BY MATT TEGELMAN
By the time a part is ready for holes, you have invested significant labour, material, spindle time, and engineering energy.
You may also be running short on time and
stock to make adjustments or recover from
mishaps. That’s why the importance of
meticulously prepping, starting, and finishing holes can’t be emphasized enough.
Another consideration at this critical stage
of production is that bores in a part often
require another part to mate perfectly with
them or travel cleanly through the hole.
These requirements demand that sizing,
straightness, concentricity, and finish are
all to specification.
Manufacturers often get stuck somewhere
in the boring phase of a job. In addition to
their cutting challenges, they have real con-
cerns about profitability, on-time delivery,
customer relationships, and how problems
from a production stalemate may trickle
down to other scheduled jobs and resources.
Questions about fine boring typically fall
into four categories:
1. Inconsistent hole size
2. Poor hole roundness
3. Tapered bores
4. Unsatisfactory surface finish
Let’s discuss each issue, its unique chal-
lenges, and a few tips and tricks for produc-
ing a fine bore.
INCONSISTENT HOLE SIZE
When a fine-boring job is set up, the plan
typically involves leaving a certain amount
of stock, say 0.020 inch, on the diameter for
passes with the finishing tool. Regardless of
whether you measure and set the tooling for
this cut in the spindle or on a presetter, the
forces exerted on the tool while cutting will
cause a certain amount of deflection, and the
size you set offline is not going to be what
you get in-cut.
A frequent response is to use a preparation
cut as a test cut, setting the tool undersize
by 0.003 in. or 0.004 in., measuring the bore,
and adjusting for the final cut. But this
essentially leads into a spring pass. You are
not going to have the same cutting forces
from the first to the second cut.
It is recommended that you keep the stock
allowance consistent from measuring cut to
final cut. If you are leaving 0.020 in., remove
0.010 in. with the first pass, measure, adjust,
and then finish. You will achieve much more
Using a toolholder with a worn taper can contribute to repeatability problems.
Tips and tricks for improving fine-boring operations