I got a lot of work done on the intake manifold and the car this week. I went off in a different direction again on the intake manifold, by going to a fully machined runner rather than trying to cut and weld aluminum tubing or forming runner halves and welding them together. This kept the CNC machine busy most of the week, allowing me a little extra time to get back to finishing the chassis work on the car.
Last week at my local machine shop my friend Bryan there showed me some photos of the latest sheet metal intakes for big block Chevrolet engines, and pointed out that the runners were actually CNC machined, rather than formed and welded out of sheet metal. Earlier this week I got to thinking about that, and realized it would offer a couple advantages for my project. First, rather than trying to weld thin aluminum sheet metal tubes, or build a forming tool, form a bunch of half runners, then cut and weld them together, I could just buy some thick wall aluminum tubing, write a CNC program, and let the machine do the work. Second, I could design the runners so that the were thicker where they needed to be welded to the manifold flanges, making the welding easier. Third, there wouldn't be any welding required on the runners themselves.
By Tuesday I had myself convinced that this would be worth a try, so I ordered an 8 1/2" long 1" end mill from Enco, and found some 3.375" diameter 3/4" wall aluminum tubing down at Discount Steel and Aluminum to try out the idea. I formulated a machining plan for making the parts, and started writing the CNC program on Tuesday night. Thursday the long end mill arrived, so I got started doing the actual machine work. I had purchased an 8" length of the aluminum tube, so I squared off the ends and turned it down to an overall length of 7.2", which is what my drawing of the manifold called for. I also turned down the outside of the tube by a few thousandths, in order to give it a nice appearance. I figured that the last two or three cuts on the outside of the tube would result in a diameter slightly larger than 3.375", so I wanted the uncut portion of the tube to still have a machined appearance.
I had written the CNC program in two parts, one for the inside of the runner and one for the outside. Because of the very long end mill, I had written the programs to cut very slowly, to minimize deflection issues at the end of the mill. I set up my low profile vise on the CNC table to give myself as much Z axis room as possible, clamped the tube in the vise with some room underneath so the chips would fall through the center hole, centered the coordinates on the middle of the tube, and started the inside runner program.
First pass around the tube, though, the cutter knocked the tube out of the vice. Getting the tube fixtured properly turned out to be the most difficult part of the entire process. I was trying to clamp the round tube in the vise with enough force to keep it stable, but after a few tries I concluded that this wasn't going to work. I ended up taking the tube out of the CNC machine and putting it on my Bridgeport to machine a couple of flats on either side. This solved the clamping problem, and finally on Thursday night I got the inside runner program working. Here's a couple of photos of the work on the first tube. First the program cuts the outside of the top of the runner, where it has to fit inside the plenum plates. Then it moves to the inside of the runner, and starts with an elongated circular cut, slowly reducing the elongation until finally at the bottom of the runner the cut is perfectly circular, to match up with the circular port in the head.
Late on Thursday night the program finished, and it really looked good inside. The steps inside were only .015" wide, so I can easily take a cartridge roll and smooth them out after the manifold has been welded up. Friday night I worked on the outside of the runner. In order to fixture the partially completed runner, I cut an elongated circular piece out of 1/2" thick aluminum, that just fit into the hole in the top of the runner. I used this piece as a support for the thin part of the tube, and clamped the thin part of the tube in the vice using this support. Seemed to work pretty well; the outside runner program ran without a hitch. Here's a picture of the outside of the runner being cut on the CNC:
Late Friday night the outside runner program finished. I was really pleased with the finished runner:
Now that I knew I could build these things, I decided to proceed with them for the intake manifold. Saturday morning I was back down buying enough of the aluminum tubing to make the remaining 7 runners. After I got home, I started by cutting the aluminum tube up into the required lengths and doing the manual machining operations. Each piece took about 20 minutes to cut in my bandsaw, then it had to be turned on the lathe, flats had to be milled in the sides, and then it could be mounted in the CNC to run the runner programs. In order to make best use of my time I did the steps in sequence, so that once the first tube was cut, I started the bandsaw on the second and then began the lathe work on the first tube. Ditto after the second tube was cut, but this time I added the flat milling operation on the Bridgeport. Pretty soon I had all four of the machines running at once, as I tried to turn out the tubes as quickly as possible. It felt like a real machine shop in there! In the end it only took me about three hours to get all the work done. Here's some photos of the tubes being turned on the lathe, and the flat being cut on the Bridgeport:
I was now constrained by the time required for the runner programs on the CNC machine. This meant that I could do other work on the car, while the CNC machine hummed away happily doing the inside runner program on all the aluminum tubes. I only had to take ten minutes every three hours or so to pull the finished tube off the CNC, and set up the next one. While this was going on, I decided to work on getting the roll cage completed. This involved installing the swing out side bars on the cage, including the hinges and locating pins, plus the window net mounting brackets and the seat belt, shoulder harness and submarine strap mounting tabs.
With no family activities scheduled until Sunday evening, I stayed out in the shop late, working on getting the cage finished up, while the CNC machine whirred away. I finished the cage work just as the fourth runner finished at 1:30 AM. Here's a photo of the intake manifold jig with some of the un-CNC'd tubes and some of the tubes with the inside runner program already run. I went to bed with a pretty good feeling of accomplishment for the day's work.
Sunday I slept in late, and then go out to the shop by around 10:00 AM to keep going on the runners. After getting the first one set up on the machine, I had to spend some time finishing my tax return, so that kept me busy until mid afternoon. With that rather unpleasant task out of the way, I decided to finish the chassis work on the car, which amounted to attaching the trailer hitch to the back of the 2X3 frame and adding some supports. The trailer hitch not only mounts the trailer ball, but it also mounts the parachute, so I cut it into the 2X3 tube at the back of the car, welded it in place, then added some 1" round chrome moly tubing to tie the receiver into the 2X3 back half kit frame rails that run from the back to the center of the car. I finished around 6:00, and left for dinner with my family while the seventh tube was finishing up on the CNC. Here's a couple of pictures of the car, showing the side bars and window net mount, and the trailer hitch receiver with the parachute mount installed:
I now have seven of the eight tubes machined for the inside of the runner, and should be able to get the last one finished up tomorrow night. Then I can get the outside of the runners machined this week. I have Good Friday off from work, so with luck I will be finished with the runners late Friday, or else Saturday morning. Next weekend looks like another good one for shop time, so on Friday I'll try to get the headers finish welded so I can send them off for coating the following week, and also work on getting the new door hinges and new hood hinges installed. I have changed the design of the intake somewhat again, and this will require some new plenum plates, so those will have to get done on the CNC machine next weekend as well. But I expect that by Sunday I'll be, FINALLY, striking an arc on the intake. I'll post another update next Sunday night.
