Round perimeters are bumpy when printed fast

I just turned up the perimeter feed rate from 40 mm/sec to 45 mm/sec and I notice that suddenly, my Prusa is having trouble with printing curves smoothly. I've watched the hot end moving and it looks like it vibrates every time it reaches the end of one of the facets of a curve. Here's a crappy picture taken from an in-progress print:



I know people have gotten perimeter feed rates higher than 40 mm/sec, so there must be some problem here. Any ideas?

Vital statistics:
Prusa/Sanguinololu/Sprinter/Pronterface/SFact/directly connected via USB to Mac OS X host/White PLA

Edited 1 time(s). Last edit at 10/02/2011 12:55PM by Pointedstick.

This is a product of ramping acceleration. It is the entire reason why people run their perimeter below 20mm/s. If you would like to try printing fast without the bumps try Marlin [github.com]

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FFF Settings Calculator	Gcode post processors	Geometric Object Deposition Tool Blog
Tantillus.org	Mini Printable Lathe	How NOT to install a Pololu driver

I also started a thread in the firmware forum (http://forums.reprap.org/read.php?147,99606) that address this issue. Once again, I beg to differ with Sublime. This is not just a product ramping acceleration in the firmware. I think that the microcontroller is not fast enough to keep up and so there's a pause between the small line segments that make up the circle. I get this with and without acceleration.

Anyway, as Sublime suggested, you can reduce this by printing the perimeters at 20 mm/s. It also works better if you print in the sequence: perimeter-loops-infill. If you print the perimeter after the loops, the loops will have blobs which will show up through the perimeter. While, if you print the perimeter first, then the blobs in the loops won't affect the perimeter as much.

I've actually moved to printing "Loops, Infill, Perimeter" in the last week. I found I get more accurate parts, I think its because the perimeter has a backing to hold it in place and not shrink.

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FFF Settings Calculator	Gcode post processors	Geometric Object Deposition Tool Blog
Tantillus.org	Mini Printable Lathe	How NOT to install a Pololu driver

Thanks for the info, guys. Sounds like we should continue this discussion in the other thread: [forums.reprap.org]

Edited 1 time(s). Last edit at 10/02/2011 02:23PM by Pointedstick.