Courtesy of Kennametal Facing being performed on a tubular workpiece. Comparison between calculating cutting time when facing at a constant cutting speed vs. a constant rpm.
Facing is a turning operation in which the workpiece is machined to its center. It involves moving the cutting tool perpendicular to the workpiece’s axis of rotation.
This operation can be performed at a constant surface speed (sfm, m/min.), which is recommended, or at a constant rotational speed (rpm), which is not recommended. Facing at a constant rpm decreases the cutting speed, because the diameter of the facing surface gradually decreases by the amount of a feed per revolution. The closer a cutting tool is to the center of a workpiece, the lower the cutting speed. It approaches zero at the center of a workpiece. A low cutting speed can cause builtup edge.
Therefore, end users have a dilemma: facing at a constant speed or a constant rpm. One way to calculate cutting time for facing is to use Web sites such as www.calculatoredge.com, which offers online calculators for engineers at no charge. Among the numerous categories, the lathe operations calculator provides a variety of calculations, including cutting time for facing. It calculates time for facing as a function of the constant rpm, workpiece diameter and feed per revolution. A representation of this calculator and an example of calculating time for facing at selected cutting parameters is shown in Figure 1.
Courtesy of Calculator Edge Figure 1. The lathe operations calculator page for cutting time for facing from www.calculatoredge.com. The following are the author’s comments regarding this calculator:
• The formula for calculating time for facing contains a value “Feed/Rev.,” which is confusing because it can be interpreted as feed divided by revolution. These two words should be replaced by fpr (feed per revolution), which is the most commonly used value.
• Revolution of the job/minute is an awkward definition of the machine tool’s spindle speed, which is defined in rpm.
• Required time for cut is given in five decimal points, such as 1.90476 minutes in this article’s example. Such precise “astronomical” accuracy is unjustified. Two (maybe three) decimal places are more than enough for practical accuracy.
In many of today’s CNC machines, most operations, including facing, are programmed using constant cutting speed, which is specified in sfm in inch mode or m/min. in metric mode. At constant cutting speed, a CNC lathe automatically applies the correct rpm based on the following formulas for inch and metric systems:
rpm = 12 × sfm ÷ (π × cutting diameter in inches),
rpm = 1,000 × m/min. ÷ (π × cutting diameter in millimeters).
Constant Surface Speed Pros
Mike Lynch, president of CNC Concepts Inc., Cary, Ill., described four advantages of constant surface speed in his article “The Pros and Cons of Constant Surface Speed” (Modern Machine Shop, January 2009, pp. 62, 64).
1. It simplifies programming. Speed can be directly specified in sfm or m/min., the units used by most cutting tool manufacturers for recommending speed. This eliminates the need to calculate rpm.
2. It provides consistent workpiece finish. When used in conjunction with feed per revolution (feed rate mode), finish will be consistent regardless of the diameters machined.
3. It optimizes tool life. Tools will always machine at the appropriate speed.
4. It optimizes machining time. Cutting conditions will always be properly set, which translates to minimal machining time.
This article’s author developed a formula and designed a calculator for required cutting time when facing at constant cutting speed. The calculator is developed in Microsoft Excel and available in inch and metric units. Table 1 shows its computational worksheet (inch units) before the required values for computation of the cutting time are entered. Table 2 shows the calculator after process parameter values have been entered and cutting time calculated.
Table 1. Required cutting time for facing – calculator
Enter your values in the darker shaded cells

Symbol

Value

Unit

Workpiece diameter

D


in.

Cutting speed (constant during the cut)

V_{c}


sfm

Feed rate (constant during the cut)

f


ipr

Machine tool’s maximum spindle speed

n_{max}


rpm

Calculated machining data

Symbol

Value

Unit

Spindle speed at the start of facing

n_{o}


rpm

Workpiece radius at the start of facing

R_{o}


in.

Workpiece radius at which the machine tool’s spindle is running at its maximum speed

R_{min}


in.

Required time for facing*

t


min.

* Statement “FALSE” will appear if n_{o} > n_{max}.
Calculated data will appear after all required values are entered.
Table 2. Required cutting time for facing – calculator
Enter your values in the darker shaded cells

Symbol

Value

Unit

Workpiece diameter

D

4.00

in.

