Monday, August 31, 2009

 

Polylactic Acid


It sounds nasty, but polylactic acid (PLA) is some pretty cool stuff. Up to this point I've been printing objects with ABS, but I've had recurring issues with warping, especially on larger objects (100mm or so). I've also started running my Reprap in my apartment again, until I can find a a new workshop out West; the smell of melting ABS hasn't been so popular with the other apartment dwellers. :)

I experimented with turkey bags and CPU fans to try and create a heated, sealed build envelope for ABS, but in the end it seemed more trouble than it was worth. Seeing Nophead, Vik and Adrian's results with PLA, I thought I'd give that a shot.

I'm glad I did! PLA is slightly more brittle than ABS once it's extruded, but that is more than made up for by it's almost complete lack of warping, the ease of part removal, and the lack of any bad smells. It gives off a faint odor of cotton candy when it's printing, which makes me think of Fraggles and Doozers, but so far I haven't started eating any of it.

Here's a shot of Adrian's pinch wheel extruder, printed out in tasty PLA. Even better, this stuff is more or less biodegradable, so if I don't end up building a pinch wheel extruder or recycling it, I can at least mulch it (slowly) in the garden. :)



Saturday, August 29, 2009

 

Extruder Heater update

Several months ago I posted an extruder tip design that used a transistor in a TO-220 package (http://builders.reprap.org/2009/04/inspired-by-nopheads-observation-about.html). It worked, but not for long, probably since I was running the transistor at a temperature(>230C) much higher than its rated maximum of 150C. After I had gone through a few transistors I switched to using a 10 ohm load resistor in a TO-220 package. The first resistor mechanically broke during an overnight print run. It looked like the resistor had hit the printed object. I replaced the resistor, and the second one had been working continuously ever since. I have printed more than 5 pounds of ABS with it so far. I must credit Nophead with the idea of having the barrel temperature drop over as short a distance as possible.

Because of this success, I would like to present the details of the extruder tip design and some instructions on how to make it















  1. Heatsink: This holds down the temperature at the end of the extruder barrel. I have used a 6.3mm (1/4”) thick aluminum L bracket with a total surface area of about 15100 square mm (23 square inches). Since heat flow from the barrel is important, the barrel screws into a threaded hole to so that the thread-to-thread contact can conduct heat. I have also made a second experimental extruder with an aluminum heatsink of about half the area. I think that as long as the heatsink is not too hot to touch after several hours of operation, then it would be good enough. This heatsink also plays a structural role, since it is part of the force path involved in pushing the filament into the barrel. In my extruder, I have the pinch wheel motor attached directly to the aluminum L bracket (which means that the motor also heats the bracket).
  2. 10-32 nut: This is in the heat flow path between the barrel and the heatsink, so do not use a lockwasher. I suggest brass as a material, though I have also used an aluminum nut made by cutting up a threaded aluminum standoff from my junk box.

  3. Extruder barrel: This is a tube made by putting a 10-32 thread on a 4.76mm (3/16") OD, 0.71mm (0.028") thickness stainless steel tube (K&S Engineering stock No. 7113 Stainless Tube 3/16 x 0.028 (4.76mm x 0.71mm) UPC code 14121 17113). In order to put on thread successfully, put a 3.17 mm (1/8") diameter rod in the tube. Putting the thread on the tube with a 10-32 tap is tricky, but putting a rod inside the tube to hold it straight helps. The tubing gets so thin that you can see ridges on the inside of the tube pushed up by the teeth of the die.

  4. 10 ohm resistor in TO-220 package. Stackpole P/N TR35T110 5% B , Digikey P/N TR35T110J-ND. Other manufacturers could be used. If you use something different, I suggest picking as large a wattage as possible, based on the idea that larger wattages would have a better thermal connection between the actual resistive element and the heatsink tab, and probably be more reliable. The mounting hole must be drilled and tapped for 10-32 thread. Given how little material is removed by the drill bit, it might be possible to tap it without drilling first, but I have not tried this. The material is copper, and not thick, so be careful to keep the tap straight as it goes in.

  5. Thermistor holder: I have done this two ways. The original heater has a small bracket made from copper sheet. More recently I have used a crimp lug with a #10 hole and suitable for 22-18 AWG wire. I take off the plastic insulator and pry open the crimp barrel slightly along the seam. I open the gap enough that the thermistor can be slid in and is held by the spring of the metal.

  6. Thermistor: Allied Electronics P/N 254-0019. Use your favorite.

  7. 10-32 acorn nut, with 0.5 mm hole drilled in the end (done with a PCB carbide drill).

Heating time: This extruder heats and cools very quicky. It usually can go from room temp to 225C in just under 3 minutes.

