Monday, March 30, 2009

 

Extruder thoughts

I've been extruding all weekend so I thought I'd share my thoughts.
NB I'm not quite as organised as Nophead, so no pretty graphs and figures yet.
Friction and barrel size seems to have a big effect on the force. I built two heater barrels to the same design:

One had 3.2mm PEEK hole and 3mm welding nozzle tip. Filament *just* fits into the 3mm heater section (requires a little force).

Heating to 240C allows good extrusion with hand powered force.

One to the same design had 3.5mm PEEK hole, with a short 3.2mm section near the barrel. Nozzle was drilled to 3.2mm and filament fits easily.

Heating to 220C allows good fast extrusion with less force.

I suspect that the half-melted filament expands before melting fully and presses against the sides of the barrel, causing extra friction. I suspect that's why nophead's reaming made so much difference - a little extra space allows the filament to expand as it heats.

I also think my BfB extruder probably does work if the screw thread is adjusted correctly. Nophead has reached >12Kg with the screw extruder, so mine slipping is probably due to the spring/acrylic flexing and allowing the ABS to slide. I'll see if I can tighten up the pressure (maybe use some steel reinforcement) to drive the thread properly. Then I'll have to rebuild the heater barrel.

 

Three heater barrels and a new drive

I made some extruder barrels over the weekend:
(click on the links for more pictures, design and test results)

From renoir1_extruder_quick_v0.01

quick and dirty extruder

From renoir2_extruder_v0.01

Version 2 - Welding tip

From renoir3_extruder

version 3 - wider feed channel (3.5mm through PEEK) for the filament



And I mucked around with a minimalist drive (Keep it Simple)

From Renoir1_extruder_drive

Simple - shove a screwdriver in a hole
This worked OK - as long as it was driven at a slower speed than the screwdriver default (2.4v instead of 3.6v)
I also had problems driving the screwdriver motor from the RepRap electronics

From Renoir2_extruder_drive

Version 2
used the BfB motor. Plenty of torque, but I can't drive it slow enough!
Below about 50/255 the motor stalls: above, the speed and torque is good but it stalls the filament once it hits the end and chews up the filament.
It works well (filament speed is aprox 5mm/sec at the 3mm diameter end) and as much drive as the filament can stand. The star screwdriver bits work well and seem to drive well. Perhaps I'll have to get a stepper motor and driver circuit for the extruder to get the speed down?

I'm going to try and reconnect the original BfB screw thread drive to my version3 heater barrel, and see if that drive will screw down at a slower rate.
*embarassed* I can't exactly use the original BfB heater barrel anymore- I was drilling some out stuck filament when the drill snatched and ripped off the ends of the nichrome wire and threw the thermistor circuit across the garage. I still haven't found it...

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Thursday, March 26, 2009

 

Gearmotors vs Steppers

I'm not a mechanical engineer by any stretch of the imagination, so one thing that I don't fully "get" is the choice of stepper motors for RepRap.

They're pretty much ubiquitous as a choice, so there's got to be lots of reasons they're usually picked over the often-cheaper gearmotors.

The obvious answer is that gearmotors don't include shaft encoding unless you add it yourself, where the stepper motor has this built in as a function of how one drives them. However, with the possibility looming of teaching a RepRap to fabricate gearboxes, I'd think that shaft encoding plus a sixty-cent motor would be possible, and a big reduction in the vitamin requirements. If the only benefit of stepper motors was "native" encoding, it seems there'd already be a number of reprappers scrambling to get that brass ring of a printable gearbox, even if it were large, clunky, and required finicky sensors.

So I figure there's gotta be another reason the stepper motor is better than the gearmotor. Is it a torque/speed thing? Is it power? Lupus? Is it Lupus?

Okay, okay, it's never Lupus...

The above gearbox, incidentially, is now available on Thingiverse and should be fully reprappable. Like I know. My RepRap is still a pile of old printer parts :3

Wednesday, March 25, 2009

 

Chocolate Extruder

Hello all!

Since it's my first post to the RepRap builders blog, I thought I'd better make it a good one. I've been blogging my RepRap build at Renoir's Rants and I've just started experimenting with extruders.

