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Finding the Centre of a Round Hole

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PeterB06/11/2009 22:34:35
14 forum posts
I have a part-machined V twin IC Engine crankcase on my mill bed.  The main-bearing housing has been finely finished to size during a previous operation on a different mill. 
 
I need to find the exact centre of this main-bearing hole so that I can bore other holes in the case with direct reference to the main-bearing centre position.  I have an X/Y DRO on the mill and I have a DTI.  How do I position the vertical head centrally and accurately over the hole centre please?   
 
PeterB
Billy Mills06/11/2009 23:22:34
377 forum posts
Hi Peter,  Mount the DTI in the spindle above the rough centred hole with the dial facing you. Bring the DTI into contact with the hole wall closest to you and zero the DTI. Rotate the spindle 180 degrees then read the error. Move the table front or back to halve the error.
 Rotate the spindle 90 degrees, zero the DTI and then rotate 180 degrees, read the error then move the table side to side to half error. 
 
Repeat both axis trials a few times so that the opposite side readings are the same and you should be all centred up.
 
Alan
Frank Dolman06/11/2009 23:41:13
106 forum posts

    I am not yet risen to the dizzy heights of Rank Beginner, so I am a bit diffident
 about joining in this discussion.  Nevertheless someone should point out that
  we have done nothing to see that the axis of the housing is accurately vertical.
   Someone who knows how, please say!
mgj07/11/2009 00:23:17
1017 forum posts
14 photos
Thats certainly an unusual way of finding centre, and makes no use of the fact that the mid point of any chord is at 90deg to a diameter. ie at the 1/2 chord point the centre lies due north as it were.
 
 
First set your alignment for the block - whatever you need to be at right angles to. Come down to the hole. Find an edge by any appropriate and accurate means. Traverse across and find the other edge. Use the DRO halving facility and wind back to zero to put you on the 1/2 chord point.. You are now on a diameter which passes through centre. 
 
Repeat the operation on the other axis which will be true diameter. The diameter is only the longest chord. Centre is of course1/2D. Set zero and ABS (so you can always find it again!).
 
Switch to INC, work out the co-ordinates for all the other holes. Feed them in to the memory. I imagine your DRO has that function. Munch metal. Your true vertical will be in automatically. 
 
If you have already machined in a face that has to be a datum, like say the top face of the cylinders, then you will have to set that parallel to either X or Y first by DTI or whatever means you regard as accurate enough before you start finding centre.
 
 

Edited By meyrick griffith-jones on 07/11/2009 00:23:53

Edited By meyrick griffith-jones on 07/11/2009 00:24:26

Chris Kite07/11/2009 00:42:20
2 forum posts
Hi Frank,  I should imagine that Peter has some areas on the engine that are machined parallel or at 90 degrees to the main bearing hole, other wise a simple method, similar to the one posted by Alan, is to have the DTI touching just below the top edge of the bearing hole, zero the DTI then move quill of the mill down so that the DTI is just above the bottom edge of the bearing hole, and adjust accordingly. Rotate the spindle 90 degrees and repeate the previous operations. This will require repeating several times as when you move the housing on one axis, it will probably move in the other.
 
In an ideal world you would plan to machine every thing at the same time so that everything is in register, but it isn't an ideal world and for one reason or another it's not always possible.
 
Chris
Frank Dolman07/11/2009 02:18:53
106 forum posts

      We are already slightly at cross purposes here and I fear things will only get
  worse if we carry on!  One thing that does come out very clearly to me is the
  importance of planning machining sequence carefully and with powers of
  imagination fully engaged.
       Thanks to all for help.
PeterB07/11/2009 08:52:37
14 forum posts

Alan, Frank, Meyrick and Chris,

Thank you all for the most enlightning posts that have done much to clear the fog from my mind! 

I should have added in my first post that there are indeed 2 parallel reference faces and one at right-angles - all of which were machined at the same setting along with the mainbearing housing.  But not on my mill - no, its not always an ideal world! 
 
In the meantime, I have done much to ensure that my vertical head is truly vertical to the bed; and then locked it there.  I used a parallel ground block on the bed and set the DTI on the quill to inscribe an approx 4 inch diameter circle on the block, adjusting for zero run-out - or as close as I could get it - about 3/4 of a thou.  (Or is there a better way of ensuring "verticality"?).   
 
Anyway, the good news is, the mainbearing housing is, as far as I can ascertain, exactly vertical - as read by the DTI in my chuck.  So, thanks to the most helpful replies on this forum, I am now able to proceed with some confidence.  Thank you so much guys. 
 
Peter

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