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 Post subject: Re: Let's decide on a group project! [Decided: Prototyper]
PostPosted: Fri Jul 31, 2009 11:36 am 
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Mike, the Darwin is a 3D printer not a cutter.

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 Post subject: Re: Let's decide on a group project! [Decided: Prototyper]
PostPosted: Fri Jul 31, 2009 12:55 pm 
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I think the premise is that the extruder could be replaced with other tools.

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 Post subject: Re: Let's decide on a group project! [Decided: Prototyper]
PostPosted: Fri Jul 31, 2009 2:35 pm 
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uptownmaker wrote:
I'm mixed between a Darwin as designed and trying to do something slightly, but not radically, different.

Primarily, I see no reason to adhere dogmatically to their parts list and parts design when any X-Y gantry with a Z adjustable table under it will do what we need. This opens up the possibility of scrounging a lot more parts.

Today I got from a coworker a large-format printer with a 24" print head displacement. It's a REALLY good start- we'd be hard pressed to make as good a stage as that.

I'm also going to suggest that we forgo trying to make this capable of cutting anything more durable than vinyl. Resisting the deflection of the tool tip is hard work, if you're trying to drive a tool through a rigid material.


Fine, this looks like a good starting point.
So we are looking at a mainly additive system with a 2' X , Probably a matching Y and say an 18" Z. I would still argue for the inclusion of a powder/printing ability.


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 Post subject: Re: Let's decide on a group project! [Decided: Prototyper]
PostPosted: Fri Jul 31, 2009 3:38 pm 
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jwb wrote:
Mike, the Darwin is a 3D printer not a cutter.


I know, BUT there have been some people talking about making this a modular system with cutting and printing capacities.

I'm merely stating that I think that the two goals are different enough to warrant two different systems, and I think this one should be additive for the most part with a POSSIBLE option for VERY light duty cutting (I'm thinking of the extremely pointy tip dremel tools being used to cut vinyl or etch designs in plastic or wood).

I also think our Y should be based on what we can find for ways- it's not easy to make a rigid way system with a long travel. We may find that 12" is the best we can do at a reasonable price on the Y travel end of things.

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 Post subject: Re: Let's decide on a group project! [Decided: Prototyper]
PostPosted: Sat Aug 01, 2009 5:46 am 
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Add a guy name @clothbot on Twitter -- he bought a cupcake and is pondering ways to have an extended Y -- conveyor?

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 Post subject: Re: Let's decide on a group project! [Decided: Prototyper]
PostPosted: Sat Aug 08, 2009 9:23 pm 
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On 3D powder printing.

Adapting A RepRap to do powder printing should be relatively straight forward.

You all have seen the candyfab. But there are others. Make ran a blurb on a Stainless Steel prototyper again that uses powder.

Lowering the Z platform to the bottom. A plate with a hollow tube is fixed at the top and a cylinder close cylinder is inserted into it. The Z platform is then run up until the top of the cylinder is level with the top of the plate. Now a change in Z can give you a the area to create a new layer of powder. Run a feed shoe full of the base powder over and back and you have a new layer. Depending on the material a print head jets the specific binder/glue for that layer and the process repeats. In the case of plaster or metal powders the part will still have to be fired.

We really, really could make Stainless Steel parts this way as you can sinter Stainless as low as 1600F. Lots of parts in your cars are still sintered at 1650F.


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 Post subject: Re: Let's decide on a group project! [Decided: Prototyper]
PostPosted: Mon Aug 10, 2009 5:37 am 
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Check out Don Lancaster's thoughts on this matter. He's been advocating for cheap homebrew rapid prototyping systems ("flutterwumpers") since 1994(!).

He makes a compelling argument that XYZ is a bogus coordinate system- his analogy is to rocketry, where your first stage has to lift all subsequent stages and their fuel. Think about it- X supports Y AND Z, and Y has to support Z.

When you design an XYZ gantry, you end up with a system which is prone to summation error, difficult to build, and vastly overbuilt, among other flaws.

RepRap and MakerBot cheat, ever so slightly, by moving the tool in X and Y and moving the piece in Z. Better, but you still have an X translation of the entire Y-axis system. AND you have slide bearings, which wear and are painful for deflection.

An easy-to-understand cheat is to move the tool in X, the work in Y. Z, of course, requires one or the other of those two axes to be fully moved as well, but Z-axis loading is likely to be fairly constant, so deformation is less of a problem, and the motion is less frequent and in one direction only, so backlash and summation error isn't as big a problem.

A less intuitive fix (the one I'll advocate for) is to abandon XYZ entirely in favor of RӨZ. In this model, the tool is at one end of an arm, a counterweight is at the other, and at the fulcrum (they form a first-class lever) is a stepper motor. By rotating the arm, you control your R position. The workpiece sits on a turntable- that gives Ө control. You then raise or lower either the arm and stepper or the turntable (my vote is for the arm and stepper, because it's smaller and probably lighter). Think about a record player- there are no linear sliding parts, and yet the needle travels across the entire surface of the record.