Although my RepRap isn't quite there yet ( the ABS extruder isn't quite working), I have experimented by building a chocolate extruder from copper pipe fittings and a drill:
From choc_extruder_v0.01
A copper tube and bowl hold melted chocolate which is fed to a drilled-out welding nozzle via an auger made from a drill. The heater is a standard nichrome wire and fire cement, along with a 100k thermistor, connected to the standard RepRap arduino electronics.
Drive for the auger is provided from a Tesco Value 3.6v rechargeable screwdriver. Currently, this is manually operated as I'm having a problem driving the motor using the PWM DC driver.

From choc_extruder_v0.01

The heater took a while to heat up, but the entire apparatus reached target at 60 degrees C. Shoving a few chunks of chocolate in, it melted very quickly.
Extrusion worked much better than I expected: when the motor was off, there was a very slow drip of chocolate through the 0.6mm welding tip (approx 1 drip every 5 seconds).
With the motor on, I was able to extrude a steady stream at several mm/sec and drew some lines on the plate. With the motor in reverse, chocolate was actually sucked back up the nozzle!
From choc_extruder_v0.01
All in all, I'm pretty pleased with the experiment so far. I've written up some construction details at chocolate extruder. A couple of photos are on my picasa album.

Still to do :

As soon as I've printed something, I'll post some pics. Might be a little while as I'm trying to get an ABS extruder working first...
:-)

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Saturday, March 21, 2009

 

Nozzle reconstruction

Took the heater barrel apart and reconstructed, now with help of the BfB tutorial video
The result is a slightly improved, not to say less worse, extrusion. Now without hand-force, it extrudes very slowly. That is no more than four odd millimeters per minute.

Sorry, did it again, this post was meant for my own Blog, not the Builders'

Friday, March 20, 2009

 

Having another go at milling polypropylene





In which your narrator mounts his noble steed, Rocinante, and has another tilt at milling polypropylene... do you want to read more?

Thursday, March 19, 2009

 

The $1, no tools, pillow block bearing

Now that I have a linear bearing design that's inexpensive, and easy to build with hand tools, I've started on the other aspects of my repstrap. Today I built a pillow block bearing for supporting a lead screw.

The bearings used are standard 22mm skate bearings available for a few cents each online. I've used a 3/4 inch pipe hanger (they came 20 to a bag for about $2), and some JB weld. Steel-impregnated epoxy putty might work even better. I've laid everything out on some parchment paper, along with a $1 hobby clamp. Clever use of a weighty object might alleviate the need for the clamp.

I spread a generous amount of the JB weld on the inside of the pipe hanger, inserted the bearings, and clamped them in place, making sure that both the bottom egde of the bearing and the pipe hanger were flush with the tabletop. The parchment paper prevents excess epoxy from sticking to anything.

I've used a section of my lead screw to ensure that the two bearings are aligned.

The spacing of the mounting holes on this particular 3/4 inch pipe hanger happens to be 2 inches on center - perfect for attaching it to my 1 inch square gridbeam (which has one hole every inch).

This whole process took about 30 seconds, from mixing the epoxy to clamping the assembly and setting aside to cure.

Wednesday, March 18, 2009

 

ABS Prints





I finally managed to get hold of a roll of ABS from Ian, he has been printing ABS for some time but I have only done a few test prints.

To run the higher temperatures I have made a new hot end, the old one had PVC wire insulation in the hot zone. It also uses PTFE insulator with backup ring to stop it pushing out of shape with the high temp.

I have also experimented with the finish and material of the print table to see if I could get away with using MDF. I found there was good adhesion for the first one or two prints then you have no chance. The ABS seems to stick to a fresh clean surface but when the first print gets peeled off, so does the surface finish. Options then were to turn the board to a fresh spot for the next print, this works but is tedious. I also tried taking an “old” board and resealed the surface. I tried PVA adhesive, a very thin layer (undiluted) all over the surface with the excess wiped off. This worked very well, probably better adhesion than untreated MDF. But, the first print peels off the surface again.

