Saturday, June 17, 2006



If you scrape away all the idealism what we seem to be doing here is as simple as making a CNC positioning system that mounts a polymer extusion tool instead of a more conventional tool head.

We have a brilliant design for a room temperature polymer extruder. It wants to use filament which is not a terrifically difficult technical problem to solve. The rest of the stuff that we're doing is, when you get right down to it, nothing more than the same problems that the rest of the hobby CNC community are addressing every day in a million different ways.

Cool! :-D

How did I get here? A few days ago one of the people who occasionally comments on what we do on this blog noted that what he sees CDC machines doing most often is making parts for for other CNC machines. That thought has been rooting around in my mind like a kanker ever since.

Shortly after that in responding to a comment on one of my blog postings I did a recap of a quick quantity survey on the poplar that had been used in Godzilla. What jumped out at me is that if I had a proper workshop I could have made it out of rough poplar that cost maybe US$30 rather than the milled, expensive pieces costing maybe US$100-150 that I bought that suited my hand saw and mitre box.

What didn't hit me until this morning is that if I mount my Dremel tool with a cutting head on it on Godzilla instead of a Mk II extruder... I have a proper workshop...

Letting the other shoe drop the natural question arises that if you can replicate the structure of Godzilla out of U$30 of poplar why should you want to insist on doing it instead with maybe US$75 worth of caprolactone?

I think the melts I did in my toaster oven started me thinking in this direction. It is a simple matter to melt HDPE into quite a respectable ingot (this is even easier to do with caprolactone) whereas making the same HDPE into filament that will run in something like a Mk II is a lot more trouble. Why not mill the ingot into what you want and save the swarf instead of making everything into filament and trying to extrude everything.

Similarly, Simon, if I recall correctly, was talking about setting up a furnace capable of melting aluminum. Those aren't hard to make. Why not use aluminum in a RepRap? You can make a cupola furnace that will melt steel into ingots for not a lot more. Why not use steel in a RepRap?

After all, we do already anyway, don't we?

If you think of a RepRap as a CNC machine that has a bunch of tool heads, an extruder being only one of many, the whole concept gets a lot more flexible and interesting to me. :-)

Constructive fabrication allows for much more versatility in the long run. You can make much more complex objects than with destructive fabrication. That's particularly true when you want to seamlessly mix multiple materials, such as integrating 3D electronics and so on directly into things.

Still, a CNC machine is probably a good start for constructive fabrication too. Just too expensive for most.

Also in the longer term, we would hope to make a machine that produces the raw plastic, so that will bring the cost down further than poplar. And it is more quickly renewable than poplar.
Not to be brutal or anything, but steel, even aluminum, are just dumb ideas for this project. You aren't thinking about cutting tools or their hardness, or the precision you neeed.

You need to take a look at what the real cutting forces are for mill work; Cutting poplar for your parts is maybe just barely in spec for your reprap chassis, though I'm betting not; you've got lots of long arms waggling around.

One of the things that makes reprap plausible is that by doing additive machining you only need precision, not precision and strength. Further, you don't need a whole forge infrastructure to complete the replication.

If you want to build machine tools from scratch then look to Gingery, who will sell you cheap instructions to get from wood+scrap iron to mill + lathe.
"Further, you don't need a whole forge infrastructure to complete the replication."

I would just like to point out that casting aluminium is only moderatly difficult. I suspect it is a very good choice for making heavier duty parts beyond the capabilities of plastics.

There is a wealth of very good info on the web about simple home built blast furnaces, and some are getting spectacular results with their castings. My results have been more modest!

I would still suggest keeping the RepRap project focused as is for the reasons stated above by others, but aluminium is bound to play a role in things if this machine starts to spread out into the population at large.

Just think of the fusion of RepRap and Gingery concepts ...
I would also suggest we stay focussed on additive machining - as has already been pointed out, subtractive machining is best done using the Gingery machines and a backyard foundry. If you want CNC subtraction, it'd be easiest to add stepper motors and sensors to the axes of the tried and tested Gingery machines.

