[RoboChair]

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.

You could in theory use a 4 jaw independent chuck if you want it dead accurate. But your problems are likely from other causes either in set up or asking for too tight of tolerances that are not realistically attainable, we're making FRC robots after all, not aerospace components.

Originally Posted by Aren Siekmeier

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).

You are correct, if using a 3 jaw chuck you should never really expect better than 0.003" TIR.

Originally Posted by weberr

You can easily indicate your shaft and adjust it to run true; You can tap the shaft on the side that is high, or you can use thin material such as aluminum foil, paper, plastic to put under the jaw near the high side of the shaft (indicator), etc. Personally, I would check the chuck to make sure it is running true first, as it may be dirt, a burr, etc. on the chuck mounting surface that making the chuck run out.

You might be having the gears too close together. I have found that the HEX broached holes and the shafts do have some slop; in fact we have tried many different suppliers of the hex stock until we found one that was tight, but not too tight.

You can try shimming it if you really want it perfect as is suggested in bold.
I also agree with the closeness of your gearing, how many thousandths are you adding between your centers? FRC convention is to add 0.003" between centers.

Originally Posted by Andy Brockway

It is interesting that the hex collet is also causing trouble. Check the mating surfaces between the lathe, collet closer and collet for grime/debris. Check the run out of your piece after chucking. Possibly you can tap to adjust to center.

A lot of hex collets aren't made to great tolerance on the TIR, ones that are can be very costly and won't do you any good if your spindle is not in great condition.