Author Topic: Centering a part in a four jaw chuck  (Read 1470 times)

Offline Brian Rupnow

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Centering a part in a four jaw chuck
« on: January 23, 2021, 09:29:41 PM »
Ever since I started using a four jaw chuck, about 12 years ago, I have used a 1/4" diameter rod x 15" long with a point turned on one end. The pointed end is placed into a "center punch mark" on the piece I want to center in the four jaw chuck, and the other end is held in a tailstock mounted chuck. Then a dial indicator is brought in to bear on the 1/4" rod just clear of the part in the chuck, and the chuck is turned by hand and adjusted ---and adjusted---and adjusted---until the dial indicator reads "0" for a full 360 degrees of rotation. This method has served me well, but about 3 months ago I wanted to center a piece which had a 3/8" hole that I wanted to bore out to marginally oversize. I didn't want to use a larger diameter rod, because the larger in diameter the rod is, the less willing it is to flex. I thought about it for a while and then come up with this modification to my 1/4" diameter rod. This works great, the rod can still deflect easily, and I can center accurately on holes up to 9/16" diameter. Note that in order to do this, you can't use the typical pointed end on a dial indicator. It has to be unscrewed and replaced by a larger, flat "anvil" on the end

« Last Edit: January 23, 2021, 09:46:52 PM by Brian Rupnow »

Online mklotz

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Re: Centering a part in a four jaw chuck
« Reply #1 on: January 23, 2021, 11:20:06 PM »
You wrote...

Then a dial indicator is brought in to bear on the 1/4" rod just clear of the part in the chuck, and the chuck is turned by hand and adjusted ---and adjusted---and adjusted---until the dial indicator reads "0" for a full 360 degrees of rotation.

If you're having that much trouble getting it to center, perhaps my technique will be of help to you.  For the benefit of later readers, I've included the whole writeup but you may want to skip to the section marked with the rows of asterisks.

=================================================

Probably every machinist has his own pet way of doing it. My technique works
for me. Take whatever is useful and modify as you see fit.

To easily center work in the 4J, you'll need to make yourself two tools.
First, make a dedicated holder of some sort so you can mount a dial indicator
(DI) on the tool post (or directly to the compound) with its axis
perpendicular to the spindle axis. Adjust the DI so its plunger is vertically
aligned with the spindle axis. An easy way to do this is to put a pointed tip
on the DI plunger and align the point to a dead center in the headstock. The
idea is to make something that you can drop into place, already aligned, and
lock down in ten seconds or so. Leave the DI permanently mounted to this
holder. A cheap import DI (<$15) is fine since we'll be using this only for
comparative, not absolute, measurement.

While you could use a conventional adjustable magnetic DI holder, I strongly
recommend that you make a dedicated mount that is easily installed and removed.
A general maxim of machining is that you'll be much more likely to do something
'the right way' if setting up to do it is quick and simple. If it isn't you're
much more likely to try some half-a$$ed setup that doesn't work and ends up
damaging the tool, the work, or, worst of all, you.

*********************************************************************************

The second tool to make is a clone of your 4J chuck wrench. We're going to be
adjusting two jaws at a time and it's infinitely easier to do if you can move
both jaws in and out in concert without having to swap the wrench from hole to
hole. It's another example of the maxim I mentioned above. The clone wrench
doesn't have to be anything fancy. Machine a square tenon to match the
existing wrench on the end of some suitable stock, and drill for a press-fit
cross bar. Use your existing wrench as a guide for dimensions. I've found
that, if there's not a lot of room on the back side of the lathe, making the
clone somewhat shorter than the supplied wrench is a good idea.

Ok, now for the procedure. Mount the work in the 4J and roughly center -
either by eye or by using the concentric circles scribed into the face of most
4Js. Snug the jaws down so the work is held securely. Turn the chuck so one
jaw is at the nine o'clock position as seen looking from the tailstock down
the spindle axis. Use the cross-slide to bring the DI up against the work and
reading about the middle of its range (e.g., about 0.5" on a 1" DI). Turn the
scale on the DI so its needle indicates zero. Now swing the chuck through 180
degrees. Unless you've got an impossibly good eye, the DI will now read
something other than zero. (For an example, let's say it reads 0.038.) Turn
the DI scale so the zero is halfway to this reading. (Move the scale so the
needle points to 0.019.)

