American Hollow Boring
For finish-bored holes, the standard for accuracy is 0.001-in. run-out per foot, or better. Precision straightening may be employed before or after drilling a hole. Honing is used often to provide uniform size and finish.

Beyond the Rough, Finish-Bored Hole Straightness

Jan. 15, 2014
Minimize cost, maximize control Details and RFQs Must have OD Machined features to follow

How straight will the hole be? It’s a typical question in any deep-hole boring project. We took up one aspect of this subject in the first installment of this series — i.e., what properties are acceptable for rough bored holes. Here, the focus is on finish-bored holes, and how to identify applications that require greater precision and quality assurance.

While “very straight” and “it depends” are two reasonable answers to the opening question, the industry standard for rough-bored holes is 0.001-in. run-out per inch of depth.

For finish-bored holes, the standard for accuracy is 0.001-in. run-out per foot, or better.

Example: A 5-in. hole in a 12x144-in. long may have 0.012-in. run-out at the worst spot. A 12-in. hole in a part 15x200-in. may have 0.017-in. run-out at the worst spot. These bores may be specified to meet total indicator run-out (TIR) of 0.017-in. or better if necessary.

This kind of accuracy may not be required over the entire bore. In general, to maximize cost effectiveness, specify the roughest operations and widest tolerances. To maximize control of geometry for critical parts, specify tolerances sufficiently for all critical features. (The next installment of this series will take up the details of deep bores as specified.)

The machine shop responsible for finish boring may ask several questions to qualify your project. At this stage, a well-detailed drawing and complete written request for quote (RFQ) can save time. Is the hole through the entire length, blind, or stepped and semi- blind? Are special shoulder forms or bottom geometries desired?

Start With a Hole

The finish-boring process requires an existing hole. This hole may be produced from a solid bar to provide optimal stock for the given finish-bored hole. Some projects can be supplied from hollow stock, such as tubing or a pre-drilled and heat-treated bar, or forgings. What material grade, heat treatment, size, and finish are supplied? The answer starts with a good understanding of the project inputs.

Whatever the input material, critical parts get special preparation for the boring processes, for workholding and special machining after deep boring to correct and verify geometry. As such, there must be available stock on the outside diameter to allow prep and finishing.

In some cases, precision straightening may be employed before or after drilling to address problems of crooked stock or run-out in the bore. Honing is used often to provide uniform size and finish. Given the number of secondary operations required to support exacting requirements, finish-bored parts typically involve longer lead times than rough bored holes.

Some finish-boring projects involve more than one hole. Case study: 12 in. OD x 170-in. with an 8.25-in. hole, 97-in. deep flat bottom, with a 0.25-in. radius, and a 6.437-in. hole for the balance of the piece. All diameters concentric within 0.010 TIR and ±0.003-in. tolerance, all surfaces 125-micro-inch finish or better.  The material supplied is 4340 annealed and rough turned to 12.25 x 170.5 in.

For this case study, holes must be generated by finish-boring. With a careful approach, the two bores can be made concentric within spec even at such depths. Once the deep bores are generated, other conventional machined features — such as counterbores and threads — can be added as required to make the finished part. Before shipment, run-out is measured to verify concentricity. Not all shops have the equipment and techniques to measure run-out directly in deep bores.

The first installment of this series, “How Straight Will the Hole Be?”, is available here.

For further reading on this topic, American Hollow Boring recommends “Dimensioning and Tolerancing: ASME Y14.5M-1994 (Engineering Drawing and Related Documentation Practices,” American Society of Mechanical Engineers (ASME), 1995, ISBN-10: 0791822230.

Geoff Ginader, the president American Hollow Boring, an independent machine shop specializing in deep-hole drilling and large part machining, for industrial applications that range from heavy machinery to defense projects. He enjoys solving complex machining problems, and experts at AHB are available to review specific projects.

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