More on the "Very Specific Jack"

In my last post (link) I detailed the creation of a "very specific jack" to use with my new ER32 Spin Indexer (link). Its mission in life is to support the far end of stock mounted in the indexer, far above the milling machine's table. Here's a photo:

It later occurred to me that it would be neat if it would work for materials in the vise also, but it was much too tall. But... if I just made a second, shorter center support and threaded top, I could use the same base. I looked for another piece of steel, and found a short stub I had bought at the Cabin Fever expo about a year ago. I quickly found that, compared to the hard "mystery metal" I had rescued from a dumpster in the first version, this metal turned easily on my modest equipment:

Because this center support is so much shorter, it was also easier to drill, because I could do all the work from one end. I still had to drill in stages, but the metal cut easily.  This is the last clearance hole on the back side.

My plan had been to do the turning and drilling, and then cut the piece to length. However, I couldn't see a decent way to hold it in my horizontal band saw without making a fixture. I decided to "part" it instead - using the lathe to slice the work in two. People seem to get all worked up about parting large pieces, but the Logan lathe is very good at it. I can set a very slow speed using the "back gear," and also choose a slow automatic in-feed on the cutter. That's important both because it keeps the cutter in contact with the work, and it keeps me from rushing. I am not the most patient machinist in the world....

It took almost 20 minutes to part at that slow speed, but there was no drama and the cut was quite good. It was easy to clean up when finished. I stood there minding the cut for the entire time, occasionally dousing it with oil.

Once the parted cut was done and cleaned up, it was time to tap the 5/8-11 threads. That gave me a lot of heartburn last time, but Jake R. read my last post and reminded me of something I had forgotten. I'm sure that when Phil Oles sold me the Logan, he told me this tip also, but I was so "green" as a machinist I didn't understand the import of it, and forgot it.

The problem I was having last time is that I couldn't hold the work in the lathe while trying to tap it, because I was holding the tap still in the tailstock, and trying to rotate the chuck by hand. For bigger threads like 5/8-11, it's much better to immobilize the chuck, and turn the tap in a tap wrench. And with an older lathe like the Logan, there's a special way to immobilize the chuck - that's what Jake reminded me of.

The "back gear" I referred to a minute ago is the way to really slow the lathe down, and it involves using a separate gear train. To accomplish that, you first pull a special pin out of the main gear on the spindle, to allow the new gear train to work. On old lathes, you can engage back gear without pulling the pin, effectively locking the mechanism. However, if you apply power in that state, you're going to break something, so it's a good idea to disconnect power first:

That knob pulled out above the plug is what engages back gear. The lathe is locked. Now, you can use a standard tap wrench to turn the tap, and use a dead center in the tailstock to hold everything straight. My tap wrench uses a ratcheting action, which makes it even easier:

A 5/8-11 thread is still a bear, and I wound up using an extension handle, as I did before. But it tapped easily, and unlike my last try which left the hole subtly misaligned, this one was dead on, kept straight by the tailstock.

And here's the new, shorter machinist's jack aligned with the mill vise. A good result, and a good learning experience.