# Precise Centering tool for a four jaw chuck



## Winegrower (Oct 27, 2021)

I needed to trepan a groove in a piece of polycarbonate that had to be in an exact relation to some features that I would later put in on the mill.    I started by making a reference point with a center drill while set up on the mill and locating the other features.  I then pinned the piece with a live center holding it against the 4 jaw.   But that was unsatisfactory, and I didn‘t really believe this was that accurate, nor was there an easy to prove it one way or the other.   So I remembered seeing the following somewhere…I turned a 60 degree on a short piece of steel round, and center drilled the other end.   Now I could insert that between the work and the live center, and indicate on it.   This works great, and I could move it around with the 4 jaw to whatever accuracy desired, and know it.

You can also set the edge of a trepanning tool off the known diameter of the steel round and any kind of gauge…I had a 1.000” micrometer reference.  So the inside edge of the trepanning tool is at 1” plus half the rod diameter, thus you can move it to set any diameter of groove.

My explanation is way more complicated than it needs to be…you folks could glance at the picture and get it.

it’s a dirt simple tool that I imagine I will occasionally need.


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## BROCKWOOD (Oct 27, 2021)

That's cool to know! After having watched a few Bar-Z 4 Jaw competitions, I refuse to revert back to the 3 jaw. My excuses are numerous; but, the top 2 are an aching back & I can only improve with time!

I've never trepanned a square into a circle on the lathe. To date, I've preferred using a rotary table on the mill instead. Good to see this option & I thank you for sharing.

Of note: I often copy a word that I'm unsure of how to spell into a Google search just to check myself. Yes, I doubted my spelling of 'trepanned'. The resulting answer is befitting Halloween:

Trepanning, also known as trepanation, trephination, trephining or making a burr hole is a surgical intervention in which a hole is drilled or scraped into the human skull.


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## tjb (Oct 27, 2021)

Great idea.  Could have used that yesterday on a project.


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## Manual Mac (Oct 27, 2021)

Learned two things today.
I also like your toolpost holding of the DI, so simple, i’ll be using that idea often.
Thanks.


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## B2 (Oct 27, 2021)

Nice!

Also, 


BROCKWOOD said:


> spell into a Google search just to check myself


For those of us who really need a crutch.....you might want to try out a small piece of handy software called "WordWeb".  It is a windows add-on that lets you just highlight a word in a document and then click on its icon to check for spelling etc.  https://wordweb.info/free/


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## markba633csi (Oct 27, 2021)

Trepan my troubles away...
-M


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## Doug Gray (Oct 27, 2021)

A dead centre is often used for this, Rather than making up a special pin. Then I call it the double centre centring method.  

Canadian spelling of center = centre, centred, centring, plural = centres


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## MrWhoopee (Oct 27, 2021)

markba633csi said:


> Trepan my troubles away...
> -M


Trepanning doubles my troubles.


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## Winegrower (Oct 27, 2021)

Doug Gray said:


> A dead centre is often used for this, Rather than making up a special pin.


Yes, for sure that would work for centering, Doug.   I have a couple dead centers, but they all taper in all directions.   The advantage of the pin, as I saw it, was that I could use the known and constant diameter of the pin to establish a good centerline reference and I could position the trepanning tool at an accurate radius from the center of rotation by using a gauge between the pin and the tool edge.   With a tapered dead center, it would be harder to do that.


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## Doug Gray (Oct 27, 2021)

Oh now I got ya, you mentioned that in your OP.


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## benmychree (Oct 27, 2021)

A refinement of the tool is to make a large enough diameter pin and drill and ream a hole in the back (unpointed ) side of it to insert a smaller piece of round stock that has a center drilled end; a spring is inserted in the hole in the larger pointed pin, so that in use the spring is somewhat compressed, which eliminates the binding which occours when the part is moved in the chuck and also serves to push the part against the chuck jaws.  I think that I got the first one that I have from someone who had worked at Mare Island Navy Yard, where they were made.  I left it at my shop when I sold out and since made another for myself.  A note; a small hole is cross drilled through the large part to relieve air pressure when inserting the smaller part.


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## benmychree (Oct 27, 2021)

A refinement to this device is to make it two parts, a larger diameter part with the point and a smaller part with the center drilled end; a hole is drilled and reamed in the back end of the larger (pointed) part to receive the smaller part and a spring, the spring tension eliminates binding when the chuck is adjusted and helps in keeping the part up against the chuck jaws.  A small hole is cross drilled through the large part at the bottom of the reamed hole to relieve air pressure when inserting the smaller rod.  I think this device was made at Mare Island Navy Yard originally.


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## woodchucker (Oct 27, 2021)

then again, that's what this tool does


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## benmychree (Oct 27, 2021)

Yes, after a fashion, not as precise ---


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## Winegrower (Oct 27, 2021)

How would that work, Woodchucker?    There are two issues I was concerned about, how to position the part on center and then use that center reference for cutting tool placement.   Please explain?


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## keeena (Oct 27, 2021)

I also remember seeing this technique somewhere a while back. If I'm not mistaken, the one I saw had a ball (ball bearing) somewhere in the arrangement. It may have been one of Ca Lem's videos. Anyway - thanks for sharing!


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## woodchucker (Oct 27, 2021)

Still uploading the video... don't be too hard on me..




This only solves the problem of centering. I was just showing another tool used in the old days. It works, but so does what you have done.
I have used an indicator on a center. And I have also used this a few times. The indicator is probably easier, as you get an idea how much to move 1/2 the reading. But in the old days, they did it by eye. And it was accurate. You are only looking for motion at the TS end with this.


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