Holding On

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.

Bushings, Collets

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.

Drill Chucks

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.

Adjustable Holders

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.

Using Forethought

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.