Conrod order of operations
- square up the blank 1″ X 3.5″ X .438″
- machine cap bottom shape on CNC
- machine 3/16″ counter bore on bottom of cap
- Center drill two holes
- drill 2 each .089 holes on bottom .5″ deep
- follow up with and drill both .113″, but only .125″ deep
- tap the 4-40 holes using a spring tap guide in the mill
- machine off the end cap
- drill out holes in cap to .113 and install end cap
- drill and ream the .501″ main bearing hole
- drill and ream the .251″ piston pin hole
- Mount to CNC machining fixture
- Go to the CNC for contour machining both sides
Squared up blanks. The thickness is the only critical dimension.
Now I need to machine the cap bottom. Solidworks allows for “alternate configurations” which is a variant of a part with differences. I use it to crate simplified models to ease the generation of the CNC tool paths. Below I create a shadow solid that isolates the bottom of the rod so I can machine the curved bottom feature. I overlay the desired shape over the part and create an alternative configuration for the rod.
Sketch creating model for machining the conrod end
Below is the finished model, notice the small cut between the main body of the conrod and the end section. this allows me to isolate just this end part to target the machining and stay clear of the vise.
The CAD tool tells me the smallest radius I need to machine is .133″, so I can use a 1/4″ end mill to create this feature
Time to export the model for the CAM work. I use fusion360 and highly recommend it. It is free for us home hobbyists and does everything I need it to. It has a nice user interface, but getting all of the settings needed to create a tool path can be frustrating. It has taken me some trial and error to get the CAM to do what I want it to. Fortunately it has a “template” feature that allows you to save a setup for use in later parts. So once you get it dialed in, you are good to go.
I export an IGES file and import it into Fusion 360. I select “Manufacture”.
This green highlighted geometry selection tells the cutter to only cut the area of interest and not interfere with the clamping on the rear portion of the work piece.
Contour Parameters
- 1/4″ flat end mill
- 8000 RPM at 13 IPM
- Step Down of .1″
- Climb milling for smooth finished surface
- 2 Passes, the first will leave .020″ of radial material, the second will be a finishing pass.
The tool path is simulated to verify it is doing what we want, then exported to a Gcode file. This I put on a USB drive and carry out to the workshop and load onto my CNC router. I use LinuxCNC and am quite happy with it. So much of this software stuff it is finding something that works, learn its idiosyncrasies and making it work for you.
Machining
Finished end cap feature. One down, One to go.
Over to the mill, establish the center of the holes with an edge finder.
Counter bore 3/16″ and center drill
Drill the two hole sizes. I like to drill the clearance hole in the same setup as the tapped hole to insure they are truly concentric.
A quick update on the completion of the manual machining for the connecting rods.
Here I am machining off the conrod end cap. As Paul suggests, it is better to tap the holes before cutting off the end cap. A slitting saw could also be used. I am using an 1/8″ end mill and I am offsetting each pass .005″ so that each face, both the conrod and the end cap, are climb milled for a nicer finish.
The final manual machining is pretty straight forward: using the edge finder to locate the two holes, center drill, drill and ream.
Here are the two conrod blanks ready for the CNC router. Behind them is the beginning of a fixture to locate the conrods during the CNC routing operations. The conrods will need to be flipped so we can machine both sides and we need to maintain proper orientation with as much precision as we can.
Next installment I will detail some more CNC tool path generation in Fusion360.