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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It's a bit obvious, but the first thing a woodturner needs is a lathe. It's actually the world's oldest 'power tool' ' see the panel (left) for a bit of history. As you begin to investigate what's available, you'll soon discover that there's a bewildering choice of models, with very different specifications and prices, and it's very easy to become confused about exactly what you require. If this is your situation and you need some help, then this feature, originally published at hobby magazine getwoodworking.com highlights the main factors you need to consider before you buy.
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Woodturning has progressed enormously in recent years and is now no longer just a means of producing functional items, but is fast approaching the status of an art form. To fuel this massive increase in popularity and demand, many woodworking machinery manufacturers have hurriedly added lathes to their range, but a good machine requires a number of essential features that are often overlooked in the rush to get new equipment onto the market.
There are lots of very cheap lathes on the market that all look the same and appear to give you a lot for your money, photo 1. These machines are very much entry-level models, being sufficient to get you started, but if you progress much beyond very basic work you will soon find them very limited. Some even come with a stand and incorporate variable speed, but there is a definite correlation here between the price you pay and the quality you get.
As a general rule, the heavier and more substantial the machine, the better. Vibration is the woodturner's worst enemy, particularly if the workpiece is long or out of balance, and there is nothing to beat sheer weight to minimise this vibration.
For this reason, it's better to buy a lathe that is cast rather than fabricated, but this inevitably often involves greater cost. However, do remember that unlike a lot of other machines, you will often spend hours working at the lathe. Consequently it needs to be as smooth and quiet in operation as possible, so always think carefully about the quality and buy the best you can afford.
When you are buying a lathe, there are two capacities you need to consider. The distance between centres determines the maximum length of workpiece you can turn. For general work you will need at least 30in, and preferably 36in, photo 2. Incidentally, lathe dimensions are still mainly given in imperial measurements; for metric buffs the equivalents are 760 and 915mm. Longer beds are available, and whilst these may appear attractive, they can add to any vibration problems unless they're properly braced, so this is one instance where it is better not to buy too much capacity unless you have the specific need for it.
Another useful measurement of size is the 'swing' of the lathe, photo 3. This is the height of the main spindle above the bed and therefore determines the maximum diameter of work you can turn, unless the head rotates round. For anything remotely approaching serious work, you need a swing of at least 9in (230mm), but in practice the bigger the better.
A professional turner will probably need a heavy-duty floor-standing lathe, but for the home woodturning enthusiast a bench-mounted model will probably be quite sufficient, photo 4. These bolt down to any convenient work surface, but this must be really solid. The advantage of mounting it on your own workbench is that you can get the centre height just right ' a serious consideration if you anticipate doing a lot of turning in the future.
Several manufacturers provide leg-stands as an optional extra if you don't want to build a bench. These stands vary from a rather crude folded steel or tube arrangement, to a more rigid affair with provision for a tool shelf, photo 5. Bear in mind that the performance of your lathe depends on how well it is mounted, so buy a leg-stand only if it looks man enough for the job.
A homemade wooden bench is often better at absorbing vibration than a crude metal stand, and it can change the whole operation of the lathe. If you are short of space in the workshop, you can build in a lot of storage for tools or raw materials under the lathe, which also helps to give the structure a bit more mass, photo 6.
The bed is made from either heavy metal bars or tubes, photo 7, but some machines still feature a flat cast bed. This may be quite crude on the cheaper machines, or a work of art on top-of the-range models, photo 8.
Whatever the construction, it must be strong enough to support both the tailstock and the tool-rest without any flexing, and allow free and easy movement of them both. It must also permit shavings to fall through unobstructed and should sit well clear of the bench, so you can slide the tools underneath it without banging the sharpened edges.
This is the heart of the machine and needs to be really solid, and preferably cast. Fabricated headstocks are rarely heavy enough if you need to turn large or out-of-balance work. The headstock also needs to have a good spread between the bearings to ensure maximum rigidity of the spindle.
Some imported models have what looks like a huge headstock, , photo 9, but when you remove the belt cover the two spindle bearings are actually quite close together. A small bearing spread like this will causes problems with rigidity, particularly on large diameter work, so always look for a machine where there is plenty of distance between the bearings.
The bearings themselves should be good quality heavy-duty sealed ball races, photo 10. Some machines have a tapered bronze sleeved bearing which gives much greater support than ball races, although it does require occasional adjustment. However, when set up correctly this arrangement supports the spindle over a much greater length and provides totally smooth and vibration-free running, photo 11.
The headstock may be permanently fixed in line with the bed, or it may have the facility to swing round so that the spindle ends up at right angles to the bed for bowl turning, photo 12.
I would rate this swinging head as an essential feature on a lathe; the real advantage is not only for bowl turning, but also for any turning where you have to work over the bed. Swinging the head just a few degrees off-centre allows you to work with the tool handles clear of the bed, photo 13. Although you can still work off the standard tool-rest with the head swung a little, for big diameter bowl turning you will need an additional bowl rest to maximize the capacity, photo 14.
If you are restricted with regards to space, many of the swinging head machines also allow you to move the headstock bodily along the bed, which is a great advantage if the end of the machine has to be up against a wall, photo 15.
The headstock spindle is threaded to take a range of screw-on accessories such as chucks, photo 16, so you need one with a standard thread or you'll be limited in the range of extras you can buy. On many smaller lathes, a thread size of ¾in x 16 tpi is the industry standard, which makes upgrading your machine less costly.
