A look at toolholders for lathes.
An assortment of single- and double-angle collets and collet chucks. Inset: A single-angle collet keeps a good solid grip on cutting tools.
Years ago, before CNC lathes existed, machinists made parts “by hand.” By that I mean they pushed levers and turned cranks to operate the machine. In today’s high-tech world, some might find this hard to believe, but it’s true. I know because I was one of those crank-turners and lever-pushers.
My machining career began on a Hardinge Brothers DSM, Five-Nine, Super-Precision, second-operation lathe (a lofty name for a machine, to be sure). We simply called it a handscrew, and I was a handscrew operator.
This odd title generated some interesting questions from my in-laws, but I learned many valuable lessons during my tenure as a handscrew operator. Chief among them was that good tools and the proper toolholding are at least as important as a good machine.
Tooling for a handscrew consisted of hand-sharpened tools, adjustable holders and bushings. There was no cutter compensation on a handscrew, and offsetting tools required a well-placed tap with a brass hammer. Obviously, things have changed since those long-ago days. But handscrews are still around—and so is a lot of the same tooling I used.
Take bushings, for instance. When I was a handscrew operator, I used bushings to hold drills, reamers, chamfer tools—you name it. Back then, I was unaware that bushings, in many cases, are not the best way to hold drills and other center-cutting tools.
But tooling selection is limited for a handscrew, and sometimes I just had to make do with what I had. And, to be fair, bushings have their virtues. They’re cheap and in a pinch are easy to make, which probably accounts for their popularity.
Before getting into a discussion of bushings, though, let me clarify the terms. Drill bushings are used in jigs and fixtures for accurately guiding a drill into a workpiece. Available styles include “press fit” and “slip fit.”
Toolholder bushings are an entirely different animal. There are many types: B, C, J, Z, etc. They are all similar, differing only in style (split or solid), size and hole placement. Both split and solid bushings rely on set screws for clamping.
A split bushing has a slot running lengthwise through its body. This feature allows it to collapse slightly when the set screw is tightened against the body of the bushing, thus clamping the tool in place. Unfortunately, set screws tend to force the cutting tool off-center. The best solution to this problem is to always use a clean, snug-fitting bushing.
Left: The solid-style bushing has holes for setting tool shanks. The split bushing also accommodates set screws but has a slot running lengthwise through its body.
One advantage split bushings have over solid types is that they come in a broader range of sizes, increasing the likelihood of finding the correct fit for the drill or reamer. If you don’t have a split bushing, then use a solid one.
Solid bushings, also known as “sleeves,” come with a series of holes or slots that allow the set screws to pass through the bushing and push against the tool. Sleeves are readily available in fractional sizes and typically hold boring bars. However, if you end up using a sleeve to hold a drill, be sure to grind a small flat on the tool’s shank. This will prevent it from spinning in the sleeve and keep the set screw from marring the drill shank.
Small split bushings can be purchased for as little as $20, while larger ones are usually in the $40 to $60 range. Boring bar sleeves are slightly more expensive than other bushings, with a 1"x1/4" sleeve costing around $70.
At this point, you might be asking yourself, “If I’m not supposed to use bushings to hold drills, what should I use?” A good choice is a collet chuck.
Collets come in many shapes and sizes, but one of the most popular is the extended-range collet. ER collets have a long, relatively shallow angle on the taper. A “nut” on the collet chuck retains and compresses the collet, clamping the cutting tool in place.
Lathe use requires a straight-shank collet extension. These extensions come in standard 1/2" to 1 1/4" shank sizes and fit most lathes. Their 6"- to 8"-long bodies allow adjustment of the tool length to accommodate shorter tools.
The extensions also have an adjustable backup screw. By seating the drill against this backup screw, it allows you to quickly swap out a dull drill without having to retouch the tool length. It also keeps the drill from pushing back in the collet.
For drills that are too short to reach the backup screw, I have tried placing a spacer between the end of the tool and the screw. DON’T DO THIS! On the few occasions that I’ve taken this shortcut, the drill welded itself to the spacer, which in turn welded itself to the backup screw. The whole mess damaged the collet extension and created a new art form that only a machinist could appreciate.
Another type of collet chuck is the double-angle style. DA collets cost about 25 percent less than ER collets, making them an attractive and versatile alternative. The advertised accuracy for DA collets (0.0005") is comparable to ER collets, but my experience has been that DA collets are less accurate, less rugged and less capable of keeping a good solid grip on a cutting tool than their single-angle brothers.
Whatever collet system you decide on, plan ahead. Try to find a collet series that will meet the majority of your toolholding needs. Most collets will collapse up to 0.030"-0.040", allowing you to buy fewer collets. But don’t be cheap. Buy enough to avoid having to use the collet’s entire clamping range.
If you collapse a collet to its limit, some of the clamping force, which is normally directed toward gripping the tool, will get used up in compressing the collet. This results in less clamping force and a greater chance of tool slippage. It also tends to distort the collet somewhat, providing a less accurate grip.
