A Guide for the Construction

Operation of Drill-Up

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Manufacturing Engineering

Washington, DC 20234

and

October 1982

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U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 82-2590

A GUIDE FOR THE CONSTRUCTION AND

OPERATION OF DRILL-UP

NATIONAL BUREAU Of STANDARDS

1982

Kenneth W. Yee

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Manufacturing Engineering

Washington, DC 20234

October 1982

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
IMAWU lO

'V mi.:
TABLE OF CONTENTS

Abstract ill

1 . Introduction 1

2. Block Diagram Description 2

3. Flow Chart . 5

4. Circuit Diagram 8

5* Testing and Setup 9

6. Changing the Programmed Drilling Speed 12

7. Operating Instructions 13

8. Potential Operating Problems 16

9. References . . 18

Appendix A A-1

Appendix B . B-1

i
LIST OF FIGURES

Page

Figure 1. Drill-Up 3

Figure 2. Block Diagram of Drill-Up 4

Figure 3. Flow Chart of Main Program 6

Figure 4. Flow Chart of Calibration Subroutine 7

Figure 5. Circuit Board 10

Figure 6. Test Setup 11

Figure 7. Waveforms 15

LIST OF TABLES

Table 1. Constants and values for selected alternate drilling speeds . . 14

ii
A GUIDE FOR THE CONSTRUCTION AND OPERATION OF DRILL-UP

Kenneth W. Yee

ABSTRACT

This guide provides detailed information for the construction of a singlespeed version of Drill-Up and instructions for its installation and operation.

Drill-Up is an instrument designed to prevent breakage of small-diameter

drills used on automatic-feed drilling machines with a spindle retract

ability. The method and applications have been previously described in the

references given. The hardware and software necessary to construct an instrument for use at a single selected drilling speed are described. The

circuit diagram and source program are included. The description is of a preliminary design implemented at the National Bureau of Standards (NBS) and

in use in the NBS central machine shop. Sufficient details are included for

those equipped to use the common 8048 family of microcomputers to build a "cookbook" instrument. The information is also adequate to implement the

instrument on any existing microcomputer or microprocessor.

iii

A GUIDE FOR THE CONSTRUCTION AND OPERATION OF DRILL-UP

1 . INTRODUCTION

Drill-Up is an instrument designed to prevent breakage of small-diameter

drills used on automatic-feed drilling machines with a spindle retract

ability. The method and application have been previously described. ,2 This

system will predict failure of small drills on constant-feed-type drilling

machines. It is presently optimized for drill sizes of about 1/8 inch (3 mm)

or smaller, where the expected failure is breakage. It may not work on machines that use constant force, such as manual drill presses or special airover-hydraulic feed machines. The reason for this is that the detection

method depends on deflection of a long slender column with one rigid support

on the end opposite an axial load; that is, a drill held in a chuck. When the

drilling is improper, due to factors such as a dull or worn tip or a hard spot

in the work piece, the material cannot be removed as fast as the drill is

being fed. When this occurs, the drill deflects as a column and induces a vibration signal as it scrapes on the side or bottom of the hole. If this is

allowed to continue, the column will totally collapse, resulting in drill

failure. The detection method recognizes that the scraping in the hole will

be synchronous with the rotation of the spindle. Of course, as the drill goes

further into the workpiece, the material provides an instantaneous stabilizing

transverse-load which, in effect, shortens the column and increases the load

required for failure. In this case, the drill failure may be caused by

seizing, due to material caught between the drill and the side of the hole.

This may also result in a synchronous vibration.

Drill-Up is not perfect, but can prevent a large number of failures; more

than 90 percent of the potential failures have been prevented. The system has

been used most frequently with drills about 0.04-0.05 inch (1.0-1. 2 mm) in

diameter. The system was tested drilling 0.04-inch (1-mm) holes in a 1/4-inch

(6.4-mm)-thick mild steel (C1015-C1020) cold-rolled plate. Failure was encouraged by removing the coolant. In 47 out of 50 cases, the system

retracted the drill before failure occured. Additional details are given in

references 1 and 2.

