Precise Centering tool for a four jaw chuck

[Winegrower]

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.

 

[BROCKWOOD]

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.

 

[tjb]

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

 

[Manual Mac]

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

 

[B2]

Nice!

Also,

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/

 

[markba633csi]

Trepan my troubles away...
-M

 

[Doug Gray]

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

 

[MrWhoopee]

Trepan my troubles away...
-M

Trepanning doubles my troubles.

 

[Winegrower]

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.

 

[Doug Gray]

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