Tri-Polar Bot Concept -- Meet Wally

@DaveGadgeteer - without having either part in front of me to measure, I think that the diameter of the 'standard' 20 tooth GT2 pulley and the current printed pulleys are close enough not to cause any problems for calibration or resolution. Wally doesn't need any mechanical advantage on the arms, they move with almost no resistance. I think Nicholas is planning to redesign the printed pulleys to use the same kind of center-unwinding setup as on the original Kossel, though, so they won't be the same as are on the prototype (and those aren't in the zipfile of STLs anyway).

Yeah, I wasn't worried about the mechanical advantage, I was thinking about the resolution. And I worry about backlash, too, of course, though I think one of the belt types (GT2?) is supposed to be pretty good for that.
I guess with new string pulleys the advantage for belts will be small.

@DaveGadgeteer: There really is no motivation to use a belt over a string except for one thing. Belts will need virtually no maintenance. Strings will need to be periodically tensioned more often and replaced more often. The lifetime costs of a string version will still remain lower. So the question is... Do you mind checking the tension between every print?

Nophead has mentioned that he sees little difference between the XL and the GT2 belts in terms of backlash. I will be using XL but it will be a simple matter to use whatever is handy at your locale.

owens is also right that no mechanical advantage is needed for movement. However, I am going to increase the mechanical advantage to get better precision. Right now I use a mechanical advantage of 5.333 but I hope to change that to 6.25 or more.

Any backlash should happen only in the small pulley, so increasing the mechanical advantage also reduces whatever backlash there might be.

I've got the starter files, I've got a appointment with a laser cutter next week.
Any idea of BOM?

Nut Bolts, board, nema17(x4?)

@glennswest - This is based on my personal shopping list, which is completely UNOFFICIAL. Use it at your own risk, though Nicholas has read over it and didn't yell at me for any obvious mistakes. Where possible I've tried to describe what the prototype and beta kits will use, as well as some straightforward substitutions. Obviously you can make more radical changes if you are willing to deal with the side effects (modified motor brackets, for example).


Motors: The beta kits will use four of these 40 mm deep NEMA 17 motors from QU-BD .You will need 40 mm motors in order to fit in the brackets for the arms and Z lift, but you could substitute a different motor for the extruder if you want (see below).

Extruder: The beta kits are direct-drive, using one of the QU-BD motors, the QU-BD MBE block and a Trinity Labs hobbed pulley (the QU-BD Raptor gear didn't work well on 1.75 mm filament). The extruder simply needs to mount somewhere on the back board, so if you're willing to work out a different attachment you can substitute any kind of extruder. Personally I haven't had a lot of luck with direct-drive and I plan to start with a modified Greg's accessible geared extruder.

Hotend: The center hub is designed with a standard groove mount, so in theory any hotend can be used. However, since there is no provision for cooling the hotend barrel, the suggested version is a Printrbot Ubis or QU-BD anUBIS clone. The power supply in the betas is 24 volts so you may want to adjust PID_MAX in your firmware in order to avoid overdriving the heater.

Bowden: The beta kits will use a 1.75 mm Bowden setup with 4 mm OD PTFE tubing. The tubing extends through the center hub to the top of the hotend, and is clamped at the top of the hub with a printed force cone and nut. On the extruder end, the beta kits will either use a clamp or an M4 nut (TBD). If you use a different extruder, it just needs to be able to accept the 4 mm tubing.

Electronics: Azteeg X1 with SureStepr SD8825 on the beta, set up for 1/32 microstepping. Any controller should work fine, but if you use stepper drivers that only support 1/16 microsteps you'll have to keep track of the difference for your calibration values. One reason for choosing the X1 is because it can handle the 24 volt power supply and drive the bed heater; some other boards won't. If you run with an unheated bed or a 12 volt heater that won't be an issue.

Bed heater: QU-BD 200 mm round silicone, which runs on 24 volts and can draw up to 12 amps (hence the beefy power supply). A 200 mm square heater would work fine too. If you are happy with a cold print surface, the bed heater can be left out.