I have now switched the board to Acrylic, this seems to be working, the first few prints were very difficult to remove, but now it seems to be settling down, I will let you know if this is a long term solution. I should also say that I am very late getting into ABS, have any others tried Acrylic for more than a couple of prints? The same spot on the Acrylic has been used about 8 times.

Note, on the new board I ran the first prints at 200C and these were very hard to get off, now the surface is matt finish I can run the first layer at 225 to 230C.

ABS boards are always an option but price is an issue plus we are already cutting acrylic so to run another board is very easy.

Print Temp is 235DegC measured at the tip, print speed/extrusion speed – 16mm/sec
No raft, continuous extruder running.
(V3 BfB machine with PIC32/SD control)

Its still early days with ABS, these first items show promising results, I need to get some more material through the machine to fine tune the settings and explore higher speeds.
The new V3 machine is capable of running 64mm/sec, Ian has printed at this speed, quality definitely suffers but the machine and software keep up. I don't intend to go anywhere near to 64mm/sec it is extremely hard on the machine. 24 to 32mm/sec would seem a more realistic maximum, I will post results as time permits.

Tony.

Tuesday, March 17, 2009

 

Skate bearings and bike chain

I've been meaning to document my experiments with bike chain and skate bearings for a while now. I've used bike chain from the beginning for my handmade almost-darwin, but only started using skate bearings because I was unhappy with the wobble and the friction that wood-steel bearings gave. I shall split this post up a little, because there is no reason the two have to be used together. Here goes:

(Various people name their x and y axes differently, on my reprap it is the y-axis which moves the x stage)

Bike chain:

When I started my reprap, I couldn't find the belts and sprockets for less than £60 shipped to the uk. A quick look on some cycling websites showed that I could get the equivalent amount of bike chain and jockey wheels (the smallest sprockets I could find) for about £30. This wasn't just a cost saving measure, I liked the fact that bike chains are unbreakable (with the loads present here), they won't stretch, and I felt I was getting something more significant for the money. Before I go on, here are the drawbacks of bike chain you should be aware of:

-Reduced precision due to larger sprockets. I'm getting 4 steps/mm, which I believe is lower than you'd get with belts. This didn't matter for me personally.
-You need a sturdy frame and good bearings. Since the chains have to be tight, you'll get a lot of friction without good bearings, hence the skate bearings I'm using.

My reprap uses 14 jockey wheels, 8 for the z, 4 for the y, 2 for the x. They cost around £1 each, and seem to be made of some nice HDPE. For the z axis, it was sufficient to trap them with m8 nuts, as the z axis needs very little rotational force to turn. For the z axis, I found it was easiest to use 4 belts instead of a single long one, because it meant many more teeth meshed correctly. The white discs were fashioned out of polymorph to prevent the belt from slipping.
To attach them to 8mm steel rod, I tended to bodge it, with suprisingly good results. I put an m3 bolt through the rod just shy of where I wanted the jockey wheel, and then drilled three small holes through the sprocket. Then, I smothered the whole thing in polymorph. The m3 bolt prevented the polymorph from turning on the rod, and the polymorph seeps through the holes in the sprocket, preventing that turning. It doesn't look great, but there was no movement whatsoever even when gripping both with pliers and turning as hard as I could. To tension the chain, I used a machine screw I had lying around. It would have been a good idea to put a split-ring/sprung washer on as well, but I couldn't find one at the time.

Roller bearings:

My reprap uses 30 roller bearings, but at only 30p each from ebay, I don't think its much more expensive considering there are very few extra parts. Bear in mind I will probably replace a lot of the parts with better wood, and cut things more accurately, but it shows the design works with badly made parts.

As I mentioned, I was getting too much friction, so I decided to make new x and y axes. My favorite aspect of the darwin is that few of the parts must be made to a great deal of accuracy because the threaded rod can be adjusted to suit. I tried to design the axes in the same way, so anyone can build it with simple tools:

Tap and die set - One of the most useful set of tools, you won't realise how great they are until you get some. I got a set of really nice carbon steel ones from aldi for £8.
Drill - I used a cheap pillar drill, but a hand drill would be just as good.
Saw - Whatever's appropriate for the material you use.

