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Molding & Casting, HTMAA-style

Dropbox = Intelitek tool library and CAD/CAM examples will be added/updated here

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Your assigned material: one block of machineable wax @ 3x6x1.5 inches, Oomoo 25, and Hydrostone/Drystone gypsum-based casting material.

The constraints: everyone must cut a 'positive' into the wax- 1 or 2-sided mold. Tool geometry must be included. Then pour Oomoo silicone into the wax to make a 'negative' in which to pour the third material, a type of plaster.

This is an intentionally specific set of steps for the materials for this week. Please keep in mind that this is not meant to be the one and only correct way to mold/cast- it does maximize experience in a small package, and without necessarily requiring super-expensive software or machines.
You may want to try larger, more complex, different types of molding and casting, including other materials, and that's great - but materials provided to each student for this week are very specific, so even if you have previous casting experience, please work with these for the assignment. The constraints peculiar to this setup can still be an interesting challenge to those who have done other kinds of molding/casting.
Once you have done the required process, then you can progress from there.

If you advance beyond Oomoo/plaster, Do Not Forget that other liquid chemicals must be approved and used under supervision of shop management in our space - do not ever bring in other casting plastics or 2-part liquid chemicals without contacting us first.

I've saved several samples from previous years so you can see some of the details you'll be focusing on. They'll be out on the table in 3-412 by the Intelitek over this week, and I can pull them out for scheduled training groups.

There is a significant limit to what can be squeezed into these little volumes, especially considering the different parts that need to fit inside (only one of which is going to be your finished object), and also the widths of the end mills also needing space to move around between all those parts.

You're not just designing an object - you're designing a tool to use to make an object, and the tool needs to fit inside a fixed volume- and the tool's features require space of their own. Keep these size limits in mind while coming up with your designs.

I've taken a lot of notes to help you stay within the boundaries- illustrated in the relatively dense toolpath setup - please check all details carefully.

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We have several machines that are capable of 3-axis milling. For the scale we're working at this week, the Onsrud is not ideal. We have the Intelitek (in 3-412), the Prototrak (in N51), and the Shopbot (in 3-410). The Modelas are also capable of 3-axis milling using Mods, but are by far the slowest of the bunch.

We use Mastercam to set up toolpaths for all these other machines - they should be found under the machine navigation menu. If a computer has only been used to set up Onsrud files for a long time, the machine setup for the others may not have been fully installed. The files are available on this site (software page), and need to be copy pasted into the correct folders - then in Mastercam they need to be added.
Only the Intelitek and the Onsrud have preset tool libraries (saved as machine-specific tooldb files). The Onsrud has its automatic tool changer - which is a lot of work on the part of shop staff/TAs to maintain so users can just pop in and use them without more setup. The Intelitek has a set of tool holders set up, but they still need to be inserted/removed from the machine manually.
The Shopbot and the Prototrak need tools installed completely manually, each time. For those we just use a generic tool library.

The Intelitek is a good size for a machine for a job like this. It's got a mini tool library set up, which is convenient as long as it's used correctly. It's not an automatic tool changer like the Onsrud, but there is not too much manual work that needs to be done to change and verify tools.

There is an Intelitek tutorial page on the shop website, geared towards this wax assignment. There will be a current Intelitek.tooldb (tool library) file in the Dropbox, as well as the Rhino/Mastercam drawing that I have set up as an example.



It's more complex to set up a template for 3-axis jobs - I've made my Mastercam file as universal as possible, but depending on the object that you draw, you definitely might want different finishing toolpaths to get the best results. We will have to go through this with you unless you have a significant amount of previous CAM experience.

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The Rhino drawing I set up as an example has a model of the wax block with maximum space hollowed out, while still leaving .25 inch walls. It's possible to eke out a bit more from the walls, but not much without risking cutting through them. The wax blocks are cut on a band saw, so not all edges are perfectly straight.



I've copied the interior space and pasted next to the block, just to show a positive representation of the bounding box that we can work within - if you eliminate the wax walls, they can be a little bigger, but then you have to build (hot glue chipboard?) walls yourself to contain the Oomoo with a watertight seal.

Remember that this negative space must fit the Oomoo mold with its registration marks and sprue, enough breathing room for the tools to move around in to cut the parts out (remember that tiny end mills have very short cut depths and that for any deep cuts in these, our smallest endmill is usually going to be 1/8 inch diameter), and the even smaller piece - the object being cast.

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Modeling for CAM setup is not like modeling for printing.

Do not make your objects meshes- we do not want polygon surfaces here unless there is no other choice. If you have a scanned object, use the template or draw NURBS surfaces to create the rest of the wax geometry.

Material geometry is necessary for CAM setup, especially for us here because we are using part of the wax block, left behind, as part of the tool.



On the left in the screenshot above is the view from the top. This is what the 3-axis machine can 'see' from above and can conceivably reach with the edge or tip of existing tools if we've designed with their sizes in mind.

The right side of the previous screenshot and the next screenshot are the same drawing seen from the side/below.



The dimensions and locations of the wax geometry are still exactly where they were drawn to accurately describe the material block, but I've deleted all the unnecessary vertical and hidden surfaces. I've also added curves (yellow) that we will need in some places to tell tools where to cut to clean up edges. If we only have surfaces to refer to in toolpaths, the edges will often not cut clean - depends on the geometry and the toolpath pattern.

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If you make a 2-part mold, it will be simplest if you can split your model with a simple 2D plane so the 2 halves of the silicone molds will have flat mating surfaces.



I drew a .125" circle which is the width of our smallest end mills on the various routers - just to have an easy visual to show how an 1/8 inch tool fits (or doesn't) just well enough to be able to cut along one edge of an object, and still shift over a tiny bit to cut the other side. These toolpaths will insist on having a little more than the diameter of the tool to move within, to cut in these types of situations.



If your tool is .125", you cannot easily force it through a space that is exactly .125" wide. There are ways to try to deal with that, but require extra modeling steps that we would be better off avoiding this week, considering the long list of steps required to get this assignment done in a week.

Also, see how the curved side of the tool won't fit into the sharp corner between the 2 objects? Remember that when we mill out the wax (see samples for this kind of detail), sharp interior corners like that will be rounded. The best thing to do is attempt to model something with as few details like this as possible for better-looking results.

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What to consider when modeling for 3-axis Milling for this assignment

I've got samples of some previous years' projects saved on the table by the Intelitek in 3-412.

Do not convert your file to an STL - send a 3dm, step, or other NURBS model type., Meshes are more difficult to work with in CAM.

There are some important details to take note of when looking at these.

The geometry should be recessed low enough that when you pour the silicone, there's a decent thickness at what will become the bottom of your negative.
Pick up the silicone samples in 3-412, and check out the really thin 'bottoms' on the house molds, where the negative shape of the house comes very close to that face.
Students tend to either draw their models a little too close to the upper surface of the wax like these house models, or further down than necessary.



Here, I moved my 2 halves down from the tops of the boxes to leave enough space for decent silicone thickness without going further than necessary. The original blue box was just for reference before I decided what object to model - I didn't plan on cutting all the way down to .25" Z unless my object made it necessary.







I drew especially tall registration marks which got reduced to a more efficient height in the final Rhino file - because they are positive and negative spaces that must fit together cleanly (almost like legos but not quite so tight), they need toolpaths to clean up their sides as well as the sides of the outer walls of the boxes, to clear out that narrow space between them without shredding or otherwise making a mess of anything. If your model requires tight spaces like this, it might be okay- just keep in mind that we do have minimum tool sizes, and more fussy details will mean more fussy toolpathing. Good experience, but takes time.

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