IMBSIR 78-1423

ICST/IAT Automation Project

Experimental Investigation of

Drill-Bit Wear

John S. Hilten, Carol F. Vezzetti, J. Franklin Mayo-Wells, and Paul S. Lederer

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D C. 20234

February 1978

Prepared for Office of Developmental Automation and Control Technology,

Institute for Computer Sciences and Technology,

National Bureau of Standards

NBSIR 78-1423

ICST/IAT AUTOMATION PROJECT

EXPERIMENTAL INVESTIGATION OF

DRILL-BIT WEAR

John S. Hilten, Carol F. Vezzetti, J. Franklin Mayo-Wells, and Paul S. Lederer

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

February 1978

This is an interim report. The work described constitutes the first stage of a more complex effort, results and conclusions are not necessarily those that

would be included in a summary report.

Prepared for Office of Developmental Automation and Control Technology,

Institute for Computer Sciences and Technology,

National Bureau of Standards

Contribution of the National Bureau of Standards, not subject to copyright

U.S. DEPARTMENT OF COMMERCE, Juanita M. Krepa, Secretary

Dr. Sidney Harman. Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

Experimental Investigation of Drill-Bit Wear

J. S. Hilten, C. F. Vezzetti

J. F. Mayo-Wells, and P. S. Lederer

Electronic Technology Division

Institute for Applied Technology

Washington, D.C. 2023^

ABSTRACT

The development of an experimental instrumentation system for a small drill press is described. The

parameters measured are spindle speed, vertical spindle

displacement, vertical spindle load, drilling torque,

and drilling time. Several test fixtures were instrumented and used in drilling experiments. These experiments were conducted to examine the relationship between

variations in the measured parameters and drill performance, more specifically to drill wear. Experimental

data show a 10-percent increase in drilling time from

the first hole to the last for a single set of 500

holes drilled in cold-rolled steel at a nominally con- stant load, although the drilling time began to decrease

slightly after hole 300. Changes in drilling torque

were also detected during the test runs, and in similar

runs with a brass workpiece. It is suggested that with

respect to the anomalous results in steel under the un- lubricated, constant-force conditions employed, the cutting surfaces of the drill bit were in a sense being

renewed as microflakes of material departed to reveal

fresh, sharp unburnished sites.

Key words: cutting force; drill bit; drill press;

dynamic; force; load; machine tool; measurement; online; tool wear; torque; transducer.

1. INTRODUCTION

Metal removal is a mu 1 1

i -b i 1 1 ion dollar business in the United

States; nearly all manufacturing operations require metal removal at some stage of manufacture. Cutting tools wear as various machining operations

proceed and must be replaced or sharpened. The dulling or wearing of machine tool bits and cutters may often not take place in a predictable

fashion: one tool may last three or four times as long as a similar tool

in a similar application. In production, tools are generally replaced

after a given number of operations determined on the basis of experience

in performing the given operation or a similar operation; this empirical

approach usually has a conservative bias to avoid the risk of damaged

workpieces and catastrophically failed tools, and thus may result in tool
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rep 1 acement before it is necessary. The most efficient utilization of a tool would probably involve use to incipient failure or degradation of

surface finish. In addition, it is often desirable to perform a machini ng operation at as high a speed as possible, consistent with finish re- quirements and the need not to overheat the cutting edges of the tool.

These considerations suggest that it would be desirable to have an online means of monitoring the condition of the cutter or tool bit being

used. The availability of condition information as input for a digital

computer (microprocessor?) could then permit near-optimum machining con- ditions to be achieved.

The "Study on Automation" prepared by the NBS Visiting Committee in

1975 recommended an NBS-wide program on automation technology having as one major emphasis a concentration on developing the basis for the measurement of performance and calibration parameters for dynamic sensors

having fast response, and the development of standards for such sensors.

Late in FY 197&, the Office of Developmental Automation and Control

Technology (DACT) of the Institute for Computer Sciences and Technology,

and the Electronic Technology Division of the Institute for Applied Technology, began a cooperative task supported by the Director's Reserve Fund.

One task element was to instrument a small drill press as a test bed and

to conduct a variety of experiments with it in order to determine preliminary performance measures for this machine tool. As part of this task,

the suitability of selected sensors was to be evaluated, particularly in

regard to measurements of tool wear. The intent is that this preliminary

work lead to further experimental work, in turn resulting in the development of specific calibration and evaluation methods for selected sensors.

