Centering a punch mark in the 4-jaw

[Pete O]

G'day guys, wondering what method people use for centering a centre-punch mark in the lathe. I've just completed threading my new crossfeed nut, this is the technique I came up with- an old field-tipped arrow shaft held in the tailstock chuck, with the point located in the punch mark and a DTI against the arrow shaft. Worked a treat, but wondering what a more conventional method would be?
       
I had more success with the internal acme threads this time around, made a spare whilst I was at it. The above photos are setting up the second nut, which is why the boring bar with the acme tool are already set up in the toolpost whilst centering the work. 2 good threads from 3 attempts, still a good thing I'm not trying to make a living from this!
   

[Mayhem]

I use the same method although I made a pump centre, as I didn't have an arrow on hand!  



So are you happy with the fit of the nuts?

[Pete O]

Happy in a qualified way...there is a tight area in the middle of the shaft, I must have had some deflection when cutting the external thread despite using the travelling steady (or follow rest), I brought the major diameter of the nut out to where I had an acceptable fit over the tight area, still pretty confident the backlash will be reasonable over the rest of the shaft. I had to come out about an extra .004" from where the end of the shaft screwed in easily, to get the middle section to a point where it will be okay to turn by the feedscrew handle. I guess all will be revealed when it is installed and I can see what the backlash is like.

I assume the fat end of your pump centre has a spring in it?

[Mayhem]

Sorry - I left out that detail. Yes there is a spring in the fat end.

[stevec]

(05-11-2015, 04:44 AM)Pete O Wrote: G'day guys, wondering what method people use for centering a centre-punch mark in the lathe.

Pete, I've always just used a center drill in the tailstock to start the hole. .

Others may point to my shoddy work habits.

Steve

[Mayhem]

Steve - the question isn't how to start the hole, it is how to accurately locate the hole in a 4-jaw chuck.

[Pete O]

Yes, the hole had to be in a precise position that was not concentric with the outside of the workpiece.

[EdK]

I also use a pump center with a DTI.

Ed

   

[stevec]

Thanks Darren, Apparently I am able to read but not able to understand .
It was early in the morning for me. (Like that's an excuse )

Steve

[f350ca]

Like the arrow Pete, recycling at its best. I've been going to build a pump centre, its on the list. For now its a 1/4 inch steel rod about a foot long with a point, same as your arrow.