Aerospace Expertise
radii becomes critical in this respect because they take the guesswork out of shoulders. Today, standard boring tools can usually be supplied with radii as small as 0.1 mm, or up to 0.79 mm. But oſtentimes with aerospace parts, radii from 0.79 mm up to 3.18 mm, are designed to eliminate stress risers. Not every boring tool company can claim to have inserts and in- sert holders, as standard (or at least semistandard), to produce these profiles. And traditional 90° shoulders aren’t the only profile in an aerospace machinist’s repertoire. In the case of a larger bore lead- ing into a smaller bore, aerospace manufacturers are likely to eschew a 90° shoulder in favor of a 30–45° taper, improving the component’s strength. Not only does an opera- tor then have to machine that 30° lead angle, but he will want the radius, let’s say, on a 54-mm bore to have a 6.35-mm radius. To produce a chatter-free tapered hole—especially with
large-radius tapered holders—rigidity is extremely im- portant. And to achieve a large radius, each portion of the modular system must work in concert, with no float or play to diminish rigidity. Many companies do quite well with their modular tooling, maintaining good rigidity through several connection points. But the area in which all but the highest-end cutting tools tend to lose rigidity is within the boring head itself, which includes the clamping screw, the tool carrier, and a rigid insert holder to fit into the tool. Tis is the business end, where the rubber meets the road. To achieve a bigger radius, rigidity must be maintained right through to the cutting area. Many lower end modular tools have difficulty doing this
kind of work—they just aren’t strong and rigid enough. In fact, some of these companies will limit the radius size of inserts that can be put into their tools to 0.4 mm because the tool just isn’t rigid enough. BIG Kaiser makes a point to carry standard inserts for their Kaiser boring line up to 0.79 mm, or even larger in some cases. It can do so comfortably because Kaiser’s extremely rigid tool assemblies allow opera- tors to blend large radii, shoulders, and tapered shoulders without chatter.
Mirror Finish Aerospace bores are almost always held to tight tolerances, but surface finish—another function of tool rigidity—is an-
124 Aerospace & Defense Manufacturing 2014
other consideration. OEM’s want parts that are toolmark-free, and surface finishes that aren’t smeared—operators hope to see their reflections, oſten in material that is difficult to cut cleanly. In other industries and other boring situations, opera-
Standard insert holders that fit into Kaiser boring tools accommodate the 30 and 45° lead angles that are common to aerospace requirements.
tors can cut an undersized bore, and plan to correct it on the next bore—this isn’t feasible in aerospace. An operator has to make very tight, incremental changes to the tool. A tool has to be very rigid, and be able to cleanly cut a small amount of stock without any rubbing or smearing in or- der to hit surface finish require- ments. Tere are tools currently available on the market that can make these ultra- precise adjust- ments, but many don’t have the rigidity to back that precision
up. When this is the case, the tool bends instead of cuts, or it doesn’t have the right insert geometry and sharpness to cut to the necessary surface finish standard. Tis is a common occurrence among operators using
carbide without a specialty grind. BIG Kaiser does carry the general geometries that large carbide companies provide. But it also carries a variety of standard inserts that have specially ground, highly sharp cutting edges that permit making a rebore at stock allowances of less than 0.0254 mm, which require a larger radius. When there’s a shoulder that doesn’t require a larger radius is when smart operators turn to these types of specialty inserts.
Cost/Benefit Metrics Specialty carbide inserts are expensive, and aren’t cheap
to carry in inventory as standard. Without high positive sharp cutting treatment, an insert might be $7, but with the additional high positive sharp cutting, with coating, we have prices approaching the $30 per insert range. But when dealing with a part that’s more than $100,000, an operator is wise not to concern himself with an insert that costs $30. Scrap simply isn’t an option. A large landing gear company recently shared with BIG
Kaiser a special OD boring project for raw forgings that cost $60,000 each. Consider making an oversized bore on a $60,000 raw component. Te consequences for scrap are just huge—ramifications go to the top layers of corporate manage- ment to ascertain what happened. An operator can’t rework
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