To return to the initial proposition — How straight will the hole be? — this is a question that you must consider when you plan to produce a deep-hole machined part. The planning process should verify that boring can produce the hole as accurately as the application requires, without excess processing. Here, I will try to describe the range of properties of form that may be specified and how they apply to manufacture of machined deep holes especially in round bar applications. This may promote some communication among engineers, purchasing professionals, and manufacturing managers. Sound planning and communication will contribute significantly to a positive outcome in terms of parts that meet spec and thus work as intended.
First, consider the simpler case of rough-bored holes. Rough drilling operations generally allow a wide tolerance for drilling, with no finish requirement, and straightness considerations are minimal. Very deep holes should allow enough stock for clean up after a worst-case runout of 0.001 in. per inch. Therefore, a part that is 180 in. long should allow more that 0.180 in. in the bore for clean-up, or more depending on the subsequent operations.
If such stock is not available, options should be discussed with the manufacturer. Options may include drilling a smaller initial hole, or specifying finish boring with runout of less than 0.001 in. per foot. It is a very good practice to supply a desired hole size and size of the finished ID, as well as size of the OD and finish OD size, when requesting a quote.
When tolerances become tighter and size becomes more critical, it becomes more important to understand the general requirements of straightness or concentricity. To illustrate this, suppose a 4 in. hole has a 0.003 in. tolerance on size and is 10 feet long. Without specific definition on runout, the hole technically should be within 0.003 in. in runout. It may be a simple operation to hold a size of 0.003 in. round and true within 0.003 in. in 6 in. of length, but it’s quite another matter to do it in 120 in.
If requirements allow a larger runout, the feature may be manufactured more readily with a 0.003 in. size tolerance, and a special attribute of 0.012 in. total indicator runout tolerance.
The complete article can be accessed at American Machinist.
