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(c) 2003 Stephen Redmond. All Rights Reserved
During the winter of 2002 I cast the parts for, and built, a 7" x 12" metal lathe.
It was an exciting project. I recommend it to anyone interested in owning a lathe and learning machine and casting work in the process. Besides the rush of personal capability you get from something like this, building a lathe makes tangible the historic development of machines and thought early last century. There's also the perverse enjoyment of converting thrown-out, burnt, oil encrusted lumps of broken metal into a precision piece of equipment through the use of the most basic skills and tools. All of my lathe's castings were made from scrap pistons which had been chucked into the discard barrel of an engine rebuild shop They were melted down in a charcoal briquet -fired furnace made out of a Christmas popcorn can.
There is a wonderful absurdity in this, or if not, a sad one by contrast in the idea that technology is special, is modern, has class, has complexity beyond the reach of normal comprehension, and must be schooled for and bought. Or that the word technology even means something. None of it is true. In this case you just bring some metal from here.... to there. Through a focused desire to do so. And with the assistance of a little earth, fire, wind, and water along the way.
The project was made possible by a wonderful book by Dave Gingery, a genius at calm encouragement, financial prudence, aesthetics, perseverance, grade school math, and mechanism.
My Gingery lathe, shown at the top of the page cutting its first test bar, proved out with an initial taper of less than one half of one ten thousandth of an inch over twelve inches. This was of course blind luck, and a good start, it has reverted to a more normal level of accuracy of about a thousandth on a good day, since I adjusted it.
One of the great pleasures of building the lathe was in molding the parts. Shown below is a greensand mold for the lathe's tailstock, complete with a baked sand core. The core is the brown stick laid across the sand cavity. The stick is made from wallpaper paste, molasses, water, sand and a little wire. And it was baked in a regular oven. It smelled like molasses cookies at the beach. Its function is to leave a long hole in the finished casting.
The box around the lighter unbaked sand is called a "drag." in foundry jargon. It fits another box called a "cope". The cope and drag are joined together when it comes time to pour the mold.
"Greensand," the stuff in the box is not green. It's sand colored. It is mainly sand with a little clay and moisture mixed in so that it packs well.
After the cope and the drag are re-assembled, the furnace is fired up with charcoal. Kingsford (tm) is a little high-priced so I rely on off brands. You can make your own charcoal, too, but that is another story. A fan of some sort is needed to bring the charcoal up to brimstone heat, others have used hairdryers, hand-blowers, vacuum cleaners, and car heaters. I personally prefer a mattress inflator.
When the pour is finished the long wait for the metal to cool begins. This takes about a month, or seems to. Actually an hour will do it for small castings. Below you see the cope with the casting separated from the drag. The baked core is in the center of the casting and the molasses has burned out of it from the high temperature of the metal. What's left is black burnt sand, which pours out easily once you release the part from the mold.
Below you see the tailstock cleaned up and mounted on the lathe ways prior to testing the overall taper. I did use a lathe to help build my lathe, but the lathe I used was the one I built. This is an odd concept, but all made entirely possible if you go about things in the right order.
The main task in lathe construction is to get the headstock in line with the bed ways and the tailstock. If these vary by maybe even a thousandth of an inch, you can't do accurate work. It would seem to be impossible to drill the headstock and tailstock castings to align in this way unless you have high precision equipment to start with.
Actually, it's easily possible, and this is how, historically, precision equipment was first created before precision equipment was available to make it with.
In the case of a lathe, the process works like this: a temporary headstock is set up on the bed ways with a boring bar extending from it. The bar is carefully aligned with the ways, and then the real headstock casting is mounted on the ways with a close sliding fit.
The boring bar is set in motion and the real headstock is driven along the ways into the boring bar. This results in a hole which is exactly parallel with the ways, even if the boring bar itself is not perfectly aligned. Any misalignment will show up as a very slightly egg-shaped hole, but this hole will still be exactly parallel to the ways.
After the hole is bored, the temporary headstock is removed and the real headstock is mounted permanently in place. Bearings are fitted, and the boring bar is mounted again. Now it's time to bore out the tailstock. It is mounted, again with a close sliding fit, onto the bed ways, and driven onto the boring bar. The resulting hole hole is now concentric with the headstock, and parallel to the ways.