Making Replacement Parts - With a Lathe

lathechuck's picture

I have a modest hobbyist-level metalworking shop in my basement: a drill press, bandsaw, and metal-turning lathe. These are all bench-top machines, not stand-alone, and they all run on conventional 115V AC, less than 15A. People say that "the lathe is the machine that can reproduce itself", because it has a motorized spindle and a platform that can be precisely moved on two axes. With a modest accessory, you can acquire three axes of motion. Lathe work often involves clamping something big and round in the spindle, and spinning it around while a cutting tool is clamped to the platform in such a way that it cuts into the spinning material. Think axles, spokes, bowls, candlesticks, etc. You can also clamp a cutting tool into the spindle, and clamp a piece of material onto the platform, so as the platform moves, the cutting tool removes material along a path. Think "boring out a combustion chamber for an internal combustion engine", or "cutting a slot in a sheet of metal". Between the two operations, if you have a way to cast chunks of metal to the approximate shape you want, the lathe (within the dimensions of its motion) can reshape things quite precisely.

My most recent project seems modest: I bought an old measuring instrument (a "vernier height gauge" to be precise) which requires three thumbscrews for proper operation. Mine had two. So, I used one that I had as a model, and machined the third that I needed (and one more, as a spare). If you've walked through the fastener section of a hardware store, you might think that every possible screw in the world can be had for a few cents, but I couldn't find ANYONE who could sell me a #4 screw, with 48 threads per inch, 3/4" long, with a 1/2" diameter head. Of course, it had to be 4-48, or it wouldn't fit into the instrument. It could have been longer than 3/4", but I couldn't find longer. And it could have had a smaller head, but then it would have been harder to adjust. But, in the end, I made the screw I needed, learned some new skills in the process, and turned a $20 bench decoration into a layout tool for future projects that would have cost hundreds of dollars to buy.

If you happen to need a special screw, or some other special nugget that does something "infinitely improbable", let me know. We'll talk.

Admin Note: Moved to "Green Wizard Book of Skills" 3/29/19

ClareBroommaker's picture

A while back--maybe it was a year ago-- I had a post asking if anyone knew what I might do with some remains of stainless steel rods, about 1/8 in diameter. They are short pieces, maybe 2 inches to 6 inches; I don't recall now. But is that the kind of material you might be able to use? Do you scrap a lot for your hobby needs, or is new material preferred? What metal do you most commonly use? Can you lathe rocks? Glass? Do you have any way to cool your materials, or do they not get so hot as to warp or even melt? Questions, questions!

lathechuck's picture

My favorite material is brass, but I've also worked with steel, cast iron, copper, aluminum, PVC plastic, wood, fiberboard, and even styrofoam. I haven't had a reason to try turning minerals, but I've seen a chess set turned out of onyx which was very nice. Glass would probably fracture, but there is a way to mount a small abrasive wheel on the tool-holder of a lathe so that the workpiece can be ground into shape, rather that cut with a sharp-edged tool.

Most industrial turning operations use water or oil coolant pumped into the working area. This reduces the force needed to cut, somewhat, and extends the life of the cutting tool (by preventing heat damage), and also maintains precision, since the tool and work are not expanding due to the heating effect of the process.

(Sorry to take SO LONG to browse back to this area of the site!)

BTW: I think that the verb "turning" is probably most familiar to people in the Shaker hymn: Simple Gifts. "To turn, turn will be our delight; till by turning, turning we come round right." Who could not be inspired by the sight of a chunk of rough-sawn lumber being "turned" into a round, graceful spindle for Shaker furniture?

jlg4880's picture

Pictures! You got pictures?!

Was the thumbscrew winged or knurled? Threads cut with a die or single-point threading? Did the bar stock have to be turned down to the external threads' major diameter before threading?

Is currently a milling machine taking shelter in your shop, or possibly plans to acquire one?

And how much lathe work thus far? Turning? Boring? Drilling? Reaming? Facing? External threading? Internal threading? Knurling? (I've done all of the aforementioned save for internal threading and knurling.)

Have you tried making your own tooling from drill rod--what the English call "silver steel"--and then heat treating?

lathechuck's picture

I've attached a shot of the surface gauge, with an original screw in the lower position, and the DIY screw above it. The head is round and knurled. The threads were cut with a die, after several attempts proved that single-point thread cutting put too much stress on the shank. Starting with a piece of 1/2" stock, for the head diameter, there was a LOT of turning down the part to be threaded.

BTW: The function of the tool shown in the photo is to use the sharp pointy part on the right to scribe a scratch at a precise level (i.e., +/- 0.001") above a flat level surface. To set it, both screws are loose, and it slides up or down until approximately the right position. Then the top screw is made snug, and the thumbwheel on the right makes fine adjustments of the bottom part. Then the bottom screw can be made snug, to secure the pointer at that height. How do I know that it's right? I have a steel block with a diagonal cut through it, such that the two pieces can slide to make the overall thickness vary (over some range). I set the thickness of the block with a micrometer gauge (or dial caliper), then set the surface gauge to match (under magnification).

No milling machine, due to lack of space.

I have made a few tools from drill rod. Most recently, I needed a 60-degree conical hole, about 1/2" max diameter, tapering to a 0.020" straight hole. I turned the drill rod to a 60-deg angle and ground half of it flat on top, so I had two sharp edges up to the point. I drilled a conventional hole most of the way down, about 1/8" (IIRC), then used the tapered D-bit to enlarge it to the desired dimension. Got the job done.