Now, insert both chuck wrenches and adjust the jaws so the DI needle points to
zero. Swing the chuck 180 degrees and check the reading - it should be close
to zero.

[Aside: If the part you're centering has the same dimension in both jaw axes
(i.e., it's not rectangular), the DI zero you established above will also be
the zero for adjusting the other two jaws below - another advantage of this
technique.]

Repeat this entire process for the two other jaws. [What we're doing here is
treating the 4J as two two-jaw chucks. We can do this because the jaw pairs
are orthogonal and, to first order, adjustments of one pair will have very
little effect on the setting of the other pair.]

If you've been careful, the total runout on the part should now be only a few
thou. Depending on your esthetics and the part requirements, this may be good
enough. If not, repeat the entire process until the runout meets your needs.
After centering, check to ensure that all the jaws are clamped down tightly.
It's easy to leave one loose. If you have (left one loose), you may need to
rerun the centering procedure after you've tightened it.

*********************************************************************************

With this procedure, you should be able to center something to +/-0.001" in
ten minutes on your first try. With not much practice, you can get that
number down to one or two minutes. Soon your three-jaw will be gathering
dust.

One of the most common uses of the 4J is for drilling/boring offset holes in
eccentrics (i.e. cam drivers for model engines). In this case, you aren't
centering the stock itself (as we were above) but rather need to center the
location of the hole.

First centerdrill the location of the hole in the milling machine. Mount the
stock in the 4J and roughly center this centerdrilled hole. [A fast way to do
this is to use the pointy end of an edge finder held in the tailstock drill
chuck.] Now you need a PUMP CENTER. This is a longish rod (mine is ~10"
long). At the tailstock end is a spring-loaded female center. At the
headstock end is a rigid male center. The male center goes in your
centerdrilled hole. The female center is supported by a dead center in the
tailstock and the tailstock is adjusted to lightly compress the spring. The
DI is made to bear on the rod near the male center. Using the procedure
outlined above, adjust the jaws until the DI shows little or no runout.
Voila, the location of the offset hole is now centered.

=================================================
Regards, Marv
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Offline Brian Rupnow

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Re: Centering a part in a four jaw chuck
« Reply #2 on: January 23, 2021, 11:25:08 PM »
Thanks Marv--I will try your method tomorrow, and I have another similar method to try. I can get stuff centered, but it seems to take me longer than it should. Stay safe!!----Brian

Offline pgp001

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Re: Centering a part in a four jaw chuck
« Reply #3 on: January 24, 2021, 12:17:42 AM »
Brian

If you mill a flat on the side of your pointy stick you could use a standard DTI tip so long as it is set square to the DTI.

Phil

Offline propforward

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Re: Centering a part in a four jaw chuck
« Reply #4 on: January 24, 2021, 02:09:27 AM »
Marvs method works exceptionally well. I can true a 4 jaw mounted part very quickly with it.
Stuart

Forging ahead regardless.

Online crueby

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Re: Centering a part in a four jaw chuck
« Reply #5 on: January 24, 2021, 02:24:47 AM »
Marvs method works exceptionally well. I can true a 4 jaw mounted part very quickly with it.
Same here, that way works well for me too. I had not thought about using a second wrench, that will speed it up even more.

Offline propforward

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Re: Centering a part in a four jaw chuck
« Reply #6 on: January 24, 2021, 02:47:06 AM »
That part of it was very much a “game changer” for me. Actually I need to make a couple of short chuck keys just for even easier access.
Stuart

Forging ahead regardless.

Online mklotz

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Re: Centering a part in a four jaw chuck
« Reply #7 on: January 24, 2021, 03:28:08 PM »
That part of it was very much a “game changer” for me. Actually I need to make a couple of short chuck keys just for even easier access.

While you're making chuck keys, make some* that allow you to operate the jaws on your 4J and the scroll on your 3J using your electric screwdriver.  For both chucks this makes the process of removing the jaws for cleaning much faster.

