The first precision machine tools were small ones—like the lathes used by watch and clock makers in the early 1800’s. Since then, industry has continually demanded larger and more accurate machines, but there are also now many choices for those who need small, precision tools for hobby, prototype, lab, educational and other uses. In the early 1960’s there was really only one small machine that fit the bill. The original Unimat DB200 was inexpensive, offered a big accessory line, and despite the fact that it was not very rigid, still sold tens of thousands of machines and created a market for tabletop machine tools in the home shop. It is no longer available, but since then, many more machines have entered the marked from all over the world. They range from very inexpensive machines that are little more than “kits” that will need a lot of tweaking before you could expect to use them for precision work to extremely precise machines made from hardened, ground steel that can cost upwards of $10,000 or more for a tiny manual lathe. Somewhere in between are the machines that really make sense for most people, but when you are just starting out, how do you sort out the myriad of claims from the various manufacturers? There are a few important questions you need to ask, and the answers will help you find the right machine for your needs.
On large machines, operators have a favourite speed where they do most of their work. The machines have plenty of torque so getting the speed exactly right is often not critical, and they rarely turn extremely small sizes where high speeds are needed. Going from 2" diameter stock to 6" diameter is only a 3x difference in size. Small machines, however, are often asked to turn a relatively large variety of materials and sizes. One minute you might be turning 1" diameter stock and then next you might have to turn something that is only 0.010" in diameter. That is a 100x difference in size! Therefore, being able to easily adjust speed electronically without gear or belt changes is a real advantage.
Most tabletop machine tools utilize AC/DC motors. These motors are relatively inexpensive and produce their rated power at high RPM. At low RPM they are easily stalled. This is usually addressed by adding cumbersome gear and belt arrangements with several speed ranges in order to manually adjust spindle speed while keeping motor speed as high as possible. This is time consuming, so people often just use the wrong speed rather than make the changes. Some machines are even offered without a motor, so make sure you know what you are getting when you compare prices.
Very few machines offer a DC motor simply because it is more
costly, but a DC motor produces its maximum torque at start-up and low speed,
which is where you need it for cutting metal. (In machining it is torque, not
horsepower that you are really concerned with.) It also eliminates the need for
gear or belt changes. A DC motor also packs that power into a more compact
housing. The low-speed torque of a DC motor and a sophisticated electronic
control make belt and gear changes virtually unnecessary, meaning you have
full, useable control over the entire speed range with just the turn of a dial.
A number of lathes offer a
calibrated hand wheel only on the crosstie. The carriage is moved by means of a
rack gear that offers fast travel but no precision control over your tool
position. Some lathes have a lever-actuated tailstock that offers fast
movement, but offers no precision control of depth. The same goes for a mill
with a quill feed on the Z-axis. This speeds up hole drilling but can make
accurate depth control difficult if not impossible. Make sure all axes of the
lathe or mill you select are driven by precision rolled lead screws with hand
wheels calibrated in your choice of true inch (.001”) or metric (.01 mm)
increments. Some imports that claim to be inch machines actually have metric lead
screw pitches but hand wheels marked in a close approximation in inches. The
number of divisions can be an odd amount rather than .050” per rotation on true
inch machines or 1 mm on metric machines, making it very hard to keep track of
Precision rolled threads are also a key to precise travel. Rolled threads are far more accurate than cut threads although they require a special machine to manufacture them. If a company doesn’t tell you how their threads are formed, they are probably not rolled.
Some inexpensive lathes come with a chuck, but if you need to hold work in collets accurately sized to your part, you are out of luck. A collet adapter sticking out of the chuck with just a few choices of collet sizes is not going to do the job for you. Some lathes that claim to have a fairly large chuck may have such a small hole through the centre that you can’t really take advantage of that potential capacity. Also, the chuck may be bolted to the faceplate rather than threaded onto the spindle, so you don’t have the option to easily change to another kind of chuck. This also moves the face of the chuck fairly far out from the end of the spindle, increasing the chance for run out.
Look for a machine that has a spindle that is threaded on
the outside to quickly accept a range of high quality chucks including 3- and
4-jaw versions (independent and scrolling) in various sizes. A
When removing large amounts of metal quickly, mass and horsepower are important in a big machine. However, when precision is your main objective, rigidity and tactile feedback for a good “feel” for the cut are of primary importance. Cast iron is heavy and cheap. Properly cast, aged and machined or ground it can be used to produce a very accurate machine. Cheaply cast and machined while still “green” it can warp before it even gets off the boat. Scraping in a warped cast iron bed to get it flat again takes a lot of effort and skill. If you have looked at web sites devoted to some imported mini-lathes, you may have noted that they are mostly devoted to how to turn the lathe into a useable machine rather than how to use it to make things.
You should demand a machine that is accurate right out of the box. This is particularly true for a beginner, because a poor quality machine may lead you to believe that you don’t have the skill to be a machinist, when in reality it is the machine, not you that is at fault. Look for a rigid ground steel bed and aircraft grade 6061-T6 aluminium tables. The lathe should have a high quality cast base with plenty of depth and webbing to keep it rigid while keeping unnecessary weight to a minimum. If both portability and feedback from the cutting process to your fingers on the hand wheels are important to you these are important advantages. How rigid a machine must be is determined by the power of the motor, and for motors in this small size range, the cross-sections and materials used should be engineered to give the optimum in rigidity while keeping weight and cost to a minimum.
Some imported machines come packaged with a few accessories and a basic threading capacity as part of the base price, but if you want more than this, you are out of luck. You will need not only a choice of basic machines or packages in various levels of completeness but the availability of a complete line of accessories in case your future needs call for them. There should be specialized accessories available so you can complete just about any machine shop job with ease. A well designed threading attachment, for example, offers a far greater range of both metric and inch threads than could ever be offered in a set of built-in gears, and the gears should be hobbed from billet metal, not moulded in plastic.
Low cost imported machines arrive by the boatload, but replacement parts are often not available. The same goes for technical support. Whether you purchase a machine through a dealer or directly from the factory, make sure parts and technical support are only a phone call or e-mail away. Make sure also that the machines are designed properly from the beginning, so that the parts that wear out first are the least expensive and easiest to replace.
Any modern machine tool should be able to be purchased new with these items already installed or upgraded at a later date to include them. Some companies offer this capability and some don’t. If you think this might be important to you at a later date, be sure to ask if this capability is designed in and available from the factory or if you have to go to search for an aftermarket source for it.
Remember that an unskilled operator can still make a bad part on the best, most expensive machine available, but your skills will advance faster and farther if you start out with a machine capable of doing high quality work right from the start. They say, “It’s a poor craftsman who blames his tools,” but even the best craftsman can take his work to a higher level by using better tools. A slightly larger investment up front can assure that as your techniques improve you have a machine capable of taking you to the highest levels of craftsmanship. If you choose wisely to begin with, you can enjoy a quality tool for life.