The present paper describes the hardware and software necessary to

construct an instrument for use at a single selected drilling speed. The

testing, operation, alterations required for other speeds, and some pitfalls

are included. A preliminary design is described, shown in Figure 1, which was implemented at the National Bureau of Standards (NBS) and is in use in the NBS

central machine shop. This design has been found suitable for a wide variety

of applications, but changes, such as amplifier gain, may be required for an unusual setup (e.g., a very rigid workpiece). No effort has been made to

optimize the circuit described in terms of component count or cost. The

components used were those at hand and are by no means unique. The instrument

uses a low-cost, single-chip microcomputer to implement its function.

Sufficient details are included, for those equipped to program and construct

devices using the common 8048 family of microcomputers, to build a "cook-book"

instrument. This information is also adaquate to implement the instrument on any existing microcomputer or microprocessor. The instrument described is

covered by an application for patent with rights assigned to the U.S.

1
Department of Commerce. Those desiring information on licensing should

contact the Office of Government Inventions and Patents.

2. BLOCK DIAGRAM DESCRIPTION

When Drill-Up determines that drill breakage is imminent, it outputs a command to the drilling machine to retract the drill. The input sensor is a piezoelectric accelerometer which is mechanically coupled to the workpiece as indicated in the block diagram in Figure 2. Potential drill breakage is

determined by time-domain analysis of the accelerometer signal. A calibration

routine in the microcomputer automatically adjusts Drill-Up to the normal

amplitude of the signal. The system to be described operates at one programed

drilling speed (4800 rpm for this example) and relies on the machine to be

within the allowable tolerance, +5 percent of the nominal speed. The circuit

detects large-amplitude accelerations, caused by vibration of the workpiece,

which are synchronous with the drill rotation.

As shown in Figure 2, the accelerometer signal is amplified by a highinput- impedance voltage amplifier with a selectable gain of one or five. A

remotely-selectable adjustable attenuator has been included to accomodate a second tool (e.g., a center drill) operating at the same drilling speed, which

causes significantly larger normal vibrations. The attenuator is switched

into the circuit by a signal from the machine controller, indicating that the

second tool is in use. The signal is amplified further by a variable-gain

amplifier which can be adjusted from about 4 to 24. The signal is ac-coupled

to one input of an analog voltage comparator. The reference "threshold" is

supplied by a digital-to-analog (D-A) converter which is driven by the

microcomputer calibration routine and will be described later. Signal peaks

which exceed the threshold trigger a monostable multivibrator (one-shot) which

generates a pulse with a width 40 percent of the drill-rotation period (5 ms

for the present 4800 rpm). Two pulses per revolution are possible if both of

the drill bit cutting edges hit the workpiece. These pulses are applied to an input of the microcomputer (an 8748 has been used), called test input zero

(TO). This input is used because the 8748 has software branching based on the

signal level of this input. The microcomputer continuously monitors this

input until a pulse is found. It then delays for half a pulse width (2.5 ms)

and resamples the input at time intervals equal to the period of rotation

(12.5 ms). If pulses (caused by successive accelerometer signals greater than

the threshold) are found, the input is repetitively sampled at rotation period

intervals until four pulses are found. If, at any sample, a pulse is not

found, the system resets. When four pulses in a row have been found, the

system outputs a signal called Up-Z on port one, bit zero (P10). This signal

triggers another one-shot which generates a pulse approximately one-second

long. This time was selected to ensure easy perception of a front panel

light, indicating "retract," which is operated from the same signal. The Up-Z

signal activates a relay which can be connected to the emergency "retract"

circuit of a machine controller. For the controller in use, this relay is

normally closed and is opened when Up-Z occurs. When this is detected, the

machining center in use retracts the spindle and interrupts the N/C machine

program. In most cases this will occur before the drill breaks, but delay in

the retract mechanism may result in occasional breakage during the retract.

The drilling process may be continued at any time without adjustment to DrillUp.

2
3

Figure

1.

Drill

-Up
<

4

Figure

2.

Block

diagram

of

Drill-Up.
The calibration routine in the microcomputer, which allows the instrument

to automatically adapt to the signal during normal operation, is activated by

a front panel switch that provides a hardware interrupt signal to the

computer. The routine reduces the threshold from approximately 1/5 of full

scale (10 V) in increments at fixed time intervals. For the present

application, the steps are 0.4 V at 1.5-second intervals. This continues

until the peaks of the signal begin to exceed the threshold. The

microcomputer determines that the threshold has been exceeded by counting the

pulses appearing on the input terminal. When a predetermined number ( 1 60 for

4800 rpm) have occurred, the level of normal drilling has been found. The

system then outputs a value, on port 2, to the D-A converter which sets the

threshold to five times the level found for normal drilling. This threshold,

in volts, is displayed on a digital voltmeter. The system is operating to

detect potential breakage immediately upon termination of the calibration

cycle.

