[KwartzLab] 3D Printer progress, July 26th

Sat Jul 30 00:20:30 EDT 2011

Hey Kevin, do you have an account on the Kwartzlab website that would let you post this info as a blog post? This is really good info, and I'm sure there's other people out there that would love to see the work your doing on the new extruder head. I noticed that the RepRap wiki is linking to my 3D file format converter now, so it sure seems like they're paying attention to the community.
Alex

From: kpmartin at thinkage.ca
To: discuss at kwartzlab.ca
Date: Sat, 30 Jul 2011 02:47:55 +0000
Subject: [KwartzLab] 3D Printer progress, July 26th

Since the last message I have made all the other parts: the pinch roller shaft (lightly heat-shrunk into the pinch roller bearing), the pinch roller pressure plates, and the motor mount:

Despite being simple parts, the pressure plates seemed cursed. Twice they shifted while machining them, ruining them, so it took me 4 tries to make two parts. On the other hand the motor mount turned out well: the boss on the motor was
 a hand-press fit into the hole in the mount.
Other parts not shown are a collar for the drive shaft to separate the drive gear from the bearing, and a steel plate to place under the base block to retain the nozzle.

Also, to accommodate other future extruders whose bases might have different thicknesses, the mounting studs on the X carriage were convert into M4 tapped holes by making threaded inserts from 1/4-20 hex socket cap screws. They were drilled
 and tapped on the lathe and their heads turned to a smaller diameter because of tight clearances; for some reason one went fine, but the other caused problems due to the metal hardness. a couple of minutes with a torch to anneal it cured that. I re-drilled
 and tapped the holes in the carriage for 1/4-20 and screwed in the inserts. A dremel cut off the excess length and the M4 tap was run through again to clean up the inside threads.

Now for the final assembly:

The third photo shows the spacer collar plus a pair of shim washers which were needed to get the knurled portion of the drive shaft properly aligned with the filament channel. The last photo shows how the drive shaft gear rubbed on the
 head of two of the motor screws; this was cured by reinstalling slightly shorter screws in these positions without washers.

Somehow I managed to forget to get a photo of the completed extruder re-installed on the printer.

Once re-installed on the printer we fired everything up again and tried it out. One thing we noticed was that the extruded plastic seemed to contain bubbles. MY suspicion is that the pinch roller pressure is too high and the knurling of
 the feed shaft is marring the filament, causing it to entrain air into the melt chamber. It seemed to be feeding ok so after resetting the Z home position we tried printing something again.

It quickly became clear that there were still problems. Just as before the feeding would get stiffer and stiffer over a period of several minutes until the motor stalled. Turning up the current adjustment seemed to cure the problem for
 a short time, but another oddity turned up: the feed motor seemed to jump back now and again. This is not the controlled reverse motion used to limit drooling at the end of a bead, but a quick snap back by perhaps a quarter turn of the stepper shaft. I tried
 helping the feeding with my fingers and found that the snap-back seemed to be caused by the motor coils being de-energized, letting the motor free-wheel. The compressed segment of filament between the feed shaft and the nozzle was the source of the force causing
 the snap-back. Soon thereafter, (with my manual intervention) the feed shaft chewed a notch in the filament and lost all traction, and also, the motor went into permanent free-wheel.

Geez that sounds familiar. feeding gets worse and worse, turn up the current to try to cure it, something breaks. The only difference is that this time it was not the feeder or nozzle that broke!

I suspect that the motor driver chips went into some sort of shutdown, although they did not feel the least bit hot. I must admit though that it was several minutes before I checked their temperatures. I hope this was a deliberate shutdown
 and that there was no permanent damage. A little digging around at specifications finds the motors are rated 12V 3.1Ω which means they carry a current of 3.87A (and have to dissipate 46W in the process), but the A3949 driver chip data sheet specifies Iout
 at only 2.8A with a note stating "Output current rating may be limited by duty cycle, ambient temperature, and heat sinking. Under any set of conditions, DO NOT exceed the specified Iout or Tj." Clearly this chip does not have the capacity to drive the motor
 to its full low-speed torque. The data sheet claims that the chip has internal shutdown circuitry to protect against excessive temperatures, so I'm hoping that they have not been damaged permanently.

As for why the filament feed still has problems, I have a (yet another) theory. The PTFE liner of the nozzle has a linear thermal expansion rate about 5 times that of the stainless steel nozzle body that contains it. Because the liner is
 confined lengthwise and on its outside diameter by the body, and because its Poisson ratio is a high 0.45 , almost all of its volume expansion goes inward, reducing the size of the hole the filament must pass through. A quick calculation estimates that the
 inner hole that must pass a 3mm filament would be reduced to 2.6mm diameter at the 240°C our nozzle runs at. Next time I will test this theory by using drill bit shanks to measure the hole diameter cold, and again after running at temperature for an hour.
 If this is the case the problem could be alleviated at least partially by turning down the outside diameter of the liner tube a bit on the lathe. The ends would still have to remain full diameter to maintain their alignment, and the liner would still be constricted
 lengthwise, but it would be a big improvement.

This large thermal expansion coefficient also provides further explanation why the all-PTFE nozzle body failed to retain the nozzle.

-Kevin Martin
the Papertrail Handmade Paper & Book Arts
New Dundee, Ontario
518-884-7123
www.papertrail.ca

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