A few other rules to follow are:
- Always try to use the collet size closest to, but not smaller than, the cutting tool’s shank.
- The closer the fit, the less runout you will have at the tip of the tool, and the more accurate your machining operations will be.
- Finally, if others in your shop are already using one type of collet, consider using the same type to save money. Your profit-sharing plan will thank you.
One alternative to the collet chuck is the drill chuck. Open most tooling catalogs and you will find a bewildering array of drill chucks: plain bearing, ball bearing, ball bearing super duty, microchucks, taper mount, integral shank—the list goes on. But for all their apparent variety, there are just two basic styles of drill chuck: key and keyless.
As their name implies, key-type chucks require the use of a chuck key to tighten (and loosen) the chuck, while keyless chucks don’t. Key-type chuck manufacturers usually describe their products as being “rugged” and “dependable.” Quite often, this means inaccurate and cheap. You get what you pay for. These chucks have no place in a precision machine shop, except for maybe on the business end of the shop’s hand drill.
Their keyless cousins, on the other hand, are a favorite of many machinists. Keyless drill chucks are a versatile, one-size-fits-all solution. I say “drill” because these chucks are just for drilling. Don’t succumb to the temptation to use a drill chuck to hold an endmill or any other tool that exerts radial tool pressure. These radial forces can cause the tool to come loose during cutting.
Keyless drill chucks start around $200, while ball-bearing drill chucks sell for as little as $40.
Despite the convenience of keyless chucks, they are less accurate and can’t grip as tightly as collet chucks. Drill chucks require a mounting shank so that they can be clamped in either the lathe turret or machine spindle. Using a drill chuck attached to a shank, which in turn is attached to the machine, increases the likelihood of tool runout. It also creates the risk of the drill chuck falling off the shank during use.
Some manufacturers have solved this problem by offering integral-shank drill chucks. They are more expensive—sometimes 50 percent more than replaceable-shank chucks—but they are more rigid and accurate, making them a good investment.
The integral-shank chuck does have a drawback, though. If you accidentally bend its shank, plan on throwing it away. Straightening the shank to factory specifications would most likely cost more than buying a new one.
Many lathes, whether they’re manual or CNC, suffer from turret misalignment. If a lathe’s turret is out of alignment, it will be nearly impossible to drill a straight hole. Obviously, the best solution is to fix the machine. Some machines, however, are chronically out of alignment.
Poor machine design or frequent operator error can make drilling straight holes unlikely no matter how often the service technician visits. Removal of the hapless operator is one solution. But a less drastic alternative is to utilize an adjustable drill holder, a solution that proved invaluable when operating a handscrew.
Because handscrew turrets are notoriously out of alignment, adjustable holders allowed me to compensate for this problem with a 1/4" open-end wrench and a small brass hammer. This may seem like reverting to the Stone Age, but sometimes a little gentle persuasion is the only way to fix something.
Another type of adjustable tooling is the floating reamer holder. Reamers, unlike drills, don’t make their own holes; they simply follow existing ones. A floating holder allows the reamer a small amount of radial play, thereby helping it enter the hole. Of course, if the lathe turret and workpiece are perfectly aligned, the reamer may not need to float, and a rigid holder will work just fine. But, in most instances, reamers are much more effective when held in a floating holder.
Often, toolholders are an afterthought when the purchase order for a new machine is placed or when a contract for thousands of parts is quoted. And, too often, this requires the machinist to improvise.
Tapping, Broach Holders
There are three types of tapping holders: rigid, tension-compression and self-releasing. Rigid styles hold the tap in a fixed position, allowing very little axial movement. They require a machine equipped with a rigid, or synchronous, tapping capability. In the old days, this was called lead-screw tapping.
Machines that come with this capability synchronize the movement of the Z-axis with the rotation of the spindle, which makes for an accurately tapped hole. Unfortunately, very few lathes incorporate this feature, an option typically found only on machining centers. This leaves lathe operators with two choices: tension-compression or self-releasing tapping heads.
The former utilizes a spring-loaded mechanism that allows the tap to pull out (tension) or push back (compression). This mechanism allows the tension-compression head to work well on through-holes and when thread-depth tolerance is loose. But since the mechanism relies on a spring to determine thread depth, it’s probably not the best choice for close-tolerance threading.
A tension-compression head also tends to fail at higher spindle speeds, because the spring mechanism relies on the lathe spindle to reverse before the spring runs out of travel. If the spindle doesn’t reverse in time, the tap (or tapping head) will break. For these reasons, I recommend self-releasing tapping heads for lathes.
A self-releasing tapping head also has a spring arrangement that allows the tap to pull out of the holder. But instead of breaking the tap when the spring runs out of travel, self-releasing heads employ a simple clutch that permits the tap to pull out slightly and then spin with the workpiece. As a result, the cutting action ceases.
