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| Stuart No. 4 Build |
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| smfr:
[Moved from homemodelenginemachinist.com] I thought I'd take a break from the bearings, and made some progress on the standard over the weekend. After some cleanup with a file, I wrung it onto lap that I had lying around: which allowed me to do some initial cleanup on the feet. This setup isn't rigid enough to do a good job, but I just wanted to get the feed good enough for the next step. I'll finish them off later. I made a couple of feet clamps: and used them to attach the standard to a faceplate. Now I used the same lap to get it mostly centered: I could then take a facing cut across the top. However, when trying to take the top diameter down to size, I was getting a lot of chatter, because things weren't rigid enough, so some support with a live center in the tailstock was used which helped a lot. Now came the part that I was a bit nervous about: boring. I pretty soon found out that chatter was a serious issue here too, but worse because the boring bar has some flex also. I bolted some 1-2-3 blocks to the faceplate, and to each other with some Al bar spacers. This worked pretty well, once I'd adjusted things to avoid pulling the standard out of alignment. I also tried to reduce resonance in the boring bar by wrapping it in electrical tape, but that didn't seem to do much. (I suspect that chatter often results not simply from lack of rigidity, but also from the part and tools resonating or "ringing".) The new setup worked pretty well, though the interrupted cut in the middle of the standard taught me that not all carbide inserts are created equal ;D (Yeah, I know, I should't be using carbide for an interrupted cut, but I have no HSS boring tools or inserts.) After boring to a few thou undersize, I made an Al lap: My poorly conceived plan was to screw a pipe tap into the lap to expand it, but that didn't really work very well. Luckily the lap was tight enough at the start to still be useful. I applied the lap with a couple of grades of diamond paste: which resulted in an acceptable bore. This bore is spot on 1 1/8", but I would have liked to be able to take perhaps another thou off by lapping to improve the finish. Maybe I'll figure out a way to make the lap work better and apply it again. Next is to flip the standard around on the faceplate and finish off the feet to take it to final height. I'm noodling over the best way to hold the thing to keep those feet nice and rigid. I'm also not sure if I should clean up the windows in the bore with a mill, or just leave them as-is. Simon |
| smfr:
[Moved from homemodelenginemachinist.com] I decided to clean up the feet bottom on the mill. A bolt down through the bore was a convenient way to hold it down, and I used a heavy angle plate and some clamps just to add a bit of mass at the feet to avoid vibration: I did quite a bit of checking with the indicator to make sure that everything was straight, since my milling table has 50 years of wear on it ;) I took off 18thou with a sharp 1/2" end mill, and the final height is pretty close. The flange around the feet also needs some cleanup, which I'll just do with a file, and I'm using this setup to make sure that the ends of the feet are roughly equidistant from the center line. The arbor I used to lap the bore makes a convenient holder, in a V-block. I held it down by hand while scribing the top foot, then swiveled it 180° and scribed again. That's the standard almost done, other than drilling some holes and cleanup with files. |
| smfr:
[Moved from homemodelenginemachinist.com] Did some work on the cylinder today, and in the process, discovered an error in my machining of the standard >:( When I bought this casting set, it came with a photocopy of the original plans (dated 8-12-53). Since parts of those were dark and hard to read, I bought a new set of plans from Stuart Models, which, of course, go with the more modern castings. Well, it turns out that some parts have changed! The 1953 cylinder is 2" long, whereas the modern one is 2 1/8" long. The 1953 cylinder flanges have a 2 3/8" diameter, and the modern one is 2 9/32", etc. What this means is that my cylinder is too long, and the top flange on the standard is a bit small. Nothing I can't make up for, but annoying nonetheless :-\ Anyway, on with the build. I'll work from the old plans from now on! The cylinder was chucked up in the 4-draw, and the non-critical top surface faced off. Then I flipped it around, taking care to seat it so that the ends would be parallel, and faced the bottom end, taking it close to what I thought was the correct length ;) [Not enough care taken, apparently; the ends aren't parallel, but the bottom end is perpendicular with the bore, which is what matters.] Without removing it from the chuck, I then