How to find max retraction speed  

Re: How to find max retraction speed

I'm wondering how to find the fastest possible retraction speed for the Original PRUSA i3 MK2.

It seems to me (with a whole couple of weeks 3D printing experience) that the recommendation for setting the retraction speed is to set it as fast as the printer can handle it, and to approach the limit 'by ear'. As in, it will start to make different noises once the retraction speed is set too high.

Has anyone here done this, and what was the outcome?

I'd be quite happy (for now) to use a maximum retraction speed that is only almost as fast as possible, if I can get away with using someone else's test results...

Cheers and thanks for any enlightening hints, Mik

I'm quite curious about this as well, and I read similar notes as yourself regarding how to tell if it's too fast. I have frankly no idea what such a difference in sound would be like, and I wish there was a video about it.

This is setting aside the limitations of a given filament, of course. Certain filaments will not handle retraction as well as others, nor do they need them as much as others.

Re: How to find max retraction speed

The maximum extruder speed hard-coded in 3.0.8 is 120mm/sec however when retracting you will never reach that speed when retracting less than 1mm due to acceleration.

#define DEFAULT_MAX_FEEDRATE {500, 500, 12, 120} // (mm/sec)


There are 3 acceleration settings:

#define DEFAULT_MAX_ACCELERATION {9000,9000,500,10000} // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for Skeinforge 40+, for older versions raise them a lot.
#define DEFAULT_ACCELERATION 1500 // X, Y, Z and E max acceleration in mm/s^2 for printing moves
#define DEFAULT_RETRACT_ACCELERATION 1500 // X, Y, Z and E max acceleration in mm/s^2 for retracts


I am not certain which of the 3 is used - I would guess that the last value is used for hardware retraction - but would assume that the middle value of 1500mm/s^2 is the relevant figure. Basically to reach a speed of 120mm/s^2 you would have to retract about 4.8mm.

Please note that my maths is not great and my estimate may be incorrect. I also do not take account of the jerk value (currently set to 2.5mm/sec) which I believe would be used as a starting value.

Peter

Re: How to find max retraction speed

I recall seeing the same notes regarding firmware maximums before. The related question then become is the firmware maximum speed of 120mm/s practical? Assuming the filament doesn't break/deform are there reasons not to operate retraction that fast?

Re: How to find max retraction speed

There is no reason not to use that retraction speed; it was increased in 3.0.8 to facilitate unloading and therefore under most circumstances is attainable, but it does occasionally fail (I recall Patrik making comments regarding this).

Peter

Re: How to find max retraction speed

There is no reason not to use that retraction speed; it was increased in 3.0.8 to facilitate unloading and therefore under most circumstances is attainable, but it does occasionally fail (I recall Patrik making comments regarding this).

Peter

When it does fail due to the speed being too high, what is the failure mode? What happens?

Re: How to find max retraction speed

Extruder skips. Ends up with the filament not being ejected.

I think it has also happened to me twice since I installed 3.0.8. and worked fine the rest of the time.

Peter