In addition, self-releasing heads can run at higher speeds and the clutch mechanism makes thread-depth accuracy easier to maintain. This makes them a better toolholder—especially for blind holes, where it’s sometimes difficult to achieve the required number of threads.
Rotary broaching tools need a special toolholder—one that allows them to spin in unison with the rotating workpiece. It also must hold the broaching tool at a slight angle, causing the broach to “wobble” slightly upon entering the workpiece. This wobbling action prevents the broach from cutting the final shape all at once, resulting in more efficient cutting than just plunging the broach into the part.
Unfortunately, rotary broach holders are expensive, costing upwards of $400 or more, depending on the size. Because it’s often difficult to convince a shop owner or foreman to fork over a few hundred dollars for a wobble broach holder, many machinists end up using a collet or bushing to hold a broaching tool. Doing so degrades the accuracy of the broached hole and diminishes tool life.
For good or bad, much of the tooling in shops today is the same that was used on cam-operated lathes many years ago. As mentioned earlier, tooling is just as important as machinery and machinists when it comes to a successful operation.
Often, however, toolholders are an afterthought when the purchase order for a new machine is placed or when a contract for thousands of parts is quoted. And, too often, this requires the machinist to improvise.
Don’t get me wrong, improvisation is a fine skill, especially if you’re a stand-up comic. But it’s not funny when a $100,000 machine sits idle while the machinist is rooting through his toolbox for a homemade bushing. Don’t let toolholding be an afterthought.
About the Author
Kip Hanson, a regular contributor to CTE, is general manager of Allen Co., Edina, Minn.
Related Glossary Terms
Enlarging a hole that already has been drilled or cored. Generally, it is an operation of truing the previously drilled hole with a single-point, lathe-type tool. Boring is essentially internal turning, in that usually a single-point cutting tool forms the internal shape. Some tools are available with two cutting edges to balance cutting forces.
- boring bar
Essentially a cantilever beam that holds one or more cutting tools in position during a boring operation. Can be held stationary and moved axially while the workpiece revolves around it, or revolved and moved axially while the workpiece is held stationary, or a combination of these actions. Installed on milling, drilling and boring machines, as well as lathes and machining centers.
Tapered tool, with a series of teeth of increasing length, that is pushed or pulled into a workpiece, successively removing small amounts of metal to enlarge a hole, slot or other opening to final size.
Operation in which a cutter progressively enlarges a slot or hole or shapes a workpiece exterior. Low teeth start the cut, intermediate teeth remove the majority of the material and high teeth finish the task. Broaching can be a one-step operation, as opposed to milling and slotting, which require repeated passes. Typically, however, broaching also involves multiple passes.
Cylindrical sleeve, typically made from high-grade tool steel, inserted into a jig fixture to guide cutting tools. There are three main types: renewable, used in liners that in turn are installed in the jig; press-fit, installed directly in the jig for short production runs; and liner (or master), installed permanently in a jig to receive renewable bushing.
Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.
Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.
Flexible-sided device that secures a tool or workpiece. Similar in function to a chuck, but can accommodate only a narrow size range. Typically provides greater gripping force and precision than a chuck. See chuck.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
- cutter compensation
Feature that allows the operator to compensate for tool diameter, length, deflection and radius during a programmed machining cycle.
Milling cutter held by its shank that cuts on its periphery and, if so configured, on its free end. Takes a variety of shapes (single- and double-end, roughing, ballnose and cup-end) and sizes (stub, medium, long and extra-long). Also comes with differing numbers of flutes.
- flat ( screw flat)
flat ( screw flat)
Flat surface machined into the shank of a cutting tool for enhanced holding of the tool.
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
Rotating cutting tool used to enlarge a drilled hole to size. Normally removes only a small amount of stock. The workpiece supports the multiple-edge cutting tool. Also for contouring an existing hole.
Main body of a tool; the portion of a drill or similar end-held tool that fits into a collet, chuck or similar mounting device.
Cylindrical tool that cuts internal threads and has flutes to remove chips and carry tapping fluid to the point of cut. Normally used on a drill press or tapping machine but also may be operated manually. See tapping.
Machining operation in which a tap, with teeth on its periphery, cuts internal threads in a predrilled hole having a smaller diameter than the tap diameter. Threads are formed by a combined rotary and axial-relative motion between tap and workpiece. See tap.
Process of both external (e.g., thread milling) and internal (e.g., tapping, thread milling) cutting, turning and rolling of threads into particular material. Standardized specifications are available to determine the desired results of the threading process. Numerous thread-series designations are written for specific applications. Threading often is performed on a lathe. Specifications such as thread height are critical in determining the strength of the threads. The material used is taken into consideration in determining the expected results of any particular application for that threaded piece. In external threading, a calculated depth is required as well as a particular angle to the cut. To perform internal threading, the exact diameter to bore the hole is critical before threading. The threads are distinguished from one another by the amount of tolerance and/or allowance that is specified. See turning.
Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.
Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.