23 December 2010

Christmas came early

So... ran into another problem on the Jetta. Tried to press out the old wrist pin bushings from the connecting rods, using one of my sockets to press on the bushing. No go, one size of socket was too big and the next size too small with no in-between. So, our options were go to a machine shop again, or make a press mandrel of the proper size. Since we didn't want to wait days and pay $$ for a shop and had a perfectly good lathe sitting in the garage, Lemontree gave me special dispensation to turn a press mandrel on it!!

First we needed some metal to machine. The first place we went to was closed, even though it was during their normal business hours. We assumed they were on a holiday break and just failed to hang a sign or change their voicemail greeting (grr). Fortunately, Lemontree noticed an old phone book on the floor of the car, and managed to find another place that sold metal. That place had moved, but they had kindly put a sign up listing their new location (the first place could learn a lesson from these guys!). So it was we finally arrived at Gem State Metals, where I got a 1.5" diameter bar of leaded 12L14 steel, 12" long. 12" was way more than I needed but hey, there was a minimum purchase requirement and I can always find uses for it later. Cost was $7, if you were curious. Here it is standing next to the lathe, as I clean the shipping grease off:

Next I needed to cut a suitable piece from my bar stock, as my 10" lathe really wouldn't be able to do much with 12" of steel. I clamped it in my vice and started cutting with a hacksaw, but ended up with a tired arm and only a shallow slot. Wising up, I grabbed the recip saw, loaded it up with a metal cutting blade, and made better progress. It still took a good 15 minutes to make it through that thing, but at least I was just standing there letting the saw to the work. Here's all the metal "sawdust" I made:

Before I get any further, here is a diagram of a lathe so you know what I'm talking about:

Next I had to mount the workpiece in the lathe chuck (which mounts on the spindle). This wasn't as easy as it sounds, because both ends had been rough-cut so it would wobble around if I seated it solidly back against the chuck jaws. Fortunately, I had watched the entire series of MIT machine shop videos, so I knew what to do -- whack it with a mallet. No, really. I got it straight within .004" of wobble ("runout", in machine-speak) which was not bad for a newbie.

Now I got to make a facing cut to square up one end. I made my first chip and had to stop to take a picture before getting any further.

Then I started making pass after pass to flatten the end, but after many passes my hand got tired and the lathe didn't seem to be cutting any more. Watching the entire machine instead of just the cut as I wound in the cross-slide, I saw the entire carriage was winding itself out under the pressure of the cut! OK newbie, lock that carriage down and try again... yup, now we're cutting metal again!

With one end faced, I flipped it around -- easier this time, as my newly flat end could simply be seated against the chuck jaws without wobble (or at least, no more than the .004" I had when I faced the first end). This was quickly and easily faced, and I moved on to drilling.

My intention was to make two mandrels, a top and a bottom. This piece was going to be the bottom, so it needed a hole for the old bushing to fall inside as it was pressed out. I started the hole with a 1/4" drill, then moved up to a 1/2" (my biggest). Drilling with the 1/2" was scary easy, just the slightest pressure on the tailstock and the bit dug right in. This leaded steel is nice stuff to machine, which is good as it's designed to be easily machined.

Next was to bore out the 1/2" hole to 1.087", but the cheap little Harbor Freight tool bits I was using wouldn't fit in the hole (well, they would, but not in the right position to do any cutting). I didn't expect to be using the lathe this soon, so I haven't gotten all the tooling I needed for it, such as a proper boring bar. Ah well, plan B! I'll use this piece for the top mandrel, and bodge something together for the bottom mandrel later. The top one is the critical one anyway, as it's the one that needs to be perfectly straight and centered. It'll have an extra hole in the middle now, but that's ok.

I needed to turn down a section to fit inside the old bushing. This will center the mandrel and guide it straight. I needed to go down to 0.940", from 1.500", so it took a lot of work. I made even more work for myself by mounting the tool a little crooked in my tool post, so I ended up with a taper instead of a nice sharp shoulder. Ah well, live and learn, re-adjust the tool and take pass after pass to make the shoulder square. Finally, I turned down the guide section to the final diameter and tested the fit with a connecting rod -- eh, a couple thousandths of an inch smaller than I might have liked, but it will do nicely.

As I worked, I even managed to improve my surface finish by adjusting the lathe speed and how fast I cranked the handles. Steel makes a neat "shhh" sound when you cut it right, though the chips are quite hot (yes, I got burned a few times when they landed on skin).

That's it for the day... will do more tomorrow!

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