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Rough Boring vs. Finish Boring

Jack Kerlin Applications Engineer BIG Kaiser—Americas
By Jack Kerlin Applications Engineer, BIG Kaiser—Americas

There’s more than one way to finish a hole. The most effective option will depend on the number of parts, cycle time and tolerances. One of the most effective options is boring.

Boring is basically internal turning. The main goal is to enlarge a drilled hole to final diameter, which usually has a precise tolerance. A capable boring tool will also clean up after the drill and produce a fine surface finish. Finish boring can be a delicate operation. After all, it takes only one oversized hole to scrap an entire part. The drilled hole often needs to be further prepped to improve the odds of success of the final cut-to-size. This is where a “rough” boring tool becomes a necessity.

The question is, when is it appropriate to use a rough boring tool, a finish boring tool, or both? BIG Kaiser rough boring tools are guaranteed to be accurate within ±0.004" (0.1 mm), so at least with our tooling, anything tighter than that requires a finish boring head. That part is simple enough. If boring has already entered the conversation, you can usually assume that the tolerance will be much tighter than this anyway. So, it is rare that only a rough boring tool will be used.

The next question concerns material stock. The stock allowance of a finish boring tool will depend on the nose radius of the insert, but for conventional applications it should never exceed 0.04" (1 mm) in diameter. In other words, unless you take multiple finish passes, any amount of stock greater than this amount will require the use of a rough boring tool. Regarding the initial hole, this question also depends on whether the part is cast, the length-to-diameter ratio of the hole, and the required surface finish.

Casting is a dominant process in many industries and will continue to be so, despite advances in 3D printing. However, one thing casting is not known for is tight geometric tolerances.

Fresh out of the mold, a hole diameter can potentially be a few 0.01" (0.25 mm) away from a nominal value, which is why they’re typically cast well undersize. But equally as concerning (when it comes to boring) is the hole will almost always be slightly curved or ovular. A rough boring tool can correct these issues before finish boring.

When Walking is a Problem

Another cause of crooked holes is when a drill “walks” off center while creating the initial hole, which mostly happens if the drill is slender, fed too hard or has a damaged tip. While the drilled hole might appear straight to the naked eye, measuring equipment often reveals the opposite.

A finish boring tool is sensitive to curved/out-of-round holes mainly because it’s one-insert effective; it will only experience radial force acting from one side. A finish boring tool is more likely to bend from radial cutting forces because it isn’t supported by an insert on both sides (as with a rough boring head) and takes off very little stock.

Feeding one of these tools into a crooked hole will encourage it to follow the path set before it. The longer the hole, the easier it is for a boring tool to bend when cutting (and also reaming). So, it’s good practice to be cautious and make sure a long hole is already “true” before a finish pass. In contrast to a finisher, a rough boring tool doesn’t care how straight the initial hole is; it will bore true regardless because most forces are axial.

Though it probably goes without saying, surface finish is critical. Typically, a rough boring pass surface finish isn’t terrible, but some applications will specify a finish that isn’t practically attainable by a rough boring head. Obviously, a rough boring head isn’t as precise as a fine boring head, so this will show up in the surface finish, among other places. Surface finish only depends on feed and nose radius (by definition), but if it’s specifically called out in a bore, this is a good sign you’ll want to use a single-insert finisher regardless of how you’re cutting.

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