Centering HELP needed!!!

[Sshire]

OK. Now I'm officially confused. Help me out, please!

After centering a cylinder bore with the Blake Co-Ax indicator, I set up a bolt hole circle on the DRO.

My usual procedure is to use a spotting drill and just tap it on the part to make a dimple (just as a visual check) at each bolt hole. *The "circle" looked off center.

I put the DTI in a collet and the bore was NOT centered. So, I indicated it with the DTI until it was dead nuts on.

Just for my sanity, I tried recentering 4 ways. My procedure was to use the DTI centering as a baseline (ABS 0,0), and then recentered with
A. Blake Co-Ax
B. Starrett center finder
C. Haimer ZeroMaster

So here are the results
* * * * * * * * * * * * * * * * * * * * *X. * * * * * * *Y
Interapid DTI. * * * * * * * .000 * * * * * .000
Blake. * * * * * * * * * * * * *.0006 * * * * .0152
Starrett. * * * * * * * * * * * *.000 * * * * *.0116
Haimer. * * * * * * * * * * * * .000 * * * * .0098

Help!!! I'm off somewhere between 10 and 15 thou. My assumption is that the DTI is a correct center. *I did compare the DRO with the dials and both agree perfectly, so that would ( I think) remove any DRO issues from the equation.*

 

[ncmachinist62]

I don't know the mill you are using but it sounds like your heads not square. The different readings you get are based on the difference in quill length when you indicate the bore. Your center spots will be off also due to the length of spot drill. Thats my hunch. Use the interapid to zero everything, you can't go wrong. Tony

 

[Sshire]

Tony
PROBLEM SOLVED.
THANK YOU!!!!
The front to back tram was the problem. I put the ProTram on the Bridgeport and the nod (front to back) tram was out by 12 thou over the 6" width of the ProTram. Got it back to zero now everything (DTI, Blake, Starrett and Haimer ((sounds like a personal injury law firm))) agrees to under a thou.
Thanks again for fixing this. After I thought about the geometry, it made perfect sense - both the head tram and the length of the measuring instrument. The Haimer's short probe is shorter than the Starrett which is shorter than the Blake's probe. The numbers bear this out. In any case, all is well. Thank you again.