DC motors because 120vAC is dangerous to the end user, and synchronous motors don't handle varying load well (though they do have good open loop speed control!).
- I want to share my (limited) experience.

Limited experience? Your the one with the functioning company making and shipping product.  From one that operates a factory, that fact stands for a lot!

Motors I had a question/comment on.  (Note: I agree on the 120 volt dangerous thingy, you are selling a product and liability needs to be addressed.  I on the other hand, work with AC all the time from fractional HP up to 300 HP motors so I do not have these limitations.  I thought about powering it with 3-phase through a freak drive since I have plenty of spares sitting in the store room, but then I would only be able to use it here at work.)

AC Synchronous motors are designed to be synchronized to the driving frequency without any "slip".  That is why they are used in clocks or timing devices.  As you mentioned, "open loop speed control".  I wouldn't think RPM would vary up to rated torque but have to admit I am not real well versed here.

AC induction motors do have a slip to produce the torque.  This is where the time lapse between studying this stuff and now hurts, as I need to read up on the particulars.  Good motors will have a slip of 3% and will have very little rpm change from 25% to 75% of their rated torque.  Some induction motors can have up to 15% slip I believe, and this could be an issue unless you oversize the motor to keep the torque range in a consistent area.

DC motors.  A DC motor will produce torque linear to amperage.  Need more torque, it just pulls more amperage.  But if the motor is permanent magnet or series wound, the rpm will vary with load and amperage.  Max torque at 0 rpm, zero torque at max rpm.  A parallel or shunt wound DC motor will be pretty consistent for rpm, approaching an AC induction motors 3% slip.

The change in the rpm to torque is the reason I wanted to go with an AC motor.  I was hoping a constant rpm on the feed auger would produce a constant mass flow equating to a constant filament output rate and diameter.  Now I am wondering if that would be true?  I guess what I am actually looking for is a consistent pressure in the nozzle (given other characteristics like plastic and temp are constant).

All things to think about and consider while I try to over design something that is already working!

Brian

DC motors because 120vAC is dangerous to the end user, and synchronous motors don't handle varying load well (though they do have good open loop speed control!).
Pipe couplers because they're cheap, slight mixing advantage, 1" heaterbands are smallest commonly available size.
Modular nozzle/breaker plate components are more expensive and harder to seal.
There is definitely room for improvement in the nozzle design. I designed one I knew would work well and did not have high machining cost, but plan to iterate it once I have time.

Hope that helps, and these are great questions - I want to share my (limited) experience.

I like the idea of keeping your area clean, so I will post in the projects area.  Go figure, I search the rest of the web for forums on the topic, but not soliforum. Duh!  I will change the design portion of this to over there.

Second question: I do not intend to do a machined extruder screw.  The auger bit seems to work pretty well and adding a tapered variable pitch screw is beyond my simple lathe and mill abilities.  The point was not to outsource stuff, but to make my own stuff.  Which leads to your question why.

I would say it is probably 61% design exercise, 38% aesthetics, 18.7% looking for quality improvements, 4.5% boredom and 3.2% as an exercise to improve my math skills!

You see, I used to be a ME working as a manufacturing engineer and it was really cool.  Then the company I was working for gave me 23 acres of land, a list of production goals and several million dollars to build a new production facility.  As an engineer, almost a dream project.  Unfortunately, they made me the plant manager when it was done.  Granted, it is a smaller facility at 50k sq. ft. and a staff of under 20, so not really bragging here, but it lays the ground work for the first 61%, I miss the design exercise.  I still do the engineering here, but the majority of what I do is now direct management and that stinks.  There are lots of things that I do that would be easier to just buy, but I have this problem that I have to make my own stuff.  Definitely not knocking what you have accomplished, just have to try a version with my twist to it.

Second, I want it to look like it was made in a machine shop, it is just me.  This is the vain part, I do not want it to include piping or flanges, blocks of spare wood or hot melt glue.  I am not knocking those, I do that at home with my children also to teach them the design thought process.  I just want what I am experimenting with to look like a piece of machinery.

Being the engineer, I have see if small process improvements can be made and measured.  As new materials to print in come out, being able to make custom or new filaments might be important.  If I am going to experiment and try new things, I want to start modifying a machine that I know from the beginning and is a bit more modular.

Working on my math skills should fill in the boredom part, as if 4 years of calculus wasn't enough!

