using a four jaw chuck on a lathe

Generally when you say "four jaw chuck" you mean a chuck with fully independent jaws like yours. This allows you to either chuck round pieces with zero runout, to chuck square workpieces on center and to chuck round and square workpieces with an intentional offset, for example to bore and offset hole in a workpiece.

However, there is such a thing as a scrolling four jaw chuck where all the jaws move together. This is a special chuck that is only good for chucking square work pieces on center, you wouldn't want one as your only four jaw chuck.

That's pretty much the way its done, the only thing I'd add is if you need best accuracy use a dial indicator instead of your eyeball. As you use the chuck more you'll get faster at chucking things on center. However, if you need to chuck a lot of workpieces on center and don't have a 3 jaw chuck or a collet setup, I'd consider getting both as you can afford them.

Let's see if anybody else has a tip for faster centering.

Paul T.

A dial gauge can refine the "eyeball" measurement considerably, especially my little 0.0001 unit. Gerry :-)} London, Canada

A short piece of angle iron with a rod welded to it sticking up works better than the eye. Angle is face down across the ways, rod sticking straight up. Keep dialing the jaws and rotating the chuck until you quit moving the rod and angle, then get out the dial indicator or DTI to finish it off. Always works for me.

snip----

That's the procedure, but don't fail to take advantage of the concentric rings that are cut on the face of the vast majority of 4 jaw chucks. They're cut for your convenience so you can judge where the jaws are relative to the centerline of the spindle.

When you're trying to get an object running true, end to end, it's a good idea to chuck it short (1/2" or so) so it can be tapped about in the chuck. If your jaws are not dead parallel and you hold parts deep, it's quite hard to convince them to allow the part to wallow about enough to get both ends on center, especially if the jaws are finely serrated. When they are, they tend to dig in, making it difficult for the part to shift. Unless the part is extremely long, when properly snugged up, the typical 4 jaw has more than enough holding power to allow you to machine the part once it's running true, even when it's chucked short.

Harold

As others have said, that's the way its done. I've found the quickest way is to use two chuck keys - each one on opposite jaws. This allows you to easily keep the jaws in contact with the work piece at all times. If you don't have a 2nd key, make one. You won't regret it.

Use the 'eyeball' method to get the piece pretty central, then use a dti to make it as accurate as you like. Work on the jaws that are horizontally opposed and use both keys to move the piece to halve the dti error when you turn the chuck through 180 degrees. You can ignore any wobble caused by the other 2 jaws. When you are happy with the 1st 2 jaws, turn the chuck through 90 degrees and do the same with the 2nd two. Repeat until you are happy.

Ayup. Done with two keys, and some practice over time, it can be done very quickly

Gunner

Rule #35 "That which does not kill you, has made a huge tactical error"

Your .0001" unit is called an indicator, and is the only way to accurately center a piece in the chuck. A previously drilled center drill pilot can be centered with a wiggler attachment too. Stone should find a good machinist's book to learn all the techniques available. Bugs

There's a very simple way to get the work within a half-thou without a dial indicator.

It's not that the indicator isn't easy to use, or more accurate than this method, but the extra few minutes you might spend setting it up and tearing it down before machining are often just "lost time" when you don't need that accuracy. A mag base might make it quicker, but some small vee-bed lathes don't even have a convenient place for the base-- not even on the cross-slide.

For round or square work -- Instead of the run-out gauge, just move your tool as close to the "high" side of the work as you can get it without touching. Rotate the work 180, adjust by eyeball to eliminate about 1/2 of the gap. Run the tool in again to whatever has become the high side, and repeat, halving the error each time. In three or four iterations, you'll have the tool pulling swarf over two thirds-or-so of the circumference of the work. By then it's within a thou or less of centered, and ready for any rough surfacing.

Just quicker... that's all. A bright, narrow-beam work light helps see the gap.

LLoyd

And a white 3x5 card on the cross slide, under the work, makes it easy too. Shine the light on the card, which is then viewed through the gap.

Jim

...

The Master had this to say about centering in a 4-jaw:

Quick tip for setting work true in a 4 jaw in two revolutions!

(I learned this, setting up roll turning lathes with 108" swing and a jogging speed of two to four MINUTES per revolution.)

1. Revolve the work through one revolution..noting the TOTAL swing of the indicator needle. Bring the spindle to a halt at the MIDPOINT of the swing and ZERO the dial to the needle.

1. Revolve spindle to bring jaw one to be "on the plunger". Adjust jaw one and jaw three to ZERO the needle again.

3 Rotate 1/4 turn and adjust jaws two and four to re-zero needle.

Job is done!

Robert Bastow

Something implicit in all of the replies so far is that a 3-jaw chuck is for automatically centering round or hexagonal work -- but they generally only center to a few thousandths (mine are more like 5-10 thou, but they're cheap Chinese stuff).

A collet will center much more accurately and will be, if anything, quicker than a 3-jaw but will only fit a small range of work sizes, and will generally only fit round work. I can't quote an accuracy figure for you 'cause I've never had the $$$ to buy a collet set.

A 4-jaw will let you center things to the limits of your skill and patience. It will let you center just about any straight-sided profile: round, square, hexagonal, pentagonal, elliptical, extruded aluminum in Bill Clinton's profile -- anything. It takes more time to set up, but you only need to get one of it, and it'll do for many, many tasks.

Thanks to all who answered this thread.

I have a PITA 4 jaw for my wood lathe. your tips have assisted greatly.

:-)

--=20 Will R. Jewel Boxes and Wood Art

power of accurate observation is commonly called cynicism by those=20 who have not got it.=94 George Bernard Shaw

I think that I have the same chuck on my clausing lathe, four jaws and foud adjustment points. Very strange. Why would they even want to make them like that.

i

four jaws and

want to make

You'll find out : )

Not necessarily true. Hex and square collets are readily available, and soft collets as well. For production runs, it's not uncommon to machine a profile in a soft collet so irregular parts can be routinely held with precision and speed. You are, of course, limited to size by the collet, but for large diameter short pieces, step chucks (collets) are available.

Harold

Could be that in wood working it makes little sense, but when machining metals, it makes all the sense in the world. A 4 jaw chuck is the most precise of all holding devices *because* it allows for independent adjustment. The level of precision is up to the operator, unlike the typical universal chuck (typically a scroll type device that adjusts all jaws simultaneously). With such a chuck, you're at the mercy of the level of precision inherent in its manufacture, plus the results of use and abuse. Because all the jaws work together, you can't make adjustments when they're off, although there are three jaw and six jaw chucks on the market with an adjustable body. If one had but one choice for a lathe, it would have to be an independent 4 jaw, otherwise many jobs would be impossible to hold. They are often slower, but very flexible.

Harold

Don't overlook the fact that irregular items can be held as well, and parts can be held such that diameters can be turned eccentric with one another. Very important if you're trying to build a crank shaft or eccentric device.

Wouldn't that mean, though, that every time I insert something into the jaws, I need to perform a complicated centering operation??? That seems rather painful.

i