3. FLOW CHART

The flow chart of the main computer program is shown in Figure 3. The

complete listing of the program "DRLLUP" is given in Appendix A. The

microprocessor, after power-up, immediately jumps to an initialization

procedure which sets the D-A converter (threshold) to about 1/3 full scale (an

arbitrary initial value) and enables the hardware interrupt which starts the

calibration routine (described below). The "start" sequence then occurs. The

Up-Z signal is cleared, the hardware timer is stopped, and the "count" which

is the number of pulses to be found after the first is loaded with N (3). The

input TO is then continuously tested for a "1" indicating that the signal has

exceeded the threshold. When the first pulse is found, the timer is loaded

with "delay" (20 percent of the period of rotation) and started, and the timer

flag is tested until it indicates that the time is up. The timer is then

loaded with "wait," which is equal to the period of rotation, and started.

When time is up, the input is again tested for a "1." If no input is found,

the program returns to "start" and looks for a new first pulse. If a second

pulse is found, the "count" is decremented, and, if not zero, the timer is

reloaded with "wait". The process is repeated until "count" reaches zero

(four pulses in a row at one-period intervals have occurred). If the third or fourth pulse is not present, the program returns to "start". When "count" is

zero, the Up-Z signal is output and the timer loaded with "tzup". At the end

of this time, which is the maximum timer cycle (20 ms), the program returns to

"start," where Up-Z is reset and the program begins to look for a new pulse.

A flow chart of the calibration subroutine is shown in Figure 4. When

interrupted, the microcomputer automatically goes to address 3 where it is

instructed to jump to the calibration subroutine labeled "level". The

initialization steps load the memory location "fulbmp" with the number

"levful" which is the number of pulses to be counted before the normal level

is assumed to have been found. Memory location "dancnt," which holds the D-A

converter normal-level count, is initialized at the number 50 which is less

than the full-scale (255 for 8 bits) count divided by 5. This ensures that

the threshold cannot exceed full-scale after the 5-times multiplication which

will occur later. A flag is set on bit 7 of port 1 to indicate that a calibration is in progress. This bit operates a front panel indicator light.

The "dancnt" value is output on port 2 to the D-A converter. The timer is

then started for the time period between level changes, which has been

5
INITIALIZE DAC TO

1/3 FULL SCALE

ENABLE INTERRUPT

Figure 3. Flow chart of main program.

6
Figure 4. Flow chart of calibration subroutine.

7
selected to be 1.5 seconds; this time is a number (xt) of maximum timer

periods (20 ms) as counted in memory location "xtimer" (see Appendix A). The

program sequentially tests for the timer flag and then for a pulse on input

TO. Whenever time is up, the value in "dancnt" is decremented and the new

value output to the D-A converter. The timer is then restarted for the time

between level changes. Whenever a pulse is found on TO, the program waits

until the end of the pulse (to avoid counting it more than once) and then

decrements the count in "fulbmp," checks for a zero count, and, if zero is not

found, goes back to test the timer and input. The D-A converter continues to

reduce the threshold level every 1.5 seconds. As the threshold approaches the

peak values of the normal signal, an occasional pulse will be found.

Eventually, a significant fraction of the peaks will exceed the threshold and

the count in "fulbmp" will be rapidly reduced to zero. At that time the

existing value in "dancnt" is multiplied by five (actually, added to itself

repeatedly), and the resulting value is output to the D-A converter to set the

operating threshold. This completes the calibration. The flag is reset and

control is returned to the main program. Normal operation resumes

immediately, and a retract could occur immediately following the calibration.

During the calibration, drill breakage is a possibility since the main program

is not operating.

