Here's the setup I came up with.
I purchased an 8" four jaw independent chuck to attach to the rotary table. I had to mill the T slots deeper into the rotary table to get the attachment holes of the chuck to fit on the rotary table. Then of course I couldn't find Allen head bolts the right size, so I made Allen head bolts by drilling out the bolt head to the minimum diameter of a 1/4" Allen wrench, then heated the bolt head with a torch and drove a 1/4" Allen drive head into the heated/drilled out bolt head. The result was a bolt with a perfect Allen head socket in it.
On the right side of the tube you can see I made a tail stock from some 1" steel plate and a length of 3" square x 1/4" tube. The problem with the tail stock is that it has to be centered exactly at the same height as the rotary table. I measured very, very carefully, then center punched the stock before drilling. I milled out the final holes in the tail stock with custom ground end mills, that gave me the required taper and hole sizes to insert my 4" bull nose live center on a MT3 taper. Much to my delight I found I had only .004" difference in height between the rotary table and the tail stock, well within the tolerances I'll need for this purpose.
Setting up the tube was a tedious, time consuming project. I spent about 2 hours just in set up, but this requires absolute accuracy to make sure the slots milled for the fins are perfectly aligned. In the picture you can see I have a center finder in the mill collet and a dial indicator on the table. Once everything was perfectly aligned, I started milling the four 1/8" slots needed to mount the fins. I had just started milling the second slot and I promptly broke my last 1/8" end mill. At the moment I'm waiting on a new shipment of end mills.
But a little more about what I'm planning. I was originally going to use a tube with 1/2" wall thickness, mill four slots for fins to insert into, then machine away the extra thickness between the slots, leaving enough of a "hump" around the slots to drill into the hump, through the fin and out the other side. Then I could bolt the fins in place. But that would be a lot of machining, maybe not so bad on a 3.5" fin can, but future larger fin cans would get to be a lot of work and expensive. So I decided to use .25" walled tube, I'll still mill away excess material, but rather than bolt the fins on, I'll solder them on using aluminum solder. But first I had to test the aluminum solder...
Here's a picture of a test using the aluminum solder.
I milled a slot into .2" aluminum stock, cleaned the area with a stainless steel wire brush, then inserted a scrap piece of 1/8" aluminum. The Alumiweld product I used requires heating the base materials to about 760 degrees F. Once the base material is hot enough the soldering rod melts and flows into the joint. I wasn't sure how well this would actually work, the solder is supposed to have a tensile strength of 40,000 psi, which is stronger than the now annealed base stock I'm soldering to. I can say now it works very well. After cooling I put the part in my vice and pushed, pulled and pounded on it. I bent both pieces of the base material but the joint never weakened or broke. This seems to be a viable option to welding and better than bolting, with that aspect covered I turned to working on the fins.
Here the 4 fins have been rough cut to size.
I had a partial sheet of 1/8" aluminum stock, but didn't have a great way to cut it. I used to have access to a shop with a large metal shear, but alas no more. So I had to do it the hard way. I cut the sheet into 4 rectangles in my wood table saw with a non ferrous metal blade in it. Noisy, but it worked. To cut the leading and trailing edges I used a wood power miter saw with a metal cutting abrasive wheel in it (pictured above). Abrasive wheels don't work the best on aluminum, but for small cuts like these it does work. I certainly could have used my 14" abrasive chop saw to cut these fins on, but the wood saw has just a little more accuracy and control.
Milling the tube between the fin slots.
My new 1/8" end mills arrived and I finished the fin slots in short order. I did break one more end mill, but I ordered 4 just to be safe so I had plenty of end mills. Milling the slots for the fins was pretty straight forward, I just needed to slow down and take shallower cuts. With the tube in the rotary table it was easy to set up for each slot, just turn the rotary table 90 degrees and lock it down. I milled the fin slots about .13" deep.
In the picture above I'm milling out the excess diameter between the slots. I found that by rotating the table 12 degrees from dead center of the fin slot, I'd leave the material .15" thick on either side of the slot, that's the material that supports the base of the fin so I didn't want it too thin. After leaving the .15" on either side of the fin slot I started milling the tube between the slots down to .13" thickness. I used a 3/8" end mill and took 2 cuts of .06" to achieve the final depth. I found moving the rotary table 10 degrees between cuts gave me about optimum cutting width. So after milling between the slots I don't have a true cylinder, it's basically a 36 sided polygon of sorts. I considered using a wider end mill and cleaning up between the fin slots using the rotary table to mill the surface into a true convex, and I could still do that. But I decided to leave this fin can as is, and perhaps try the other method on my next attempt. One thing I did do on the very last 1/4 between the fin slots was to move the rotary table 5 degrees off the last cut and do a return cut, this really cleaned up the surface nicely and effectively took the tube from a 36 sided polygon to a 72 sided polygon.
Milling a 45 degree angle on the fin slot edges.
To reduce surface area and weight, I cut each of the outside edges of the fin slots at 45 degrees. This was easy to do, just turn the rotary table 45 degrees and move the milling machine table in/out to center the cut over the edge. I took each edge down .06"
Just off the milling machine with one fin in a slot.
Here's a picture with an end view that really shows what I was doing. I'll need to do a fair amount of sanding and the end that was in the chuck needs to be cut off, but aside from brazing in the fins the machine work is done.
Here's the fin can set up and ready to have the fins soldered to the can.
Here's the first fin soldered in place, looks great so far.
But Wait! After soldering both sides and a closer inspection, the solder joint has cracked.
I thought perhaps the jig I had the fin can in was causing the cracks as it cooled. I tried soldering the next pair with the fins loosely clamped, it didn't matter as the joints still cracked. I tried more heat, heating the entire fin can as evenly as I could, but as it cooled after soldering I could hear a sharp snapping sound as the joint cracked. In a last ditch attempt to salvage the project I used a very thick solder joint on only one side of each fin. It was a messy looking joint but did seem to be fairly strong.
The last job was attaching the fin to to the rocket body tube. Keep in mind I did a test fit of the fin can tube over the body tube before I even started, I had expected some warping of the tube, as expected when I tried to slide the fin can on the body tube it wouldn't go on. I thought perhaps the can material was thin enough I could still get it on using a bit of gentle force. You can about anticipate how that went. I got the fin can about half way on the body tube, then it locked up and wouldn't budge. Of course I couldn't get it off either. So I ended up mounting the rocket body tube in my milling machine and cutting the fin can off with an end mill. I saved the body tube but the fin can is trashed.
The ruined fin can...
Well, it was worth a try. It wasn't what I intended when I first started this project, I had intended to start with a thicker can tube so I had deeper slots to hold the fins. Then I could drill through the slots and bolt the fins in place. For my next attempt that's what I'll try, likely that'll be a project for Spring of 2011.