Bed surface: The beta kits will have the beautiful black basalt plates, waterjet cut to size and laser-engraved. Nicholas has some available to sell as well, due to an accidental order mixup. If you don't want that, any appropriate material will work (glass, mirror, etc.)

Power supply: 24 volts at 14.6 amps for the beta, because of the powerful heater. If you substitute a 12 volt heater, just make sure to size your power supply appropriately.

Metric hardware: I don't have a good list of what's needed yet. The joints are all held together with M8 bolts and nylon locknuts, passing through the arms and bearings. M8 washers are used between bearings for spacing. There is a considerable amount of M3 used to hold the printed parts to the wood plates, with ordinary nuts and washers. I suspect that you could substitute 5/16 and 6-32 for most things if you're willing to drill out some of the mounting holes (motors will require M3, obviously).

Bearings: You'll need about 30 608ZZ skate bearings, and three 6702 bearings for the center hub.

Endstops: The prototype has no endstops; the arms have a physical stop at maximum extension, and the bed sits at its lowest point. There's been some discussion of an endstop for the bed, though; it will probably be a miniature snap switch (13 mm x 6.5 mm x 6 mm or thereabouts). Those mount with very small screws, M2.5 or 3-48, which can be difficult to find.

Braided line: The prototype used 100 pound test braided Spectra fishing line, available at any sporting goods store. It can also be bought online, either as fishing line or kite string, but be sure it is Spectra or Dyneema, not dacron, nylon, polyester, etc. The 100 pound size is probably overkill, I'll be testing with line in the 50-85 range as well.

Other parts: In order to keep the bed from lowering unexpectedly when the motors are shut off, the design may be changed to include a constant-force spring as a counterbalance.

@owens: Great, thank you for this! It'll definitely help!

Now all we need is the officially released STLs and DXFs and all of us can get going on this.

These numbers may be off by 1 or 2 here and there but this is the specific bolt and nut run down. I don't disagree with anything that Owens has posted above except... He overestimated the bearings. You need 26 608's and you need 2 6702's. However, it is never bad to have extras on hand. 6702's is about the only part you can't run down to the store and pick up.

(14) M3-20
(44) M3-16
(1) M3-10
(2) M3 NUT
(2) M3-50
(2) M3-5ish GRUB SCREW
(4) M3-6
(57) M3 NYLOC NUT
(4) M4-12
(13) M8 WASHERS
(4) M8-55
(3) M8-65
(2) M8-70
(2) M8-90
(1) M8-105
(4) M8-35
(16) M8 NYLOC NUT

Edited 3 time(s). Last edit at 09/27/2013 07:17PM by nicholas.seward.

BTW, There are a lot of different M8 lengths in the list above. If you are in a non-metric country and you can't get cheap metric bolts locally then you can replace them with 5/16" equivalents and nothing bad will happen. I just looks at McMaster-Carr and all the minimum quantities are 25 or 50. However, for $100 or so you can get an order from McMaster-Carr that will have enough for about 8 Wally's. This obviously means that you have to build 8 machines. Keep 2 and sell the other 6. :-)

Someone on the Printrbottalk forums discovered a place that sells individual metric bolts for reasonable prices, which he used for his TAZ build: Bolt Depot. I haven't ordered from them yet, but will as soon as I have time to put the list in their system. On a quick glance through their lists, the only thing they seemed to be missing is the M8-105. I was trying to figure out where that is used and whether it was a critical length, and since it's the only single bolt in the list I am guessing that it's in the spool holder. Would an M8-110 work there?

110 is good or 4.25"+. It is for the spool holder.

Edit: I just checked the prices from the link above and they are about as good as can be expected. Additionally, if you go with non-metric bolts you need 1/4" washers instead of 5/16" washers. For some reason metric washers hug the bolt much closer than non-metric washers do.

Edited 1 time(s). Last edit at 09/27/2013 02:23PM by nicholas.seward.

