in a scenario where I would notice if steps were lost, so I will post here if these turned out to be too high.

Brian, did you confuse /min^3 and /sec^3 in your video test? (I note that the Fabmo Configuration uses /min^3, not /sec^3.) 25 IPS/S/S I think would be 38,100 mm/min^3.
Re: Feed slowing due to data bottleneck?
August 14, 2019 07:46AM
Ah you’re right! I do all my vCarve speed settings in ips— 1 inch per second is so much easier to visualise than 60 inches per minute. I forget that FabMo uses ipm . I’ll fix video
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