Saturday, May 25, 2019

First Steps On the New Engine

The first steps have all been done with CAD. 

The design for the Webster internal combustion engine was released to the public domain by designer Joe Webster in 2004 with 2D drawings of the parts as .DXF format files.  My CAD program, Rhino3D ver 5, imports DXF format files but they're still 2D shapes; from there, I have to convert it to 3D if I want a 3D drawing.  If I'm going to cut it out with CNC, this is the first step.  Let me show an example.

The first part is the base, a roughly 8x5" plate of 3/8" thick aluminum with a handful of holes drilled in it.  For something this simple, I'll just drill all the holes using the mill as a precision drill press and then cut out the plate's outline with an end mill in the CNC mill.  Most of the holes have far side features (countersinks) and I'll do those after the plate is cut out.  The DXF looks like this:

I turn this drawing into a 3D solid object with some Rhino operations.  I highlight all the curves that make up the outline of the part and combine them into one curve.  Next, I Extrude the Curve to the 3/8" thickness, and then cap the top and bottom to make it into a solid.  The part looks like this as a 3D solid:

I save this solid model in the .STL (stereolithography) format the CAM program can use, import that file into my CAM program and generate tool paths for cutting the outline.  This is a screen capture of the part with the tool paths shown (horizontal red lines).

The part is ready to be fabricated at this point. 

The second part I've made a model of is a bit more intricate and will require more operations.  Making a model like this starts out as taking the outline, extruding it to the right thickness, capping it and then creating all the holes.  Each hole has to be created by the tedious process of creating a cylinder to the dimensions of the hole, putting it where it belongs in the part, and then doing a Boolean operation to subtract the cylinder from the hole.  In the end, it looks like this:

You can see that it has holes along the two sides that can be seen.  The vertical face on the bottom right in this view is the bottom of the part; it's overall size is 2.750 in the long direction; the width is 1.500, that notch on the left is 5/16 wide by 1.25 tall.  The part is 3/4" thick.  The large hole in the top section is for the spark plug on the engine, so its exact dimensions depend on which plug I can get.  The far side of that hole, though, has bore that's 1.000" diameter and 0.500" deep. 

This one has a simple shape, the challenge is the holes on the different surfaces, top, bottom and left side.  I think it's enough like other parts I've made that I know how to get there, and won't do CAM tool paths for this one. 

This is a few afternoons worth of work before I can cut metal. 


  1. Way cool. I had some AutoCad courses, but never had a chance to use them.

    I've done tons of PCB layout, from the old red-and-blue tape on a clear grid, to PCBExpress, and then on to Eagle.

    1. I had drafting in college long ago, with pencil and paper, but only got a chance to use AutoCad at work in the last 10 years. It was about the time I started to learn CAD for hobby use, and the one day AutoCad class at work helped quite a bit.

      One of my wishes would be to take these 2D drawings, import them into some program and have that export solid models. That would save me weeks worth of work. Unfortunately, the drawings don't always follow standard practice for views, so it would be some fancy software to be able to do everything.

  2. It's pricey, but Autodesk Inventor (the LT version is now a yearly subscription, dammit, but I got mine standalone in 2009 for $750) is worth it if you end up doing a lot of this. I could have modeled that second part in about ten minutes and exported directly to whatever format you need.

    Unfortunately, toolpath software is even more expensive, but I can suffer through a cheap one like MeshCAM. For a lot of my CNC work I just wrote a C++ program to generate G-code directly (G-code is actually a pretty capable, if klutzy, language if you use all of it).

    Good luck with this! One of these days I might be tempted to try to build an engine, but I have even less time now that I'm retired...

    1. The "new hotness" is Autodesk's Fusion 360, a really professional program that does 3D Solid CAD and CAM toolpaths. The feature that makes it so popular is that it's free for hobbyists and companies with less than $100K in sales.

      It has its strong points and weak points. From my standpoint, there's quite a bit of learning curve associated with it, at least if you're used to other CAD programs. After that, it's "in the cloud" and not on my machine. On the plus side, it does so much. And did I mention it's free? ;-)

      Last year, Rhino released version 6. The emphasis in 6 is creating renderings and the inclusion of software called Grasshopper, which is a mathematical engine for creating really complex shapes. The end result is they made the package less attractive to me. Grasshopper is a whole new learning curve, and I really don't see me needing or wanting it. So even if they fixed things in ver 5 that are bugs, I'm not taking advantage of it.

      I've written a lot of Gcode by hand or in Excel. For the base plate, there's no reason to do it the way I did. I could just get the coordinates of all the corners and use G01 statements to go around the perimeter. Those "optional 1/4" radius" corners can be done by driving a 1/2" diameter end mill properly with G02 or G03 arc statements.

    2. Free??!! I need to go check it out. Not fond of the cloud aspect, though. Thanks!

    3. Reading your blog just informed me of a free 3D program?
      I can't afford 2d Autocad so I use Dassault Systems, which is free.
      And a 3d Auicad is free?!?!

    4. Ed, it’s not AutoCAD, but it’s one of their products. It’s called Fusion 360.

      It’s very full-featured. Even does CAM.

  3. +1 for Fusion360. I'm also iffy about cloud storage but it's a full-featured professional CAD/CAM suite for free. I use it for my business under the startup license. If my widgets ever take off I'll have to pay for a license, but for now it's many thousands of dollars saved.