Filament jamming: Others in the reprap community have noted that filament can jam in an extruder barrel if it is allowed to soften back up the filament and form a plug pressed against the inside of the extruder barrel. I have noticed this to some extent with my extruder under experimental conditions, but under normal operating conditions it is not a problem. By normal operating conditions, I mean starting the extrusion process right after the thermistor shows that the tip is up to temperature. It takes a few seconds for the filament to start coming out, but this is on the edge of the print raft so that it does not matter. I use skeinforge, and it does not turn off filament extrusion long enough to allow the filament to jam. I also set the file that skeinforge appends after the print to turn off the heater. This allows good startup the next time.

Frank Davies

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Thursday, August 27, 2009

 

FiveD endstops

Aaaaargh, finally found why the max stops didn't work, where they were dearly needed!
Escaped from Java Host to replicatorG and Eriks' 3D-to-5D script. Though building here sends all axis way beond the Max!
Anyhow, this is the code adjustment made in cartesian_dda.pde (code taken from svn #3252):

bool cartesian_dda::can_step(byte min_pin, byte max_pin, long current, long target, byte dir)
{
bool canStep = true; // 2009.08.27 JvO
//stop us if we're on target
if (target == current)
canStep = false;
#if ENDSTOPS_MIN_ENABLED == 1
//stop us if we're home and still going
if(min_pin >= 0)
{
if (read_switch(min_pin) && !dir)
canStep = false;
}
#endif
#if ENDSTOPS_MAX_ENABLED == 1
//stop us if we're at max and still going
if(max_pin >= 0)
{
if (read_switch(max_pin) && dir)
canStep = false;
}
#endif
// All OK - we can step
return canStep;
}

Next puzzle is why the stepper-extruder doesn't wait for the temperature. Hm, expected to find such a thing in e_can_step.

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Printing test-part

Inspired by a paper from Kruth et al. (2005, Benchmarking of different SLS/SLM processes as rapid manufacturing techniques) I also designed a benchmark model for the RepRap. It looks quite random but by printing it you can check whether your RepRap prints geometrically correct. I included the following features:

-vertical holes for M3, M4, M8 screws
-vertical hexagons suitable for M3, M4 M8 nuts
-horizontal teardrops M3, M4 M8 and hexagons for suitable nuts
-sharp edges with angles of 15, 30 and 45 deg
-thin walls 0.5mm, 1,0mm, 1.5mm and 2mm horizontally and vertically
-stair effect at sloped walls with angles of 15, 30, 45, 60 and 80 deg
-round corners, radius 4mm to 16mm
-gear teeth with 2mm wide teeth
-in addition to this, you can also check perpendicularity and parallelism of your printed objects, which were the general intentions of designing a benchmark.

You can download both the STL and the AoI file from SourceForge

/mendel/mechanics/test-parts



Printing it took about two hours, so why not watching a fancy movie while your RepRap prints its benchmark.

The very first version included a complete horizontal hexagon for M8 nuts, but apparently this made the object to big in height, so I just cut it off, leaving just a small offset ;)


The gear teeth were not built as expected but as a round corner with a bubbled rim. I am also missing the 0.5mm thin walls. The 1.5mm walls consist of only two parallel strings and are not solid, there is a small gap between them in both directions.

Monday, August 24, 2009

 

Identifying backlash

I was printing parts when I noticed that the diagonal lines on the foundation were not evenly spaced and the infill overlapped badly. Since the lines are diagonal, I wasn't able to figure out whether the backlash was in the x-axis or the y-axis. Watching the machine carefully didn't provide any clues.

I decided to write a custom gcode file to reveal where the backlash was.

G21 ;metric is good!G90 ;absolute positioning
T8; select new extruder
G28; go home
M104 S220.0 ;set temperature
G4 P307 ;delay
M107 ;cooler off
G4 P100 ;delay
G1 F5.0; feed for start of next move
G1 Z0.4 ;z move
T8; select new extruder
G1 F1500.0; feed for start of next move
M108 S1400.0 ;extruder speed in RPM
M101 ;extruder on, forward
G4 P400 ;delay
G4 P200 ;delay
G1 X100 Y0 F2000
G1 X100 Y5 F2000
G1 X5 Y5 F2000
G1 X10 Y10 F2000

;
G1 X10 Y30 F2000
G1 X30 Y30 F2000
G1 X30 Y28 F2000
G1 X12 Y28 F2000
G1 X12 Y10 F2000
G1 X14 Y10 F2000
;
G1 X14 Y26 F2000
G1 X30 Y26 F2000
G1 X30 Y24 F2000
G1 X16 Y24 F2000
G1 X16 Y10 F2000
G1 X18 Y10 F2000
;
G1 X18 Y22 F2000
G1 X30 Y22 F2000
G1 X30 Y20 F2000
G1 X20 Y20 F2000
G1 X20 Y10 F2000
G1 X22 Y10 F2000
;

