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Tips & Techniques Pages
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Lathe Operation: Workpiece Holding |
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Summary |
The following table has been useful to me when comparing the different means of holding the workpiece:
|
Method
|
Precision
|
Repeatability
|
Convenience
|
Notes
|
| 3-Jaw Self-Centering Chuck |
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). |
| Collet Chuck |
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. |
| Expanding Arbors |
High
|
High
|
High
|
These work from the inside out rather than the outside in but are otherwise much like collets. |
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.
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.
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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") are extremely expensive for the hobbyist market and so are seldom seen. I think it is interesting to note that very few 3-jaw chucks are pictured in Harvey's book, yet the web sites catering to the hobby market show that this is the most popular means of workholding. The Harvey book shows more 4-jaw chucks than anything. Of course this is the chuck that comes with your lathe, it's the one everybody has seen, and so it will be the one that gets used firstest and mostest.
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 of the piece in the jaws. Considering its accuracy and repeatability issues, this should be avoided anyway.
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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. Eventually I must build one, and it is on the project list. 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.
Using a Wobbler with a 4-Jaw Chuck:
Some people swear by wobblers, but this seems to be old school. I have a wobbler, and have tried it, but frankly can see no particular advantage over a dial test indicator, and its one more thing to learn. Mine is gathering dust in a shop corner somewhere. However, there is a way to use something like a wobbler to spot a point on a part that is otherwise hard to indicate to. See for example this photo:
I came across this nice shot of dialing in an irregular part (or a feature of a part) on a 4-jaw.
Frank Ford has a nice version of this over on his site, as well as a nifty indicator in a QCTP holder.
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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 costly. I would not throw a piece of rough stock into one of these elegant expensive beasts. I was finally able to afford one by haunting eBay until I got a good deal on a 5" Buck 6-Jaw. The chuck is a little "experienced", and I had to machine a backplate for it, but I sure do love it!
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.
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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.
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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!
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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...
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Collet Chuck: High Precision, High Repeatability, High Convenience |
Straight A's on this report card, so how can we lose? There are basically three disadvantages to collets. First, is capacity. You can only use a 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. 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 much more expensive and harder to find. 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.
Still, if you want the most efficient and precise macining of small parts, collets are the way to go. Incidentally, 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 collet will also grip with more force spread over a greater area, and is less likely to mar a workpiece than many other methods.
In CNC applications, you can set up the collet so bar is fed in, machined, and parted off as a continuous operation.
I purchased a full set of 5C collets and a rack on eBay for a great price, so I had to follow up with a collet chuck. I got one of those from Lathemaster. When I get it, I'll have to fit a custom backplate similar to what I did with my 6-jaw. I'm thinking of trying to create my own Set-Tru capability to allow me to adjust the chuck's concentricity.
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Expanding Arbors |
TBD
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Use of Rests and Centers |
TBD
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Mandrels & Cat Heads |
TBD
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Unique Setups: Offset Turning |
If you are going to build model engines, you'll need to get used to offset turning. Creating 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 gallery here as well.
Surface Plate and Height Gage Used to Create An Offset Fixture...
Using a cutoff tool for clearance to turn the crank offset...
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Soft Jaws |
TBD
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Adjust-Tru and Set-Tru Chucks |
TBD
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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.