The use of the drill press as a readily available and at the same time

representative machine tool was based on DACT- i ndustry contacts and a tool-wear sensing survey [1].*

A second task element was to conduct a limited, English-language

literature survey to gather information on the types of sensors used and

measurements made in support of worldwide efforts in machine-tool automation. The resulting review and bibliography are published separately as NBS I R 78-1424 [2] .

2. EXPERIMENTAL INVESTIGATION OF DRILL-BIT WEAR

2.1 Instrumentation of Drill Press

2.1.1 Selection of Characteristics to be Measured - A number of drillpress characteristics that are considered to relate to tool wear are available to be measured. Parameters that have been measured include:

( 1 ) force of dr i 1 1 bit against workpiece,

(2) torque of drill bit with respect to workpiece,

(3) downward displacement of drill bit during cutting process,

Figures in brackets indicate literature references in section 5.
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(4) drill spindle speed,

(5) time required for a given drilling operation;

(6) time-averaged drill-press motor current,

(7) amplitude and frequency of vibration of the workpiece or drill

bit, and

(8) temperature of the drill bit at the cutting edges.

An a priori objective was to identify that parameter or combination

of parameters most sensitive to or indicative of tool wear or condition.

Selection of parameters for initial investigation was subject to considerations of measurement ease and cost. Since it was not clear on the basis

of the literature reviewed which characteristics would prove the most sen- sitive, it was decided to concentrate on the set of mechanical parameters:

force, torque, displacement, and drilling time, with spindle speed as an incidental measurement. Vibration frequency and amplitude were not measured per se because the variations in force between the drill bit and

the workpiece yield the same information. Non-mechanical measurements

were postponed as being more difficult of interpretation as well as ac- complishment; temperature in particular is regarded as an important parameter in considerations of wear mechanisms.

It is important to note that both static and dynamic measurements

provide useful information.

2.1.2 Description of Drill Press - The drill press used in the drilling

experiments i s a 3^-cm ( 1 5" i n) fToor model with a 1.3~cm (0.5-in) drillbit capacity. Spindle speed is selected by means of the conventional

belt and stepped-pul 1 ey arrangement to provide nominal no-load speeds of

725, 1300, 2500, and 4650 rpm.

2.1.3 Drill-Press Modifications - To provide a nominally constant feeding force, the drill press was modified by removing the hand lever which

raises and lowers the spindle and replacing it with a large aluminum disk,

or pulley, 46 cm in diameter and 1.3 cm thick. This pulley is mounted

rigidly to the pinion shaft in the same plane as the hand lever, and is

shown in figure 1 at F. A machined groove in the outer edge provides a guide for the drive rope. The rope transmits a constant force to the

pulley and hence a constant force is developed between the drill bit and

the workpiece. For different experiments, the force is adjusted by re- moving or adding weights to the pan attached to one end of the rope, as shown in the figure at D. The rope is fastened to the pulley groove at

a point that becomes the top of the pulley with the spindle raised ready

for drilling. The other end of the rope is led around the back side of

the pulley and attached to the shaft of a low-speed, geared motor. This

arrangement permits lowering of the spindle at a controlled rate until

the workpiece is engaged in order to prevent excessive force and torque

transients, which may be unrelated to the actual process of drilling.

The rate of lowering is adjusted to be greater than the rate of penetration of the workpiece by the bit.
-AThe modifications described provide a constant- 1 oad mode of operation. This mode was chosen for initial investigation as being simple to

effect and easy to analyze. Another mode frequently used in machining

is that of constant feed rate; it is intended that later investigations

be extended to include this mode. 2.1.

A Measured Parameters

2.1.

A.1 Spindle Speed - A 60-tooth steel gear is mounted on top of the

drill -spindle pul ley; a magnetic sensing coil mounted as shown in figure 2 generates a pulse each time a gear tooth is detected. Spindle

speed is read directly in rpm with the sensing pulses as input for an electronic counter set to a counting interval of 1 s.

2.1.

A. 2 Spindle Displacement in Vertical Plane - The sensing coil of a

1 in ear- variable-differential transformer ( LVDT) displacement transducer

with a range of + 2.5 cm and an output of + 1 0 V dc from its associated

s i gna 1 -cond i t ion i ng instrumentation is mounted concentric to the armature

by means of a bracket attached to the head casting. For convenience, and

to avoid the need for permanent modifications to drill-press components,

the transducer armature is attached to a bracket mounted rigidly to the

bottom of the vertical stop adjustment screw, as shown in figure 3« Thus,

as the drill spindle moves up and down, the armature moves with it.

2.1.

