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| Stuart No. 4 Build |
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| smfr:
[Moved from homemodelenginemachinist.com] OK, after mucking about with chucks and soft jaws, I got back to the Stuart this weekend. I'm waiting for some tooling to arrive to finish the cylinder covers, so I started on the crankshaft. I've been somewhat dreading this, but it's turned out fine so far. The first job is to set it up with the steady rest, in order to face and center-drill each end. I used a bit of bar stock of a similar diameter to get the steady rest fingers in roughly the right spot. Just the tip of the casting is gripped in the 3-jaw, since we can't assume that the shafts of the casting are aligned (and they are not!). I'd cleaned up the spot under the steady rest with a file, but this thing was still wobbling all over the place. However, I managed to face and center drill: even though the drill was flexing. With one end done, I flipped it around and did the other. From now on, it will be turned between centers. I used a sharp-tipped HSS tool to take off the bulk of the material. I only worked right-to-left, so flipped it around for the other side. I figured this sharp tool would place less stress on the casting, but the finish was lousy. I also used it to rough out the throw sides and ends. Cleanup was done with some indexable tooling, including a radius insert on a Nikcole mini-system tool, which gave a great finish. I used wet-and-dry backed by a parallel to get to final diameter, and was able achieve a nice finish. Next up was making a couple of fixtures for the offset turning. One is longer than the other to act as a lathe dog. I actually made up these fixtures after finishing one end of the crankshaft, so I could use the longer fixture as a lathe dog to avoid marring my just-finished shaft. Making these gave me a chance to try the 7/16" reamer that I'll use for the main bearings. Good thing too; it's a spiral hand reamer, and I discovered that it has a significant taper over about half of its length. I won't be able to go straight through both bearings with it, but I think it should be fine to finish them off by hand. You can see my awful wandering hacksawing (I had to use a junior hacksaw when starting from the reamed hole). Holes were drilled, part clearance-drilled and part tapped M6 for the socket screws to clamp the fixtures. What you can't see is that I messed up when center-drilling the points that will be used to turn between centers. I forgot that these fixtures needed to be mirror-imaged. I'm not sure yet whether to center-drill the other side, or shim the few thou of offset when I flip one of the fixtures over as I clamp them onto the crankshaft. It's pretty critical that everything is lined up when turning the journal. I also hope that the Al isn't too soft for these fixtures; I'm using a live center in the tailstock, but there's a chance the cutting forces will cause things to loosen up. I'll have to keep an eye on the tailstock pressure. Before turning the journal, I think I'll mill the throws to thickness (using the fixtures to keep things square). That's for tomorrow! |
| smfr:
[Moved from homemodelenginemachinist.com] Well, I ploughed ahead with the crankshaft, using the fixtures I made already. First up was to clean up the throws on the mill: This is the part where I go through my small end mill selection, looking for one that hasn't been blunted already :) With one side done, it was flipped over and the other side done. I was careful to clamp on the non-slit ends of the fixtures, since those remain square. Now we end up with this (with some tape to protect the spindle): Next comes the critical setup, to get the journal parallel with the main axis. Since my fixtures were milled as a pair, I can be pretty sure that the ends are square. So I used a square to align the crankshaft with the fixtures: To check the setup, I used an indicator once the assembly was set up between centers. Indicating on the main bearing journals, I checked with the crank in the upright position on either side of the crank webs. Any error here would indicate that my fixtures had mismatched distances between the bearing hole and the center-drilled spot. Luckily, error here was under half a thou. I then turned it 90°, and checked again. Offset here indicates that my fixtures are not aligned with each other, and I had about 1.5 thou offset, so I loosened one of the fixtures, tapped to rotate it on the shaft until the delta was under half a thou, tightened it up again and re-checked. The next thing that was tricky here was the tool setup. Because of the bulk of the right-hand fixture, you can't get the tool post close in, so the tool needs to stick out quite a bit: This is an HSS bit that I purchased partially ground already, but had to modify on the grinder for more clearance. Since this was the first time I've done any tool grinding, the result wasn't very pretty. My attempts at radii on the corners was particularly bad, so I had to resort to other tools for final cleanup. Anyway, with this tool I reduced the crank webs to almost the required 3/8", and took the journal to size. The finish on the sides of the webs was pretty bad, so I swapped in a Nickole tool with a radius insert, which gave a great finish but could only be used on the left side. This forced me to undo the fixtures and flip the crankshaft around to clean up the other inner face, which then meant that I wasn't quite concentric on the journal. It's not a critical surface, however, so I think it's OK. The webs needed quite a bit of cleanup on the file to remove the milling marks, and the edges were deburred. Here's the end result: Not too bad for a first crankshaft, but I sure need to work on my tool grinding skills! |
| smfr:
