Printing at constant speed

Hello! I just bought a reprap (an i3x) and am wondering how to set the printer to have a constant print speed? I want the nozzle to go at a constant slow or medium speed the entire time.

I'm not sure if this has to be done in Marlin's configuration file in the firmware, or if I need to do this in Pronterface. Ideally I'd do it in Pronterface somehow so that I can choose this setting whenever my heart desires.

... set the speed equal to the min start-stop frequency, then no acc-/decceleration will occur ...

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Viktor
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... How and where do I set the speed ...

Edited 1 time(s). Last edit at 02/20/2015 04:09AM by jessicabrenner.

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jessicabrenner
... How and where do I set the speed ...

There are two places. One is the firmware, in Marlin everything you need is in configuration.h. You can only define maximums, minimums make little sense really from a firmware perspective.

The second is in the slicer. You don't say what you're using, but as you're using PronterFace I'll guess it's Slic3r. There are a lot of settings in there that can affect the speed, so you'll need to find all of them and ensure they're either set to 0pc, 100pc, or the specific max speed you want.

While that (sort of) answers the question of how and where, I'm curious as to what you're trying to achieve. I'm guessing it's a noise issue?

Imagine driving somewhere but only allowing yourself to drive at 20MPH. Even disregarding the physical fact that you can't instantaniously go from 0 to 20MPH and back down to 0 without some accelleration/decelleration, any time you had to start or stop you'd have a sudden jerk based on your cars momentum (or lack thereof) resisting sudden changes in speed or direction. The same thing applies to your extruder. Acceleration and decelleration help prevent defects in the print job by minimizing the sudden changes. It's essentially impossible to maintain an absolute speed as there's going to be direction changes and any of those changes is going to result in a change of speed, even if just for a moment.

If, as Uklan guessed, it is a noise issue, then your print speed isn't even what you would need to be looked at. If you only drew a straight line in either the X or Y direction, then the print speed would equal the motor speed resulting in a constant sound. But most of the time the lines are going to be at an angle. Worst case drawing a circle your print speed is constant, but your motors are constantly changing speeds at a sinusoidal rate, producing the tell tale music that steppers make.

Nope not a noise issue, but good guess! I want to pneumatically extrude stuff like the frostruder, but due to the nature of the pneumatic extrusion, the speed of extrusion is either 0 or X (the speed is not dynamic like with plastic extrusion where the extrusion rate probably adjusts proportionally to the speed of the nozzle). If I make it so the nozzle moves at constant speed always, I won't have to worry about adjusting the pneumatic pressure dynamically. Make sense?

So I guess what I need is to set a maximum speed and a minimum speed and a fast acceleration. I was hoping there would be a simple place in configuration.h or more ideally slic3r, where I can set max/min speed and acceleration.

Yes Uklan, I'm using Slic3r and Pronterface!

... better use a motor-driven syringe - here you can set the extruded volume in relation to speed and acceleration ...

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Viktor
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Yeah motor driven syringe would avoid this issue. I'm not using it for various reasons... I'm using air pressure.

So is there any way to set it so that the speed is fairly constant? It doesn't have to be perfect.

You can set a high acceleration. Setting it to 5000 mm/s2 and moving at 5 mm/s means you have an acceleration distance of just 2.5 micrometers. That's less than one step, so you effectively have "instant" speed.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

... extruding/dispensing with air pressure is not precise enough, to handle high accelerations - the viscosity of the paste material is varying with ambient temperature, humidity and air pressure over the day, so you won't get uniform filament dimensions.

Some years ago I was involved into the development of a CNC-machine, equipped with a dispenser for thermo-curing glue -- they contacted me, after their 4th or 5th reiteration to solve this sort of problems, which didn't work as expected

The team leader too declined motor-driven dispensers, so my options were strictly limited -- the only halfway working solution was to set the ambient temperature to defined values with minimal variations:
- enclosing chamber with 45degC (+/- 3 degC) ambient temp. and constant humidity
- heated bed with 50 degC (+/-2 degC)
- heated syringe with 65 degC (+/-2 degC) and thermal isolation+enclosure to hold the paste at this temps over 3-4 hours, what was the normal life-time of a load

The results weren't perfect though, but in the allowed variation window ... the next generation of this glue-dispensers used motor-driven syringes with much better uniformity

Here is an image with several dispensers:
Dispenser family

The one with the brass head is from this project (but without head heater and temp. enclosure) -- the two smaller types in the middle are common pressure-driven solder-paste-dsipensers --- the leftmost one shows a small stepper-driven dispenser ...

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Viktor
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I just saw that pic in the reprap magazine Viktor. I realize that motor driven has its advantages.

Thanks Traumflog, thats a good solution. I've changed to a slow speed, and even at a slow acceleration, I still have constant enough speed. I'm just worried that even though I set a constant speed of 30mm/s in Pronterface, the Marlin Firmware might override this setting at some time during the print, or perhaps during slicing, slic3r may decide that more or less than 30mm/s is better.

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jessicabrenner
I'm just worried that even though I set a constant speed of 30mm/s in Pronterface, the Marlin Firmware might override this setting at some time during the print, or perhaps during slicing, slic3r may decide that more or less than 30mm/s is better.

Acceleration isn't part of G-code, it's done entirely inside the firmware. If you set a high acceleration and your firmware accelerates slowly anyways, that's a bug.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

I didn't realize this, but there are m-codes for minimum and maximum speed. Using these, the printhead moves at a constant speed (maybe with acceleration/deceleration - I haven't checked) and I get pretty decent results using air pressure for paste extrusion. (I wrote code to insert m-codes to turn on and off an I/O pin to control the solenoid through an h-bridge.)

Viktor, I've built a crude motor-driven system as well (based on a crappy open source syringe pump that I will redesign). It is having oozing problems. For instance, the paste will continue extruding for a minute or two through the fine tip after the motor has stopped turning. I'm working on a couple ways to fix this: using more stable (less flexible) parts for holding the syringe parts, and minimizing the amount of air in the syringe. Do you know of any other things that I should be worried about to prevent oozing? (I'm extruding highly viscous material like silicone and frosting)

Edited 1 time(s). Last edit at 03/28/2015 04:43PM by jessicabrenner.

... you should try with retract - when stopping extruding, then the syringe motor will run some turns backward, to release pressure ... this has to be reverted, before starting to extrude again -- for this the piston has to be fixed to the rod.

Most RepRap-firmware has this option, so read through the related infos ...

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]