A. 3 Force and Torque - In the case of the displacement and spindlespeed measurements, it is possible to locate instrumentation where it

does not interfere with the drilling process. Fnvisioned instrumentation

for load (force) and torque, however, has to be positioned close to, or concentric with, the drill bit. While for later work this instrumentation will have to be designed and located so that its interference with

the drilling process is held to a minimum, the initial emphasis is on making measurements and not on the convenience or practicality of a test

f i xture

.

In the course of this investigation, two different test fixtures

incorporating force and torque sensors were built and used as described

in the following paragraphs.

In the first test-fixture embodiment, an LVDT load cell with a range of 0 to 30 kgf is mounted on a base plate; this base plate is in

turn mounted on the drill-press table so that the direction of motion of

the load-cell armature is aligned with the axis of the drill bit. The

exposed top of the armature is fitted with a machined cap that is slightly rounded on its top. A machined block has a shallow hole in its under

surface sized to fit loosely over the cap and carries the jaws from a small machinist's vise on its upper surface. The vise assembly is free

to move up and down and to rotate on the rounded cap as a bearing, but

is constrained from moving in a lateral direction by brackets which en- gage slots machined in the sides of the block. This arrangement is

clearly seen in the photograph of figure A. The vise assembly is fitted

with an arm that is spr i ng- 1 oaded to contact the armature of a second

load cell mounted so that its armature motion is parallel to the drillpress table. This second load cell is similar to the first, but with a
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measurement range of 0 to 9 kgf. In use, as the drill bit engages the

workpiece held in the vise assembly, load (force) is measured by the

load cell supporting the assembly, and torque is measured indirectly via

the arm bearing against the second load cell. The spr i ng- load i ng of the arm is intended to prevent chattering.

In a second design, the load cell is mounted between the two sides

of a length of steel U-channel and preloaded to ensure that the cell always sees a downward force. The preload of about 10 to 20 percent of the

full-scale range of the load cell is achieved by inserting shim stock between the top of the load-cell armature and the inside of the upper side

of the channel. The bottom side of the channel is bolted to the drillpress table and positioned so that the drill-bit axis and load-cell axis are aligned. A result of this arrangement, shown in the photograph of

figure 5, is that the effective range of the load cell is reduced.

A 1.3~cm diameter brass rod is threaded into the top surface of

the channel; the axis of this rod is also aligned with the axis of the

drill bit. A small vise equipped with a projecting arm is attached rigidly to the upper end of the brass rod. As with the first model, the

level arm is loaded to contact the armature of a second load cell,

mounted so that its armature motion is parallel to that of the drillpress table. The loading is accomplished by forcing the load cell

against the lever arm; the brass rod thus serves as a torsion bar. The

preload is usually set to about 10 to 20 percent of the load-cell range

and is adjusted by inserting shim stock between the load cell armature

and the arm. The two load cells used in model 2 are bonded strain-gage

instruments with a range of 0 to 45 kgf. As the drill bit engages a workpiece held in the vise, the force is transmitted through the brass rod

and through the cantilevered top side of the channel to be measured by

the load cell. The torque is measured indirectly, as before. A strain

gage is mounted on the brass rod at an angle of 45 deg to the rod axis

as an additional means of checking the torque calculated from the loadcell data

.

2. 1.4. 4 Drill Penetration Time - No specific instrumentation is required

for measuring the time required to drill through a given workpiece. This

time is determined by scale measurement of a signal trace from the load

cell measuring downward force, as displayed on the screen of a storage

oscilloscope. The times at which touch-down on the workpiece and breakthrough occur are readily determined from such a trace.

2.2 Test Procedure

2.2.1 Drilling Considerations - Starting with a new 1-cm diameter drill

b i t (high-speed type, as obtained from the NBS storeroom), a series of

500 successive holes was drilled in "1/4 hard brass" stock 1.3 cm thick.

The nominal constant force applied by the drill bit to the workpiece was adjusted to be 260 kgf. The nominal drill spindle speed was 725 rpm.

The nominal feed rate (intentionally not held constant) was 425 ym per

revolution. These values were selected with guidance from tables in Machinery's Handbook [3]. This first experiment was followed by a similar
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one in which 500 successive holes were drilled in 1 .3~cm thick type 1015"

1020 A I S I steel. The nominal constant force applied by the drill bit to

the workpiece in these tests was adjusted to be 620 kgf . The nominal

drill spindle speed was agajn 725 rpm. The nominal feed rate (aqain intentionally not held constant) was 90 ym per revolution. Because

drill wear was not expected to change rapidly, and to reduce the data - collection task, data were not taken for every hole drilled. The instrumentation was active during the drilling of 15 pairs of holes. These

holes were 1 and 2, 10 and 11, 20 and 21, 40 and 41 , 60 and 6 1 , 80 and

81, 100 and 101, 150 and 151, 200 and 201, 250 and 251, 300 and 301, 350

and 351, 400 and 401, 450 and 451, and 499 and 500. It was intended that

in the event that any measured parameter showed sudden change, the interval between instrumented pairs would be reduced accordingly. However, in

neither experiment was any sudden change observed.

