Coach Bolt Hack

The M8 bolt in my BfB extruder hasn't been gripping the filament well. I've tried the standard tricks, sharpening the thread, increasing the force (until I cracked the acrylic), but I'm still able to pull the filament out by hand against the thread. While shopping around, I came across some coach bolts with much deeper, sharper threads. Simple replacement of the M8 bolt was difficult - there aren't nuts that fit and it was impossible to attach the existing gear or provide vertical support.
I needed a bolt with m8 at one end for the gear and bearings, and coach bolt screw at the other for the filament bite. So I made this:

From Coach Bolt drive

It's soldered together ( see pictures for more detail of how), but if I had an extruder I could easily have made a coupler, since the joint is in compression.

From Coach Bolt drive

It seems to fit OK into the BfB assembly, with a little modification of the bearings (one bearing, rearranged nuts to fit). It grips the filament lovely now, and hand-winding the gear drives the filament well.
I then attached my rebuilt extruder nozzle.

From Coach Bolt drive

I'll post later when I've tested the nozzle.

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 snached and ripped off the ends of the nichrome wire and threw the thermistor circuit across the garage. I still haven't found it...

Extruder progress

I spent the weekend building extruder nozzles and drives and finally managed to build one that might work.

I'll write up the details of the working/not working designs over the next few days.

Chocolate Extruder

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 :

• Drive the motor from the RepRap electronics DC motor driver board


• Design/build a mount (probably when I get my reprap working)


• Print a chocolate object!

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...
:-)

New parts arrived

I ordered some bits'n'pieces for extruder experimentation and modification. I grabbed some 8mm PTFE, 10mmPTFE and 8mmPEEK rod from DirectPlasticsOnline , and I ordered some wirewound resistors from Farnell, along with some thermistors and a set of 10-way pins and headers so that I can unplug and change extruders easily.

I spent some time in the garage cutting and drilling to make some simple insulators - and found that it's hard to drill vertical holes without a lathe or pillar drill. I also collected together some scavenged parts - some large washers, plumbing bits and drilled out a few 0.6mm welding tips to 3mm.

Sunday afternoon I spent soldering on the 10-way headers, and wiring up a couple of extruders. I'll test them out soon.

BitsFromBytes slow extruder

Detroit RepRap Builders

Notitieblokje

I have a similar issue with the BfB extruder, very slow extrusion. Checking out the alternatives, the heater and nozzle seem to be similar to other designs, and with additional force it seems to extrude OK. The obvious difference seems to be the drive mechanism.

There's two main possibilites : the screw drive and the heater resistance.
My guess is the screw thread can't generate enough force to push the extrusion through and slips. Once it slips, it chews up the filament and can't get going again.

Give that Nophead's produced some useful figures for the extruder nozzles and forces produced by the pinch wheel design, I'll see if I can measure the output and identify the problem side.