Solving Holemaking Issues: Boring or Reaming?

Reader Question

At what point (diameter, tolerance, surface finish, volume, operator experience, material…) should I switch from single-point boring to reaming?

Miller’s Advice

You are starting with the right mindset, but unfortunately, “it depends” is the best answer to that deep question. Each of these constraints comes with arguments for both reaming and boring, and every shop will weigh these constraints with varying degrees of importance. A small prototyping shop may lean toward the flexibility of a boring system, while a high-volume shop may want the “set and forget” benefits of a reamer.

Drilling and end milling are also viable methods for general hole making, but if you are making the decision between boring and reaming, the tolerances and finish requirements likely dictate a dedicated finishing tool.

Diameter ties into availability and the physical limitations of boring and reaming. For example, it is very difficult to find a reamer for an 8.000” bore. Equally challenging is finding a boring head for tiny holes — and a custom solution for either may be cost prohibitive. Physically speaking, you should also consider the planned RPM range for the tool size and surface speed. Boring heads may be difficult to balance for high RPM applications, and running them slower to compensate may not be ideal for tool life.

Tolerance discussions include obvious diameter considerations, but less obvious may be the geometric tolerances related to holes. Boring bars and reamers can produce the same diameters and roundness when the hole is properly roughed. If the hole is imperfectly prepped, boring bars tend to feed straighter and are a little more forgiving for squareness and position, while reamers tend to balance the flutes and follow the pre-hole.

Surface finish can be a tricky topic. Is there a specific call out on the print, is it a min/max requirement or is it for visual appeal? For min/max requirements (roughness between two values), boring bars will be more successful because their nearly infinite choices for insert shape, corner radius and feed rate provide many opportunities to dial in surface finish. Reamers tend to burnish the hole as well, and so are better for pure visual appeal.

Volume constraints prove interesting. For job shops with orders at 10 parts or less, boring bars are the easy answer, requiring less tool inventory and providing higher flexibility. As order volume increases, new variables like cycle time, backup tools, tool life and changeover time enter consideration. Reamers are better for cycle time with their additional flutes and faster feeds, but some shops may find it easier to keep inserts on standby. That said, changeover times will be longer for the boring bar, and you will need to confirm insert size after indexing or changing inserts.

Operator experience also plays a factor in this decision. How comfortable is the operator setting up the operation? A special-purpose guide pad reamer can be extremely difficult to set up compared to a boring bar, while a precision-ground solid carbide reamer is easier than both. Given enough time, any operator can get it right, but long setups eat into crucial spindle uptime. That said, at high volumes you will likely have a dedicated tool crib attendant who can specialize in this task, instead of an employee who handles all general setup duties. Be sure to play to the strengths of your people and operation.

The material you’re cutting impacts every aspect of the previous topics. It establishes the expected tool life, cost per part and need for backup tools. It can impact roughing conditions, surface finish and ability to hold size and geometric tolerances. It also dictates your potential RPM range. Generally speaking, reamers will get better tool life in properly roughed materials — the cutting is lighter and spread over more flutes. However, if the hole has cross-holes or other interruptions, the reamer may chip quickly. A boring insert will be much more forgiving in these instances, and you can easily change to a tougher grade or corner radius to help. Stringy materials may also require chip breakers to assist with potential rat’s nests — this simply isn’t an option for most reamers.

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Want Miller’s help solving your machining challenges? John Miller, owner of Way of the Mill CNC training and consulting, will use his machining expertise to answer a reader-submitted question in his column each month. Submit your question here!