Simply put, I like spending time on the lathe and mill, gets me away from the desk, CAD and Excel.  I want something with tighter tolerances to see if it makes any improvements.  I want to answer questions in my mind like: Why DC over AC motors? Why pipe couplers with their large diameter voids compared to the auger verse machined bores?  Why not modular components to allow separate breaker plate designs from separate nozzle designs?  Why not different nozzle designs to allow for temperature gradients in the nozzle? Why not play with different heat zones along the auger?  All kinds of stuff.  I got out of the automotive plastics industry about 20 years ago and never looked back, so my knowledge there is very little and was based on injection molding.  My fascination with 3D printing is an extension of my hobby work with silicone molding and urethane casting.  The possibilities are endless.

Thank you Tim for your questions.  I will start a thread over on the projects forum and would appreciate any input or feedback as I post my project.  My budget and forecast just got approved for next year, so I have some time to go make some chips now in the machine shop!

Brian

I'm not aware of any other, other than perhaps this one:

http://forums.reprap.org/list.php?70

Extrusionbot has a google group with ~10 posts and the creator doesn't post there.
Filabot pulled their forums offline.
Filafab and Strudittle have no forums to my knowledge.

I'm okay with discussion of extruder theory here, and people sharing their designs. I'd rather not have discussion of other commercial units in order to avoid confusion, as it is titled Filastruder. The other option is posting in the Projects section of the forum. Either is fine by me.

Second question:

I'm not aware of anyone that has machined a extruder screw. I think that's really the big difference between home-built and the pro machines. The industrial machines also have a lot more control over barrel smoothness, screw temperature, etc. The operating theory is a little different between the two as well - the Filastruder produces over 90% of its heat from the heaterband, while an industrial extruder produces over 90% of its heat from shear. I read a lot in the process of designing the Filastruder, learning what I could and couldn't compromise on in order to produce a machine that output filament at a reasonable rate and with quality equal to or greater than offered commercially.

That brings me to my last point - why? If you spend $1k+ on a more "less homebuilt" homebuilt extruder, what would you be looking to improve?

Output rate? (Filastruder is 1-2kg/day, more than a 3D printer can consume)
Filament diameter? (already on par with industrial extruder)
Aesthetics? (sure, room for improvement here - but is it worth the money?)
Another area I am not aware of?

I am thankful to have another ME here, especially one with previous polymer experience - I'd just ask what it is that needs to be improved - or is it simply a design exercise for the fun of the journey?

You can go faster - but it isn't simply a bigger motor. Eventually you have to look at melt fracture, a vacuum port, keeping the viscoelastic flow stable, etc. Its a design balance, especially if cost is a variable to be optimized (though it sounds like it is not in your case)

If I had $1k to blow on parts, I'd get a custom barrel and screw, (I got quotes from here: http://www.flitetech.com way back) I'd get a 0.5HP DC motor with a big ole gearbox, and I'd probably be overbudget by then.

Don't get me wrong, I welcome the discussion and think there are areas for improvement - I just think it is important to set the goals early.

I am new to 3D printing and currently waiting for my printer kit to arrive, hopefully in late December from Kickstarter.  So in the meantime, I have been researching extruders which eventually brought me to this site from kickstarter also.

Let me say up front, what a fantastic job others have done on the home based extruder, especially the information that elmoret (Tim) has shared here with his creation/ evolution of the lyman extruder.  It is really cool to see how this is taking off.  I do not mean to take anything away from their accomplishments, but I want more.  You see, I suffer from a problem...   I am a mechanical engineer.  Even worse, an engineer with a project account!

So my first question (yeah, 8 lines later.  I did say I am an engineer), what is the best forum for those wanting to build their own extruder?  I have searched and soliforum seems to have the most information and activity, but it is titled "Filastruder", so I do not want to talk about fabrication and design on someone else's forum.  Is there a better site, a better section of this one, or has this become the defacto go to forum for extruder design?

Second question: Has anyone played around with extruder designs without the homebuilt limitations?  Lets say you have machining ability with mills and lathes, a fabrication shop for welding, bending, cutting, and a material budget that exceeds 3 figures if need be (that sounds pathetic! LOL!).

So, is this my new home, or can someone provide me with directions on where to go? (be nice).

Background: I am a middle aged mechanical engineer that has specialized in manufacturing from injection molding, vinyl and powder coatings, production machining to now playing the part of a small plant manager in manufacturing.  While I could (and probably should have) just bought a filastruder, the engineer part of me craves to design and build stuff.  I guess that is why the maker movent is so fascinating to me!

Again, Kudos to Elmoret for what he has created here for the home user, I am impressed.  I do not mean to take anything away from or diminish what they have accomplished.

Thank you and I await a response before I expand on the second question,

Brian