2.2.2 Load (Force) Measurements - In order to obtain both low- and h i g

h

frequency information, the instrumentation signals from the second hole

in each measured pair were processed somewhat differently from those

from the f i rst hole.

As the first hole was being drilled, the output signal of the load

cell measuring downward force was fed through a 5

- Hz low-pass filter and

then recorded on a storage oscilloscope (the resulting trace is shown in

figures 6 and 7 for steel and brass, respectively). Neglecting vibration and friction effects, the load applied to the drill bit was constant. A 500-ms portion of the load-cell output signal was also captured by a transient recorder with 1024 eight-bit data points. The timing of this operation was intended to capture data just after the bit

had fully engaged the workpiece. The output signal from the transient

recorder was first fed through a 16-Hz low-pass filter and displayed on

the oscilloscope, 16 Hz being chosen so that the 12-rps spindle frequency could be readily detected. The same output signal from the transient recorder was then fed through a 32-Hz low-pass filter and again

displayed on an oscilloscope, 32 Hz being chosen so that the 2 times 12

rps flute frequency (the bit has two flutes) could be readily detected

if present. Finally, the same signal from the transient recorder was fed

into a frequency analyzer set to monitor frequencies from 0 to 200 Hz and

the results photographed.

The second hole was then drilled, and again the output signal from

the load cell recorded after passing through a 5“Hz low-pass filter; the

data for consecutive drillings could thus be compared. With brass test

pieces, the transient recorder was set to record a 10-ms portion of the

load-cell output signal; the recorded signal was fed into the frequency

analyzer set to monitor frequencies from 0 to 10 kHz. The 10-ms recording interval was used only for brass; for steel, a 50-ms recording interval was used and the frequency analyzer set to monitor frequencies from

0 to 2 kHz in an effort to improve the resolution of the test data.

2.2.3 Torque Measurements - The output signal from the load cell used to

determine torque was processed in a manner analogous to that described in

2.2.2 for the signal representing downward force. In order to provide a
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convenient means of comparing the timing of drilling events, as detected

by the two load cells, load and torque traces are recorded on the same photograph

.

2.2.4 Other Measurements - As described previously, drilling time was scaled directly from the photograph of the load-cell signal trace. A

trace representing spindle displacement was also displayed on the oscillo

scope (along with load and torque) and was used to monitor the drilling

rate. The no-load spindle rotation frequency was displayed on a frequency meter and was used at the beginning and end of each test series as a check that the drill motor speed had not changed by more than 2 percent.

Table 1 is a summary of measurement conditions for the set of eight

photographs obtained in the drill wear test for each pair of holes.

2.3 Test Results

2.3.1 Results with First Test Fixture - Load and torque information was provided by the first test fixture but was accompanied by considerable in

stability and vibration at the workpiece. Excessive play in the vise as- sembly holding the workpiece was caused primarily by efforts to reduce

friction in the torque-measurement apparatus. Redesign of the vise did

not appreciably reduce the vibration. Although a considerable number of

tests were run, the data are regarded as invalid and are not presented

here. The second test fixture was designed to eliminate or reduce the

problems encountered with the first model and proved to be successful in

this regard. All experimental data presented in this report were obtained with the second test fixture.

2.3.2 Results with Second Test Fixture - The test data are largely in

the form of photographs of oscilloscope displays. As an example, see photograph A of figure 6, in which the top trace shows the output signal

from the load (force) sensor as fed through a 5"Hz low-pass filter. The

traces shown in figure 6 were obtained during the drilling of holes 1,

100, 200, 300, 400, and 500. Although a constant force was applied by

the masses, this force was transmitted through the drill-press gearing so that the force between the drill bit and the workpiece varied from hole

to hole because of friction. The total variation in downward force attributed to friction as determined from the 30 monitored holes in the

500-hole series in steel was estimated to be about ± 5 percent of the

nominal value. This estimate refers to the near steady-state values of

load and not to the v i brat ion- i nduced a-c ripple superimposed on top of

the steady-state load. Observation of this ripple is discussed later in

this section.

