Remote direct drive extruder question August 23, 2020 06:22PM |
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Re: Remote direct drive extruder question August 24, 2020 02:51PM |
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Oh ok, you accounting for flex shaft error thru the gear ratio. The way you described it previously I thought you were using gear to dampen the movement of the shaft itself or something. The motor ability to deal with rapid change is related to the motor spec it can be heavy but if it is powerful it can still change speed rapidly but the same goes the other way less power but light = rapid change.Quote
obelisk79
I'm going to use some hypothetical numbers to help explain. Every flexible driveshaft will have different characteristics.
Let us presume your have a flexible drive shaft which can twist up to 90 degrees at one end compared to the other. This is known as torsional deflection.
90 degrees of 'sloppy' rotation can be cause for a great deal of inaccuracy in a positioning system. But when you divide that up using a reduction gear of 1:30 now you've reduced the effective deflection down to 3 degrees, you've also reduced extruder step distance which provides an even further reduction in impact to extrusion accuracy.
The drawback, is that your motor now has to step/pulse/rotate 30 times as fast for the same amount of extruded plastic, these higher speeds also result in high speed direction changes for functions such as retraction. The step motors ability to deal with these forces (from my humble understanding) is called its moment of inertia. Smaller steppers have lower mass in their rotors (thereby moment of intertia) allowing for those high velocity direction changes. I thought the same as you, higher torque = better, right? When it comes to these types of remotely driven extruder setups that just isn't the case. I run a smaller weaker stepper at lower current and it far outperforms the much larger 84oz-in motors I attempted to use at first.
I think the only setup I have seen like this was in an Ultimaker some time ago.Quote
obelisk79
If you wanted to do something like a 1:1 gearing on a remote direct drive type setup, you'd need a gimballed motor with a telescoping spline shaft to drive the extruder hob, that is likely possible, but would come along with its own uniquedesign/engineering challenges.
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