Total Guide to Lathe Workholding

 

Summary

We can characterize lathe workholding solutions according to their Precision, Repeatability, and Convenience. This Guide will help you choose the best workholding solution for your lathe or turning application.

The following table provides a quick comparison of the strengths and weaknesses of the different means of holding the workpiece on a lathe:

Method	Precision	Repeatability	Convenience	Notes
Collets	High	High	High	Fast, high precision, high repeatability, grips well, unlikely to mar workpiece, grip spread over a wide area. Expensive chucks and collets. Handles limited lengths. Workpiece must be round and must fit nearly exactly to the collet size.
3-Jaw Chuck With Soft Jaws	High	High	High	For larger workpieces, 3 jaw chucks with softjaws are the norm in the CNC world.
3-Jaw Self-Centering Chuck with Hard Jaws	Low	Low	High	Common, cheap, simple. Low precision, low repeatability if you remove the workpiece and have to put it back.
4-Jaw Chuck	High	High	Medium	Can be time consuming to individually adjust the jaws, but will result in high precision. Can hold pieces offset for turning cams or eccentrics. Can hold irregular shapes and square or rectangular stock.
6-Jaw Self-Centering Chuck	Medium	Medium	High	Best for thin wall work or to grip finished edges of workpiece. Obviously good for hex stock.
Faceplate Turning	Varies w/ Setup	Medium	Low	Great for irregular shapes. Involves clamps like a milling setup. May need counterweights to keep things balanced.
Turning Between Centers	High	High	Low	Great precision, allows part to be put back between centers with very high repeatability.
Constant Face Turning	High	High	High	The modern alternative to turning between centers. Instead of using lathe dogs, which are kind of a nuisance to set up, the constant face system uses hydraulic or other force to grip and drive the spindle end. You can now buy these systems relatively cheaply on eBay (see seller 800Watt who sold me mine).
Expanding Mandrels	High	High	Medium	These work from the inside out rather than the outside in but are otherwise much like collets. They're ideal for thin walled parts that need the support, or also for holding parts where you need access to the whole outside surface and have no place to clamp other than an ID bore.

Method describes the particular technique or tooling to be used.

Precision describes how precisely the workpiece will be held, or how close to concentrically it will run with the spindle before taking any cuts.

Repeatability describes how easy it is to take the workpiece out and then get it back in precisely again.

Convenience describes whether the solution is a time-saver versus other solutions with similar advantages.

Notes provides narrative summary of the method's strengths, weaknesses, and uses.

Below we will consider each method in more detail. You should be able to choose an appropriate method pretty well from this table based on your needs. For example, if you can carry out all the operations on the part with removing it from the lathe, repeatability should not be an important factor.

 

Collets: High Precision, High Repeatability, High Convenience

Checking runout on a 5C Collet Chuck. That's an Interapid 10ths indicator, so we're seeing not quite 0.0004"...

Hardinge built a mighty lathe empire on machines that used collets very well, and we've got straight A's on our precision, repeatablity, and convenience report card, so how can we lose? There are basically three disadvantages to collets.

First, is capacity. You can only use a 5C collet up to perhaps a 1" diameter workpiece capacity and the collet will only take a similar length inside the collet. By definition this limits us to small parts. Of course there are larger collet systems, such as 16C. But by comparison to the size lathe you generally see a 16C collet on, this still begs the question of how to hold even larger workpieces. It's generally accepted that collets are best for parts 3" or less in diameter.

There are various ways to escape this limitation:

- Expanding collets act like mandrels. They expand inside a hole on the workpiece rather than gripping the OD of a workpiece.

- Step Collets and Pot Chucks) have to be machined for each diameter but can hold larger workpieces. They're also useful for holding workpieces off center.

Second, you will need a collet that fits the workpiece with an accuracy of 1/64", at least for the very popular 5C collet system. The ER system tolerates more error, but is less amenable to automatic operation.

Lastly, collets are expensive. You will need an expensive collet chuck and an even more expensive collet set. Given how closely the collets have to fit the workpiece, you will need a lot of them if you expect to cover a wide range of sizes. In addition, collets are a consumable--they will wear out and need to be replaced.

Still, if you want the most efficient and precise machining of small parts, collets are often the way to go. Here are their advantages versus jawed chucks with soft jaws.

Higher Spindle Speeds and Less Vibration

The total mass of a collet workholding system is often less than conventional chucks allowing you to run higher spindle speeds with less vibration. The spindle can also accelerate to speed faster since it is spinning less mass.