did the bore with a boring bar with carbide insert. I was trying to hit about 1.498", but I think I ended up with the tool just rubbing when I was trying to take the final few cuts. Still being undersize, I went to remove another thou, and the tool then started taking off material again, resulting in a bigger cut than I wanted. It ended up around 1.499" before lapping: Not a great finish or diameter, but not too bad. I'd earlier splashed out on an expending brass lap for this build, so then spent some time lapping the cylinder with 3 grades of diamond paste: The cylinder is held by hand and gentled moved to and fro, so the lap extends about half way out on each side, trying to avoid making a bell-mouthed bore. After perhaps 45 minutes of lapping, and 3 grades of paste, the result is reasonable: and measures about 1.502". I think to get a really smooth finish I'd have to go to about 1.505", and it would take forever with a lap. Maybe a cylinder hone would be better for this? I would also worry when going much larger about whether the piston rings will still fit OK. I think I'll leave it as-is for now. So, because of my plan mix-up, I need to take 1/16" off each end of the cylinder. I'm going to have to be really careful to keep the bottom face perpendicular to the bore, so I'm thinking of making an arbor, and using that to trim the ends. Hey, at least that lets me correct the non-parallel ends :) |
| smfr:
[Moved from homemodelenginemachinist.com] I also cleaned up the port face today. First I set up the cylinder on a nice angle plate that I got recently, with the base of the cylinder against the plate, so that the face is parallel with the bore: Not ideal workholding with with just one bolt, but it worked fine. I'm using the square to set the rotation correctly. I also left a bit of cylinder bore peeking out so that I can pass a small bar though that and through a hole in the plate, which lets me pick up the top of the bore with a height gauge. I can then use that to scribe a line so I know how much to take off the face. Over on the mill, I used a older end mill to get through the crust, then a sharper one to take it down to final level: That was it for today. Now the plans call for the cylinder flange diameter to be reduced to 2 3/8", and they should flow nicely in to the inlet and exhaust port faces. I guess I'll trim up the flanges using a rounding table, but blending with the faces is going to be tricky! Also, the ports in the casting are already a bit oversize, so I'm not sure how to account for that in cutting the exhaust port. It's too bad that the top of the standard is 3/32" undersize because of my earlier boo-boo with the plans, but I don't think I want to reduce the cylinder and cylinder cover diameters to match. Or do i? |
| smfr:
[Moved from homemodelenginemachinist.com] Today I made an expanding arbor to use for trimming the cylinder casting down (I made it too long after reading the modern plans, which are different from the older ones). This was my plan for the arbor: There's a tapered section of the bore, to take a tapered plug that would push out the sides. The drawing doesn't show it, but I had an M8 bolt going down the middle, which I could clamp down. Luckily I had a bit of 1.5" Al stock lying around, but I only had to skim it before the cylinder slipped on. I wish I'd had a bit more to remove: Once the outside diameter was close, I started on the bore. Time to get out the big drills :) After drilling to 7/8", I used a boring bar to do the rest of the bore, leaving material for the taper. It was a pain to get behind that shoulder, and a lot messier inside than in the diagram. Oh well, no-one will see it ::) Lastly I set my compound to about 7°, and did the tapered section. I then removed the chuck, with the workpiece still in place: Then I put in the 3-jaw and do the plug. Here I'm cutting the tapered plug to the same taper as the arbor: The plug was a bit of scrap that was already tapped 3/8", which was fortuitous; I drilled it out to 17/64" for clearance for an M8 bolt, but that left enough of the 3/8" thread that I could screw in a 3/8" bolt to pull the plug out of the arbor. Here are the final bits: I was expecting to have to cut slits, but actually it expanded just enough to hold the cylinder without them. So, finally, I could slip on the cylinder, tighten it up, and work on the cylinder. I checked the previously "good" end to see how far out it was, and saw less than half a thou wobble on the end. I don't think it was that good after I'd done one end and flipped the cylinder around; I suspect that not being able to skim more off the arbor allowed the cylinder to sit slightly askew. I'll be careful to find the "best" end, and make that the one that attaches to the standard, since being perpendicular to the bore is what matters. |
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