This blog post is adapted from an article that appeared in the February edition of the Machine/Shop print supplement to Modern Machine Shop.
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One of the key characteristics of a lathe, unlike a vertical or horizontal milling machine, is that the workpiece turns, as opposed to the tool. Thus, lathe work is often called turning. Turning, then, is a machining process used to make round, cylindrical parts. Lathes are commonly used to reduce the diameter of a workpiece to a specific dimension, producing a smooth surface finish. Basically, the cutting tool approaches the rotating workpiece until it begins peeling away the surface as it moves linearly across the side (if the part is a shaft) or across the face (if the part is drum-shaped).
Very few lathes today are not controlled by a CNC, although you can still buy a manually controlled lathe. When equipped with means for changing tools out automatically, such as with a tool turret, the CNC lathe is more properly called a turning center. CNC turning centers are available in a wide range of sizes and capabilities, from simple two-axis lathes, which move in only X and Y, to more sophisticated, multi-axis turning centers capable of handling complex four-axis turning operations, milling, drilling, tapping and deep-hole boring ' all in one operation.
The basic two-axis lathe consists of a headstock with spindle, chuck for holding the part, lathe bed, carriage and cross-slide, tool turret and tailstock. While most lathes have a moveable tailstock to support the workpiece at the end, away from the chuck, not all machines come with this feature as a standard. A tailstock is particularly useful, however, when the workpiece is relatively long and slender. Failing to use a tailstock in this case can cause 'chatter,' which leaves telltale marks on the surface of the part. Unsupported, the part itself can become tapered, because it may bend excessively from tool pressure while being cut.
When considering adding a tailstock as an option to a lathe, pay attention not only to the current job being run, but also the size of future work. When in doubt, include the tailstock with the initial machine purchase. This recommendation will likely save the headache and expense of installing one later.
Regardless of how many axes of motion are required, in evaluating the purchase of any lathe, a shop must first consider the size, weight, geometric complexity, required accuracy and material of the parts being machined. The expected number of parts in each batch also should be taken into account.
Common to all lathe purchases is the question of the size of chuck to hold the intended parts. For turning centers, chucks generally range in capacity from 5 to 66 inches in diameter, or even larger. When parts or barstock must extend through the back of the chuck, maximum spindle through-hole or barstock capacity is important. Machines designed with 'big-bore' options are available if the standard through-hole size is not large enough.
The next critical spec is the swing diameter, or maximum turning diameter. This figure indicates the largest-diameter part that could fit in the chuck and still swing over the bed without hitting. Equally important is the maximum turning length required. This workpiece dimension determines the necessary bed length of the machine. Note that maximum turning length is not the same as bed length. For example, if the part being machined is 40 inches long, the machine bed will need to be much longer to effectively turn the full length of that part.
Finally, the number of parts to be machined and the required accuracy are prime factors for specifying the capability and the quality of the machine. Machines for high production call for high-speed X and Y axes, with rapid-travel rates to match. Machines for close-tolerance work are designed to control thermal drift in ballscrews and key components. The machine structure may also be designed to minimize thermal growth.
This is part one of a four-part series about buying turning machines.
Find more insights about acquiring a new machining center by visiting the Techspex Knowledge Center, 'Guide to Buying Machine Tools.'
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Are you thinking about buying a lathe? One of the first things to determine is what you want to do with it and what kinds of materials you will cut. Are you a hobbyist looking to make some wooden candlestick holders and salad bowls? A benchtop wood lathe might be all you need. However, if you wish to hold greater accuracy or cut metals, a more rugged and versatile machine is in order. If you're among the latter group of people or just want to know more about precision lathes in general, keep reading.
Lathes are among the oldest of all metalworking machinery. In fact, many refer to them as the mother of machine tools for their ability to make parts for other machines. These include threaded fasteners, shafts, bushings, and much more, including the candlestick holders mentioned earlier. Historians will tell you it was the early Egyptians who invented the first crude lathes, yet it wasn't until that they gained thread-cutting capabilities. That invention is attributed to Englishman Henry Maudslay, although others certainly laid the groundwork for his important work.