Higher Accuracy of Second Operations

Since collets clamp around the entire circumference and not just where jaws meet the part, they can be more accurate. A jaw chuck with bored software jaws repeats within 0.0006 to 0.0012 inch TIR. But a collet chuck can typically do 0.0005 inch TIR or better. That can be a big deal for precision second operations where the additional features must be aligned with features machined in an earlier operation.

Since the collet will also grip with more force spread over a greater area, it is less likely to mar a workpiece than many other methods. A collet is also less likely to distort a thin-walled part.

Advantages in Setup Time

For very large and very small lot sizes, collets have an advantage. Swapping jaws takes around 15-20 minutes for a standard jawed chuck. There are chucks especially designed for quick changes that can do it in 1 minute, but a quick change collet chuck allows the collet to be changed in 15-20 seconds.

With large lot sizes, consider that a collet chuck takes less time to open or close than a jaw chuck, which will reduce the non-cutting time on a cycle. That adds up for very large lot sizes.

We can summarize a lot of this by generalizing that collet chucks trade flexibility for speed. If part size is consistent and the parts are small enough, the collet chuck is faster. But if you have a lot of variation in workpiece size, the jawed chucks are likely faster.

Another consideration is type of material. Hot rolled steel, castings, forgings, and extrusions have more diameter variations in the raw stock. These materials can be better suited to jawed chucks. Cold rolled material is much more consistent on size.

Because of all this, collets are the default workholding for smaller CNC lathes and many precision toolroom lathes such as the Hardinge. Larger lathes typically use 3 Jaw Chucks with Soft Jaws to gain many of the same advantages.

There are a variety of ways to get started with collets. One of the simplest is to purchase a collet chuck. Slightly more involved is to use a nosepiece and drawbar setup. With a lever operated collet closer, you can quickly pop parts in and out of the collet, which is a tremendous productivity increase when making runs of identical parts.

The logical conclusion to all this is a pneumatic or hydraulic collet closer and bar feeder or bar puller. In CNC applications, you can set up the collet so bar is fed in, machined, and parted off as a continuous operation.

Using collets is an art form unto itself. Hardinge has or used to have some excellent publications on techniques.

 

3-Jaw Chuck With Soft Jaws

If the principle shortcoming of collets is workpiece size, the most common answer to that for larger CNC lathes is to use a 3-Jaw Chuck with Soft Jaws

Soft Jaws have a couple of large advantages over hard jaws.

First, they align the workpiece to the centerline precisely. The problem with hardjaws in a 3 jaw chuck is the chuck scroll does not perfectly align the workpiece on the centerline at different diameters. Softjaws are precisely bored at the diameter they'll be used at so they're accurate and repeatable.

Second, they can provide a backstop that precisely aligns the rear of the workpiece as well.

Their main disadvantage is a new set of jaws has to be made for each workpiece size, but that's not very hard to do.

Here is a two-part video series from Haas that talks about how to make and use Soft Jaws:

Part 1: Fundamentals and OD Gripping

Part 2: ID Gripping, Recutting, and More

 

3-Jaw Self-Centering Chuck: Low Precision, Low Repeatability, High Convenience

If your mission is to chuck up a cylindrical piece of material and get started making chips quickly, the 3-jaw chuck is your tool of choice. They are possibly the lowest precision option for holding your workpiece because there are many ways for them to be off in terms of concentricity with the axis of spindle rotation. Any little chip or dust between the jaws and workpiece or in the scroll mechanism will throw one off. Because they are so easy to use, they often have a lot of wear.

Once you turn a workpiece in the chuck, the physics of turning will ensure that your workpiece is now concentric with the spindle. Unfortunately, if you take it out of the chuck, the low precision means you are right back to an imprecise situation. Always try to finish all the operations, both rough and finishing in one setup when using a 3-jaw chuck! Conversely, if you can finish a piece entirely in one setup of the 3-jaw chuck, it is probably the absolute best choice because it will let you get started the fastest.

Harvey (Machine Shop Trade Secrets) says there is no point in using one unless it can be adjusted for concentricity. Such chucks (often called "Set-Tru") cost more, but the feature adds a lot of flexibility.

Note that the 3-jaw chuck, having the fewest jaws of the chuck types (thank you Albert Einstein!), will therefore apply the most pressure to the workpiece yet with the least surface area. More jaws will spread the pressure more evenly. This can result in marring of the surface, so a 3-jaw is not ideal for gripping a finished part in the jaws. They're also problematic for machining thin-walled parts because they apply too much pressure and distort the walls.