That said, of course, deposition machines do have the ability to make certain parts that subtractive machines cannot - one option with the existing apparatus that I recently suggested would be to use the reprap to make low temperature wax shapes which can then be surrounded with plaster of paris or green sand or whatever and used in lost-wax casting. Of course, I'm still interested in cutting out this step and adding a molten metal deposition head directly to the RepRap machine - the melting part's easy, you just need a high current transformer, say from an arc welder. The tricky bit is getting the layers of metal to adhere properly without oxides causing problems. For that, I was considering an ultrasonic welding head to follow behind the deposition head to finalise the layers - driven by magnetostrictive iron transducers as opposed to piezoelectric, so no exotic materials are needed. Unfortunately that branch of development is on hold until I get my foundry finished, which is necessary for melting the raw material and casting it into feed rods. If anyone's interested in the progress of the foundry itself, there are new photographs on my weblog.
seagrol: I'm very glad that you decided to misbehave. Thanks for the commentary. :-)
One approach I'd been wondering about was if one ought to think of putting down a layer as fast as one could and then switch to a much slower cutting tool to trace the perimeter of the layer much more accurately and slowly.

Similarly, it seemed to me that you might want to map the surface of the extrusion periodically for uneveness and either fill in the dips or cut down the high points.
//Why not mill the ingot into what you want and save the swarf instead of making everything into filament and trying to extrude everything.//

I had wondered about that to. I think I mentioned it to someone and they indicated that, while you can mill down blocks of plastic, there are limits to the shapes possible with that method. I agree, though, that a large number of parts could be milled and maybe a few would need to be printed.
It would be possible to compromise and design a Reprap that could be made from recycled HDPE sheet - or chopping boards as I have seen elsewhere. We could even print the plans so they could be stuck down on the boards to be cut out.

Vik :v)
Sand... Hourglass... Hmmmm... Nice...

No lateral support though. Vik is already having problems with the extrusion thread dragging his extruded object back and forth across the work surface. :-(
The great thing about RepRap (if I may say so) is that, once we get our first release out there, anyone can take it in any direction they like, including down a subtractive route.

But as others have said, there are two big advantages to additive manufacture over subtractive: 0 force, and no tool collisions. As someone involved in automated path planning, I can tell you that avoiding tool collisions is a very very very difficult problem in all types of subtractive manufacture.

I sometimes wonder where we would be now if, at the start of the industrial revolution, people had taken something like a Jacquard loom (additive, of course) and made it cut out and lay down thin veneers on top of each other, gluing them together...
Well, we are going for cold-melt-metal in the form of Wood's metal of course; that will allow us to build 3D electonic circuits...
On the subject of melting metal, have a look at the Stovetop Metalcasting site.
Wood's metal? Humm... that contains about 28% lead. Over the years I've researched lead and it's infamous neurotoxicity a number of times. Great stuff for permanently reducing the IQ of children. And all this at a few microgrammes per -decaliter- of blood.

I hate to say it, but I would think it is a very risky choice for anything other than supervised lab use, never mind third world application. There is also the possibility of legal issues in other countries where they are on the ball about polution and public health, take for example lead in car battries. As things stand right now, there is also a push to gradually ban lead solder in electronics...

I would suggest seriously investigating the new alloys being proposed to replace lead solder. Yes, there are a number of vexing downsides, such as higher melt temps, but none of them are as substantial as brain damage in kids.
The problem that I've seen with low melting temp alloys is that if you want them non-toxic (no lead or cadmium) and you want them to melt at temperatures lower than about 140 degrees Celsius you've got to include a fair fraction of Indium. That runs the cost of the alloy way up. :-(
If you want to go readily available, I at least can get an alloy of 95% tin and 5% silver for a few dollars US for a half-pound over at the hardware store. I can't quite find that particular mix's melting point, but tin with 3.5% silver and .9% copper melts at 217 C and is supposed to be eutectic.

Might be worth working out an extruder-like device that can run at 250 C.
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