The middle trace of photograph A of figure 6 represents the output

signal from the torque load cell through a 5“Hz low-pass filter. As the

drill bit first touches down on the workpiece, there is little torque be- cause the tip of the bit constitutes a point contact; as the drill-bit

flute area in contact with the workpiece increases, the torque increases

similarly. The torque signal shows spikes in output at the beginning

and the end of drilling with a plateau in between. The cause of the beginning spike is not known, but it, and some contribution to the spike at
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the end of the drilling, may result from the fact that the surfaces of

the workpiece are "... cold worked and therefore tougher than the undisturbed central zone" [3]. The greater part of the end spike is probably

the result of the drill bit's breaking through the workpiece and tending

to grab. Curve A of figure 8 is a plot of the peak values of the beginning spikes as taken from the 30 monitored tests of the 500-hole series

in steel. After about 80 holes, these beginning spikes show a decrease

in amplitude (with a good deal of scatter), until after about 400 holes

the response tends to flatten at an amplitude of about 70 percent of the

initial values. Curve B of figure 8 is a plot of the torque amplitudes

in the "plateau" region; the amplitudes are relatively constant. Amplitude values of the terminal, or breakthrough, spike are not plotted as they were too large to be recorded with the oscilloscope vertical deflection required for other traces.

The bottom trace of photograph A of figure 6 represents drill-bit

displacement. The initial part of the curve corresponds to the lowering

of the drill bit by the slow-speed motor. As noted, the drill bit must

be lowered faster than the drill cutting rate but not so fast as to shock

excite the load cell. Toward the end of the drilling process, as anticipated, the displacement rate increases rapidly as the drill approaches

breakthrough

.

The time required to drill through a 1.3 - cm thick steel workpiece

with a 1 -cm diameter drill increased from about 11.5 s to about 13*5 s by the 200th consecutive hole; then the drilling time began to decrease

so that by the 500th hole it was down to 12.5 s. The data for the 30

monitored holes are represented by curve C of figure 8.

This discussion of the results has been concerned with quasi-static

measurements of load, torque, and drilling time. The analysis of dynamic

data is exemplified by figures 9 and 10, which together show a set of

eight photographs of traces on the oscilloscope screen taken during the

drilling of holes 60 and 61 in steel. Photograph B of figure 9 shows

the a-c ripple on the load signal (top trace) and the torque signal (bottom trace) after passing through a 16-Hz low-pass filter (sweep time 50

ms/div); a signal at the 12-rps spindle rotation frequency can clearly be seen. For the traces of photograph C of figure 9 the same signals are passed through a 32-Hz low-pass filter; earlier tests had indicated that

a signal at 24 Hz, or the "two-flute frequency," would be detected. In those tests, however, there was little evidence of a 24-Hz signal. Photograph D of figure 9 shows a sample spectral frequency analysis covering

the frequency range 0 to 200 Hz of the load signal (unfiltered except for

transient recorder response) in which the contributions of the spindle

frequency of 12 rps and the drill-press table resonance frequency of 117

Hz can be seen. Curves A and B of figure 11 show the analyzed amplitudes

at these two frequencies as a function of the number of holes drilled.

The amplitude of the 12-Hz component rises sharply until about 80 holes

have been drilled; beyond 80, the amplitude levels off for the remainder

of the test. The amplitude of the 1

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- H z component appears virtually un- changed during the entire 500-hole test, with possibly a slight rise in

amplitude as a function of the number of holes drilled. Photograph E of
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figure 9 shows a sample spectral frequency analysis of the torque signal;

again, components are evident at the dri 1 1-spindle frequency and the

drill-press table resonance frequency. In this example, the component at

the spindle frequency is much more pronounced than that at the table re- sonance frequency. Curve C of figure 11 shows the data plotted for the

entire test, and shows little or no evidence of change. Photographs B and C of figure 10 show a sample spectral frequency analysis over the

range 0 to 2000 Hz for the load and torque signals, respectively. The

most pronounced component is in the torque signal at 750 Hz. The amplitude of this component is shown plotted as curve D of figure 11; again,

very little change in amplitude can be detected, although there is con- siderable scatter in the data.

None of the four plots of frequency-analysis data in figure 11 show

any significant changes in amplitude during the entire series of 500 holes.

An earlier test series of 500 holes in brass preceded the series in

steel. The test series in brass showed so little change that a new series

was begun with steel; because changes from the first to the last hole do

not appear to be significant, only a few trace photographs are presented

for the test in brass. Figure 7 shows photographs of the displacement,

load, and torque traces for hole numbers 1, 100, 200, 300, 400, and 500.

The drilling time increased with number of holes drilled from about 3-5

to k s .