 

4-Jaw Chuck: High Precision, High Repeatability, Medium Convenience

Next up in popularity is the 4-jaw chuck. Many lathes include one. This is your weapon of choice if you have to take a workpiece and start out with it being as close to concentric as possible right after you put it in the chuck before taking any cuts. It's ability to do this speaks to the repeatability of the chuck should you need to remove a part and then put it back. The fly in this ointment is convenience gets sacrificed. Each time you put a part into the chuck you need to center it by adjusting the jaws and using a dial indicator or wobbler.

4-jaw chucks grip the workpiece more tightly than a 3-jaw, which may yield benefits in rigidity, especially for small parts. They also allow a part to be turned eccentrically (i.e. off center), which is essential for making cams and similar kinds of parts. The other great use for a 4-jaw is holding non-cylindrical pieces that have to be turned. Since the jaws are individually adjustable, you can make them fit a piece well. I have seen an amazing amount of work get done on non-cylindrical pieces using a 4-jaw from machinist's who don't have access to a mill.

I also use mine if I started with the 3-jaw and have to put the piece back in for futher machining. You won't get the 3-jaw lined up properly again, but the 4-jaw is made for realignment.

Many of the old hands feel a beginner should force himself to work exclusively with the 4-jaw until it becomes second nature, and there is much to be said for that. The act of indicating in a 4-jaw is something that ought to be second nature to a machinist, and it resembles many other measurements you will have to make, particularly if you have a mill.

Indicating in a 4-Jaw Chuck so the Work is Centered:

Here, fresh from another great HSM thread is the world's fastest way to set up a 4-jaw chuck. The technique requires no more than 2 revolutions of the chuck, and is performed as follows:

1. After rough aligning to the chuck rings, using a dial indicator on the work-piece, rotate the spindle through one complete revolution noting the highest and lowest indicator readings;

2. Continue rotating the spindle and halt at exactly Midway between the above two readings, then zero the indicator bezel to the needle;

3. Rotate the spindle to bring jaw #1 ‘on plunger’ and adjust jaws #1 and #3 to re-zero the indicator; finally

4. Rotate the spindle 90 degrees and adjust jaws #2 and #4 to zero the indicator once again.

This is one of those things where I read it, it made total sense, it was elegant, and it made me feel stupid for not having thought of it myself. Here is a nice video by David Lemereis showing how to perform this technique:

Fastest way to indicate a 4-jaw...

QCTP Indicator Holders

You can slap a magnetic indicator holder down on the cross slide, most people do, but having a QCTP indicator holder seems like such a wonderful luxury. Meanwhile, here is a photo to show you what I mean:

 

A QCTP Indicator Holder...

Indicating Square or Hex Stock

Indicating in square or hex stock? Try my height gage method.

I use a bubble level to get the face level, then I drop my height gage onto the cross slide platform (mine is flat) and measure the height of the face. This is compared to the height of the opposite face. It really helps to know how far the jaws move in one revolution. On my 4-jaw, it's 0.140" per turn. Once you know how far off you are numerically and how much a turn gives you, the process goes much faster. As you can see, I got it lined up within a thousandth.

 

6-Jaw Self-Centering Chuck: Medium Precision, Medium Repeatability, High Convenience

I personally love the wonderfully complex look of a 6-jaw chuck. Their true forte is in holding thin walled tubing or finished parts without marring or collapsing the walls. Aside from their ability to grip with more surface area, and to spread the gripping force more evenly, they're pretty similar to 3-Jaw Chucks.

Given the 6-Jaw advantage, and the tendency to reserve them only for finished workpieces or thin wall tubing, they achieve somewhat higher precision and repeatability than a 3-jaw with the same high convenience. Their biggest disadvantage is that they are more costly.

I love my 6-jaw chuck!

Some of the machinists I most respect swear by the 6-jaw chuck. Swede, the master machinist behind the popular 5Bears site says it is his favorite chuck. If you are a fan of Harvey's book, Machine Shop Trade Secrets, you will note there are very few pictures there of 3-jaw chucks. Most are 6-jaw or 4-jaw models.

 

Faceplate Turning: Variable Precision, Medium Repeatability, Low Convenience

Think of the faceplate as a milling machine where we spin the work instead of the cutting tool. We can bore a large hole in a piece of rectangular stock, for example. We need to take care to set up the work accurately and clamp it firmly using techniques very much like that off a mill. An additional complication is that you may need to also clamp a counterweight to the faceplate if the piece is off center in shape or mass to keep the vibration down.

This is all a lot of trouble, hence the low convenience rating. If you have a mill, you are probably going to use it to do these operations rather than futzing with a faceplate.