This is a good system for removing or adding material: if more cutting force is desired, place extra weight on the tool and the counterweight end. The motor can be placed above the arm while a rotary bearing of arbitrary strength supports the weight of the arm. The motor can be geared (no belts required) to arbitrary strength, providing cutting force as desired. Another advantage: easier feedback. A linear feedback system is NOT trivial- either you need an artifact (like a ruler or graduated stripe) which provides pulses that can be counted (although this can be replaced by an optical mouse, these days), or you need a wheel-to-linear conversion and pulse counting on that. Oh, and you need to home the system, usually, to get any good accuracy at all. XYZ needs that for all three axes, too. In RӨZ, your Ө position is tracked using a rotary encoder, geared down for arbitrary resolution. R is tracked by using a potentiometer- there's no need for full rotation on that motor. In fact, R can be realized with a fancy-dancy RC servo motor, for small loads. Z doesn't REALLY need feedback at all- stepper motor pulse counting is probably sufficient, since motion is in one direction and gravity is in your favor.

There's one other option which can be considered, and that's a third-class lever for the tool arm. In this case, you put the tool in the middle, a motor at one end and a bearing at the other. A track of some sort is provided for the bearing at the end opposite the motor. This has the benefit of providing more rigidity to your tool in the Z-axis, and of keeping the entire tool arm within the footprint of the machine (although by choosing a large enough counterweight for the first-class lever option, the back end of that arm can be pretty short). It has the drawback of making the Z translation harder: instead of a single screw which lifts the entire tool arm at once, you now need several screws, placed quite far apart, in the case of a machine with a large desired workpiece size.

The main drawbacks of this system is size (it needs to be square, so if it has to handle, say, a 4'x8' sheet of plywood, the turntable needs to be 4.5 feet in radius, meaning a nine foot square(-ish) footprint, as opposed to a 4-ish by 8-ish footprint for a gantry machine. In terms of complexity, it thrashes every other solution out there, hands-down. No belts required, no precision linear slide bearings required, easier geometry (consider that a RepRap needs four quite parallel axes, one per corner of the table, plus the X-Y axes must be perpendicular to each other AND the X and Y planes must be parallel with the table)(whereas RӨZ requires only that Z be orthogonal to Ө and the tool be parallel with Z), cheaper parts (think about all the bits and pieces we just eliminated from our system- imagine a record player with an arm that moves in Z versus the RepRap).

Plus, it'd be wicked cool, and WAY cheaper.

Time to get ready for work. Anybody have any thoughts on this?

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 Post subject: Re: Let's decide on a group project! [Decided: Prototyper]
PostPosted: Mon Aug 10, 2009 8:49 am 
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uptownmaker wrote:
A less intuitive fix (the one I'll advocate for) is to abandon XYZ entirely in favor of RӨZ... it'd be wicked cool, and WAY cheaper. Time to get ready for work. Anybody have any thoughts on this?
I have had the same thought running in my mind for quite some time. I can see definite advantages. I like this!:
Quote:
Even more elegant is a revolutionary new virtual ways
system. Described in the August 15, 1994 issue of Design
News. Take a base plate and place six ballscrews or linear
steppers uniformly around a large circle on it. Now lean
each ballscrew by 45 degrees or so and attach them all to a
smaller upper head plate. The head plate can easily assume
all of the normal lathe or mill motions. But there are no
precision sliding contacts anywhere! All bearings are plain
old round ones. All forces are pure tension or compression
with zero side loadings.
Lancaster provides plenty of helpful references too!
Quote:
Additional support can be found on the Flutterwumper
Library shelf of my http://www.tinaja.com web site. And in my
Blatant Opportunist reprints.

Consulting services are also available on the concepts
shown. Let’s hear from you.
This article was written in '94. Perhaps there's even a lot more information on this now.

Let's get started on this! Here's a YouTube video of a flutterwumper; from this page there are of course related videos as well:
http://www.youtube.com/watch?v=NQNWIKGeg68

Everyone: Remember that we have project page on our wiki for this; please contribute!

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 Post subject: Re: Let's decide on a group project! [Decided: Prototyper]
PostPosted: Mon Aug 10, 2009 9:36 pm 
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uptownmaker wrote:
Time to get ready for work. Anybody have any thoughts on this?


Admittedly I have a vested interest in XYZ as I would find it hard to rotate a wall. But...

First I can translate an XYZ coordinate system in to integers to any arbitrary precision, I can't really do that with a rotational system. While this probably wont come into play with our project real math errors could easily accumulate in software. It will be much slower when cutting or producing straight lines as except for arcs and circles with origins common to the platform all motions will need to be simultaneous on two axis. There really is no way to position the part to minimize tool movement and no software fix that can compensate for tool maladjustment.

But it is certainly cool and we should do it.


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 Post subject: Re: Let's decide on a group project! [Decided: Prototyper]
PostPosted: Mon Aug 10, 2009 10:17 pm 
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Judeling wrote:
uptownmaker wrote:
Time to get ready for work. Anybody have any thoughts on this?


...There really is no way to position the part to minimize tool movement and no software fix that can compensate for tool maladjustment.

But it is certainly cool and we should do it.
I think this problem has been addressed with closed loop (feedback) mechanisms, but I don't know how much that would elevate the cost. Lancaster talks about using a dedicated computer to do complex math and just leaving the core or kernel operations to be handled by the flutterwumper.

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