Monday, June 06, 2005


Extruder feed mechanism image

Adrian is currently printing the first draft of the pinch wheel feed mechanism for the extrusion head. It consists of two interlocking pinch wheels that push the 3mm Polymorph rod down a guide and into the opening in the 10mm PTFE rod:

Small holes in the pinch wheels can take metal points if additional traction is required, and the symmetrical clamp should hold the PTFE rod centred.

The strange lumps near the axles of the pinch wheel are where ArtOfIllusion seems to fall over when trying to poke a hole using its boolean operator. It was much faster to add drill centring points and drill manually than fix the obscure bug! I did actuall try to fix the bug, but no joy. Still, onwards and upwards we go.

It is possible to RP the gears on axles, but we won't be because we want nice, strong, smooth shafts and 4mm steel is a better choice. Might even need 6mm steel if the forces get much higher. Ultimately we'll need to make something similar in Polymorph, and even thick axles made out of that will probably bend too much.

The feed head won't melt because the hot parts are machined from PTFE - frying pan coating. This is an insulator of heat as well as electricity, and the Polymorph parts will clamp on to the cold end of it.

I hope to document the process of making parts like the PTFE rod and hollow bolt without a lathe. Maybe I'll start tomorrow after I've tidied the workshop up a bit and it's in a fit state to photograph once more :)

Vik :v)
Blender's user interface has always been in for a bit of a bashing, and I tend to agree with the bashers.

On a more positive note, the AoI crew are extremely helpful and cooperative. The ease with which they seem to rustle up plugins for things like STL fixups and mesh manipulation gives credit to the Open Source movement. It's under active development and the developers are not mysterious recluses.

While not a true CAD application AoI seems to be quite capable of heading in that direction. As I'm more of a modeller than a CAD person, I get on very well with AoI for doing CAD work.
Vik, any chance you'll be posting this file for download? would like to try something on the CAD end of things.
Sure, but please bear in mind that it is a work in progress. It's a good idea to read Ed's FDM Primer first, too.

Adrian, can we put Ed's RP manual up as documentation?

Vik :v)
"The ease with which they seem to rustle up plugins for things like STL fixups and mesh manipulation gives credit to the Open Source movement"

I've been looking at the STL file format and it appears to have two odd limitations. First, it allows for eaxctly one, contiguous solid. Second (actually a logical conclusion of the first), it allows for exactly one material.

The reprap device doesn't have either of these limitations, though. In fact, a typical circuit board produced by one would have a number of solids and at least two materials.

I wonder if there's a way to manipulate STL files so that they can handle this. I guess the easiest way would be to create an encapsulating file format that contained multiple solid-endsolid blocks, each with a material tag.
The STL issue of dealing with multiple materials is a bit of a limiting factor. Others have had the same problem of course. There are suggestions for "coloured" STL files, eg

Colour STL Spec

None of them are particularly good and there's no agreed standard. I doubt there's much software out there that supports it. Of course if the AoI people are keen to add STL colour support it might work quite nicely.

I have to wonder if maybe another format would be a better choice.
We're limited to what formats the StrataSys can take for the initial parts. I'm not sure what it does in the way of multi-material formats but because of the support material it uses I guess it must have one.

Vik :v)
There are multi-material formats, but it should actually not be too hard to have a file collection - one STL file for each material - and then to have our machine build from them all at the same time.

For example suppose you make a block and then carve some channels in it for metal circuitry, as we do. If you then do a boolean subtraction of the result from the original block you end up with the circuitry as a solid. You can create two STL files - one for the block, one for the circuit - and have the machine make both concurrently.

Remember that all RP machines are working in slices. If you take a planar slice through two STL files at once you end up with a set of polygons of two 'colours'; the machine colours in one set in material A, and the other in material B.

All this is the advantage of having complete control over all the open-source software...
"You can create two STL files - one for the block, one for the circuit - and have the machine make both concurrently."

I think I must have missed something in the STL file format. It looked to me like you couldn't have more than one solid per file. For a circuit, that would mean having a separate file for each trace.
Yes - that's so. But maintaining a list of files isn't so hard; one can store them in a file...

I'll have to check if OpenRP has a solution to the problem.
This comment has been removed by a blog administrator.
That's true. If the STL exporter could take a multi-object selection and export each as a separate file into a specified directory, bundling the files for processing would be fairly easy. You could even have it push files to different subdirectories according to the object's texture, allowing you to separate the objects by material. That would give you everything you need and still be 100% STL compatible. If I understand how AOI works, you could probably use the STL plugin in its current state and write a script that would loop through all of the objects in a model, sort by texture and export.
Post a Comment

<< Home

This page is powered by Blogger. Isn't yours?

Subscribe to
Posts [Atom]