Cutting speed (constant during the cut)

V_{c}

550

sfm

Feed rate (constant during the cut)

f

0.002

ipr

Machine tool’s maximum spindle speed

n_{max}

3,000

rpm

Calculated machining data

Symbol

Value

Unit

Spindle speed at the start of facing

n_{o}

525

rpm

Workpiece radius at the start of facing

R_{o}

2.00

in.

Workpiece radius at which the machine tool’s spindle is running at its maximum speed

R_{min}

0.350

in.

Required time for facing*

t

0.98

min.

* Statement “FALSE” will appear if n_{o} > n_{max}.
Schematic of a facing operation, where n is the spindle speed, R_{o} is the workpiece radius at the start of facing, R_{min} is the workpiece radius at which the machine tool’s spindle is running at its maximum speed (n_{max}), V_{c} is the cutting speed, and f is the feed rate. Courtesy of Kennametal A common facing application is finishing aluminum alloy wheels. The author also developed a similar calculator based on constant rpm. A sample calculation using the same workpiece diameter, feed rate and rpm instead of the cutting speed is shown in Table 3. Comparison between the two methods of facing shows that machining at a constant sfm (0.98 minutes) requires significantly less time than machining at a constant rpm (1.90 minutes).
Table 3. Required cutting time for facing – calculator (at constant rpm)
Enter your values in the darker shaded cells

Symbol

Value

Unit

Workpiece diameter

D

4.00

in.

Spindle speed (constant during the cut)

n

525

rpm

Feed rate (constant during the cut)

f

0.002

ipr

Calculated machining data

Symbol

Value

Unit

Cutting speed at the start of facing

V_{c}

550

sfm

Length of cut

½ D

2.00

in.

Required time for facing

t

1.90

min.

Terry Ashley, training manager for Walter USA LLC, Waukesha, Wis., provided the following technical information on facing aluminum alloy wheels:
• 15 " and 17 "dia. aluminum alloy wheels are the most common for automotive applications, although there are truck wheels that are much bigger, starting at 19 ".
• Recommended cutting speeds are from 1,000 to 1,300 sfm.
• Common feeds for “beauty face” are from 0.004 to 0.006 ipr.
• PCDtipped groovingtype inserts are applied.
• A 45 to 55hp machine tool’s maximum spindle speed is 3,000 rpm.
• Okuma makes the most common machine for this operation.
• Special workholding is required.
Based on this information, the author calculated the required time for facing using his calculator (Table 4.)
Table 4. Required cutting time for facing – calculator
Enter your values in the darker shaded cells

Symbol

Value

Unit

Diameter of the aluminum alloy wheel

D

17.0

in.

Cutting speed (constant during the cut)

V_{c}

1,300

sfm

Feed rate (constant during the cut)

f

0.004

ipr

Machine tool’s maximum spindle speed

n_{max}

3,000

rpm

Calculated machining data

Symbol

Value

Unit

Spindle speed at the start of facing

n_{o}

292

rpm

Workpiece radius at the start of facing

R_{o}

8.5

in.

Workpiece radius at which the machine tool’s spindle is running at its maximum speed

R_{min}

0.828

in.

Required time for facing

t

3.67

min.

If the facing of the same wheels were performed at a constant rpm (equivalent to the same cutting speed) and the same feed rate, the cutting time would be significantly longer—about twice as long (Table 5). CTE
Table 5. Required cutting time for facing – calculator (at constant rpm)
Enter your values in the darker shaded cells

Symbol

Value

Unit

Workpiece diameter

D

17.0

in.

Spindle speed (constant during the cut)

n

292

rpm

Feed rate (constant during the cut)

f

0.004

ipr

Calculated machining data

Symbol

Value

Unit

Cutting speed at the start of facing

V_{c}

1,300

sfm

Length of cut

½ D

8.5

in.

Required time for facing

t

7.28

min.

About the Author: Edmund Isakov, Ph.D., is a consultant, writer and frequent contributor to CTE. He is the author of the books “Mechanical Properties of Work Materials” (Modern Machine Shop Publications, 2000), “Engineering Formulas for Metalcutting” (Industrial Press, 2004), “Cutting Data for Turning of Steel” (Industrial Press, 2009) and the software “Advanced Metalcutting Calculators” (Industrial Press, 2005). He can be emailed at edmundisakov@bellsouth.net or reached at (561) 3694063. 