G1 X22 Y18 F2000
G1 X30 Y18 F2000
G1 X30 Y16 F2000
G1 X24 Y16 F2000
G1 X24 Y10 F2000
G1 X26 Y10 F2000
;
G1 X26 Y14 F2000
G1 X30 Y14 F2000
G1 X30 Y12 F2000
G1 X28 Y12 F2000
G1 X28 Y10 F2000
G1 X30 Y10 F2000
;
M103 ;extruder off
M104 S0.0 ;set temperature
G1 F5.0; feed for start of next move
G1 Z1 ;z move
G1 F2000; feed for start of next move
G1 X0 Y0 F2000


The long lines on the resulting print should be evenly spaced. If there is any backlash, it will alternate between a small gap and a large gap. The magnitude of the backlash is half the difference between the large gap and the small gap.

Looking at the image, I see that the horizontal lines are not evenly spaced. Using my digital calipers I measured the y-axis backlash to be 0.6 mm. I went ahead and measured the x-axis backlash as well to be 0.2 mm.

Tuesday, August 18, 2009

 

Precision Reprap Prints

Here is a collection of other peoples reprap prints that I thought were of "high quality", enough to bring together in one place as a display of what's possible with reprap/repstrap technology!

http://davidbuzz.googlepages.com/random_precision_reprap_prints


Sorry in advance for not giving proper credit for each of the photos... but I've at least provided links to the overall site/s I got them from.

Got other really great quality reprap prints? post a URL in the comments.....

Buzz.

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Tuesday, August 11, 2009

 

Simple Pinch-Wheel Extruder Plans

Hi all,

Following up from my recent post [here] about our success with a simple pinch-wheel extruder (made from things around the workshop), some folks suggested it would be a good idea to post plans since the design seems both functional, and doesn't necessarily require any printed parts or precision metal working.

The measurements will depend on the toothed gear and idler pully's that you use -- we don't have any part numbers for ours, since they came from the "random bits we've saved" box. The extruder's DC gearmotor is the same used in the Makerbot Plastruder, which they list as a Kysan 1156006 [Makerbot Partslist] [Makerbot store link].

Here are some illustrations:



























And a few pictures that focus on the extruder:




















Hope that helps! :)

 

About the RepMan

Hi, I 'm Batist Leman. I follow the RepRap project for a couple of years now, and this is my first post to the builders blog.

I made a short introduction video about the Bits From Bytes RepRap:

A while ago I also made a dutch-speaking video about the RepRap in general: http://www.vimeo.com/5973368

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Wrench-built machine update

We made stunning amounts of progress at the first and second MIRUG meets, on several machines. Unfortunately, it's taken a while to squeeze pictures out of the members who took them. That said, here are a couple pictures of the wrench-built machine from the first meet:







































As you can see, Ben and Bill figured out how to make decent motor mounts for the standard NEMA-23 RepRap stepper motors out of our 1" square perforated tube. Lead screws were also cut to length, collared, and coupled to the steppers. Also, in the second photo, you can see the horizontal stage attachment to the drive nut on the lead screw.

At the second meet, Ben re-worked the Z stage and added a motor mount and lead screw to it. I'll get photos for another update as soon as I can.

Sunday, August 09, 2009

 

Success with Simple Pinch-Wheel Extruder


Hi all,

I thought I'd take a second to post some exciting success with our new pinch-wheel extruder. My first post (a few months ago) is here.


Our pinch-wheel extruder is extremely simple, and has gone through a few design iteratations. We have tried both some random DC gear motors as well as some steppers that we had laying about, before finally replacing the motor with the very torquey Kysan DC gearmotor motor used in the Makerbot plastruder. The other motors that we tried usually didn't have enough torque, and in the case of the stepper, didn't produce a constant flow of plastic (the flow would be quick immediately after a step, then slow down considerably).

We changed the toothed gear from one with very small teeth to one more similar to the Makerbot plastruder, with very large teeth. The remarkable thing we found is how tight the gap has to be between the toothed gear and idler wheel -- our initial designs had a much larger gap, and didn't extrude very well. Last night we reduced the gap to the point where we have to taper the plastic filament to get it to initially feed, and it's actually *crushing* the filament quite a bit, but it has a wonderful extrude rate -- the plastic POURS out of it. We're still tinkering with feed rates and speeds, and aren't running at maximum speed just yet, so I don't have any numbers.

Since last time we've replaced our dsPIC controller with a full set of Makerbot Gen3 electronics, and we've switched from HDPE (which we didn't have much luck with) to ABS.

Here are our first prints (we found random interesting things on Thingiverse):







We are using a piece of dense cardboard (from a binder) as a build platform, with some double-sided tape on the spot where we print.

thanks for reading :)

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