4. CIRCUIT DIAGRAM

The circuit diagram is included in Appendix B, but does not show the 5-V

and + 15-V power supplies. For the instrument shown in Figure 1, the majority

of the components, other than the controls and indicators, are located on the

printed-circuit board shown in Figure 5. The input sensor is an accelerometer. The piezoelectric accelerometer used has a sensitivity of

about 90 mV/g, an unmounted resonant frequency of about 25 kHz, and a capacitance of about 1200 pF. It is connected to the input connector J 1

,

shown in the circuit diagram, by 10 feet of 50-ohm coaxial cable which has a total capacitance of about 300 pF. The input amplifier U7 is a FET

operational amplifier operating in the noninverting mode. The input impedance

is essentially determined by the 10-megaohm resistor R20. The amplifier has a voltage gain of XI or X5, selected by switch S3. The remotely-selectable

adjustable attenuator consists of R12, R 1 3 , and R15. The attenuator is

selected by relay K2 which is connected for operation by a 24-V dc control

signal from the machine controller through connector J6. Shorting resistor

R 1 9 will allow operation from 5-V dc. If the relay is not activated, the

attenuation is essentially zero. It will divide by a factor of about 2.5 to

6, as determined by R12 when the relay is closed. The second amplifier U6 is

the same type of operational amplifier as that used in the first stage. It is

connected in the noninverting mode with an adjustable gain from 4 to 24, as determined by potentiometer R8. The signal is then ac coupled through

capacitor C12 to one input of the analog comparator U3. When switch S4 is in

automatic mode, the reference voltage to the other comparator input is

determined by the D-A converter U2. The current into the D-A converter output

pin causes a voltage drop across resistor R5, which subtracts from the voltage

across zener diode CR1, which is a nominal +10 V. The resulting reference

voltage is the threshold voltage and is read by the digital voltmeter. The

threshold voltage is zero for a zero output from the microcomputer U1 and a nominal +10 V for a 255 (FF hex) output. When switch S4 is in manual mode,

the threshold voltage is determined by variable resistor R21 and is not

affected by the D-A converter or, therefore, the calibration routine. This

8
manual setting is useful when a calibration has been run for a specific setup

and material. If set manually, it will be retained even if power is lost.

When the peak signal exceeds the threshold, the comparator output

triggers the one-shot U4. Timing components R1 and C8 are selected for a pulse width that is 40 percent of the drilling period of rotation. This pulse

is applied to the TO input of the microcomputer and is available at a test

point J4. The Up-Z output signal from the microcomputer on port 1, bit zero,

triggers one-shot U5 which generates a pulse about one second long. This

pulse opens the retract relay (K1) contact through a driver transistor Q1. A

front-panel red light-emitting-diode (LED) indicates a retract. A positive

going Up-Z pulse is available at test point J5. Switch SI which is across the

retract relay contact will disable the retract function when desired. Recall

that, for the controller used, the retract circuit is normally closed. The

retract circuit connects to connector J2. The calibration routine is initiated by switch S2 which produces an interrupt signal to the microcomputer.

The calibration flag lights a yellow front-panel LED through driver transistor

Q2. The computer clock frequency is determined by crystal Y1 which is

presently 6 MHz. The power supply requirements are 5 V at less than 200 mA,

+ 15 V at less than 50 mA. This does not include any power used by the

digital voltmeter, should it operate from these supplies. The commons of the

5-V and 15-V supplies (digital and analog grounds) must be connected together

at one point. If an 8748 microcomputer, programmed as shown in the listing in

Appendix A, is used for U1 along with the component values as given on the

circuit diagram, a Drill-Up system for use with a drilling speed of 4800 rpm

will result. The tolerance on the drilling speed is about + 5 percent from

the nominal speed. Software and hardware changes, to be described later, are required for operation at any other speed.

5. TESTING AND SETUP

The test setup shown in Figure 6 is suggested for testing the circuit and

adjusting the variable controls. This type of method must be used to generate

a variable number of pulses that occur at time intervals corresponding to the

period of drill rotation. A gated square-wave or pulse generator is required

along with another pulse generator to produce the gate pulse. The output signal level must be variable from about 10 mV to 1 V peak-to-peak for a square

wave (peak, if a pulse). The gated generator should first be adjusted for a frequency equal to the rpm/60, if a square wave, or a time between pulses of

60/rpm. For 4800 rpm, this is 80 Hz or 12.5 ms. The gate pulse must be

adjustable in width from one to four or more periods of rotation (up to 50 ms

for 4800 rpm). The repetition rate should produce a perceptible pattern on

the oscilloscope (about 5-10 Hz). Connect the generators and oscilloscope to

Drill-Up, as shown in the figure, and apply power. If 3 oscilloscope inputs

are not available, a voltmeter may be used to monitor Up-Z since it is a one- second-long pulse. Set the gain switch S3 to low and set the threshold

selector-switch S4 to automatic. Operate the gated generator in the

continuous mode and adjust its output level to about 0.1 -V peak. Adjust the

gain by varying R8, until the signal at the amplifier output is 15 to 20 times

the input level. Reduce the level as necessary to less than 2 V at the

amplifier output and set the gain switch to high. Verify that the level in- creases by a factor of five. At power-up, the threshold should have been set

to about 3*3 V (1/3 full scale). With the continuous input still applied,

reduce the level at the amplifier output to about 1 V. Push the calibrate

9
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5.

