I started with a 3" x 3" x .125" wall piece of aluminum square tube, then I centered it in the 4-jaw chuck (a good bit of work). Then I used a boring bar to bore 2.00" deep. (Remember that a 2.75" OD round tube is actually more like 2.758", and the interior dimensions of that tube is closer to 2.745" than 2.75".)
Then I drilled and tapped a 3/8"-16 hole (the one on the right), and I used the vertical mill to cut a 1" long slot slightly wider than 3/8" in the top, exactly in the center. The tapped hole holds a bolt that locks the tube in place by friction. The final product has three 3/8"-16 holes in it that are used to secure these to the telescope tripod legs, so the the slot lets me use a 3/8"-16 bolt to secure the tube in that dimension as well.
In practice, I probably only needed one of these screws. Previously, when cutting 2" OD aluminum tubing, I used a somewhat similar clamping scheme, but with only the bolt going into the threaded hole in the tube--and the square material was acetal, not aluminum, which is less stiff.
The next step was to use the chop saw's workpiece clamp to lock the square tube in place, and then I did my 30 degree slice. It went extremely well, producing an astonishingly clean cut, probably because there was no vibration. I still needed to sand the cut, but it was really quite beautiful, compared to the usual ragged mess.
This particular set is for something called the Meade Giant Field Tripod, which is aptly named. These are at the upper limits of what I can make on this size of machine tools. In fact, this was a pretty major undertaking for me. I previously had made these out of acetal cylinders, not aluminum tubes. This required boring out the interior using a 2.5" Forstner bit on a drill press. I actually need the interior to be 2.510" (because the OD of the tripod leg is 2.500") so I relied on the fact that the Forstner bit tended to cut a slightly larger hole in acetal than it does in wood. The reason is that acetal actually vaporizes from the heat of cutting, so the holes come out slightly oversized.
The same does not work when trying to bore out aluminum--I really do not want to run any cutting tool at a temperature where aluminum vaporizes. The problem is that the standard boring bar that you buy from Sherline is only good for a bit more than a 2" deep bore, and here I needed a 2.705" deep bore. For that reason, I bought a 6" boring bar from Little Machine Shop. Of course, the longer the boring bar, the less stiff it is once it starts turning, which is the reason that you don't get a 30" boring bar and use that!
There was an interesting effect from using the longer boring bar: instead of the beautifully polished interior that the shorter boring bar produces, this is what happened instead:
It is actually rather pretty--almost like a jeweled metallic effect! I believe that the cause of this is that flexibility of the longer boring bar means that the cutting tip is bouncing as it does this, and gives an effect that looks like a pattern, because it probably is a pattern! This sounds like an assignment in a mechanical engineering class: look at the pattern, use your knowledge of the stiffness of carbon steel and aluminum tubing of these dimensions to figure out how fast we were turning the workpiece!
I spent part of the weekend organizing the ScopeRoller workshop. There is a reason that a proper machine shop has drawers and shelves for everything--so that you can sweep or vacuum up all the debris when you are done with a job. That was not practical in my mess. In addition, having the lathe and the vertical mill on the same workbench meant that when my son was doing part of the job on the lathe, it was crowded to be working on the vertical mill at the same time.
I bought a rolling tool cabinet at Home Depot to get everything off the shelves and the top of the workbench. I also made it the new home for the vertical mill, so that it is no longer cheek by jowl with the lathe.
Everything has a place. I have also had my son going through and sorting through the enormous collection of screws and bolts, and putting them in little plastic drawers. My son had no idea that there could be so many variants of 8-32 machine screws in length and head type! This is so much more elegant!
little trick to dampen the vibrations in a boring bar. if the bar is hollow, fill it with lead shot. this will add enough mass that it will change the vibration frequency, as well as since the shot is loose and not a solid chunk of lead it dampens the vibrational energy. sand works too, but not as well. you might also consider making your own boring bar, not from steel, but from ductile cast iron. Ductile/malleable/spheroidal/graphitic (all variations on a theme.) cast iron soaks up and dampens vibrations quite well. a company called Durabar sells cast iron barstock of various types and hardnesses.
ReplyDeleteI really do not want to run any cutting tool at a temperature where aluminum vaporizes.
ReplyDeleteOh, you're no fun anymore!