The other use for the faceplate is when turning between centers. The spindle engages the workpiece by means of a lathe dog which tracks in a groove on the faceplate.

 

Turning Between Centers: High Precision, High Repeatability, Low Convenience

Turning a piece between centers means you have to drill the holes the centers run in, hence there is setup overhead resulting in low convenience. However, if want a part to be well supported and hence rigid, and if you will need to take that part on and off the lathe more than once so needing repeatability, turning between centers is a great approach to take.

Chuck center button for turning between centers...

And away we go...

Tip: If you don't own or want to deal with a faceplate and center at the spindle end, turn a "center" by chucking up some round stock in the 3-jaw or 6-jaw and tapering it to a point. Try for about a 60 degree taper. As long as you leave the center in the chuck, it will be perfectly aligned to the spindle. You can even put a lathe dog on the workpiece and let the chuck jaws drive it. If you have a 4-jaw or Set-Tru chuck, you can even make this new center a permanent fixture in your tooling because you'll be able to dial it in with the 4-jaw so it is properly centered again.

Tip: No lathe dogs? I've seen guys use geared hose clamps (like on your car's radiator hoses) to good effect for this purpose. They will not deal with incredible amounts of force, so go easy!

 

Constant Face Turning: High Precision, High Repeatability, High Convenience

The modern alternative to turning between centers. Instead of using lathe dogs, which are kind of a nuisance to set up, the constant face system uses hydraulic or other force to grip and drive the spindle end. You can now buy these systems relatively cheaply on eBay (see seller 800Watt who sold me mine). More on this when I get a chance to experiment with the rig I purchased. Meanwhile, if you are curious, do a little Google footwork. I found these interesting links:

http://www.riten.com/technical_support/face_drivers/default.asp

http://www.riten.com/products/face_drivers/default.asp

http://www.rohm-products.com/: Check the live center w/ pressure gauge! Nice diagram too.

This constant face business is some cutting edge CNC-type stuff. Pretty cool!

Face turning a part on my Lathemaster 9x30 lathe...

 

 

Expanding Mandrels: High Precision, High Repeatability, Medium Convenience

Expanding Mandrels are ideal for:

- Holding thin-walled parts that would be distorted when too much pressure is applied by an outside clamping force.

- Holding parts whose outside surfaces must be machined in a single operation.

- Holding parts that have many different outside diameters and features that would be otherwise be difficult to clamp from the outside.

They are limited in the amount of cutting force that can be applied without spinning the part on the mandrel.

Expanding Mandrels grip the part by expanding against the walls of an inside bore.

They typically work by tightening a screw in their center that causes the leaves of the mandrell to expand and grip the ID bore of the part. It's possible to machine a profile on a mandrel to fit a part with complex internal geometry. It's also common to see them with shoulders to locate the end of the part and other similar features.

Mandrels don't have a huge range of motion, so it is common to machine them for a particular part's needs.

It's possible to obtain expanding mandrels to fit collet systems such as this one intended for 5C Collets:

Royal even offers a power operated mandrel setup if you need to automate a lot of turning operations with mandrels on a CNC lathe:

A power operated mandrel like this can be a good choice for a sub-spindle on a CNC lathe because they grip without any possibility of damage to the outside surface and provide complete access to finish that surface in one operation.

Unique Setups: Offset Turning or Eccentric Turning

If you are going to build model engines or do repair work on odd-shapes, you'll need to get used to offset turning. Coming up with the right tooling to make this possible is a creative art. I find old metalworking books and magazines have a lot of ideas, and I have collected a small page of techniques as well.

Surface Plate and Height Gage Used to Create An Offset Fixture...

Using a cutoff tool for clearance to turn the crank offset...

Truing a Lathe Chuck

You can increase the accuracy of a lathe chuck by truing the jaws. Doing so involves grinding them with the chuck spinning. You will also want the jaws to be loaded, so place a washer or other piece deep inside the chuck and clamp down on it. Then, use a toolpost grinder (rig one up if you have to) and a small stone to grind the inside of the jaws until you get down to the washer. You are only looking to take off a tiny bit of material, which makes the lathe jaws concentric.

It is also suggested that for maximum accuracy, you always tighten with one of the multiple tightening holes in a 3-jaw chuck. Check which one works best, mark it, and stick to it.

The other way to true up a lathe chuck is to use an Adjust-Tru or Set-Tru Chuck. These chucks have set screws in the backplate that allow the chuck's orientation to the spindle centerline to be precisely adjusted.

 

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