Tag Archives: gearbox

Revisiting machining the sun gear holder

My very first sun gear holder I machined myself was something of an artistic feat.  Each operation was re-run many times, and as a result the part was largely a one-off.  The final part properties were not really indicative of the final program.  My next step in my learning adventure was to up my Pocket NC game and get to a single reproducible program that would emit a part that I could use, then be able to run it over and over again.

The biggest problem I had in making this happen was pull out problems that manifested intermittently, but persistently.  When machining the part in a single operation, the mill needed to be able to reach all the way to, and slightly past the center of rotation of the B axis of the machine.  With the Z travel of the Pocket NC, that means that you need a stickout of almost 27mm or sometimes even more.  Standard length tools can kind of do that, but they don’t result in much of the shank being in the collet.  All my testing with them resulted in occasional pull out at some point during the hour or two of machining.

I tried getting a Kyocera 2 inch endmill 2 flute end mill, which has a minimum stickout of about 34mm.  To run it without mind numbing chatter required dialing the feeds and speeds so far back that the part would take twice as long to complete.  That hardly seemed worth it.

Next, I stepped back and decided to try a 2 operation approach using the Sherline 4 jaw chuck.  This has the advantage that the part is always kept well within the Z travel of the mill, so that standard length end mills can be used, but the disadvantage that you have to manually flip the part over and try to keep things registered.  I don’t have great, or really any, metrology that would allow me to measure the resulting concentricity errors so I was really trying to avoid using this approach, however, I was kind of at a loss at this point so decided to give it a go.

For this configuration, I used a 1.5″ round stock cut to approximately 1″ long.  The first operation used a Datron 3mm end mill to do nearly everything on the back side, with a final chamfer mill pass to break the edges.  The second operation used a Datron 2mm end mill to do everything there, with the chamfer mill once again to do the countersinks and break edges.

The parts that come off here are usable, and about what I expected to achieve without heroics in terms of final accuracy from a Pocket NC.  All the diameters and dimensions are around at most a thousandth off from what I intended.  The walls aren’t as vertical as I would like, but they are serviceable.

Final part
Final part
dsc_1534
My various experiments, single op in back, multi-op in the front

Granted, I probably won’t be using these parts for much of anything going forward, but it was a great learning experience in making the Pocket NC do what I wanted.  Next in this adventure is probably machining a planet input, which I forsee continuing to use in future iterations of the gearbox.