Hex Turning - Runout Question

[Aren Siekmeier]

Originally Posted by Darren Collins

Over the past few years, our team has moved to using many of the hex bored gears, sprockets and wheels that seem to be ubiquitous within the FIRST community. There is one challenge that we have encountered and I’d appreciate some advice. When turning down a hex shaft to round the end for a bearing, we’re having difficulties ensuring that the round end is co-axial with the hexagon. We have tried both a 3 jaw chuck and a hex collet in various lathes, but our tolerances are still limited to the runout in these holders. For many applications in our drives, this small runout is a non-issue. However, we are finding some slight binding in some of our gearboxes due to this runout. I’m curious if other teams have encountered this issue and how they have addressed it. I’m also wondering if some teams have chosen to just avoid using hex stock for some of their precision gearboxes. I would like to add that I am not a machinist and that I may just be missing something fundamental.

Thanks in advance for any input.

Shouldn't this be just as good as the runout from turning anything else while held in a 3 jaw chuck? If the chuck is not holding the part concentric, the turned diameter won't be concentric, regardless of the shape being held in the chuck (round or hex).

2175 has also turned down the ends of Vex hex shaft for gearboxes, and doesn't usually have issues. However, I wouldn't be surprised if the few issues had something to do with shaft concentricity. There hasn't been any effort to measure these sorts of things until very recently.

Curious to hear what some experienced machinists have to say about this.