The goal for today was to complete the tube holder and bore the seat tube up to size so the seat post would fit. From the supplier the seat tube was slightly undersized so that the bike builder could ream up the tube to fit a standard seat post. After a quick Google search revealed that most seat reamers cost in the neighborhood of 200 bucks it was obvious that an alternative solution was necessary. Previously I welded a broken center drill to the end of a steel rod in an attempt to fashion a boring bar. Today the steel was shaped on the grinder and considering I have no idea how to grind lathe tools I am a bit surprised it worked as well as it did. The finish was not quite the 32 finish I would have liked but the steel rod cost around a dollar so less than ideal finish on a part that will never be seen and a savings of 199 bucks seems like a good trade. And I got to make a bunch of sparks!
With the seat fitting nicely in the seat tube it was time to refocus on the tube holder for the lathe. I left off previously with the lower portion of the fixture containing the threaded holes and the T slot features. After cutting another section of aluminum bar to a similar size as the lower portion I drilled a couple thru bolt holes to hold the two blocks together as well as some threaded but counter bored holes between the two bolt pattern. The counter bored holes allow a couple socket head cap screws to function as jacks jamming the T slot in place and preventing any movement. The next step was to make a big hole in the center of the two blocks. In order to determine where to drill the hole the fixture was bolted together and installed on the lathe as it would be used then using the live center as a scribe I marked the height where the hole should be drilled. (not a generally practiced machining process) A second scribe line marking the center of the part resulted in a nice target to aim for when setting up everything in the four jaw chuck. After some tedious adjustment the part was in the right place and the long and boring process of making a 1.75" hole could be completed. Even though this was quite a slow process I did end up making a huge pile of chips and some horrific noises as the tool chattered through the material. (my cuts may have been a little aggressive) The only thing left to do was make a slot in the collets so they could be compressed around the tubes which was easier said than done. I have never been very good at math, hence my avoidance by becoming an engineer, but when you measure an end mill and it reads .5000 on your digital calipers and the shank on the key cutter you want to use reads .4990 you might assume if you can get the end mill in the tool holder you would surely be able to get the key cutter in as well right? WRONG! I tried for a few minuets to find a bur or something on the shank of the key cutter but it would not go into the tool holder or rather I could have gotten it into the tool holder but then there for ever more it would stay and not even Arthur would be able to remove it. So back to the grinder where I precisely ground on the shank until I could get it into the tool holder. Words of wisdom when using a key cutter on aluminum. The aluminum alloy 6061 melts at over 1000 F which may seem like a lot but it is easily attainable with dull tools and excessive tool pressure which will result in much frustration while you try to pick/chip aluminum out of the cutter teeth. Once the collets were slotted a small amount of shim stock was used to get just the right compression on the tube and as you can see in the picture the tube holder turned out very nicely. Now if only I did not weld all the aluminum tube into a big mess I could have tried the holder out, next time.
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