METHODS OF TOOL LIFE TESTS

TEAM :- ABDUR RAHMANAROMAL ASHOKANFEBIN BABU ABOOMOHAMMED RASHIDPRANAV PADAVILSHAMITH C.PYADHUKRISHNAN M.M

TOPIC :

TOOL LIFE TESTS• Crucial practical importance in machining

– To arrive at economical cutting conditions -> cutting speed, feed and depth of cut.

– To obtain optimum tool geometry.– To assess performance of tool material, cutting fluid, work

material and machine tool.

• Practicing engineers need to carry out Tool Life Tests for their particular requirements.

• For type of work material, tool and machines available

CLASSIFICATIONS

• Conventional tool life tests• Accelerated tool life tests • Based on variable cutting speed

CONVENTIONAL TOOL LIFE TESTS

• Performed under special conditions.• Tool life is high, of the order of 60 to 90 minutes.• Factors effecting tool life are :- speed, feed, and

depth of cut.• Tool life is measured on the basis of tentative

failure criterion.• For e.g. :-

– Width of flank wear– Maximum depth of crater

• Taylors tool life equation :-

•VTn = C• Tool life equation (generalized)–

•VTnfn1dn2= C• Tool life exponents n, n1, n2 are found by

plotting experimental data on log V – log T, log T – log f and log T – log d scales.

• Long and expensive test, involves considerable amount of material, labor and machining time.

• Recourse is taken to experimental design techniques such as factorial design, multiple regression analysis and response surface methodology to reduce cost and no. of observations.

Determination of tool life constants n, n1, n2

• Limitations :-– Large scatter of experimental data.– Identification and elimination of extreme values

and assessment of tool life variability.

Variability of tool life exponent n

ACCELERATED TOOL LIFE TESTS

• Quick and less costly tool life testing

A. Extrapolation on the basis of steady wear rate– Test carried out for Small period of time.Let VB - the failure Criterion V – wear rate in the steady wear rate phasethen,

Where V0 is width of wear land at the end of initial nonlinear wear

phase lasting for period τ0

• Test time is very small τ1 < T and V0 and τ0 are found out

• Rate of steady wear V is calculated as

V = (V1 – V0) (τ1 – τ0)

B. High speed tests– Tool life test carried out at much higher cutting

speeds, -> shortening tool life to few minutes.– Assumed that constants of tool life equation

determined by this, also apply to practical cutting speeds.

• Assumption must be verified as higher speeds cause higher tool chip interference temperatures -> change the character of wear from flank wear to crater wear.

• Thus the constants obtained do not apply to lower cutting speeds

• Spot checking done by extrapolation method.

VARIABLE SPEED TESTS

• Cnvntnal tests carried out at constant cutting conditions, including cutting speed.

• Many constant dia w/p required for a turning tool.

• Cost of w/p can b reduced by taking multiple machining passes on the same w/p.

• Based on the assumption that portion of tool life ηi , lost in time Ti for cutting speed Vi is given as,

from Taylors eqn ,

• Three types of variable speed tests :-A. Facing test,B. Multi –pass turning , and;C. Taper turning.

A. Facing Test• One or more facing passes taken on a work piece

with internal dia D1 and external dia D2

• w/p rotated at constant speed, N1

• For any dia D, cutting speed ,V = πDN1

• Proportion of tool life lost Δη in small interval of time Δτ is given as,

Where ΔD is change in dia in facing in time Δτ

• For m1 facing passes, total proportion of tool life lost is given by,

OR

• Similarly taking m2 passed on the same w/p at another speed N2, the fraction of tool life is η2.

• Substituting in the previous eqn, and taking the ratio, we get,

• Now from actual measurement of tool wear (say flank wear) values of η1 and η2 can be obtained and thus, the tool life exponent can be evaluated.

• Hence the constant C of Taylor’s tool life equation can be found out from the previous equation.

B. Multi pass Turning

• When solid w/p machined, dia decreases w/ every pass depending on the depth of cut.

• Speed of w/p rotation, constant - cutting speed, V vary in each pass.– Let Di - initial dia– Li - length of w/p– di - constant depth of cut– fi - feed rate

• Vi in the ith pass when w/p rotates at Ni rpm is given as,

Vi = π(Di-idi)Ni

• And time of cut Δτi in the ith pass is expressed as,

So we have τ1 = τ2 = τi

• In m1 passes, the proportion η1 of tool life lost is the sum total of proportions of life lost in each single pass, i.e.

• llly in m2 passes at N2 rpm, with initial dia D2, length L2, feed f2, and depth of cut d2 , the tool life lost η2 is,

• Taking ratio,

– For a special case where, D1 = D2, f1 = f2,d1 = d2, L1= L2 and m1 = m2

OR

value of C can be found by substituting value of n in previous equation.

C. Taper Turning• Solid w/p of uniform taper is machined let DA - dia of smaller side DB - dia of larger side

then dia at a distance x from smaller end is,

if depth of cut is d and rotational speed is N1 rpm,Cutting speed at x is,

and machining time τ =

Portion of Tool Life for infinitesimal length dx, dη1 =

Upon integrating,

llly for spindle speed N2, the portion of tool life η2 ,

Taking ratio,

And hence,

Which can be used to find the constant C of Taylor's tool life equation.