NEW TOOL ANATOMY FREES MILLING CAPACITY

When your spindle load meter is pegged at 100 percent on a bottleneck milling job running on a low-power machine, would you take the risk of trying to speed things up? Would you even entertain the possibility?

In April 2008, Luke Perusse of BVM Corporation (Denver, CO), took precisely that risk, milling heavy steel forgings on a Hamai vertical CNC mill. And he came out a winner. Milling throughput doubled and tool life tripled while protecting the machine, thereby eliminating the need for a big-ticket capital investment. His secret: retool with a positive-rake tangential mill. Called the S-Max High Feed, this cutter from Ingersoll Cutting Tools (Rockford, IL) is specifically designed to hog off material faster under unfavorable conditions while reducing the power required to do it.

The workpieces are huge and heavy type 4340 forgings for oil rigs. Some of these parts measure 20 ft long and need 800 lb of stock removed, much of it involving interrupted cuts. One such early job, a barbell-shaped link that required skinning over its entire exterior, took 56 hours to complete at 100 percent spindle load and required edge replacement every hour and a half.

?We had to do something because management needed to free up machine time for other work and avoid buying another big-ticket machine,? says Mr. Perusse. ?Capacity was our main problem. Machining cost was secondary. Strategically, our ability to deliver a multi-million dollar oil rig component a week sooner benefits our business much more than saving a few bucks in machining.?

The planned debottlenecking was part of a plant-wide lean manufacturing effort in the 43-employee captive machine shop within the BVM construction-crane plant. The machine shop runs eight hours a day, five days a week. Their basic approach to lean manufacturing is part-by-part: improve processing of a problem part and apply what you learn plant-wide, as quickly as possible.

The big barbell-shaped link was an obvious target because of its 56-hour machining cycle. Even a small improvement stood to free up literally hours of machine time. Besides, rough milling is pretty ubiquitous at BVM. Almost every other big forging, and there are many of them in a typical construction crane, needed hours and hours of milling too.

Mr. Perusse and his team did their own Internet research of tooling alternatives and narrowed the field to the S-Max and two others. ?We knew of their involvement in heavy milling, which was exactly our problem,? he adds. Perusse also recalled past success working with local application engineer Steve Cross and ?Mac? Allsup of the Ingersoll tooling distributor MSC Industrial Supply, so he called Cross and Allsup in to run a trial along with machinist Nate Osterberg.

?We could hear the difference almost immediately,? says Osterberg. ?Even at moderately higher settings, the operation ran much quieter, with no hammering.? Aware of the power limitations on the Hamai, Cross ramped up the feed rate gradually until they were removing material at least twice as fast as before.

A closer look at the before-and-after stages of this heavy milling process shows that the original standard tool for this work was a 4 in zero-rake five-flute conventional radial face mill. Water-based coolant was used to keep the cutter cool and avoid heat transfer to the spindle. The machining parameters were 300 rpm, 30-40 ipm, 0.080 in DOC. Under those conditions, the spindle load meter pegged at 100 percent almost all the time.

That tool was replaced with a 4 in positive-rake nine-flute S-Max High Feed tangential cutter with helical double-positive, on edge inserts. The coolant remains unchanged, but the machining parameters are now 360 rpm, 80 ipm, with the same 0.080 in DOC. With this drop-in tool replacement, cycle time to remove the 800 lb of chips dropped from 56 hours to 30. As proof that the cutting action itself is gentler, the spindle load meter rarely reads above 45 percent and the edges now last more than twice as long. Earlier, the edges needed changing every 1.5 hours, now they last 4 hours or better.

All of this was achieved with a simple drop-in tool replacement. Retooling required no changes to the program or datum offsets, just ramping up the feed and speed settings. ?At these rates, we?ve reduced machining cost per part by $1000 to $1500 at zero cost to us because the old tools were almost at their replacement point anyway,? notes Perusse. ?More important is the fact we?ve freed up nearly thirty hours of machining time per part. That?s like getting almost another complete machine for free with a zero footprint in a pretty crowded plant.?

According to Steve Cross, the big gain in removal rate here stemmed from five key physical differences between the old and new tooling: tangential tool design, positive-rake presentation geometry, helical cutting action, and higher pitch.

?Tangential cutters are stronger than conventional radial cutters for two main reasons,? states Cross. ?First, the inserts are oriented so that the main cutting force vector is distributed over their strongest cross section. Second, tangential cutter bodies are stronger because less metal is cut away for the insert seat pockets. This is a main reason why the inserts last twice as long as before on the BVM jobs.?

?Positive-rake geometry induces more of a shearing than scraping action between the cutting edge and the work,? he continues. ?The higher the rake angle, the more you are shearing and the lower the cutting forces. In fact, cutting forces vary inversely with rake angle. With a helical cutting action, impact forces especially are reduced. On a microscopic level, the insert enters the work like scissors shearing paper, rather than slamming into it all at once as do straight-edge inserts.?

?The higher pitch using nine flutes rather than five reduces the chip load per tooth, other things being equal,? explains Cross. He points out that the tangential cutter design allows for higher pitches because more inserts can fit around a given size pitch circle. ?Without the tangential design, you couldn?t squeeze nine inserts into a four in diameter cutter yet maintain the same amount of insert backing and chip gullets.?

Based on the success on the original link milling job, BVM standardized on the S-Max Hi feed cutter for all rough milling on their big parts. ?We?ve freed up about three new machines worth of milling hours with the retooling,? says Mr. Perusse, ?and we?re not done yet.?

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Ingersoll Cutting Tools, 845 S. Lyford Road, Rockford, IL 61108-2749, 815-387-6600, Fax: 815-387-6968, www.ingersoll-imc.com.