Saturday, January 13, 2007

 

Modified Mk II polymer pump working...

A bit of concerted shopping at Orchard revealed that 4-40 threaded rod (studding) replaces M3 pretty well. I was able to get lock nuts and springs to do the job properly. Whereas I was considering building a complete bootstrap Mk II I got anxious and decided to make the Mk II that Adrian gave me work with a few simplifications. I dispensed with that nasty milling job on the threaded polymer pump (M5 studding on the original specification and 1/4-20 on the American version) and fitted the bushings to the outside of the threads instead. I then built a brass thrust collar to contain the thrust towards the gearmotor and burned off the fingerprint on my left thumb when I underestimated how hot 1/8 inch brass gets when you drill a 1/4 inch hole in it.

I sawed a slot in a 1/4 inch coupling nut to turn it into quite a nice little gearmotor coupling. You can see the result...


I was able to get a stainless steel 1/4-20 long bolt to serve as the pump. I haven't sawed it off yet and won't until I get the whole assembly working together properly.

Hooking up a 6 v Solarbotics GM8 gearmotor to the assembly I was able to pump both HPP and HDPE without problems. I expect that I will have to sand off a half millimeter or so from the filament side of the pump to insure a snug fit with the welding rod.

I will be finishing the pump in the morning and building up the 2 amp heated extruder barrel. I will also be installing a dual darlington chip (754410) on the extruder test board so that I can run the pump as an ensemble.

Comments:
Nice going plaas, I better get to bed so I can get up tomorrow and read what happened next. Being a yank, I love all that fractional thread size stuff :). I finally have some parts ordered! One each x,y and z stepper and 4 pic16f628a. How do I get added to reprappers blog, looks like i'm going to start building soon. Where did you get your PTFE, or Teflon(r)(c)(TM) or whatever goes along with it.
 
***How do I get added to reprappers blog***

Click the people link on the reprap.org website.

Click on Adrian Bowyer.

Click on his personal web page.

Get his email address.

Request to join the RepRap Builders group.

Wait about a week for him to get time to fiddle the blog entries and you should be on.

***Where did you get your PTFE***

Funny, I just wrote an email to the RepRap group a few days ago talking about where to get stuff, including PTFE, in the US. I'm going to paste it here so that you can get a look.

***********************************
I wonder if we're not making this parts list a lot harder than it needs to be, for Americans at least. The problem with looking for a lot of "lowest price" vendors for electronics components, for example, is that you soon find that you are being eaten up on shipping and minimum order charges.

For most of the electronics in the US you can get virtually all of it from either Mouser or Glitchbuster at very attractive prices. Digikey tries to screw you over on PIC chips.

Exceptions are the Hamamatsu limits detector chips, which you can get at...

http://www.acroname.com/robotics/parts/R64-P5587.html

Shipping would be a problem except, iirc, you have one of these bad boys at both ends of each axis which means six which will cost $13.20 so shipping won't eat you alive.

I gather that you all are sticking with unipolar steppers since the last I heard the bipolar board didn't work that well or people simply were avoiding buying them, I don't know which. Until you have a clear idea of how much torque will be required to drive Darwin shopping for those will be a problem.

As to metric threaded rod and the like, Brett long ago recommended

http://www.mcmaster.com/

...as having both good prices and high quality. They do.

For the Mk II, you can get the gearmotor from either Pololou in Las Vegas or direct from Solarbotics in Calgary. If you buy direct from Solarbotics have them ship by post in that the courier service charges in Canada are extortionary compared to US charges... and that's saying quite a lot.

The PTFE rod can be had from...

http://www.usplastic.com/catalog/category.asp?catalog%5Fname=USPlastic&category%5Fname=12077&Page=1&gclid=CJau5Nmf0YkCFQRDYwodyRxxRQ

...at decent prices. The only problem is that sizes are in inches, so you've either got to mill down their half inch PTFE rod or talk Adrian to redesigning the clamp on the Mk II to accomodate half inch instead of 10 mm rod. Maybe you could drill out that 10 mm hole in the clamp. I don't know.

Power supplies with lots of amperace can be had from...

http://www.tigerdirect.com/

for anywhere from $15-25 for a PC power supply with plenty of punch to drive those steppers. I've found power bricks a bit pricey for what you get in the States.

Now that Adrian has designed that universal board for Darwin the question comes up as to how you get one made. I doubt Adrian wants to make hundreds of these darned things and needing five of them for every Darwin it will get to be hundreds very quickly. I was thinking that it might be worthwhile to farm out the board fabrication and perhaps the assembly as well to one of the many hobby robotics companies in the States. They could sell you the board plus a sack of components for the DIY crowd or sell it too you completely assembled. You'd just have to shop the designs around till you found somebody who was willing to do it at a price that wouldn't break the budget. I know Pololou does things like that, though they do most of their designwork in-house as best as I understand.

That leaves the belt for Darwin. I got Adrian a link to a US supplier for that a long time ago. I don't know if he ever used it, but I'll chase it up in my correspondence or track it down on the web again if that's necessary.

One last little thing you will need is a 3 mm drill bit to tap the PTFE thermal barrier.

http://www.smallparts.com

has those in proper lengths for a good price. You will see them cheaper but be careful about buying drill bits in "jobber" lengths. The fluting on those is only 10 x diameter, so you only get 30 mm of fluting on your drill bit, which isn't enough.

You need a #76 wire gage drill bit to do 0.5 mm holes in the end of your extruder barrel. You can get those from...

http://www.gyrostools.com/Default.asp

...or from the better hobby shops, which are getting quite rare these days. You'll need a dremel tool to drill with these, preferably with a drill press add-on. The chucks on regular drills are too big to seat these hair thin drills.

http://www.woodcraft.com/family.aspx?FamilyID=784

If you're in no hurry you might also want to try a spiral hand drill...

http://www.hobbylinc.com/cgi-bin/a3.cgi?back=http://www.hobbylinc.com/htm/gyr/gyr4510272.htm&add=zon37-160

It's cheaper and you probably have more control than with a dremel.

You can probably get that thermistor from Mouser. I wouldn't know since so far my extruder upgrade of the Mk II doesn't use one.

You can get your #32 insulated nichrome 80 wire from

http://www.pelicanwire.com/index.htm

You can get 3 mm plastic welding rod in a variety of engineering plastics from any plastic welding supplies distributor. You have to specify coil rather than cut rods but otherwise no problems. If you want CAPA (polymorph) you're going to have to special order from manufacturers.

Other sundries you can get from Radio Shack and your nearest hardware store.

************************************
 
How will we get thick individual sized pieces? For example for the thermal barrier on the low thermal inertia extruder design? Those sheets are huge and expensive.
 
Yeah, the 1x1 inch PTFE bar at $29.50/ft is a bit pricey. You need about 1 cubic inch and have to buy a foot of it to get that. I expect that builders need to club together to get this sort of nonsense and then share it out.

I just got through drilling and tapping a 1 cubic inch block for the extruder. It is finicky work. I thought that PTFE was slick. I had to ream it out a bit to be able to handle a 2.9 mm polypropylene filament, though.

I'm living and learning. :-)
 
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