COM­MON INSERT FAIL­URES

DEMM Engineering & Manufacturing - - FRONT PAGE - By Don Gra­ham.

Insert fail­ure and its neg­a­tive im­pact on man­u­fac­tur­ing equip­ment is sim­i­lar to an ath­lete ex­haust­ing a good pair of run­ning shoes. Much like a shoe un­der the weight of the run­ner wear­ing it, an insert en­dures tremen­dous stress over and over again, cre­at­ing wear and tear. If not ad­dressed, wear can cause pain for an ath­lete and in­ac­cu­rate pro­cesses or poor pro­duc­tiv­ity for a man­u­fac­turer.

Man­u­fac­tur­ers, how­ever, can an­a­lyse used tool­ing to achieve max­i­mum tool life and pre­dict tool us­age, thereby main­tain­ing part ac­cu­ra­cies and re­duc­ing equip­ment de­te­ri­o­ra­tion. Early insert ex­am­i­na­tion is im­por­tant in de­ter­min­ing the root cause of its fail­ure as is care­ful ob­ser­va­tion and reporting. By not tak­ing these im­por­tant steps, it's pos­si­ble to be­come con­fused be­tween the dif­fer­ent types of fail­ure modes.

To as­sist in the insert ex­am­i­na­tion process, a stere­o­scope, with good op­tics, good light­ing and a mag­ni­fi­ca­tion of at least 20X, can pay great div­i­dends in iden­ti­fy­ing these eight com­mon fail­ure modes that con­trib­ute to pre­ma­ture insert wear.

Flank wear

An insert will fail due to nor­mal wear in any type of ma­te­rial. Nor­mal flank wear is the most de­sired wear mech­a­nism be­cause it is the most pre­dictable form of tool fail­ure. Flank wear oc­curs uni­formly and hap­pens over time as the work ma­te­rial wears the cut­ting edge, sim­i­lar to the dulling of a knife blade.

Nor­mal flank wear be­gins when hard mi­cro­scopic in­clu­sions or work-hard­ened ma­te­rial in the work­piece cut into the insert. Causes of such wear in­clude abra­sion at low cut­ting speeds and chemical re­ac­tions at high cut­ting speeds.

In iden­ti­fy­ing nor­mal flank wear, a rel­a­tively uni­form wear scar will form along the insert's cut­ting edge. Oc­ca­sion­ally, metal from the work­piece smears over the cut­ting edge and ex­ag­ger­ates the ap­par­ent size of the wear scar on the insert.

To help slow down nor­mal flank wear, it's im­por­tant to em­ploy the hard­est insert grade that does not chip, as well as use the freest cut­ting edge to re­duce cut­ting forces and fric­tion.

Rapid flank wear, on the other hand, is not de­sir­able, as it re­duces tool life and the nor­mally de­sired 15 min­utes of time in cut will not be achieved. Rapid wear of­ten oc­curs when cut­ting abra­sive ma­te­ri­als such as duc­tile irons, sil­i­con-alu­minium al­loys, high temp al­loys, heat-treated PH stain­less steels, beryl­lium cop­per al­loy and tung­sten car­bide al­loys, as well as non-metal­lic ma­te­ri­als such as fi­bre­glass, epoxy, re­in­forced plas­tics and ce­ramic.

The signs of rapid flank wear look the same as nor­mal wear. In cor­rect­ing for rapid flank wear, it be­comes key to se­lect a more wear re­sis­tant, harder or coated car­bide insert grade, as well as make sure coolant is be­ing ap­plied prop­erly. Re­duc­ing cut­ting is also very ef­fec­tive, but coun­ter­pro­duc­tive as it neg­a­tively af­fects cy­cle time.

Cra­ter­ing

Of­ten oc­cur­ring dur­ing the high speed ma­chin­ing of iron or ti­ta­nium-based al­loys, cra­ter­ing is a heat/chemical prob­lem where the insert es­sen­tially dis­solves into the work­piece chips.

A com­bi­na­tion of dif­fu­sion and abra­sive wear causes cra­ter­ing. In the pres­ence of iron or ti­ta­nium, the heat in the work­piece chip al­lows com­po­nents of the ce­mented car­bide to dis­solve and dif­fuse into the chip, cre­at­ing a 'crater' on the top of the insert. The crater will even­tu­ally grow large enough to cause the insert flank to chip, de­form or pos­si­bly re­sult in rapid flank wear.

Built-up edge

Built-up edge oc­curs when frag­ments of the work­piece are pres­sure-welded to the cut­ting edge, re­sult­ing from chemical affin­ity, high pres­sure and suf­fi­cient tem­per­a­ture in the cut­ting zone. Even­tu­ally, the built-up edge breaks off and some­times takes pieces of the insert with it, leading to chip­ping and rapid flank wear.

This fail­ure mech­a­nism com­monly oc­curs with gummy ma­te­ri­als, low speeds, high-tem­per­a­ture al­loys, stain­less steels and non­fer­rous ma­te­ri­als, and thread­ing and drilling op­er­a­tions. Builtup edge is iden­ti­fi­able through er­ratic changes in a part's size or fin­ish, as well as shiny ma­te­rial show­ing up on the top or the flank of the insert edge.

Built-up edge is con­trol­lable by

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