Has anybody tried 3mm filament in airtrippers filament drive for rostock?

Has anybody tried 3mm filament in airtrippers filament drive for rostock?
I've heard that 3kg is neccesary to get 3mm filament to extrude on a good extruder.
My motors are rated 4.8kg/cm at 2.5amp max.
My stepper drivers should handle up to 2amp with heatsinks and fan but hopefully we won't need all that amperage.

Max torque when the motors are running on 2amps should be as follows: 4.8kg/cm * 2amp/2.5amp = 3.8kg/cms of torque.
The filament drive gear I was looking at buying is 8mm in diameter, so it has a 4mm radius (shaft on motor is 5mm diameter = 2.5mm radius)
So 3.8kg/cm / .4cm = 9.5kg of torque pushing the filament when the radius is 4mm. That should be plenty.
A 13mm gear is also avaliable. If 13mm gear is used radius is 6.5mm so:
4.8kg/cm / .65cm = 7.4kg of torque. Which should also be enough.


Any mistakes? Would this work? Is it practical?
Or rather, has anybody tried this?



TTN

Edited 1 time(s). Last edit at 12/07/2013 01:56PM by TheTechnicalNoob.

The bigger the gear the more torque required to deliver a specific amount of force. The units are length X force rather than length / force.

[en.wikipedia.org]

You are delivering a required amount of force to the filament at the edge of the gear ....

My apologies. Here, units work out properly now:
3.8kg x cm / .4cm = 9.5kg the unit cm cancels out as it should.
4.8kg x cm / .65cm = 7.38kg


To be technically correct kg isn't a unit of force, but gravity is present. So it does exert a force on it. Not that there is anything falling, but the unit kg has the assumption built in that we're on earth and gravity is 9.81m/sec^2
Force due to gravity:

Mass x gravity = weight (or rather force, in newtons)

3.8kg x 9.81m/sec^2 = 37.3 kg x m / sec^2 which is the unit newtons.
4.8kg x 9.81m/sec^2 = 47.1 kg x m / sec^2

But I tend to think with the unit kg x cm in mind when I'm dealing with stepper motors and reprap ^ ^

Edited 3 time(s). Last edit at 12/07/2013 02:02PM by TheTechnicalNoob.

It's the silly pound force thing all over, in this case it's kg used as force.

There certainly are a lot of reports on these forums of people who are unable to get those torque level motors to work on direct drive extruders with those gear sizes. With the units straightened out, that sort of leaves the 3KG starting assumption.

Quote
uncle_bob
It's the silly pound force thing all over, in this case it's kg used as force.

There certainly are a lot of reports on these forums of people who are unable to get those torque level motors to work on direct drive extruders with those gear sizes. With the units straightened out, that sort of leaves the 3KG starting assumption.

Yea that was really my question. I couldn't find any reports of people using it or trying it. But ok, so its not the best idea then. I'll go with a geared extruder then.

The best approach for a direct drive extruder seems to be a stepper with nice strong metal gears built into a gear box on the motor. That can give you massive torque in a fairly small package. The problem is finding a good motor.

Right now I'm printing with 2.7mm nylon weed eater line using a 6.7mm diameter hobbed gear directly driven. No stripping. Seems to be working, though I doubt it will continue to work when I get PLA and put it in there.
EDIT: I though it was good, and thats true, for anything slower than 40mm/sec otherwise it starts to slip.



Edited 1 time(s). Last edit at 01/05/2014 02:56PM by TheTechnicalNoob.

Hopefully I will be able to print a geared extruder in pla with the current setup

UPDATE: My PLA and ABS arrived in the mail today
The PLA prints without eating into the PLA or skipping with the current setup as posted above in the picture. Although probably not too well at higher speeds. Enough to get me printing with PLA so I'm happy

UPDATE 2: Well, it was working fantastic, but its starting to chew up the pla when doing super fast retract moves (500mm/sec) for other prints its still fine. Time to print a geared extruder drive..

Edited 1 time(s). Last edit at 01/16/2014 10:38PM by TheTechnicalNoob.

One limit case is trying to push filament with close spacing against a glass print surface. Nothing's ever flat / level so you get variable back pressure as you hit the bumps ...

One geared extruder later and it's printing much better. It appears my prints have also become more accurate, which I think is due to better plastic flow control. At least the direct drive was enough to get me repstrapping. From here its only going to get better