Author Topic: Centering in my 4 jaw chuck  (Read 5462 times)

Offline Brian Rupnow

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Centering in my 4 jaw chuck
« on: September 01, 2016, 06:45:16 PM »
I don't like using the four jaw chuck! Now I know two dozen old time machinists are going to jump in here and tell me how easy it is, and that they would rather use the four jaw chuck, but I don't really care what the old time machinists think. I don't like using it.---BUT---There are times when I have to use it, so yesterday I decided to do something to take at least some of the pain out of it.
In this picture, you see a piece of 7/8" round bar, with a center punched "divot" which is obviously "off center" in the end of it. That "divot" is what I want to be on center to drill a hole there using the tailstock chuck.

Offline Brian Rupnow

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Re: Centering in my 4 jaw chuck
« Reply #1 on: September 01, 2016, 06:47:11 PM »
These two pictures show the way I would normally approach this centering business. A 1/4" diameter cold rolled steel rod, with a point on one end is held in the tailstock chuck, and the pointed end is centered in the "divot". I set the dial indicator up so it is close to but not touching the 1/4" steel rod, and turn the chuck by hand, which allows me to adjust the four independent jaws so it is not radically off center.  Then I position the dial indicator and stand with magnetic base so that the stem of the dial indicator is on the horizontal centerline of the headstock and the plunger is positioned at about 1/2 it's normal travel. The dial indicator has a pointy little end on it, and it is difficult to ensure that it is really on the horizontal centerline of the chuck. Then by adjusting the four independent jaws and turning the chuck by hand, eventually I get to a "0" reading on the dial indicator when the chuck is rotated thru 360 degrees. You will notice that the bottom part of the magnetic indicator is not registered against anything. It is just stuck to the flat surface of the bed. I use this mag base and indicator for other things also, so each time I use it for centering a part in the four jaw chuck it has to be set up and adjusted.


Offline Brian Rupnow

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Re: Centering in my 4 jaw chuck
« Reply #2 on: September 01, 2016, 06:48:14 PM »
So--The first thing we do to improve the situation is to replace that pointy little indicator "contact point" with a 0.3" diameter flat indicator point. This was one piece of a 22 piece set from a "Select a Point" set that I bought from my local supplier for the princely sum of $19.78 including sales tax. all of these "contact points" have a #4-48 thread which matches the dial indicator stem.



Offline Brian Rupnow

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Re: Centering in my 4 jaw chuck
« Reply #3 on: September 01, 2016, 06:49:01 PM »
The next part of the "plan" was to grab an old Chinese metric mag base that was taking up space on the shelf and turn the vertical rod down to the nearest Imperial standard, which happened to be 7/16". I also shortened it up to be about 1/8" longer than the minimum requirement. Take note that fortunately, this mag base has two machined areas on one of the vertical faces. Then a piece of 3/4" square bar was drilled and tapped to slide over the 7/16" vertical shaft, and a second piece was machined from 3/4" x 3/8" steel to give me a horizontal slider which I could attach the dial indicator to.


Offline Brian Rupnow

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Re: Centering in my 4 jaw chuck
« Reply #4 on: September 01, 2016, 06:49:49 PM »
Now, here is where a bit of the "magic" comes into play. The flat bed of the lathe where the mag base sets has a "shoulder" about 1/8" high adjacent to it. With a bit of calculation, cutting, and carving, when the vertical machined areas on the mag base are tight up against the shoulder, the dial indicator is exactly on the horizontal centerline of the lathe, the plunger is exactly at half travel, and the 0.3" diameter flat "contact point" works much better for this operation than the little pointy "contact point" did. The mag base and indicator are now "dedicated tooling" and won't be changed or reset for any other function. I still don't like using the 4 jaw chuck, but with this little improvement, as I said at the beginning of this thread, it takes a bit of the pain out of the operation.

Online Jasonb

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Re: Centering in my 4 jaw chuck
« Reply #5 on: September 01, 2016, 07:07:13 PM »
I'm not sure there is any real need to have it spot on ctr line or vertical  or any need to set the dial to zero.