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David Trammel's picture

Lathechuck, I moved this post to the "Green Wizard Book of Skills" because as a old lathe operator myself I know how versatile they are. I expect that even in the Collapse machine shops will be running them still. I think from now on, when a thread gets of sizable length and covers some skill like this, I'll move it to there. Now for some pictures.


For those who don't know what a Lathe is, here is a typical metal lathe.

Here is an explanation of some of its features.

  • The large enclosed part on the left contains a motor which turns the Chuck and runs the Autofeed. The speed that it turns can be changed, made slower or faster depending on the hardness of the material and is picked with the various controls on that panel. Softer metals like brass or aluminum can be machined at a higher speed, stainless or tool steels must be worked at slower speeds. You can also choose which direction the Chuck spins.

    The Tool Post can move in towards the material with the smaller of the two hand wheels on the Block below the Tool Rest, which dictates how deep of a cut you make. It will also move horizontally along the V-Rails using the larger hand wheel either towards the Chuck or towards the Tail Stock. The long handle on the Block engages the Autofeed mechanism, which causes it to move towards the Chuck or away. You can adjust the speed of this depending on how fast you want to cut.This is how the Lathe skims off thin slices of the material. Combining these two movements dictates how the cutting tool works the material, and is important if you are doing various specialty cuts threads (like a screw) or knurling (a type of surface texture).

  • The Chuck is what holds the material in the Lathe and can be of various sizes. It has mostly two types of Jaws, either an 3 jaw or 4 jaw. That refers to the parts that actually hold the material. In this picture, see the three stepped features on the chuck? They can be adjusted in or out and clamp on the outside of the material, and center the material on the axis of spin. They can be reversed too, which allows the operator to work tubing. In that case, the jaws go inside the tube.

    For short pieces, you would only need to clap the material into the Chuck and go. For longer pieces you have two choices, either the tool which is between the Chuck and the Turret Tool Post in this picture (and for what its called I can't remember) and the Dead Center in the Tail Stock. The unknown tool has three adjustable bearing surfaces that you can place on the outside of the material. This supports it as it turns and stabilizes longer pieces. For longer material or precision cuts you need the Dead Center.

  • The Dead Center is a conical rest that goes into a small beveled hole that the operator drills at the start. It provides a rest along the axis of rotation. It rests in the Tail Stock and can be removed to add either a Drill Chuck or other boring tools. Here is a good picture of a work in process.

    You can see the tool in the Tool Rest as it is cutting the face of the material. The horizontal movement has been activated and the Tool Rest is slowly moving towards the Chuck, removing a thin layer of material. The Tool Rest has four sides and for convenience can be loaded with several different tools at the same time.

    Here is a chart of some of the tools and the types of cuts they make.

    The Parting tool is what you use to cut the finished part from the stock material, since you have to leave some in the Chuck to keep it spinning.

  • You can also cut holes in the center of material. This is called "Boring". Here's a picture of that process.

    You can do very big holes with a very big lathe.

    Lathes are very old technology and were driven by belts from a Line Shaft, usually placed in the ceiling of the early machine shop. At first driven by a steam engine, then later large electrical motors, most lathes now have internal motors. Here is a picture of a Line Shaft.

    That picture is a screen grab from this Youtube video - Lineshaft Wood Shop. Its short (3 minutes) but worth it to watch. You can see how such a mechanical system could be easily set back up, using a scavenged car motor or even a newly built steam engine. Here is a good example of the kind of precise work in metal you can do with a lathe. The cross hatch on the handle is called "knurling" and has a special tool for applying it.

    Wood lathes are slightly different from metal lathes, in that most lathing is done with hand held tools. As in this picture, you can see the operator holding the tool on a metal rest. Lots of chips too, lol.

lathechuck's picture

... to hold a workpiece that projects out from the chuck to far to support itself, is either a "steady rest" (which is clamped to the lathe bed) or a "follow rest", which is bolted to the lathe saddle, and moves along with the tool holder. The steady rest is just the thing you need so you can drill a hole in the end, to receive the point of the "center". A center can be either "dead" (non-rotating), or "live" (rotating). The dead center needs lubricant in the hole, but avoids the potential vibration of the bearings of the live center (and it's substantially cheaper).

At the other end, you can either hold the work with a chuck, or (for maximum precision), put a center into the lathe spindle, in the middle of the faceplate (which replaces the chuck), and use a "lathe dog" clamped onto the work, driven by the faceplate.

Other times, the work can be clamped into the faceplate without either a chuck or center.

mountainmoma's picture

There is a park by me that used to be a homestead and thriving dairy and some parts of the park have been restored to show what it was like to live then. One of the things that was restored was the machine shop, and it looks an awful lot like your picture with the line shaft. DIfferent tools could have their belts moved over and connec tot he power. But, at the dairy, the motive power for the lineshaft was a re-direction of part of a stream, so the water was run over thru the shop, and then back to the stream, and at the shop this moving water powered the whole machine shop. It did not take much water and has a very small, discrete water wheel.

The barn for the cows was also raised to make it easy to clean out.

And the house, in the kitchen, had a hot water tank suspended from the ceiling , plumbed thru the wood cookstove and then piped over to the sink

they say the machine shop is from 1896

alice's picture

Impressive lathe skills, what a great skill.

Folks might also be interested to hear about Pole-lathes, the operators of which are known as bodgers, which are a primitive kind of lathe a person can build themselves and the woodsman's kind is typically driven by a pedal which pulls a strong springy sapling for the turning force. Also a good skill to pass along if anyone's inspired. Some pictures here of more indoor type ones.

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lathechuck's picture

nope. I see that "avatar" photos have gone missing, too.