Boring Bar Deflection Calculator - Kennametal

Boring Bar Deflection, Cutting Forces, and Horsepower Requirement

These calculations are based upon theoretical values and are only intended for planning purposes. Actual results will vary. No responsibility from Kennametal is assumed.

Conversion of Workpiece Material Rockwell (Optional)

Conversion of Workpiece Material Rockwell (Optional)

Skip this step if you already know the Brinell hardness number(HB)
Boring Bar Dimensions

Boring Bar Dimensions

in mm
If bar does not have a coolant hole use 0
in mm
in mm
Boring Bar Material Properties

Boring Bar Material Properties

psi N/mm2


in mm
Workpiece "p" Values (hp/in.3/min) for machining conditions: "p" Values (kW/cm3/min) for machining conditions:
Type of material Designation Calculate Hardness Finishing Roughing General Finishing Roughing General
Carbon steel AISI 1018 141 0.70 0.66 0.62 0.03322 0.03094 0.02867
Carbon steel AISI 1045 195 0.74 0.70 0.72 0.03458 0.03140 0.03276
Alloy steel AISI 4140 194 0.79 0.74 0.73 0.03777 0.03322 0.03367
Alloy steel AISI 4340 214 0.76 0.72 0.73 0.03822 0.03322 0.03367
Alloy steel AISI 4140 258 0.85 0.77 0.79 0.04004 0.03504 0.03595
Alloy steel AISI 4142 277 0.84 0.77 0.75 0.04050 0.03504 0.03595
Alloy steel AISI 4340 485 1.31 1.00 1.05 0.05961 0.04687 0.04869
Tool steel AISI H11 205 0.78 0.73 0.76 0.03822 0.03367 0.03504
Stainless steel AISI 316L 147 0.81 0.73 0.73 0.03822 0.03185 0.03322
Stainless steel AISI 410 243 0.81 0.71 0.74 0.03868 0.03367 0.03413
Stainless steel AISI 17-4 PH 294 0.99 0.70 0.72 0.04551 0.03367 0.03458
Gray cast iron SAE G3000 195 0.53 0.48 0.47 0.02730 0.02184 0.02184
Ductile cast iron ASTM 65-45-12 165 0.58 0.55 0.51 0.03003 0.02366 0.02366
Titanium alloy AMS Ti-6Al-4V 287 0.64 0.62 0.62 0.03094 0.02776 0.02867
Nickel alloy Inconel 718 277 1.20 1.01 1.02 1.05961 0.04596 0.04733
Aluminum alloys AMS 2024 139 0.31 0.29 0.30 0.01547 0.01320 0.01365
hp/ln.3/min. kW/cm3/min
Machining Conditions

Machining Conditions

in mm
ipr mm/rev
sfm mm/min
Machine Tool Efficiency Factors
Direct Belt Drive .9
Back Gear Drive .75
Geared Head Drive .7-.8
Oil - Hydraulic Drive .6-.9
Your Results

Your Results

in mm 4
in. cm 3/min
Dynamics of Boring

Dynamics of Boring

lb N
lb N
lb N
in mm
Horse Power Calculation

Horse Power Calculation

hp KW
hp KW
ft-lbs Nmm

Boring Bar Deflection:

Calculating boring bar deflection, provides a means for analyzing a boring bar’s performance characteristics, prior to machining a part. Deflection can affect the surface finish or in extreme circumstances the part’s dimensional tolerances. To minimize deflection select a boring bar that offers the most rigidity with the shortest overhang.           

Boring Bar Deflection (y) Formula:

y = FL33EI

F = Cutting force [lbf or N]

Cutting force = unit of power constant x d x f x Kp x C

unit of power constant = 396,000 (Imperial) or 60,000 (Metric) 

d = DOC [inch or mm]

f = feed rate [inch per minute or mm per minute]

Kp = power constant [hp/inch3/minute or kW/mm3/minute] 

C = feed rate factor for the power constant adjustment    

L = Unsupported length (overhang) of the boring bar [inch or mm]

E = Modulus of elasticity (in tension) for the boring bar material [psi or N/mm²]

Steel Modulus of elasticity = 30,000,000 psi

Steel Modulus of elasticity = 206,843 N/mm2

Heavy Metal (Tungsten alloy) modulus of elasticity = 48,000,000 psi

Heavy Metal (Tungsten alloy) modulus of elasticity = 330,948 N/mm2 

Carbide Modulus of elasticity = 80,000,000 psi

Carbide Modulus of elasticity = 551,581 N/mm2  

Note: The stated values are specific to Kennametal boring bars.

I = Moment of inertia for the boring bar cross-sectional area

Moment of Inertia for a solid cylindrical bar = π × DOD4 ÷ 64

Moment of Inertia for a hollow cylindrical bar = π × (DOD4 - DID4) ÷ 64

To use the boring bar deflection formula you must calculate the required cutting force (F), measure the boring bar’s overhang (L), select the correct modulus of elasticity for the boring bar (E), and calculate the moment of inertia for the boring bar’s cross-sectional area (I). Once those items are input into the formula the resulting outcome will be the amount of deflection (given as a distance). 

If the resulting deflection is unacceptable; recalculate using a boring bar with a greater modulus of elasticity or a boring bar with a larger moment of inertia. 

Deflection should be minimized but balanced against what is possible, based on the intended boring operation (depth and width). 

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