Counter boring

[ttabbal]

So this has happened a couple of times now and I feel like I have to be missing something. Drilled and reamed a hole. Didn't have a counter bore the right size, so I used a boring head. Seems simple enough. I ended up offset by maybe 20 thou.

X and Y were locked and the DRO showed no movement. I did move the table up. That's never caused a problem before, but I should double check that. Perhaps hold something round in the vise and use an indicator to center it, move the table, and indicate again?

Drill chuck is an Albrecht with a good brand arbor. Boring head is a Criterion DBL-202 with a straight arbor in a R8 collet. Mill is a Bridgeport.

At the moment, I'm leaning towards me doing something dumb, but might as well test that I don't have a crooked arbor or something. Never know. I'd appreciate thoughts on things I can check to see if something needs to be fixed, or ideas for things I might be doing wrong.

 

[tq60]

Did the drill stay on point?

Or did it wander?

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[compact8]

deleted ....

 

[compact8]

20 thou is 0.5 mm which is wayyyyyy too much to be explained by drill wandering or bent arbor. Something must be loose causing the spindle to shift relative to the table. I will put a DTI on the spindle with the probe touching the vice and try to push the head and table in all directions to see if the needle moves and by how much. Yes, centering the DTI around a hole and checking again after moving the table up / down is also something I will do. I have done it on my bench mill and found that the spindle and the vertical column are tilted in the way depicted below but the magnitude of XY shift due to change in Z is far less than 20 thou so the reasons in your case must be something else.

 

[Martin W]

Could be a loose gib on the Z?
Martin

 

[jbobb1]

As you mentioned, I would check tram.

 

[RJSakowski]

If your mill has a movable head and quill, check the parallelness of the spindle to quill and to the z axis ways before adjusting tram. Adjusting tram just makes the spindle centerline perpendicular to the table. As @compact8 stated, there can be a situation where two errors cancel out for perfect tram. A modification of the RDM method can check for parallelness of the spindle to the quill, the quill to the z axis ways, and the spindle axis to the z axis ways.

 

[ttabbal]

Thanks, those are good points. I'll do some testing and verify the things mentioned.

I'll have to read RDM again, I usually use 2-collar on the lathe.

Z gib... I haven't checked that in a while. Thanks for the reminder! I need to be more diligent on gibs in general.

I did spot drill, and use a couple of steps to drill before reaming. The bit looked like it was running well. I can't be 100% sure it didn't wander though. After running a test hole, I should be able to indicate it true to the spindle, right?

 

[jaek]

I don't think there's any way a crooked arbor can cause a boring head to make an offset hole.

20 thou runout from a crooked drill, bent arbor, etc should be obvious to the eye.

Head out of tram (spindle axis not parallel to Z axis) matches the description. Chuck up a drill, center it on a spot on the table. Lower the knee, extend the quill, see if the tip of the drill still points at the spot. Or use an indicator on something round in the vise like you suggested.

 

[compact8]

After running a test hole, I should be able to indicate it true to the spindle, right?

It depends. On my hobby-grade bench mill ( Emco FB2 ), when the quill is lowered by operating the quill feed handle, the quill will tilt and shift causing the tool tip to move towards the operator by 4~5 thou. The magnitude of the error is quite consistent so it can be corrected by adding the same amount to the target Y value of the hole before drilling. If the hole is drilled by lowering the mill head with the quill locked. The error will be a lot less ( about half a thou ).

If you keep the quill feed handle at the same position by some means without locking it ( I used a hook made from an old clothing hanger ) , yes, you should be able to see the hole indicated true to the spindle.