[Moved from homemodelenginemachinist.com] I finally got some time to finish off the main bearings. First job was to clamp the soleplate to an angle plate, taking care to get it aligned, and ensure that the bearings were screwed in tight. Then onto the mill, lined up and drilled in increments: I stoned the drills to reduce the tendency to grab in gunmetal, which mostly worked. I was careful to drill slowly, and keep pressure on the quill feed to avoid the drill getting pulled in too fast. After drilling to 1/64" undersize, I switch to the reamer. This is a right-hand rotation, left-hand spiral reamer. It's a hand reamer, which means it has a taper. [Please note warning in later reply about using hand reamers in the mill.] Because of that I can't go through both bearings from the same side, so I went as deep as I could from one side, flipped over the entire setup (one of the G-clamps had to be moved, but the other kept the soleplate aligned), and relocated on the hole: I then reamed from that side, and cleaned up the edges with a light touch of the countersink. Now for the acid test: did I mess up the alignment? Phew, things seem to line up! Now the crankshaft still doesn't fit because of the radii: Oddly the plans show zero clearance between the bearings and the crank throws, but I'd reduced the bearings a little to leave some room, and the radii keep the crankshaft from moving side-to-side. However, the bearings needed some relief to fit the radii, which I did by hand with a countersink and some wet & dry: Then I did some initial settling in with some Brasso as abrasive, turning the crank by hand: The final fit seems pretty good. |
| smfr:
[Moved from homemodelenginemachinist.com] Made some good progress today. I'd roughed out the bottom cylinder cover earlier, and today almost finished it. I made use of the soft jaws I made for my newly acquired 3-jaw. First I clamped the jaws down onto a bit of bronze of a diameter which put the jaws roughly in the right location, then faced the jaws, and bored a step which will hold the part. I could then clamp down on the part, and have it accurately centered. In this setup I did the upper faces, with a 1/16" step which gives a close fit to the cylinder. I also took the outside diameter down to size. Since alignment of this face with the center hole is critical, I drilled and reamed the center hole 1/4" in this setup. I'm deviating from the older plans, which call for a 9/32" piston rod and using 1/4", since I have the 1/4" tooling already. Stuart moved to a 1/4" rod on the newer models, presumably because the material and tooling is more readily available. Then I removed the part from the chuck, and re-bored the chuck jaws to fit the step in the part I just made, then flipped the part around and put it back in the chuck. I then took a light facing cut, and checked that the flange had a similar thickness in three spots: That tells me that the part is aligned in the chuck. I also checked for concentricity with a dial indicator on the rim, which was within a thou. The steps on the lower side of the cylinder covers where then turned to size, fitting to the standard. The boss for the packing gland was drilled for threading, but my BSB taps and dies are lost somewhere between Devon and California, so I wasn't able to cut the threads today :( Here's the cylinder with both covers: and everything in place: Here you can see my earlier boo-boo of turning the top flange of the standard to the wrong size. I'm not quite sure if I want to do anything to rectify that. While I had the lathe covered in cast iron dust, I roughed out the piston casting. The piston has a groove in one face, and a recess in the other, which gave me an excuse to work on my tool grinding skills ::) Here's my attempt at a cutter for these kinds of face grooves. It worked OK, but I got a fair amount of chatter because of large cutting area, since it's radiused. I'll finish off the piston when I've made the piston rod. Sadly all my 1/4" precision-ground stainless is curved, so I need to get some better material. |
| smfr:
[Moved from homemodelenginemachinist.com] Flywheel time! While the lathe was covered in cast iron dust, I thought I might as well tackle the flywheel. I started out in the 4-jaw, gabbing the hub and adjusting until things were running as true as I could get them. The outer edge of the casting was pretty messy, and dulled an HSS tool pretty quickly, so once I got the skin off, I switched to my trusty insert tool (from http://www.latheinserts.com/). The (older) plans the hub flush with the rim on one side, and 3/16" proud of the rim on the other, so that's what I did. Stupidly I left the hub proud on the more wobbly side :( I'll probably skim the hub, and I need to do the inner edges of the rim to make things pretty. I left some material on for a later finish pass, once it's mounted on a shaft. The hub was drilled and reamed 7/16", which was uneventful. I pondered for a while how to attach the flywheel to the shaft. The older plans call for a keyway, and the newer ones a grubscrew. I hate grubscrews, so it was either a keyway or something like a taper hub (discussed here http://www.homemodelenginemachinist.com/index.php?topic=11471.0). A taper hub wouldn't look very authentic on a Stuart model, unless fancied up somehow, so I went with the keyway. First up was a practice run with the woodruff cutter on some spare 7/16" drill rod, which worked pretty well. I'm doing conventional cutting here (part moves right to left). With that experience, I made a collared broach bushing from some easy-machining stainless. It was a simple case of turning down a section of 1/2" rod to 7/16", then cutting the slot as in the photo above (noting that the back of the broach is 4 thou over the nominal size). I did the math to compute how deep the channel needed to be to give me the correct final depth of keyway on the flywheel, and figured that I only need to do two passes with the broach, first without the shim, and then with the shim. Finally it was parting off leaving a collar, and cleaned up with a file. Now for the broaching step. There's no need to clamp things down, since the broach is held in place by the bushing. Care needs to be taken to align the bushing to start off with (the slot aligns with one of the flywheel spokes), and mine was a light push fit so it stayed in place. I used an angle plate with a large hole in it to give clearance for the broach, and used the mill quill feed with an old saw arbor in a collet to push down on the broach. It took some amount of force, but I didn't feel that it was enough to harm the mill. One pass without the shim, and a second pass with the shim gave a slot of about the right depth. Phew! I was worried about the broaching, but it actually turned out well. I'm tempted to go back and apply the same technique to my Stuart Beam. |
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