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The accuracy of a reamed hole depends heavily on the procedures leading up to

the actual reaming operation.

the purpose of a reamer is to expand a hole to a precise specified size.

In some cases the process of drilling and reaming a hole is accurate enough, but

this sometimes results in a loss of accuracy in true position location, size, and roundness of the hole.

If the drill “walks”, drifts or creates a hole that is not precisely round during the

drilling operation, a reamer is not rigid enough to correct these issues and it will mimic the hole error that the drill left whether it be slightly out of round or slightly off location.

The accepted way to avoid this problem has been to “skim bore” the drilled hole

with a boring bar. This means finding, and assembling a boring head and the appropriate boring bar, aligning the boring bar, taking a test cut and measuring hole, making the adjustment on the boring head to creep up on the size desired,

and hoping that it cuts consistently after that. This usually amounts to 30 minutes plus for most operators.

A solution to this problem is to eliminate the “skim bore” operation by utilizing a

carbide endmill that is slightly undersize from the reamer size. plunge-milling a  hole after the drilling operation with a Pre-Ream Endmill, qualifies the hole leaving it perfectly round with an accurate location, correcting any error occurring from the drilling process.

This also leaves a consistent and proper amount of stock in the hole for reaming

(most often .003 per side, .006 total diameter) allowing the reamer to cut efficiently and accurately. The reamer is never experiencing a heavy cut or

an inconsistent side wall that a drill can sometimes produce.

This process also greatly extends the life of the reamer.


In the past, some machinists used this method with endmills that came back

from regrinding. However, regrinds do not always come back concentric and

it takes time to sift through regrinds to find the proper size to pre-ream and hopefully the regrind has enough length of cut. Regrinds also do not offer the same tool life as a new endmill.

Lakeshore Carbide Pre-Ream Endmills eliminate these issues and greatly improve efficiency in reaming operations.
For best results we recommend the drilled hole be .012 min.- .032 max. under the size of the Pre-Ream Endmill.