How straight will the hole be?  This is a critical question to answer in most deep hole drilling boring and machining applications, answering it with assurance cuts across the critical connections between design, project management, and manufacturing.  In the third installment of this series we explored Deep Hole Geometry as Specified, with emphasis on the balance of cost-effectiveness versus feature control, considering the range of form attributes.  Here, our attention is focused on the bore, considering the range of variation and precision available as machined.

Given the exact specification of rough- or finish-bored holes as conveyed by drawings and other controlling documents, such as purchase orders, a manufacturing plan can be developed.  Machine shop operations available for generating or improving deep bore straightness in order of accuracy include:
•  Straightness from drilling and trepanning;
•  Straightness from mechanical straightening in a press;
•  Straightness from finish boring and pull boring;
•  Straightness and roundness from deep boring in a lathe;
•  Roundness and uniform finish from honing; and,
•  Concentricity from other machining in a lathe or milling machine.

Along with a machining plan, an inspection plan enables and verifies the meeting of all requirements. 

In addition to micrometers, calipers, roughness testers, indicators, and scales, deep bores may require special equipment such as bore gages, borescopes, ultrasonic equipment, and other special gages to verify size, finish, location, and runout over the complete inside and outside of the part.

An important goal for manufacturers is to produce parts without variation.  In reality, as with most machining operations, deep-hole bores may have variations within tolerance or beyond tolerance.  It is important to understand possible variations and plan to allow or prevent them as required.  Variations can be measured and described as follows:

In rough-bored holes with wide tolerances and no finish requirement, bores may be produced in a single operation and with little concern for rings, chatter, score marks, or other imperfections.  As we observed in Part 3 of this series, stock may be required to allow for significant runout.  

In finish-bored holes, better straightness, roundness, concentricity, and tight tolerances can be generated.   Recognize that as demands of tolerances, lengths, and complex geometry increase, so does the cost of machining and verifying the range of finished attributes.

In many cases, a balance can be struck with strategies to simply drill or trepan, hone, and machine true.  For example, consider a workpiece that is Solid H13 tool steel, 14 in. diameter by 80 in. or 2 m long. 

The desired part is produced with a through hole finishing at 5.118 in. or 130 mm, and OD of 13.78 in. or 350 mm.  Tolerances are ±0.005 in, and the desired runout is 0.040 in. max., or 1 mm.  This part can be drilled at a size just under tolerance, honed for final size, and the OD can be turned true to the bore to reduce runout from the drilling operation. 

In this example, the drill removes the stock with adequate straightness, perhaps as much as 0.060 in. wall variation at the exit.  The hone improves finish and roundness while achieving a uniform finish size.  The turning operation reduces bore runout in the worst areas.  Runout is independently verified with an indicator throughout the bore.  It is very unlikely that runout will exceed 0.010 in. in any spot, but it could be as much as 0.040 in.

Given tighter tolerances, such as runout requirement of 0.003 in. or better for the same H13 part, a new process must be used.