# Drill bit wander ??



## GYoung (May 12, 2013)

I'm ready to drill and ream a main bearing journal hole.  The hole will be through to different metals at the same time.  The engine frame is cast iron and the journal cap is brass. The part line splits the through hole bearing surfaces with the center directly on the part line.  My concern is that since the metals are dissimilar and have different strength properties the drill or reamer will wander into the softer metal and spoil the hole being drilled.  

Should I have a concern that this could happen or should I just bolt the brass bearing cap to the cast iron frame and drill the hole for the bearing shaft.  Not fearing there should be be any bit wander to spoil the concentricity of the journal hole.

If I should have concern what do I do to solve the problem before it happens??

Your help will be greatly appreciated.


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## enfieldbullet (May 12, 2013)

it will wander a bit yes.

drilling is never considered for high precision/accuracy in hole alignment or location.

you should drill, then bore the hole. reaming won't guarantee the alignment.

if you have access to a lathe or mill that should be easy to do, if all you got is a drill press it might be harder.


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## GYoung (May 15, 2013)

I step sized my way up to the necessary 1/2" bore size needed with many of the boring done with milling machine bits.  Worked just great.  Thanks a lot for the guidance now I have no fear with a problem like this that may come up again.


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## Mainer (May 16, 2013)

One way to help ensure alignment is to use a center-cutting end mill to enlarge a pre-drilled hole.


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## desellers (May 16, 2013)

Standard practice in tool & die shops is to use a center drill to start the  hole then drill about 1/32" undersize. Next use a square end endmill roughly 0.01" undersize and finish with a reamer. This method is used to locate dowel pins in dies.

Dennis Sellers
Model Dimensions Machine LLC


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## MachineTom (May 17, 2013)

There will be some wander in the hole but s long as you set up the work and machine correctly, its not  big deal.

Measure from whatever is the datum point, center drill, then drill a handy size hole smaller than needed, say .750 for a .875 hole, Now set up your boring head and bore to size. If you don't have real internal hole measuring tools, turn a piece of AL stock to final size minus .0005 of so, add a couple of .100 wide steps at ,872, .870. Now you can sneak up on final size using your pin that you know is right on size.


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