BTW, regarding the basalt print surfaces, it appears they would be really difficult to melt. But that experiment is just a few miles away from my house, so I'll have to make sure my basalt beds stay well hidden

FWIW, I placed an order with Bolt Depot for everything Wally needs and a few other things that might be handy, at 3:43 PM on Friday afternoon, and they shipped the order at 5:12. I am somewhat amazed. . .

Also, if anyone is looking for 6702s, I bought a pack of 10 from Fast Eddy Bearings on eBay for $10.74 shipped. That's about what one costs on Amazon.

Bill.

Can I buy the basalt bed Nicolas?

(If you have spares)

Or get the supplier to buy 1-2 off them.

Guys I'm going to get a wally cut, what materials do you recommend.

I've got a industrial laser cutter working with me, up to 150watts.

Guys, I did a blogpost, on wally, summarising here what we've discussed, just to try to get more interest.
If there's edits, etc, or I need to give more credt to anyone, please let me know.

[3dwest.blogspot.sg]

The source files for Wally?
So it can be modified.

Also anyone know how many of each part?

Mind if I go ahead and create a git repo?
I can merge changes, as I'm changing the extruder to the kossel 1.

Just wondering if anyone looked at converting to GT2 belts, verse using the fishing line.

I'm going to go with the geared extruder used on Kossel Mini.

I'd suggest waiting for @Nicolas_Seward to create the git repo and then clone it.

@glennswest - Yes, a belt drive design is also being explored by Nicholas. It was mentioned above.

I know a few people are chomping at the bit to take off on building a Wally. However, the design is in a very rapid evolutionary stage right now after the first prototype has been built and demonstrated just a week ago. I would give Nicholas at least a couple of weeks to incorporate what he has learned into the design plans. It will make it a lot easier for the first efforts at independent replication to have all his current learnings and clean files to work from (JMHO).

I believe there will be a lot of variations on the Wally design in the next year. I am looking forward to seeing Wally grow up and have a family.

So any need for help on sourcing?

I'm already choosen my psu, the extruder/stepper for it, the 4 core steppers, silicon heatbead.

I think I can pull down the price quite a bit.

@glennswest - Wally only needs three steppers for the axes, plus one for the extruder. I haven't been able to find 40 mm steppers of similar specs for substantially less that QU-BD gets for them. QU-BD says that they're custom wound just for them and make 4.5 kg*cm of holding torque at 1.2 amps. The label says that they're Wantai 42BYGHS610P2, which isn't a standard part number in the Wantai catalog, so they may very well be special. Rattm has a 40 mm motor that claims 4.0 kg*cm at 1.3 A, and theoretically they're $7.50 each rather than the $14.50 QU-BD gets. . . but with shipping, the Rattm price jumps up to $12.65 each, and you have to buy five.

Unfortunately QU-BD is currently sold out, perhaps because of all us Wally and Simpson builders. . .

Is it me, or in the starter files, I dont see diminsions on the drawings?
Maybe its the version of the software I'm using?

Those are to scale DXF's for CNC mills or laser cutters. I am working on human readable ones along with printable templates. This is turning into quite a project just prepping the release.

nicholas.seward Wrote:
-------------------------------------------------------
> This is turning
> into quite a project just prepping the release.

I can only imagine! Definitely looking forward to seeing the release so a number of us can start printing.

Hey,

regarding string drives, the following description of a capstan drive for observatories and similar specifies a way of reducing maintanence by using springs to preload the (multiple) drive strings:
[www.sagebrushtech.com]

Cheers
Shez

What wood is recommended for Wally?
What about paint? Latex? Stain?

That is all open for interpretation. I use 1/2" birch and I sent it off to get coated. I don't know what exactly they did.

Powder Coating I think is best.

If you use MDF, you can powder coat it. That would look amazing. I keep asking local power coaters to work on MDF and they apparently don't want extra business. Before I got into 3D printing, I made a mill that could make all of its own parts. I wanted to powder coat all of these parts so it would look like a finished product.

thats because when powdercoating powder bakes in owen on 170+ degrees celsious.so every moisture in wood will results with bad powdercoating.

i have akso powdercoating on my working place, but we are dealing only with metals...