This which means that you can then take all your threaded accessories with you, rather than renewing them with the lathe when you upgrade it. This is an important point, as you will often have a substantial investment tied up in chucks and other accessories.
It is essential that your lathe is also equipped with Morse tapers in both headstock and tailstock. This is a universal means of installing centres and a range of other tooling, photo 17, and does not restrict you to using only the original manufacturer's fittings.
There is a huge range of Morse taper kit on the market, but if you buy a lathe with only screw-on fittings you are very restricted as to what you can use. Morse tapers are commonly No 1 or No 2 on the smaller lathes; the bigger the number the thicker the taper. The tapers just push into the headstock and are then knocked out afterwards with a bar that runs through the main spindle.
If the spindle is solid, there needs to be a centre ejector which screws onto the spindle nose before you insert the taper, photo 18. Take great with these tapers and keep them clean and undamaged, or they will start spinning inside each other, which as well as causing them damage, will lead to inaccuracies when you are using fittings such as drill chucks.
A small lathe will need a motor of at least 1'3hp, particularly if you envisage turning bowls, but bigger is better in this case. In order to give some speed variation, the motor is usually fitted with a three or four-step pulley and a matching one on the spindle, photo 19, to give a speed range from about 400 to rpm. This is achieved with a belt, which is moved around on the pulleys to select the required speed. The traditional V belt has now virtually been replaced with the more efficient fl at poly V type, which gives a smoother, vibration free drive as it has no lumpy joint.
Some lathes achieve the speed variation in other ways. This may be mechanical, where a lever operates two cone pulleys, so changing their diameter and therefore the speed, photo 20. This system does work, but it is prone to wear belts very quickly and is rather noisy. Also, you can only change the speed whilst the lathe is running. So if you finished the last job at top speed and now want bottom, you firstly have to switch the lathe on and reduce the speed before you can mount the work, all of which is a bit fiddly and time consuming.
The ultimate for speed changing is an electrical speed control, which gives you infinite variation of speed at the turn of a knob. This is usually reserved for the top-of-the- range lathes, but in the past electronic speed variation has suffered from loss of torque at low speeds. Fortunately modern electronic technology has largely overcome this problem, usually by operating a three-phase motor through an inverter off a single phase supply, photo 21.
State-of-the-art electronic speed controls have memory functions that can remember a selection of favourite speeds. They can also sense incidents such as dig-ins and then instantly shut down the power, photo 22.
Whatever your motor type, make sure that the switchgear is easily accessible and doesn't get hidden by large workpieces. I prefer to have the switch (or at least a separate 'off ' button) at knee height for emergency situations when you have both hands full. Some machines have a magnetic switch-box, photo 23, which allows you to move it around at will depending where you are working.
Motors with a reverse facility are a valuable aid for sanding, and are quite safe to use on between-centres work. However, if you engage reverse with a piece of faceplate work, there is always the possibility that it will unscrew itself, so lathes with reverse should feature a faceplate locking system.
This needs to be as substantial as the rest of the lathe, as it has to provide fi rm support for between-centres work. Make sure that it slides freely and locks firmly onto the bed. The tailstock barrel is moved backwards and forwards with the hand-wheel and needs plenty of travel for drilling work, photo 24. It should be bored with a Morse taper to match the headstock, and should also be drilled right through to allow for easy removal of the tailstock centres and for long hole boring, photo 25.
The tool-rest assembly is another vital part of the lathe, the main requirement being that it is quickly and easily adjustable. The actual locking mechanism varies from machine to machine; some use a simple clamp and lever under the bed, whilst others use a cam type of lock, photo 26, which is easier to use as it is accessed from the front of the lathe. Always check this point before you buy.
The tool-rest itself needs some vertical height adjustment and should lock into the holder with a simple handle that works effectively; there must be no movement possible once it's locked. For general use the rest needs to be about 10in (300mm) long, and made of heavy cast construction so that there's no vibration when you are working at the end of it.
Alternative length rests are available, photo 27. You will probably need a shorter one at some stage. For very long work there is a rest with two stems, but this requires an additional tool-rest holder.
Look out for part 2
where Alan introduces the basic woodturning tool kit, and explains how to separate the wheat from the chaff
So now you know all about the lathe, how do you choose one to suit your needs?
Firstly, consider the type of turning you will be doing. If you will mostly be turning spindles, then there's perhaps no need for a swinging head model, but rigidity of the bed and good between-centres capacity are important features.
On the other hand, if you think your main interest will be bowl turning, a swiveling head is vital but between-centres capacity is less important. You'll also need plenty of motor power for big-diameter bowls.
If you want to do a bit of everything, try to decide on the biggest diameter you want to turn and choose a lathe accordingly.
Think also about how often you will use the machine. If you anticipate being an occasional user making a few simple furniture parts, then a basic model is all you'll need. But if you think you will spend a lot of time at the lathe as your skills and ambitions grow, then you will need the extra power and weight of a larger machine.
As you go further up the range you will find this extra power and solidity allows you to turn more quickly and confidently. You can take deeper and more ambitious cuts, and the ease of use of features such as electronic variable speed control make turning more intuitive and enjoyable.
Above all, bear in mind that woodturning is an addictive hobby, so try to buy in as much spare capacity as you can afford now to save expensive upgrades later on.
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