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Model-Building Workshop - Combining CNC and Resin Casting to Create a Smokebox Front

In August 2004 I picked up the new IMService System 3 DeskCNC Sherline Servo CNC conversion package. You can see information about that on the Sherline CNC page on my site. This page is about using CNC, Freeman Machinable Wax, and Alumilite Resin to create a Western Maryland H8a Consolidation steam locomotive smokebox front.

This page is under construction at the moment. I have it up just so I can share the WIP pictures with you.

Click on any of the photos on this page to see a larger version.

If you find the information on this page useful or interesting, please take a moment to sign my guestbook. Thanks!

Step 1 : Research

Western Maryland H8 Consolidation Full view. Pete Brown Collection

H8 Consolidation Detail Front end only. Pete Brown Collection

Step 2 : Create the CAD Model

Smokebox in Rhino Pete Brown, January 6, 2005

I prefer to use Rhino 3d to create most of my CAD models. While Rhino is primarily a 3d NURBS-based program, it has decent 2d capabilities as well, although I sometimes prefer to use AutoDesk AutoSketch 9 for strictly 2d. The advantage of using Rhino, besides the addins I have written, is that I can easily convert to 3d if I want to better visualize the model. I model everything in 1:1 scale so I can use prototype dimensions instead of having to do all the 1:87.1 conversions ahead of time.

Step 3 : Translate, Resize and Export the Model

The CAM program will allow me to translate and scale the model. However, since I often have to run the same model through CAM several times, it is easier for me to translate and scale it ahead of time.

I first move the model so that 0,0 is at the bottom left of the model, rather than the center. This allows me a place where I can perform tool changes and ensure I set all the tools to the correct height (more on that later). I then scale it to 1:87.1 (a scaling factor of 0.01148).

Finally, I save the model as a DXF. I do not overwrite the original Rhino model with the smaller version. Instead, that smaller version exists only as DXF. Take care on the export, as the settings you choose will directly affect the accuracy of the model created by the CAM program. Read the Rhino help for more information.

Step 4 : Create the Toolpaths

I use DeskCNC to create the toolpaths, since that is what came with my CNC system. The toolpaths for this particular piece can all be machined in simple 2.5d, so DeskCNC is well up to the task.

I have found it easier in DeskCNC to create each operation as a separate DNC file. I'm sure this is just my lack of experience or patience with the program, but I could never get it to correctly mix different toolpaths in the same NC file.

(to do - explain the 9 or so CNC programs generated to build this mold)

Step 5 : Machine on the Mill

I use an assortment of mills from 3/8" to .005" in order to machine my molds. The smaller sizes are especially useful for rivets, done using the drill cycle of the CAM program.

Milling Wax Machining an early version of the mold. I use a large block as I keep slicing it down until I get the mold right. Pete Brown, January 4, 2005

Tiny Mills Two of the tiny mills used to machine the H8 smokebox front. See my Sherline CNC page for more info. Pete Brown, January 4, 2005

Wax Swarf For me, Freeman Wax swarf is preferable to renshape dust or metal shavings. However, each material has its advantages. Pete Brown, January 4, 2005

I mainly mill Freeman Machinable Wax. However, I sometimes mill plastic or metal, depending on what the project calls for.

Materials Freeman Machinable Wax, ABS Plastic, Delrin, High Impact Styrene. The wax round is 2" in diameter. Pete Brown, May 8, 2005

Step 6 : Cast the Piece using Alumilite Black

Alumilate Black The new Alumilite Black is very opaque, and not purple like the older formulation. You can't see through the smokebox front here. Pete Brown, January 6, 2005

Stuck to the Glass The newly cast smokebox door. I should have used a mold release or talc on the glass. I was able to pry it off with an x-acto chisel blade. Pete Brown, January 6, 2005

Mold The wax mold after the casting was removed. There was no melting or other damage. Pete Brown, January 6, 2005

As you can see, it did not harm the Freeman Wax mold at all. The rivets that stuck in the rivet holes (which I drilled too deep) blew out with some compressed air. No resin stuck to the wax at all. In fact, I found it had better release properties than silicon RTV, but without the flexibility required for complex parts or multiple castings.

The scratches and tooling marks in the mold are mistakes of mine. This was a learning mold, and by the time you read this, likely has been wiped out to be recreated.

The mold still needs several revisions in order to make the smokebox dogs more visible, and to reduce the tooling marks, and make the rivets work out.

If you found the information on this page useful or interesting, please take a moment to sign my guestbook. Thanks!