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Figure

6.

Test

setup.
button. The yellow calibration-indicator should light and the threshold

should jump to a value slightly less than 2 V (1/5 scale). The threshold will

decrease in steps every 1.5 seconds. When the threshold reaches the level

equal to the positive peaks of the amplifier signal, pulses will appear at the

pulse output J4. They should be present for about 2 seconds. When they stop,

the threshold will jump to a value equal to about five times the peak of the

signal. The calibration indicator will turn to off. The attenuator may be

tested and adjusted by applying 24-V dc to connector J6 and observing the

change in level at the amplifier output. The attenuation factor is varied by

changing R12. The gated generator should now be switched to the gated mode

and the gate-pulse width set to produce 1 to 3 pulses. Increase the level

from the gated generator until the peak exceeds the threshold. Pulses corre- sponding to the signal peaks should appear on the pulse output. Increase the

gate-pulse width to allow 4 pulses and the Up-Z signal should appear during

the 4th-pulse. The red retract-indicator should light. Verify that the

circuit detects 4 or more pulses in a row, but not less than 4. An ohmmeter

connected across connector J2 may be used to verify that the retract relay

contacts open when Up-Z is generated. The enable switch SI must be at the

"on" position (contacts open). Now, vary the gated-generator frequency or period between pulses while ensuring that the gate still allows 4 pulses. The

Up-Z should continue to be present until the frequency or period is changed by

approximately + 6 percent. If the tolerance is not symmetrical about the

nominal value, the programmed rpm (4800) and the speed simulated by the gated

generator do not coincide. If the total tolerance does not equal about 12-14

percent, the width of the pulse from one-shot U4 is not 40 percent of the

period.

6. CHANGING THE PROGRAMMED DRILLING SPEED

As previously stated, the microcomputer program and circuit diagram given

are for use at 4800 rpm. Several constants in the program and one or two

hardware component values must be changed for any other operating speed. With

the present 6-MHz crystal frequency, use of the program given is limited to a drilling speed range of 3000 to 10,000 rpm. This limit is due to the range of

the internal timer. The limitation is primarily at the low speed end, where

the timer count used is near the maximum. The low range can be extended, of

course, by rewriting the program to run the timer more than once in order to

obtain the longer time period. However, the present program can be used down

to 500 rpm by changing to a lower frequency crystal, since the length of one timer count is related to the microcomputer clock frequency. The corresponding slower execution time for any program step is inconsequential. Values of

the constants and components which must be changed are given in Table 1 for a number of other drilling speeds. Crystal frequencies of 6, 3* and 1 MHz are used to cover the range. If the computation of these values is not obvious

from the data given, scaling will provide useful values for intermediate

speeds. Note that a choice of crystal frequencies is available for some speed

values. The symbols TDY, TPD, LEVFUL, and XT are constants that appear in the

equates at the top of the program listing given in Appendix A. Note that TDY

and TPD are negative numbers. All numbers given are in decimal. The number

base is converted to hexadecimal by the assembler. The values for components

R1 and C8 determine the pulse width of one-shot U4 which must be 40 percent of

the period of the drilling speed. These component values are not unique for

the required pulse width.

12
7. OPERATING INSTRUCTIONS

Attach the accelerometer to the workpiece using a threaded stud, quicksetting adhesive, or magnetic mount (if the workpiece is ferromagnetic). The

sensor should be located so as not to interfere with the drilling operation,

but be as close as practical while equalizing the distance from the sensor to

the drilling location throughout the area to be worked. Depending on the

particular setup, distances from a few inches to about ten inches are usually

suitable. Attach the cable between the sensor and the input connector J1 on Drill-Up. Route the cable out of the way of the spindle. Set the threshold

switch to ’•automatic'1 and set the gain switch to "low." Electrical insulation

between the sensor body and the workpiece may be required to eliminate noise

from ground loops. Such insulation should be provided if the peak noiselevel, with no vibration of the workpiece, at the amplifier output test point

J3 is greater than 50 mV at the amplifier gain settings used. Any insulating

material must be nonresilient; if not, the high-frequency response of the

accelerometer will be limited. With the retract "enable" switch SI off,

connect the cable to the machine-controller retract circuit to the retract

connector J2.