2.3.3 Use of Instrumentation Tape Recorder - After the two 500-hole

drilling experiments had been completed, an instrumentation tape recorder

that had been ordered by the sponsoring office was made available for a brief evaluation. Several test runs were carried out in which the data

from drilling a hole were recorded continuously from first engagement to

breakthrough (instead of only 500 ms, 50 ms, or 10 ms) by means of a tapeloop accessory. The tape was played back into the spectrum analyzer re- petitively. Time constraints interrupted these experiments before definitive conclusions could be drawn. On the basis of the limited tests, use of the recorder would be expected to improve the frequency-analysis capability significantly for future drilling experiments.

3. DISCUSSION AND CONCLUSIONS

The principal aims of the work — to develop apparatus for measuring

the down force (load) and torque imparted to the workpiece by a drill bit

in a drill press during drilling and to gain some familiarity with the

practical problems of instrumenting a machine tool —have been achieved.

This exploratory program of work was not primarily intended to provide information on the behavior of the measured parameters; the 500-hole limit

and the use of a single bit in steel was determined by managerial considerations of available time and personnel.
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However, it may be of interest to examine the trends of the recorded

parameters keeping in mind the constant - rpm , constant-load mode of operation. A descriptive explanation of cutting-tool wear is that a sharp,

microscopically jagged edge becomes burnished with use as a result of microimpacts between the workpiece and locally high points on the edge and

as a result of complex mechanisms involving microstructural changes, plastic deformation and flow, and other aspects of the behavior of material

subjected to mechanical and thermal stress. This description is con- firmed by the common shop experience that a dull bit or cutter requires

an excessively high feeding force to thrust the tool into the work; if

the tool cuts at all, the operation time is greatly increased compared to

the normal time required with a reasonably sharp tool, and there is usu- ally evidence of considerable heating of the tool and workpiece. In a constant-load mode of operation, as the tool wears and becomes smoother,

the torque required to turn it would be expected to decrease. For drilling in a constant-rpm, constant-load mode, if the torque required to turn

the drill decreases, the implication is that there should be less material removed per revolution of the bit, and the drilling time for a given

thickness and composition of workpiece should accordingly increase.

These predictions are confirmed in curves B and C of figure 8, up to

about hole 300. After hole 300, the trends for both curves reverse. The

net change in both parameters is on the order of 10 percent. Visual ex- amination of the drill bit shows that the cutting end has experienced

temperatures high enough to cause permanent discoloration of the steel,

with dark blue-black at the tip and along the cutting edges and with gradations from this coloration to pale straw some 3 cm from the tip. Examination of the cutting edges with a zoom microscope in the nominal magnification range 70 to 1 80X shows the presence of both burnished areas decorated with small modules of apparently fused material and freshly scored areas with parallel ridges similar in microtopography to the ground sur- faces of a new bit. A possible explanation of these observations and of

the reverse trends in the torque and drilling-time curves may be that near hole 300 the condition of the drill resulted in the unlubricated bit

reaching successively higher temperatures with each successive drilling

until flakes of material were being removed at localized sites. It is

suggested that the effect of this process was to produce sites with cutting properties at least in some respects similar to those of a new bit

and that the number of such sites increased as the number of holes

drilled increased beyond 300.

Three prominent frequencies appear in the frequency analysis data,

these being (1) the drill spindle speed of 12 Hz, (2) the drill-press table resonance of 117 Hz, and (3) from an unknown source, 750 Hz. There

appears to be little correlation between the number of holes drilled (and,

presumably, drill wear) and the amplitude of any of these frequencies.

A. RECOMMENDATIONS

A.l Overall Considerations and Principal Recommendation

A major conclusion drawn from reviewing the references in the associated bibliography on machine-tool measurements [2] is that "calibration
needs have not been broadly addressed;" further, "there is a real danger

that calibration methods will not be available to match the technology."

This conclusion identifies an important element of a long-term National

Bureau of Standards program in machine-tool automation which is responsive to the Visiting Committee's recommendations. It identifies also a legitimate role for the Components and Applications Section in the

machine-tool automation field. The Section has had long experience with

the measurement of pressure, vibration, and the like, and with the development of both static and dynamic methods for calibrating such measurements. Thus, the development of calibration methods for machinetool sensing falls appropriately within an area of technical expertise

on the part of the Section staff and within the intended scope of Section activity.

The principal recommendation of this report is that resources be al

located to the Section in support of the exploratory and engineering development required to realize calibration methods for machine-tool sensing.

A. 2 Specific Recommendations for Immediate Work

The work reported represents the first step in a logical program of

effort aimed at developing calibration methods for machine-tool sensing.

