Here is my start in testing out if parts produced through rapid prototyping methods will work out well for armatures. Right now it is just the start but I will be trying new and different techniques, equipment, and designs on a regular basis and will try to keep everyone up to date. The metal parts are done in a 420 stainless steel powder that is then infused with bronze through its many steps. These pieces were designed with the known typical tolerances of the equipent it would be produced on just to get a baseline. I have other parts on their way that go beyond these to scale even smaller closer to the target designs. Right now it is hinges but everything is on the table here in the near future.

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Impressive. How does the motion of the hinge feel?

The plastic ones feel great (of course). The plastic parts are sized exact, but I over sized the metal ones on purpose since I knew the surface (at the surface areas of contact) would not be good enough and needed a bit extra to work with. I am doing different finishing tests with this material to get a better idea of what works and doesnt. So as of right now the metal hinges themselves have not been connected for testing yet. I am itching to get to that step too. Even if it does not work out (but I believe it will) I can resize the spacing to use washers and not have to finish the metal off that much. I just got the next step down prototypes today and I am working on getting pictures up here shortly.

Here are the new pictures as promised. They are sitting on the penny in the middle. The previous larger size is in the back. And again a plastic version of the new smaller ones are in front. I am working on an even smaller hinge but the design is different. I am changing the design for the even smaller one more so for the material limits and not for the machine limits.

My first thought was that it was like a cold casting, resin with metal powder, but I wasn't taking in the title of the thread.  So it's built up with all metal?   what is the strength of these rapid prototype 3d printouts? 

For a hinge, where the tension screw is also the pivot, I would think they would be strong enough.  There is a sandwich of solid material in there, so it mostly needs good compressive strength. But with a ball joint, the tension screw has to be slightly off from the ball, so more tensile strength is involved.  I will be looking forward to hearing how well it works for that.  I know there are many things cast in metal which have the weight and look of metal but very little strength at all, but I have no idea how this process compares.

One thing I would question - the thin neck between the discs and the triangular body.  I wonder if that could be a weak point.  Seems to me, the widest parts of the triangular blocks would collide first when the joint is bent as far as it can go, so there is no advantage I can see in having that neck so thin.  What if the neck were thicker, and the wide part of the block were slightly less thick?  There may be a reason for this I haven't worked out by looking at photos of course - I need to grab bits in my hands and move them around to really understand the mechanics of it.  I don't know how the 2 parts look when the joint is fully bent.

Interesting approach, anyway, ways of reproducing joints that are less labour intensive than machining each one are always worth looking into.  Keep us posted!

Yes it starts out like a cold casting, of just 420 steel powder, with an organic resin binder. From there the parts are put in a furnace for sintering. At this point the binder is burned off leaving a very porus stainless steel part. From there these pieces are put in a support material and powdered bronze is added and they go back into an even hotter furnance and the bronze just gets sucked right into the porus stainless steel parts. So they are a true solid metal trough and through in the end. It is not quite an alloy but it is a composit. The shape design allows for an up to a 30 degree flexion (think more so for the elbow) to a 180 degree extension.

I can work on getting some pictures showing flexed and extended of the plastic parts if that would help you visualize better. Also to add this metal has a 20 to 25 Rockwell hardness (HRc, C scale).

Here are some quick pictures to show the flexion and extention of these joints using the plastic prototypes as an example.

I haven't machined this sort of hinge myself yet, but my understanding is that this design usually incorporates 2 very thin phosphor bronze washers. Depending on the tolerance of the rapid prototyping parts, using those might allow you to avoid having to do finishing work on the mill.

Rapid prototyping  ... this production methods creates very accurate dimensioned objects which are prototypes for an actual 3D solid object evaluation, study, and hands on tactile feel of the product. The final product eventually mass produced using more robust and/or higher strength materials via other manufacturing techniques. Obviously rapid prototyping now being used to make custom or one off decorative type items that are perhaps not subject to structural fatigue or wear & tear. In the highly niche Stop Motion arts/crafts, it is being exploited for replacement puppet facial features or appendages. In that application, the rapid prototype model/item is not really subject to mechanical type stress. This includes prototyping items for miniature props.

I have no experience with rapid prototyping and only cursory knowledge reading about it randomly. Also, I do not know the properties of the metal-like material used for those rapid prototyped hinge joints. Of course testing would give answers. A good acid test to evaluate the strength of it, would be, using that same metal, and rapid prototype some sandwich ball joint plates that are socketed. Then insert your balls (with rod attached to balls) inbetween plates, tighten down via the tension screws, onto the plates, then grab the rod attached to ball and manipulate the ball within the sockets to feel the tension, whether it's smooth, grabby, jerky, etc. Another test with these same rapid prototyped sandwich ball joint sandwich plates ... reasonably tighten the tension screw down on the plates/balls and let it sit there for awhile and observe if the plates eventually bend or sag.

There have been toy like figure-type armatures made using Pot Metal as the material. At first the pot metal ball joints (the plates) seem to temporarily work, but afterwards, the plates slowly starts to bend and requiring more tightening. Eventually the plates will snap when they can bend no more. Some years ago, I bought a desk lamp that was armatured jointed (kind of robot looking). The lamp used only for decorative purposes (not animation). In the few years, the armatured lamp just sitting there, the plates started fatiguing and I have to re-tightened. Plates eventually break and as each one breaks, I replace with my own real steel plates. I would be curious about this metal used in rapid prototyping, whether or not it will behave similarly to pot metal.

http://en.wikipedia.org/wiki/Pot_metal

What you are concerned with would be called the yield strength.This is the stress at which a material begins to deform plastically without returning to its original shape. Pot metals of the type you are talking about have an average yield strength of around 250 MPa (for the good alloys, not the cheap one used in "toys"). This material, and it is not metal-like but a true metal trough and through, has a yield strength of around 455 MPa. Next thing to worry about is the ultimate tensile strength. This is the maximum stress that a material can withstand while being stretched or pulled before necking. The same pot metals have a UTS around 350 Mpa. As for this prototyping metal composite it has a UTS of around 680 Mpa. This material is also tough enough that you need carbide bits to mill and drill it. So this material by specs is good. I will be doing test though as well and share those as I move along.

"my understanding is that this design usually incorporates 2 very thin phosphor bronze washers"

I use 4 0.010" washers, two on each side of the tang.  That should isolate the structural part of the joint from the bearing surface.

The prototyped parts could include the additional space for the washers which, along with a relief slot, would make this fairly reasonable. Cleanup machining for threads and what not would be fast.

Here's a knee I did earlier this year showing washers. The tang is 1018, and the yoke is 2024.

This is all very interesting and I look forward to hearing about movement test!

 

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