The circuit should now be calibrated. Install a good drill bit and begin

drilling at the nominal speed (4800 rpm for the design given). Ensure that

the material, coolant, and drilling process appear normal. The drilling

operation must be continuous during the calibration cycle or an invalid

threshold will result. The system has been designed to accomodate continuous

or "peck" drilling. The threshold should have been set to about 3.3 V at

Drill-Up power application. Push the calibrate switch S2. The calibration

indicator should light and the threshold jump to about 1.9 V and begin to

decrease in 0.04-V steps every 1.5 seconds. When the level of normal drilling

is found, the threshold will increase to five times the lowest value displayed

during the cycle and remain at that value. The calibration indicator will

turn off. The time required to complete the calibration depends on the

threshold level to be found, but should be less than one minute. The

resulting threshold level should be between 2.0 V and 8.0 V. If not, try

another calibration cycle. If the values are consistently high or low, an adjustment of the amplifier gain is required. If too low, the gain switch may

be changed to high. If too high, in either switch position, the internal gain

adjustment potentiometer R8 must be varied to reduce the gain. Observing the

signal, on an oscilloscope, at the amplifier output test point will facilitate

these adjustments. The signal observed is the same signal that is compared

with the threshold. If the pulse output from J4 is simultaneously observed, a pulse will occur whenever the positive peak of the analog signal exceeds the

threshold level. These signals will be similar to those shown in Figure 7,

where the top trace is the amplifier output and the bottom trace is the pulse

output. Figure 7a, is a new drill operating normally. In Figures 7b and 7c,

the coolant has been removed to promote breakage. In Figure 7b, only two

pulses exceed the threshold and will not cause a retract, while, in Figure 7c,

there are four pulses in a row which will cause an Up-Z and a subsequent

retract.

If permitted by the machining process, it is recommended that the

calibration cycle be repeated several times or until a similar threshold value

is found two or three times in a row. An identical value should not be

expected on successive cycles but values differing by 10 to 15 percent, or

13
Table

1.

Constants

and

values

for

selected

alternate

drilling

speeds.

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less, should result. This will not be the case when hard spots or potential

breakage are encountered during the cycle. The value in these cases will be

excessively high and should be rejected.

The system is now ready for operation. Turn the retract enable switch on whenever the retract function is desired. When the retract enable switch is

not on, the red retract indicator will still show that a retract would have

been initiated. The threshold setting should continue to be suitable, if no significant change in the drilling process occurs. This includes the drill

bit, material hardness, coolant or lubricant, sensor location, etc. A new calibration may be run at any time. Remember that the drill is not protected

during the calibration cycle. The threshold value will be lost whenever the

power to Drill-Up is turned off. The power-up value of about 3.3 V will

automatically reappear. It is for this reason that the manual threshold is

provided. With the retract switch turned off, change the threshold switch SM

to manual. Adjust variable resistor R21 for the same threshold level that was obtained during the calibration. This value will be retained, even if power

is lost, and is useable after power-up without a need to recalibrate. Turn

the enable switch back to on. The threshold switch must be changed back to

automatic before another calibration can be run, or the calibration cycle may never terminate. If this should occur, turn the power switch off to reset the

calibration in progress and return the program back to the operating mode. It

is recommended that the calibration be taken on a new (or newly resharpened)

tool. If the drill bits are from the same lot, recalibration after a tool

change is generally not necessary. If a second tool, operating at the same rpm—but with a signficantly different normal vibration level—is to be used,

the remotely-selectable attenuator must be adjusted. The tool having the

lowest threshold level, when calibrated, must be the one in use when the

attenuator relay K2 is not energized. The attenuator adjustment R12 should be

varied such that the second tool, with the relay energized, has about the same threshold level when calibrated. Alternatively, it may be adjusted for about

the same peak signal level at the amplifier output connector.