Continuing effort will be required to obtain sufficient data for a meaningful evaluation of drill-press and other machine-tool performance to

provide, in turn, a basis for assessing those calibration requirements

that arise from the measurement situation as opposed to external con- straints. Specifically, it is recommended that:

1. The drilling experiments be extended both in number of drill

bits used and in number of holes drilled. Data should be taken more fre

quently and the series extended until an increase in drilling time on

the order of 100 percent is achieved. The tape recorder with loop attachment should be incorporated into the instrumentation chain to permit

frequency analysis of the data from drilling an entire hole (or at least

to provide a means of checking that data taken over a small fraction of

the drilling time are representative of data from the entire hole, with

the possible exception of touch-down and breakthrough times). Analysis

of the data should be correlated with microscopic examination of bits at

selected intervals in the series.

2. Efforts continue toward the realization of a more compact, con- venient test fixture suitable for use in the machine shop. The performance of this test fixture should be compared to that of a fixture known

to be commercially available and incorporating piezoelectric quartzcrystal sensing elements for multi-axis measurements.

3. Experiments be conducted using a drill press operating in a con stant spindle-feed mode to permit comparison between the rate of drillbit wear in the cons tant- 1 oad and constant-feed modes.

4. A continuing search of the literature, coupled with liaison between other NBS organizations conducting work in machine-tool automation

be maintained as an essential part of this effort.
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4.3 Recommendations for Further Work

1. It is recommended that, using the background developed in the

earlier investigations, a survey of machine-tool calibration needs be

carried out with the goal of identifying important parameters, such as dynamic force, for which there is little or no calibration support

available and that appropriate work be undertaken to respond to the needs

after priorities are assigned.

2. It is recommended that calibration methods for the selected measurand be developed, first in terms of laboratory procedures and subsequently as methods suitable for in-shop use, such methods to be documented and disseminated.

3. It is recommended that additional measurands be selected as subjects for supporting calibration and that development of methods proceed

as before. At this stage it should be possible to develop field calibration procedures for the sensors incorporated in the fixtures discussed

in (2) of 4.2 above.

REFERENCES

[1] Cook, N. H., Subramanian, K. , Basile, S. A., Survey of the State of

the Art of Tool Wear Sensing Techniques, Dept, of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, September 1975*

[2] Hilten, J. S., Vezzetti, C. F., Mayo-Wells, J. F., and Lederer, P. S.,

Bibliography and Brief Review of Literature on Machine-Tool Measurements for Automatic Control, NBSIR 78-1424 (January 1978).

[3] Obert, E., et al., Machinery's Handbook, Twentieth Edition, Industrial Press Inc., NY, 1975-

[4] Gautschi, G. H., Cutting Forces in Machining and Their Routine Measurement with Mu 1 1

i -Component Piezoelectric Force Transducers, Paper,

12th International Machine Tool Design and Research Conference, Manchester, England, September 1971.

USCOMM-NBS-DC
Figure 1: Drill press with instrumentation. Identified are the 1 oad-and-torque

measuring fixture - second model (A); the linear-variable-differential

transfomer (LVDT) displacement transducer ( B ) ; and the spindle-speed

sensor (C). Also shown is the means by which a load is applied to the

spindle. Masses on pan (D) exert a force on the drive rope (E), which

is fastened in the groove of large pulley (F). Pulley F thus experiences a torque which is in turn transmitted to the spindle pinion to

which the pulley is rigidly attached. Rope (G) leads to a low-speed

geared motor, as described in the text.
Figure 2: Drill spindle-speed instrumentation. Identified are the magnetic

sensing coil (A) and the 60-tooth steel gear (E?) mounted on the

spindle shaft (C) . One of the two stepped pulleys of the belt drive

i s s hown at (D) .
Figure 3

’ Displacement-measuring instrumentation. Identified are the LVDT

transducer sensing coil (A) mounted by means of bracket (B) to the

head casting (C) . The transducer armature (D) is mounted by a bracket (E) attached to the vertical stop screw (F)

.
Figure A: Force-and-torque measuring fixture - first model. Identified are the

mounting plate (A) carrying the load-measuring load cell: the machined block (6) bearing on the capped armature of the load cell and

carrying vise jaws (C): and the brackets (D) that restrain lateral

motion of the block and hence of the workpiece (E). Also shown is

the arm (F) extending from the block to contact the armature of the

torque-measuring load cell (G). The spring (H) applies a preload to

the cell so that the arm does not bounce against the armature.
Figure S' Force-and-torque measuring fixture - second model. Identified are the

length of U-channel (A) into which the load cell (B) measuring downward force is mounted: the brass rod (C) instrumented with strain gage

(D) and supporting vise (E) with arm (F). Also shown are the load

cell (G) measuring torque and shims (H) for adjusting torque preload.

