The straightness of a hole is a typical question in the course of any deep-hole drilling project. The industry standard guarantee for straightness is less than 0.001-in. run-out per inch given good material and equipment. But, common answers are “pretty straight,” or “it depends.”
The goal here is to inform readers about the properties that may be acceptable for rough bored holes and the point at which users may wish to consider finish bored parts for greater precision and quality assurance.
● A 2-in. hole in 6-in. bar x 20-in. long may have 0.020-in. run-out near the exit. (0.001-in.x20-in.)
● A 3-in. hole in 8-in. bar 100-in. long may have 0.100-in. run-out near the exit.
In practice run-out is usually well under this 0.001-in. per inch guideline.
Rough bored holes are generally produced in a single operation with little or no prep work. Rough holes may be blind (that is, not ‘through’ bored) entirely from one end, bored from either end, or step bored. Using 0.001-in. per inch as a guide, run-out can be minimized by boring from either end. In the 100-in. long example, boring from each end produces no more than 0.050-in. run-out— but note that mismatch could be as much as 0.100-in. in the middle of the piece.
Rough holes can be specified in cost-effective ways. Parts can be supplied peeled or rough turned with ends sawn or faced square. Parts can be provided with sufficient OD stock to correct the worst expected run-out, and if provided this stock can be used to turn the OD concentric to the ID. Cold straightening may be required if the bars supplied are crooked, so specify if pressing is allowed. Finally, critical areas may be identified on drawings or specs to avoid the additional cost of holding an entire bore to a close positional tolerance.
Rough bored holes may not come with a uniform surface finish. Although carbide indexable tooling offers high process reliability, rough bored holes still may exhibit marks from chatter rings, steps and other surface imperfections.
Checking for Conformity
Before shipment, rough bored holes are checked for conformity with the order. Generally, start and finish size, length, OD, wall variation, and results of traceability and visual examination are recorded. Other checks may be specified as appropriate such as use of a drift for clearance fit checks.
In many cases, a rough bored hole is not acceptable for finished parts. Specify finish bored holes if your requirements are for as close as 0.001-in. per foot of run-out. Also, anticipate having some, if not all, of the OD stock machined for bores finish-bored and finish-machined.
Remember, it is more demanding of expertise and equipment to get a tight tolerance and finish throughout a deep hole than it is to obtain similar results in a short hole. If straightness is not critical, but finish is critical, you may avoid finish boring and simply rough bore and hone for size and finish.
Use finish requirements on drawings or specs to call out required finishes that may be obtained directly from the finish bore, by honing, or machine polishing. 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.
Geoff Ginader is 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.
For further reading, 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.