Since the present design is for a retract circuit requiring a normallyclosed contact, a retract will occur immediately when power is lost. The

machine controller is likely to be locked in retract and be inoperable. To

prevent this problem, the retract enable switch should be turned off whenever

power is removed from Drill-Up.

8. POTENTIAL OPERATING PROBLEMS

As previously mentioned, ground loops may induce noise in the sensor

input to Drill-Up. Similarly, the lack of a single earth-ground to the DrillUp common may result in ac pick-up. The signal from the sensor can be

expected to vary in amplitude as the distance varies from the sensor to the

location of the drilling. This is usually not a problem, if the variation is

not more than about 1/3 relative to that from the location at which the

calibration was taken. A calibration may be run at various drilling locations

on the work piece to measure this variation. If the variation is excessive,

the sensor may have to be relocated as the drilling location changes.

The signal level also varies depending on the rigidity of the workpiece

and the mounting on the machine table. Thick materials, firmly positioned on

the table, will, of course, require higher gain settings to obtain a useable

16
threshold level. Large workpieces with long unsupported spans cause much more

variation in the signal level vs drilling location. Intermediate supports to

damp large low-frequency vibrations are recommended. Best results have frequently been obtained by positioning the sensor directly over one of these

supports.

The speed of the drill must remain within the allowable tolerance which

is + 5 percent from the nominal speed. If the expected variation for the

drilling machine in use exceeds these limits, but the change is primarily a speed reduction under load, Drill-Up can be programmed for a nominal speed

which is between the no-load and full-load speeds to make better use of the

available tolerance.

Drill-Up will not prevent every drill bit from breaking! Some will break

without being detected. Testing and operating experience have shown, however,

that Drill-Up prevents over 90 percent of the breakages that would otherwise

occur. For the machining center used at NBS, there is a significant timedelay between the opening of the retract circuit and the physical movement of

the head (a few tenths of a second). This delay results in occasional breakage during retract. The break, however, is clean and the tip of the bit is

not driven into the hole. Also, failure modes have been observed, not related

to the cutting edges of the drill bit, which may not produce the type of

vibration signature being sensed. An example is binding along the body of a bit when cutting a deep hole, which is caused by chips lodged in the hole.

17
9 REFERENCES

1. Yee, K. W., and Blomquist, D. S., "An On-Line Method of Determining Tool

Wear by Time-Domain Analysis", Technical Paper MR82-901, Soc. of Mfg.

Engineers, Dearborn, MI, 1982.

2. Yee, K. W., and Blomquist, D. S., "Checking Toolwear by Time Domain

Analysis", Manufacturing Engineering, Vol. 88, No. 5, May 1982.

3. Office of Government Inventions and Patents, Attn: Mr. George Kudravetz,

National Technical Information Service, P.0. Box 1423, Springfield, V

A

22151 (703-487-4732).

18
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NBS-114A irev. 2 -eo) U.S. DEPT. OF COMM. 1 . BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

PUBLICATION OR 2 REPORT NO.

NBSIR 82-2590

. Performing Organ. Report No 3. Publication Date October 1982

A Guide for the Construction and Operation of Drill-Up

5. AUTHOR(S)

Kenneth W. Yee

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7 . Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 7

WASHINGTON, D.C. 20234

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

. Type of Report & Period Covered

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant Information. If document Includes a significant

bi bliography or literature survey, mention It here)

This guide provides detailed information for the construction of a single-speed

version of Drill-Up and instructions for its installation and operation. Drill-Up

is an instrument designed to prevent breakage of small-diameter drills used on automatic-feed drilling machines with a spindle retract ability. The method and

applications have been previously described in the references given. The hardware

and software necessary to construct an instrument for use at a single selected

drilling speed are described. The circuit diagram and source program are included.

The description is of a preliminary design implemented at the National Bureau of

Standards and in use in its central machine shope. Sufficient details are included

for those equipped to use the common 8048 family of microcomputers to build a "cookbook" instrument. The information should be adequate for others, familiar with the use of any specific microcomputer of microprocessor, to implement the instrument on

that device.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Drill breakage; Drill-Up; drill wear; time-domain analysis; tool breakage; vibration

sensing.

13. AVAILABILITY 14. NO. OF

DTI Unlimited

| | For Official Distribution. Do Not Release to NTIS

PRINTED PAGES

26

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402. 15. Price

[X] Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $7.50

USCOMM-DC 6043-P80