On the 3J once the jaws are removed, insert an oily toothbrush bristles down into the scroll in one of the jaw slots, then run the edriver in reverse to spin the scroll.  Keep the bristles in the scroll thread and allow the revolving scroll to push the brush out of the jaw slot.  As the scroll revolves, the gunk in it is captured by the brush and carried to the top of the slot where it falls out.  What doesn't fall out sticks to the brush and can be removed easily.  The oily brush ensures that the scroll is lubricated when you replace the jaws.

One caution, however;  don't use the toothbrush to brush your teeth after doing this. :-)

--

* I made mine by milling suitably-sized tenons on a short slug of steel and then cutting short sections from a sacrificial 1/4" Allen wrench with an angle grinder; these were hammered into an undersized hole drilled into the end of the slug opposite the tenon.
Regards, Marv
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Offline Brian Rupnow

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Re: Centering a part in a four jaw chuck
« Reply #8 on: January 24, 2021, 03:57:55 PM »
I just centered a piece in my 4 jaw using the method described by JDunmyer---It was an absolute treat. Much quicker than anything I have tried before.

Center by eye first, then put your indicator against the part. Rotate by hand, noting "high" and "low" readings. Rotate so the reading is halfway between the two, then zero the indicator. Rotate the part until a jaw is lined up with the indicator plunger. Adjust that jaw and the one opposite until indicator reads Zero. Rotate 1/4 turn until another jaw is under the indicator plunger, adjust that jaw and the one opposite until indicator reads Zero. Rotate chuck one turn to verify centering, you should be very close, if not right on.​​​​​​​
« Last Edit: January 24, 2021, 04:11:33 PM by Brian Rupnow »

Offline Brian Rupnow

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Re: Centering a part in a four jaw chuck
« Reply #9 on: January 24, 2021, 04:06:16 PM »
I just read thru an alternate method of centering in a four jaw chuck by Marv Klotz. I'm sure that Marv's method works fine, except that it requires that I make a spring loaded center instead of the pointed rod that I already have.  I think I will stick with the Jim Dunmyer method, because it works so well and doesn't require that I build anything more. Thank you Marv for your suggestion.

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Re: Centering a part in a four jaw chuck
« Reply #10 on: January 24, 2021, 04:24:15 PM »
I just read thru an alternate method of centering in a four jaw chuck by Marv Klotz. I'm sure that Marv's method works fine, except that it requires that I make a spring loaded center instead of the pointed rod that I already have.  I think I will stick with the Jim Dunmyer method, because it works so well and doesn't require that I build anything more. Thank you Marv for your suggestion.

My method doesn't require the spring-loaded pump center.  That tool is nice to have but the thin, springy rod will work as well.

The method I outlined between the asterisks is essentially the same as JDunmyer's.

The key to all these method variants is treating the 4J as two independent two jaw chucks which, since the jaw axes are orthogonal, is largely true.
Regards, Marv
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Offline propforward

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Re: Centering a part in a four jaw chuck
« Reply #11 on: January 24, 2021, 06:02:07 PM »
I don't use a spring loaded plunger - just a pointed rod.

Looking at the descriptions, both methods are fundamentally the same. Using the two chuck key approach helps me easily control the jaw movement when truing.
Stuart

Forging ahead regardless.

Offline propforward

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Re: Centering a part in a four jaw chuck
« Reply #12 on: January 24, 2021, 10:11:17 PM »

While you're making chuck keys, make some* that allow you to operate the jaws on your 4J and the scroll on your 3J using your electric screwdriver.  For both chucks this makes the process of removing the jaws for cleaning much faster.



Thank you Marv!
Stuart

Forging ahead regardless.

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Re: Centering a part in a four jaw chuck
« Reply #13 on: January 24, 2021, 11:20:48 PM »

While you're making chuck keys, make some* that allow you to operate the jaws on your 4J and the scroll on your 3J using your electric screwdriver.  For both chucks this makes the process of removing the jaws for cleaning much faster.


Thank you Marv!

You're welcome.

I found a picture of the one I made; it's at the bottom of the photo in this article...

https://www.homemadetools.net/forum/shifting-chuck-jaws-26219

in the Homemadetools forum.
Regards, Marv
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