The wooden parts could also be reprapped if you had a generous friend. For the y axes, the only parts needed are 8 rectangles with holes in the right places. I chose arbitrarily, so its probably easiest to see how it works from the diagrams. I chose 3cm center-center separation for the x axes. Each pair of bearings is separated by two m8 washers, so they center really well on
the rod. Because the bearings are so good I can tighten everything so the y-axis can even be powered from only one side with no rotation of the x stage. I chose not to do this, since I already had power from both sides. I could measure no backlash whatsoever, and each axes moved with a gentle nudge. The x axes worked in a similar way, I haven't attached the extruder yet, because it obscures the photos, but it will simply bolt on.

Sorry for the long, disjointed post, if anyone wants more info, ask in the comments. Some more photos are here.

UPDATE: pritam has tried the roller bearing setup with success, wish I'd had the space on my reprap to make it so neatly.

Thursday, March 12, 2009

 

New Wine Glass




I thought this picture was worth posting, the wine glass is 128mm high and 60mm dia at the rim.
They were both printed with the following settings:
192DegC / PLA
16mm/sec print speed
No Raft, no stops on the extruder (there are a few fine hairs inside the glass)
Both objects were made using the same file with one difference, the glass on the right has the comment lines, on the left it has no comment lines.

I think it clearly shows the effect of segment pausing as the reader takes time to get past the comments, the buffer runs out and the head stops for a fraction of a second between layers. The effect is less obvious in the stem as there are fewer comments to define the small layers and worse as it get higher in the glass with more comments required to define the larger layer.
The print on the right was terminated early to investigate the cause of the defects.
The ripple down the side of the left hand glass is the due to the Z move, its not quite fast enough for the extrusion rate.


Tony

Wednesday, March 04, 2009

 

Plastic supply

Hello

I am new to builders, but I have been following the Reprap builders blog for some time.

One problem I see is sourcing the plastic, but an idea I had was to use plastic pop bottles (PET), which could be cut into a ribbon and formed into a reel by heating the ends and compressing them together. This would mean a change in the design of the printing head. but may actually make the extrusion of the plastic easier.

This would be easier for people running one of these machines as a hobby and recycle plastics.
The plastic could be cut in a spiral from the bottles maximising the length the ends could be joined in a small press that had been heated, so that a long reel of the plastic could be made.

I sent this idea to Adrian Bowyer and he suggested that I post this on the blog. He aslo suggested that the ribbon could be folded over in width in the extruded to increase the thickness of the ribbon prior to going into the heating element section.

I am currently building a CNC miller / drilling machine to be used for making small components and cutting out foam patterns for castings, but intend to fit a Reprap head to it to convert it to a Repstrap machine. Again this is to make small components but also to make patters from casting in finer detail than achievable with lost foam technique.

I am a long way off trying my suggestion because I am still building the machine, but will try it as soon as I can.

Regards

Paul


Sunday, March 01, 2009

 

Introducing the JunkRap

Or, what there is of it so far, any way.

After around a year of watching the RepRap project with quiet envy, I have finally decided to go for it and start scraping pieces together.

Having taken Forrest's post on the future of RepRap to heart, I decided that, in theory, a complete 3D prototyping table (x, y and z assemblies) could be built from the guts of only TWO 2D printers, since each has a sheet feed in addition to a print head. A repository of printer-to-reprap hacks might be a great start to a list of "locally inexpensive" RepRap parts.

JunkRap is definitely a RepStrap, but I don't think that would necessarily be true of a second JunkRap, which would have screwdriver-and-wrench-scavenged printer parts as some of the vitamins. To my regret, I don't have the printer make I'm working from here (I scavenged this assembly quite some time ago) but RepRappers who kept track of the printers they used could ultimately create a database where one enters in the available scavenge and recieves a list of printed parts necessary to create the remaining subassemblies. In the industrialized world, this might drop the cost of a RepRap dramatically.

On the JunkRap itself as it stands: I have hacked an interface in to the shaft encoder on this printer's sheet feed, which should give me about a half-degree accuracy in knowing the position of the wheel from my initial experiments. Of course, actually getting the wheel to go there may be another story, to say nothing of stabalizing the table (yet to be added) on the additional rollers (also yet to be added) and making it reliably move back and forth.

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