Henry Maudslay would recognize any of these modern metal-cutting lathes from Kent USA. All have hardened and groundway surfaces that allow the carriage to move longitudinally via a leadscrew. Sitting atop the carriage is a cross slide for axial motion, with a four-sided indexable tool post attached to that. A series of changeable gears power the carriage and give the lathe its screw-cutting capabilities, while a geared headstock provides plenty of power at the rotating spindle. For anyone interested in buying a lathe, these are just a few of the primary functions to keep in mind. The Kent USA MLX-T precision lathe is a very good example of a well-equipped modern lathe.
There's way more to buying a lathe than these basics, however. Let's start with the price tag. My father used to tell me, 'You get what you pay for, son.' He wasn't necessarily talking about machine tools, but his words remain valid nonetheless. Here's the thing'if you want an inexpensive lathe, you won't have to look too far to find one. There are plenty of entry-level machines listed on websites and in catalogs that seem to offer a good bang for the buck. And even though they might be similar in appearance and function as those listed on the Kent USA website, looks can be deceiving (another of my Dad's many adages), and a year or two of use will prove differently.
John Lennon was writing about Yoko Ono in this classic Beatles' tune, but he might just as well have been buying a lathe. Consider this: Kent USA's TRL- Lathe comes in at a hefty lbs. Compared to a benchtop or bargain brand lathe, it's a big step up, and yet, it's the lightest machine in the Kent USA lineup. Who cares? You should. A heavy machine means heavier cuts, greater rigidity, and less vibration. For the longest life and highest precision, always opt for a lathe with a heavy cast iron base, not one constructed made of welded steel.
The headstock and its rotating spindle is the heart of the lathe. It goes without saying that you should look at available horsepower'anything less than 2 HP or so will be inadequate for metal cutting. Also, make certain that the chuck is large enough to grip whatever workpieces you have in mind and that it has a standard Camlock-style or equivalent spindle mount. Beyond that, be sure to ask about the type of bearings used, how many, and how far apart they are. A cheap spindle with low-quality bearings will cause nothing but problems a year or two down the road.
Swing indicates the maximum diameter workpiece you can turn on a lathe. Measure from the top of the bed to the center of the spindle and then double that value. Similarly, the bed measurement is the maximum length of the workpiece that you can turn. It's determined by the distance between the spindle face and tailstock. Also, look at the bed width. Wider ones provide greater stability and better results in turning operations. You should consider all of these factors before buying a lathe.
What else separates the good from the not-so-good when buying a lathe? You'll want a foot brake for stopping the spindle quickly. A geared headstock with multiple speed ranges helps to make sure you have plenty of available power. Hardened and ground ways assure years of productive use. A job button makes gear changes easier. Hobbyists probably won't care about inch/metric dials, but someone starting a machine shop will. They'll also care about cutting threads, so make certain your lathe has the right thread-cutting capabilities.
The tool post should make it easy to change and adjust tools. For shaft work and other longish parts, you'll probably need a steady rest. This is an attachment that bolts to the bed and supports the workpiece while turning. You'll need a three-jaw and four-jaw chuck, as well as a faceplate, live center, and various Morse taper adapters for the tailstock. How about power? If your shop has 220-volt, three-phase electricity, so much the better. If not, be sure to pick up a phase converter or buy a lathe able to run on a 110-volt single phase. And if you have the cash, a digital readout (DRO) makes lathe operation much easier and less prone to mistakes.
Granted, you're facing a significant investment right now. Before whipping out your credit card, take a look at the big picture, and determine what will be best for you long term. Yes, a hobbyist or someone just getting started can often get by with a bargain brand machine. As their needs grow, however, this could end up being a mistake. Spending a bit more now on an industrial-quality lathe means better parts and far less hassle. That, and a larger, heavier, more rigid machine typically saves its owner money on repairs and replacement parts in the future. Whatever else you do, be sure to kick plenty of tires, and give us a call to discuss your project.
CNC Precision Teach Lathes Series allows any shop to step up to CNC turning. It's easy to learn teach-in conversational programming enables machinists of any skill level to produce higher quality parts with lower labor costs. These CNC lathes are perfect for machine shops transitioning to CNC and for machine tool technical schools. Read more about CNC Precision Lathes.
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