I just plonk my mag base on and set position the DTI approx vertical and roughly central on the rod.

Its then just a case of looking for movement in the needle, sometimes I'll just turn the bezel so the marker is near the middle of where the needle is moving but most times I'll just adjust until the needle stops moving, easy enough to see what is the high and low point and work out where zero movement would be.

If the needle is swinging between 45 and 75 you don't need a calculator to know the work needs to move 15 divisions

Offline Gas_mantle

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Re: Centering in my 4 jaw chuck
« Reply #6 on: September 01, 2016, 07:08:32 PM »
Thanks for posting that Brian, I like the idea of having a dedicated set up for this kind of thing.

I find these kind of threads very informative in seeing the sort of tools etc others have made.

Offline AOG

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Re: Centering in my 4 jaw chuck
« Reply #7 on: September 01, 2016, 08:27:36 PM »
Brian, I'm with you on using the four jaw. I do my best to avoid using it. To make life easier, I ended up making an indicator holder that I could mount on the tool post. It makes it a lot quicker to get things centered. The build log is here's:

http://www.modelenginemaker.com/index.php/topic,5001.msg95436.html#msg95436

Tony

Offline mklotz

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Re: Centering in my 4 jaw chuck
« Reply #8 on: September 01, 2016, 09:50:14 PM »
I stopped futzing with mag bases for this job a long time ago.  Made an aluminum tool holder for the QCTP and bolted a dedicated DI to it.  Easy zeroing, good repeatable placement, trivial to set up when needed.

For those of you who fear the 4jaw, I did a writeup of a simple centering procedure that many folks find useful.  If interested, you can read it here...

http://www.homemadetools.net/forum/centering-work-four-jaw-chuck-27241
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Offline Brian Rupnow

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Re: Centering in my 4 jaw chuck
« Reply #9 on: September 01, 2016, 09:56:50 PM »
Tony and Marv--those are both excellent write-ups. I wanted the dial indicator to be on the opposite side of the chuck to myself. I'll have to try using two chuck keys (actually the key for my rotary table mounted 3 jaw is the same size.) I see that method using the two chuck keys is quite popular, based on the number of "how to" threads I have seen about it.

Offline 10KPete

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Re: Centering in my 4 jaw chuck
« Reply #10 on: September 01, 2016, 10:03:15 PM »
I use the two key method with the DI in the tool post and it works well for me. The two key thing is soooo easy. Just like leveling up a transit.

Pete
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Offline Brian Rupnow

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Re: Centering in my 4 jaw chuck
« Reply #11 on: September 01, 2016, 10:08:19 PM »
I just tried the two chuck key method, and it does work better.

Offline modelman 1938

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Re: Centering in my 4 jaw chuck
« Reply #12 on: September 01, 2016, 10:11:09 PM »
I must be one of the old time machinists who like the four jaw chuck, but the reason we like it is because we need to get things concentric. The three jaw is ok for first operation work  but not much good for second operation, as the one thing self centering chuck will not do is self centre much better than .003 inch TIR. 

Offline mklotz

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Re: Centering in my 4 jaw chuck
« Reply #13 on: September 01, 2016, 10:21:59 PM »
If you need to center polygonal stock, don't forget to include space for a "flapper" on the DI tool holder for your QCTP...

http://www.modelenginemaker.com/index.php/topic,303.new.html#new
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Online Jo

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Re: Centering in my 4 jaw chuck
« Reply #14 on: September 02, 2016, 08:12:46 AM »
You will find it a lot easier if you use a larger diameter centre in the dimple in the work: personally I often use a MT dead centre on the end of a ball centre mounted in the tail stock.

Then always measure offset/adjust the movement of the work when it is on the same plane as the dial indicator and it takes next to no time to set up.

If you want to be even quicker buy yourself a Keats angle plate like Tgs has been working on for Mr Silky on my Orphans thread. Clamp you piece of bar in the Keats, stick a Dead centre in the tailstock. Use the centre in the centre drilling/dimple to hold the plate on the face plate while you clamp the Keats to the face plate. When you measure it you will find it is spot on 8).

Jo

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