Quote:
Originally Posted by Greg Woelki
How do you go about manufacturing the oversized shafts? Also, how tight are the interference fits? Hand tight? Arbor press tight?
|
We shoot for what I think is described as a transitional fit (+- a couple tenths off of nominal). We used to just sneak up on it and test fit it, but this year we got access to an optical comparator at a sponsor's lab and measured the hex size on the 1/2 hex VP broach to be ~ .5045 and the 3/8 hex VP broach to be ~.3765 (I would verify your own numbers if you choose a similar route). We ended up needing a light press to assemble, but it was easy to remove the parts when we did end up having to change a ratio. One trick we found was that it is important to put a radius on the edges of the hex to account for the radius on the broach. The numbers we cut to should be in our CAD.
We machined them using a 5C collet fixture on a HAAS mini mill (PN 3174A42 on McMaster or equivalent). This puts the shaft vertically. We machined it primarily using with a 2" LOC 1/2"D carbide endmill, and designed the shafts so that they could fit in two setups of this. We started with 5/8 precision ground 7075. We would start with a blank of known length (so we could use collet stops), machine one side, flip it over and hold onto some section that was either previously machined or a section that was designed to be left un-machined, and then machine the back side. Last year we started using slitting saws to put the snap ring groves in during the same operation, and that has been a nice time saver. Another trick we learned is it is really worth while to have a finisher endmill that you use just for the finish pass so that you don't end up with the bottom of each profile being a little tight due to the end of the endmill being more worn. We haven't had any issues of runout, although it is a concern.
Overall, I estimate that we spend 1/2 to 3/4 of a day machining all the critical shafts for both robots. The setup and CAM changeover goes relatively quick once the first part is setup. We find the time savings on the design and fiddle factor to be worth the effort.