Home Shop Machinist

The lathey guy’s guide to work holding.

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Most home shop lathe work is done using a standard three-jaw scroll chuck, but there are about as many different ways to mount and hold turnings as there are lathes, or airplanes for that matter! This month, we’ll cover the “standard” methods of work holding, as well as some unusual techniques that might come in handy for those occasional jobs that might well leave your friends scratching their heads wondering, “How did you do that?”

A typical three-jaw self-centering chuck.

Chucks

Everyone is familiar with a drill chuck, right? The three-jaw chuck is basically the same idea, just scaled up and made a bit more useful with clever design. Like a drill chuck, a key (or T-handle) turns a mechanism called a scroll to expand or contract the jaws in unison. Three-jaw chucks usually come with two sets of interchangeable jaws to allow the widest possible range of diameters for a particular chuck body. A scroll chuck is also called a self-centering chuck and can be found with as few as two jaws and up to six jaws or more.

Independent jaw chucks are typically of the four-jaw variety and they probably are the most versatile, but least used, of all chucks. As the name implies, each jaw can be adjusted independently by means of a jackscrew. This allows many interesting possibilities, including holding irregular objects such as rectangles or off-axis turnings like a crankshaft or a bellcrank. Make note that any object that is mounted off-axis is likely to be out of balance and can cause potentially destructive vibration. The spindle speed must be set to the absolute minimum rpm and tested for balance before turning. It’s a good idea to always stand clear of rotating parts when flipping the “on” switch, but especially when testing potentially out-of-balance turnings.

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Independent jaw chucks hold odd shapes.

Independent jaw chucks come in really handy when you need to re-chuck a previously made part. Even though they are called self-centering, all regular scroll-type chucks have some off-center error called runout. Manufacturers typically guarantee their chucks to be within a certain spec. For example, when describing their Model 2337 five-inch chuck, LittleMachineShop.com says: “The radial runout of a test bar held in this chuck, when the chuck is mounted true, is 0.003 inch.” This does not mean you can’t use this chuck for precision work. Most lathe work is about reducing (via turning) raw bar-stock into parts, so some small run-out in the chuck is not an issue. The parts will come out round and true to the spindle axis from which they originally were turned. It’s only after a part is made and you need to rework or re-machine it does runout become a problem. The spec tells you what to expect in terms of runout in the event you need to remount a previously turned (and concentric) part. That’s where the four-jaw chuck comes to the rescue! By using a dial-test indicator and carefully adjusting each jaw independently, you can bring most previously turned parts into near-perfect “zero” runout. The only drawback to the independent chuck is that dialing them in takes some experience and even then, they can be quite tedious and time consuming to use.

The D1-4 cam-lock system: three studs in the back of the chuck mate with the spindle. A 90° turn on each of the three cams secures the chuck.

Chucks are mounted to the lathe spindle in a variety of standard ways. Most shop and tool room lathes use a cam-lock mount that conforms to a series of standards (D1-2, D1-3, D1-4, etc.) that define parameters such as nose taper, stud location and backplate diameter. Chucks on most mini and hobby lathes bolt directly to a drive flange that is integrated into the spindle. A self-centering recess in the back of the chuck mates with the flange. Many older lathes, and wood lathes in particular, have an external thread on the spindle (usually 1×8 TPI or 11/4×8 TPI). Chucks for these lathes are either threaded or have a matching thread adapter bolted to the back.

Cam-lock chucks provide convenience of rapid change-out, good precision, and solid mounting for both forward and reverse rotation. Bolt-on chucks lack the convenience of cam-lock systems, but are lower cost, offer good precision, and provide solid mounting for both forward and reverse rotation. Thread-on chucks are as fast and convenient as cam-lock chucks, but can’t be reversed, at least not too aggressively, as they will try to unscrew themselves. Most have one or more setscrews to provide some lock to the spindle, but it’s a marginal approach at best.

A 5C collet system mounted to the lathe. This particular model uses a scroll system with a T-handle to tighten the collet.