The workpiece is in the vise at (l).
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TIME

|s)

PLATEAU

TORQUE

(N

M)

PEAK

TORQUE

(N

M)

Figure 8: Drilling time and torque signal as a function of number of holes drilled

in steel

A - Output signal of the torque load cell (measured at the peak of the

first torque spike)

B - Output signal of the torque load cell (measured in the plateau region).

C - Time to drill through a 113-cm thick piece of steel with a 1 -cm drill.

J
Figure S'- Typical output waveforms and frequency analysis of force and torque Instrumentation signals. Five oscilloscope photographs for the 60th hole drilled

in steel are shown (figure 10 shows the remaining three oscilloscope photographs for the 6lst hole). Two drillings and eight photographs represent a set of data.

A - See figure 6 for details.

B - Top trace, force load-cell output signal through a 16-Hz low-pass filter,

horizontal scale 50 ms/div; bottom trace, torque load cell output signal

through a 16-Hz low-pass filter, horizontal scale 50 ms/div.

C - Same as B, except through a 32-Hz low-pass filter.

D - Frequency analysis of force load-cell output signal; horizontal scale

20 Hz/div, vertical scale 10 mV/div.

E - Frequency analysis of torque load-cell output signal; 100 Hz center,

frequency, horizontal scale 20 Hz/div, vertical scale 50 mV/div.
Figure 10: Typical frequency spectra to 2 kHz of force and torque instrumentation

signals. Figure 10 shows three oscilloscope photographs for the 61st hole

drilled in steel; the other five photographs that make up this set are

shown in figure 9-

A - See figure 6 for details.

B - Frequency analysis of force load-cell output signal; horizontal scale

200 Hz/div, vertical scale 2 mV/ div.

C - Frequency analysis of torque load-cell output signal; horizontal scale

200 Hz/div, vertical scale 20 mV/div.

w
FREQUENCY

ANALYSIS

AMPLITUDE,

DIV.

NUMBER OF HOLES DRILLED

Figure 11: Plots of spectral amplitude at selected frequencies as a function of number

of holes drilled. Data for both force and torque are shown. The amplitude

data are scaled from the four frequency-analysis oscilloscope photographs

taken for each set of holes drilled in steel. (Typical oscilloscope photographs are shown in figures 9 and 10.)

A - Response for 12 Hz (drill spindle frequency), force load-cell output

s i gnal . B - Response for 117 Hz (drill-press table resonance frequency), force

load-cell output signal.

C - Response for 117 Hz (drill-press table resonance freauency) , torque

load-cell output signal.

D - 750 Hz, (source unknown), torque load-cell output signal.
NBS-114A (REV. 7-73)

U.S. DEPT. CF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. 2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE 5. Publication Date Experimental Investigation of Drill-Bit Wear

6. Performing Organization Code

1+25.03

7. AUTHOR(S)

Mayo-Wel

John S. Hilten, Carol F. Vezzetti , J. Franklin 8. Performing Organ. Report No. and Paul S. Lederer

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Office of Developmental Automation and Control Technology

Institute for Computer Sciences and Technology

National Bureau of Standards

Washington, D. C. 20234

13. Type of Report & Period Covered

NBS-IR

12-1- 76 to 4 -30 -7Z.

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The development of an experimental instrumentation system for a small drill press is

described. The parameters measured are spindle speed, vertical spindle displacement,

vertical spindle load, drilling torque, and drilling time. Several test fixtures were

instrumented and used in drilling experiments. These experiments were conducted to

examine the relationship between variations In the measured parameters and drill performance, more specifically to drill wear. Experimental data show a 10-percent In- crease in drilling time from the first hole to the last for a single set of 500 holes

drilled in cold-rolled steel at a nominally constant load, although the drilling time

began to decrease slightly after hole 300. Changes in drilling torque were also detected during the test runs, and in similar runs with a brass workpiece. It Is suggested that with respect to the anomalous results in steel under the un 1 ubr I cated

,

constant-force conditions employed, the cutting surfaces of the drill bit were In a sense being renewed as microflakes of material departed to reveal fresh, sharp unburnished sites.

17. KEY WORDS (six to twelve entries ; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Cutting force; drill bit; drill press; dynamic; force; load; machine tool; measurement

on-line; tool wear; torque; transducer.

18. AVAILABILITY [X] Unlimited

For Official Distribution. Do Not Release to NT1S

I I Order From Sup. of Doc., U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. CU

I I Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Price

USCOMM-DC 29042-P74