For example, just saw a video of a lathe machining a chess piece. The final product was much smaller than the starting piece. Why do you start with such large pieces of stock for small projects? Wouldn’t it be smarter and more efficient to rough cut using cnc or water jet then finish?
In: Engineering
Properly setting up work holding is a hassle. Saving half a minute on cutting material is not necessarily worth 4 minutes of tapping the piece with a wooden mallet then rotating it until the gauge stops swinging around. Also that implies the presence of at least another van sized 5000$ machine in the shop. A lathe by itself is already a rarity in a garage.
Starting with larger pieces of metal or wood when using a lathe is about stability and control. The bigger stock gives you more material to work with, which helps secure the piece better and allows for more accurate cuts. If you started with something already close to the final size, it could be harder to keep it steady and get the precision you need. Plus, lathes are great at shaping material gradually, so it’s easier to start big and work your way down. Using a CNC or water jet first might not give the same level of detail or finish.
When figuring out how to approach a machining job, either on a lathe or other machine, there’s a lot of things to consider. If you’re starting with a normal bar of material, those only come in certain standard sizes. Plus, material costs money, so for hobby work it’s not uncommon to use a leftover from a previous project, even if it’s not an ideal size or shape for what you’re trying to make. That logic even sometimes applies to oddball one off jobs in industry, but much less frequently.
Next is order of operations and work holding. You need some amount of material to be able to hold the part, and you need to hold it sufficiently well to be able to resist the cutting forces. So maybe you need to cut a blank some amount longer than the part you want to make so you have some material in your chuck jaws. Otherwise you risk the part flying out and hitting you at worst. Maybe it just shifts, and you scrap the part by cutting something you didn’t want to. And work holding only gets more complicated as you remove more material. It’s easy enough to stick an oversized saw cut disc in a 3 jaw chuck, or a block in a vise. But now you need to flip it over, and make sure everything on this next side lines up properly with what you’ve already done. If you thought ahead, maybe you intentionally used a larger piece of material so you could give yourself something to hold onto, especially on smaller parts. Or maybe you used that extra material to give yourself a feature you could use to locate something else, like having a reamed hole for pin to indicate off of.
The other answers given have been good, but also this might be a bit of a false premise. There’s no general need to start with a huge stock excess. I work in CNC machining, but we start from metal castings that are already roughly the right shape. The difference is those castings are very expensive for a complex end result (jet engine parts) compared to starting from a block of material for a simpler shape. It’s about the balance of machine capability, cost, and need for efficiency.
A CNC what? Lathes can be and often are CNC. A CNC mill? A mill works similarly enough to a lathe that if you’re doing a round part it will be no faster, but the setup time for moving it between machines certainly slows you down.
Water jets are very expensive to run. Also, making round cuts with a water jet may not be any faster than lathe work. Why use a big expensive tool when the classic one works just as well?
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