Collets

Collets are split sleeves with an outer taper that are used in conjunction with a drawbar, scroll system, or compression nut to grip a tool or work-piece. The ubiquitous Dremel uses collets to hold a variety of bits, as do all wood routers. Collets for lathes generally hold the parts instead of the tools (there are exceptions, such as when milling on a lathe—Google it if you are curious). What makes collets stand out over chucks is their precision. Collets, when directly mounted to the spindle nose, allow you to re-chuck a part in the lathe with high precision and minimal fuss.

Naturally there’s a downside to using collets, and that is you need a large collection of them to cover the smallest to largest diameters. Although there are dozens of configurations, most bench and tool room lathes are adaptable to the “5C” type collets, which go up to a maximum of 11/16-inch diameter. The 8×14 lathe I have at home is considered a mini lathe and, as such, it uses “3C” collets, of which the largest diameter is 9/16 inch.

This comparison shows the relative size difference between the “standard” 5C collets and the mini-lathe 3C collets. Collets are precision tools that should be stored in an appropriate tool holder.

Faceplates

Faceplates are a traditional lathe accessory from way back in the days before chucks. The general idea is you can use a faceplate to fix odd shapes for turning by using clamping blocks, holddowns, or by directly bolting the work to the slots in the faceplate. Remember, odd shapes usually are out of balance and can cause potentially destructive vibration. The spindle speed must be set to the absolute minimum rpm, and the setup tested for balance before turning. Always stand clear when flipping the “on” switch!

Faceplates give you a way to mount and turn parts that otherwise would be problematic, if not impossible, using a chuck.

Between Centers

When you need a shaft or tube to be perfectly concentric, the preferred method is to turn it between centers. A “dead” center (dead = non-rotating) mounts directly in the spindle (replacing the chuck) and a “live” center (live = free-rotating) is mounted to the tailstock. A lathe dog is often employed to slave the workpiece to the drive spindle to prevent slipping, especially for heavy cuts. It’s also possible to use the compression force of the tailstock to maintain engagement with the drive center. But this only works in limited circumstances where the piece is not too long, too thin, or at all flexible. Even then you are limited to taking very light cuts. The advantage to compression clamping is you can turn end-to-end in a single pass because there is no lathe dog in the way.

In this variation of between centers turning, a small sheet of Plexiglas is turned to a perfect disk (the tape is to prevent scratches). This method can be used to turn any material that is too thin or fragile to machine any other way.

Although between centers turning is usually for shafts and spindles, you can turn disks and plates between centers as well. I recently had to make a repair lens for a Hobbs meter. It was the weekend and I couldn’t order a new glass lens in time for a Sunday flight. But I did have some 1/8-inch Plexiglas that would be perfect for a temporary replacement. Using my bandsaw, I roughed out a slightly oversize blank from Plexiglas sheet. I then turned a sacrificial faceplate to mount in the three-jaw chuck and a matching tail plate, each out of aluminum (wood or a free-machining plastic like Delrin would also work). Both were slightly oversize from my final target lens diameter. I center-drilled the tail plate so it could be interfaced with the rotating tailstock center. Turning the lens to size was a simple matter of positioning the plastic blank between the two faces, applying sufficient pressure with the tailstock ram to clamp the part, and then turning both the faceplates and lens to size.

The dead center fits into a taper adapter, which fits directly into the spindle nose. Precision is assured because all the tapered parts are precision ground.

The clamping force of the tailstock live center (right) is often enough to engage the drive center for taking light cuts.

Summary

Chucks, collets, faceplates, and between centers represent the most basic methods of work holding on the lathe. For certain shapes that won’t fit in a chuck or defy even a faceplate mounting, lathe work can sometimes still be accomplished by building a special fixture to hold the part in a chuck or even between centers. These are outside the scope of basic methods, but it’s worth mentioning because it shows the possibilities are nearly endless when it comes to things one can do in the home shop!


Bob Hadley is the R&D manager for a California-based consumer products company. He holds a Sport Pilot certificate and owns the VW-powered Victory Stanley Fun-Kist.

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